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Friday, 13 November 2015

The lifestyle of Tanystropheus, part 1: was that neck too heavy for use on land?

Two Tanystropheus longobardicus tussle in Triassic Europe. There's a distinct lack of water supporting their necks in this scene, and some might suggest this makes such behaviour impossible for these animals. But does it? Read on...
One of the most famous non-dinosaurian denizens of the Mesozoic is Tanystropheus, a spectacularly long-necked reptile which lived across Europe and Asia in Middle-Late Triassic times. We've known about this 5-6 m long animal since fragmentary fossils were pulled from Italian Triassic rocks in 1855, and now regard it as a particularly large and anatomically extreme member of the Protorosauria. This is a Permian-Triassic group of archosauromorphs (all reptiles more closely related to crocodylians and birds that lizards) that spawned numerous aberrant taxa, such as drepanosaurs, Sharovipteryx and Dinocephalosaurus. Within Protorosauria, Tanystropheus can be considered a tanystropheid, closely related to similar, but shorter-necked and smaller-bodied species such as Tanytrachelos and Langobardisaurus. Tanystropheus longobardicus is by far the best known Tanystropheus representative, and the one we always think of when discussing this animal, but something like five Tanystropheus species have been named over the years. It is currently uncertain how many of these should be considered valid and, of those, which ones truly represent Tanystropheus and not some other type of protorosaur. There are hints that longobardicus might be the sole representative of this genus, but work on this is ongoing.

We know a lot about Tanystropheus because it's fossils are not uncommon, and many of them are complete or every nearly so. Its remains occur in Alpine Europe, the Middle East and China and we can conclude that, weird as it seems, the Tanystropheus bauplan and life strategy was a successful one. But exactly what that strategy was remains a bone of contention for palaeontologists. Summarised simply, opinion is divided over whether Tanystropheus was confined to aquatic habits, at least above a certain age and body size, or else capable of living terrestrially as a shore-patrolling, 'animated fishing rod'. Unsurprisingly, the principle source of this contention is its neck anatomy: clearly long and relatively stiff in life, was it so heavy that would over-balance the animal if not supported in water? Or was the neck anatomy not as heavy as commonly supposed and really no great hindrance to life on land? Other aspects of Tanystropheus form have also influenced this debate, including limb structure and tail anatomy, but it seems fair to say these discussions persist because experts disagree about the significance of that crazy neck. Renesto (2005) and Nosotti (2007) provide recent overviews and contributions to this long-running controversy.

I've been wanting to cover Tanystropheus lifestyle here for some time now, and I've ended up with sufficient material to spread discussion over two posts. In the next article I'll be discussing nuances of arguments for aquatic and terrestrial habits, but, first, I want to satisfy some personal curiosity over how Tanystropheus was constructed. Specifically, I'm interested in the mass distribution of this animal: is it really _that_ front heavy? There are plenty of terrestrial animals with very long necks - sauropods, giraffes, some pterosaurs - and we don't worry about them toppling over. Moreover, although neck mass is frequently mentioned as critical to understanding Tanystropheus lifestyle, to my knowledge, there isn't any information available on its body volumes or mass (if I'm wrong, please tell me below). I thought I'd see what I could find out about this myself using the GDI (Graphic Double Integration) method of volumetric mass estimation, a quick and easy way to get a sense of mass and body fractions of fossil animals. It basically involves chopping up drawings of animals to determine volumes of body segments, then multiplying these by a suitable density - check out this excellent SV:POW! summary for a full lowdown.

Tanystropheus longobardicus as reconstructed by Rupert Wild in 1973. Image borrowed from Palaeos.
GDI methods require a clear layout of animal form to measure and divide into segments. There is no shortage of life restorations of Tanystropheus out there, and plenty of photographs of near complete specimens, but objective, modern portrayals of its anatomy are hard to come by. Rupert Wild's skeletal reconstruction from the 1970s (above) seems to remain a common frame of reference, and a David Peters reconstruction is sometimes used as an alternative. Neither really seemed suited for my purposes here, the crouched poses obscuring anatomical details, some specifics of vertebral count being inaccurate to modern interpretations, and the latter being produced with techniques of questionable reliability. I decided to try my hand at producing a new skeletal reconstruction based on the large, near complete Tanystropheus skeleton described in detail by Rieppel et al. (2010): GMPKU-P-1527:

Tanystropheus cf. longobardicus specimen GMPKU-P-1527, as depicted by Rieppel et al. (2010).
I wanted a large animal because the Tanystropheus neck seems to increase in length disproportionately to body size (Tschanz 1988). I want to give this animal the best chance of falling over, so it makes sense to use the largest neck possible. GMPKU-P-1527 is articulated and includes most of the neck, missing only the relatively short anterior 3.5 vertebrae (of 13), the skull, and the end of the tail. I reconstructed these missing parts using smaller Tanystropheus specimens (from Nosotti 2007) and Wild's widely-used 'adult' skull reconstruction. These came together to form a skeleton measuring 3.5 m as reconstructed, and likely over 4 m if the vertebral column were completely straightened. This is not as large as we think this animal could get, but is c. 70% of maximum size, and the minimal amount of proportional inference and cross scaling means we should be looking at a fairly authentic image of Tanystropheus form. The results are below.

Tanystropheus cf. longobardicus skeletal reconstruction, almost entirely based on GMPKU-P-1527. See text and illustration below for details on which bits are borrowed from other specimens.
The length of the limbs here is quite striking. Note that they aren't depicted in a true sprawling pose, because foreshortening would impact measurements for the mass calculation, but I depicted a crouched pose which hopefully conveys something of a low, sprawling gait. I also followed Nosotti's (2007) suggestion of digitigrady, which boosts the standing height a bit. Despite the low pose, I immediately get a different vibe from this image to that of Wild's classic, sitting reconstruction. Simply putting the animal on its feet gives the impression of the limbs and body being more proportionate to the neck. The arc of the neck follows that preserved in GMPKU-P-1527 quite closely, a pose also occurring in several other articulated Tanystropheus specimens. As depicted, I don't think it conflicts with recent interpretations of Tanystropheus neck arthrology (e.g. Renesto 2005). The body outline should be non-controversial, pretty much following the outline of the skeleton and hitting major muscle landmarks.

Time to chop this guy and up see what it's made of. Ideally, we'd want full orthographic views for a GDI mass estimate, but I've not had time to produce a multi-view skeletal. This means we're going to have to make predictions of body width. For the neck, body and tail, I decided to calculate width as 2/3 of body segment height, this being indicated by the proportions of Tanystropheus neck and tail verts, and the fact the dorsal ribs straighten out as they approach the lateral margins of the body. The 2/3 figure is a little arbitrary and arm-wavy, but seems more precise than assuming a circular cross section across the entire body. Other elements - the head and limbs - were modelled as having circular cross sections, however. You can see how I chopped the reconstruction up below: note that this uses an earlier, differently posed version of the skeletal shown above, and that the limbs are somewhat straighter. The bone sizes are no different, however, so influences on mass estimation should be negliable.

GDI mass estimation on Tanystropheus cf. longobardicus. Grey portions of the skeleton show which parts were modelled on other specimens. Numbers in parentheses give mass fractions for each body component, and the grey shapes indicate the cross-sectional shape used for that part of the body.
The entire animal shakes out to 26.7 L, and using a middle-of-the-road reptile density of 0.85 kg/L, the animal masses 22.9 kg. Of more interest to us is the mass percentages of each component, which are indicated in parentheses in the illustration above. You can see that the neck and head together are a hair away from 20% of the body mass, despite accounting for something like half the length of the animal. Virtually all of this 20% represents neck, of course, the head being less than 1% of the overall mass. As is usual for tetrapods, the trunk volume dominates all, being 50% of overall bulk despite only just exceeding 50 cm long in a 3.5 m long animal.

What do these figures actually mean? 20% doesn't seem like that much in the grand scheme of things, it being balanced by the other 80% of the body. These are certainly not values which make me think this animal perpetually toppled over unless it was in water. But can we be more precise here - how does this neck fraction stand up to other long-necked animals? For brachiosaurid sauropods, Mike Taylor (2009) suggested the neck accounted for 14% of the body mass, while Don Henderson (2010) suggested 8% for the same animals, noting that this was the largest neck mass fraction in his dataset of 10 volumetric sauropod mass predictions. Mitchell et al. (2013) did not report exact head and neck mass fractions for a large set of giraffes, but eyeballing their data suggests male giraffe necks and heads account for around 14% of body mass, with females slightly less than that. These are all significantly lower than the 20% I've estimated for Tanystropheus, implying that my gut feeling might be wrong: maybe it did have quite a heavy neck and, perhaps, was at greater risk of overbalancing.

However, it strikes me that giraffes and sauropods are not particularly good analogues for Tanystropheus, because their anatomy is built around a fundamentally different set of demands: processing of plant material. Herbivores need large guts to get the most from their nutrient-poor diet, equating to proportionally larger trunk volumes. Anyone who's played with volumetric mass estimations will know that even slight adjustments to trunk proportions can have a big impact on absolute mass and tissue fractions because they represent the biggest components of most animal bodies. We therefore can't ignore the requirement for herbivore torsos to be large when comparing them to non-herbivores like Tanystropheus. Our problem here is that finding a long-necked terrestrial carnivore to compare with Tanystropheus is challenging. Such body plans have been rare throughout geological time and are entirely unrepresented nowadays. We're not entirely licked, though: following the laws of monster movie science, any challenge involving a poorly understood, freakish creature is best solved with another poorly understood and freakish animal: in this case a long-necked azhdarchid pterosaur. Azhdarchid palaeoecology has a history of contention and controversy, but no-one believes that they were aquatic animals, or herbivorous, or at risk of toppling forward without environmental aid. This is despite azhdarchids bearing neck/trunk proportions similar to those of Tanystropheus, as well as much larger heads. We just assume they could carry their heads and necks one way or another, because all indications are that they were not adapted for an aquatic existence.

Taking a GDI approach to the Zhejiangopterus linhaiensis skeletal I produced earlier this year, I attempted to gather some data on azhdarchid body volumes and masses. As before, I estimated widths rather than producing full orthogonal views. The head and neck were assumed to be half as wide as tall, with the neck widths not permitted to exceed those of the skull. All other elements are treated as having circular cross sections. Azhdarchid torsos, forelimbs and necks are all highly pneumatised, so I gave these low tissue densities of 0.7 kg/L (about the lowest density recorded for modern birds), while the legs were given a more typical density of 0.85 kg/L. The breakdown and results:

Zhejiangopterus linhaiensis gets the GDI treatment. As above, numbers in parentheses indicate mass fractions, and the grey shapes indicate cross sectional area used in the calculations. Skeletal based on data in Cai and Wei (1994).
First things first, I was happy to see the animal come out at 7.9 kg - that's in line with most post-2000 interpretations of pterosaur mass, and seems about right for an animal with a 2.5-3 m wingspan. That makes me think the constituent volumes and masses are probably in a sensible ball park. In terms of body component masses, the torso - famously small in derived pterodactyloids like azhdarchids - provides less than 25%, the neck is just over 25%, and the head and paired forelimbs are 22% each. The legs account for just under 6%, and the tail might as well not exist. This puts - wow - almost 50% of the mass in front of the shoulders in this reconstruction. But even accepting that I've been generous with neck tissue in my reconstruction (following a reptilian, rather than avian pattern of neck musculature), and that some cross sectional shapes used here could be refined, it seems unlikely we could slim the head and neck tissues down to the mass fractions seen in long-necked herbivores. Even if my estimates are out by a factor of 2, the neck and head will still account for more mass than the same components in Tanystropheus. This finding makes the neck tissue fraction of the Tanystropheus model look a lot less aberrant, as well as verifying the suspicion that lifestyles, and not just anatomy, are important factors when comparing animal bauplans.

Let's bring all this together. While the sums outlined here are provisional, back-of-the-envelope-type stuff, I find them sufficient to at least make me sceptical of claims that Tanystropheus has a terrestrially-untenable mass distribution. At least one group of non-aquatic Mesozoic carnivores seem more front heavy, and a basic model of Tanystropheus mass distribution does not raise major alarm bells about relative neck and head weight. I could be convinced otherwise, and obviously there's a lot more than could - and should - be done to investigate this issue, but I currently don't see neck mass as a significant barrier to terrestrial habits. This exercise has also brought home the fact that we might not know much about the adaptive and structural significance of extremely long necks in carnivorous animals, and that we should be careful comparing them to other long-necked creatures. Perhaps our unfamiliarity with this extinct bauplan, along with our generally poor intuitive sense of animal mass and tissue fractions (see this discussion and comment field at SV:POW!), means we should be extra cautious about gut-feeling interpretations of such creatures. I guess the bottom line is that running numbers to test our intuitions is an essential part of understanding unfamiliar animal types, especially if we're suggesting those assumptions are significant for extinct animal behaviour and lifestyle.

There'll be more on Tanystropheus in the next post, where the plan is to review recent arguments for and against different lifestyles in this animal. In the mean time, I'm very curious to know what others make of the ideas presented here. Would you interpret these results differently? Would you have reconstructed Tanystropheus in a different way? The comment field is open...

Tanystropheus was brought to you by Patreon, a weekend of downtime and the letter 'S'

I've really enjoyed putting this research-led post together, and would like to do more of this sort of stuff in future. Obviously they take a little longer than 'routine' posts but, thankfully, this time investment is possible because of my patrons, individuals who support this site via my Patreon page. From $1 a month you can be a patron too: this gets you access to cool, exclusive content and rewards, and helps me make my art and writing more detailed and interesting. Thanks to all those who've signed on already - your contributions are really appreciated.


  • Cai, Z., and Wei, F. (1994). "On a new pterosaur (Zhejiangopterus linhaiensis gen. et sp. nov.) from Upper Cretaceous in Linhai, Zhejiang, China." Vertebrata Palasiatica, 32: 181-194.
  • Henderson, D. M. (2004). Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits. Proceedings of the Royal Society of London B: Biological Sciences, 271(Suppl 4), S180-S183.
  • Mitchell, G., Roberts, D., Sittert, S., & Skinner, J. D. (2013). Growth patterns and masses of the heads and necks of male and female giraffes. Journal of Zoology, 290(1), 49-57.
  • Nosotti, S. (2007). Tanystropheus Longobardicus (Reptilia, Protorosauria): Re-interpretations of the Anatomy Based on New Specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Società Italiana di Scienze Naturali e Museo Civico di Storia Naturale.
  • Renesto, S. I. L. V. I. O. (2005). A new specimen of Tanystropheus (Reptilia, Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus. Rivista Italiana di Paleontologia e Stratigrafia, 111(3), 377-394.
  • Rieppel, O., Jiang, D. Y., Fraser, N. C., Hao, W. C., Motani, R., Sun, Y. L., & Sun, Z. Y. (2010). Tanystropheus cf. T. longobardicus from the early Late Triassic of Guizhou Province, southwestern China. Journal of Vertebrate Paleontology, 30(4), 1082-1089.
  • Taylor, M. P. (2009). A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of vertebrate Paleontology, 29(3), 787-806.
  • Tschanz, K. A. R. L. (1988). Allometry and heterochrony in the growth of the neck of Triassic prolacertiform reptiles. Palaeontology, 31(4), 997-1011.

Thursday, 22 October 2015

The Spinosaurus saga continues

A year after the 'Spinosaurus reboot' as a small-legged, early whale-mimicking aquatic quadruped, experts remain divided over fundamental aspects of Spinosaurus palaeobiology. This depiction shows Spinosaurus aegyptiacus as generally imagined prior to 2014.

The long, tragic and occasionally controversial research history of the giant, enigmatic theropod Spinosaurus aegyptiacus will be familiar to many readers of this blog*. First named and described in the early 20th century by Ernst Stromer from remains found in Late Cretaceous strata of Egypt, our principle Spinosaurus material fell victim to Allied bombing raids in World War II and was completely destroyed. Stromer's detailed illustrations and descriptions are all that remains of this material, and these have formed a variably interpreted foundation of all subsequent Spinosaurus research. For much of the 20th century the life appearance of Spinosaurus remained mysterious. Depicted as a nondescript sailed giant theropod early on, discovery of well represented spinosaurids like Baryonyx and Suchomimus, as well as fragments of new Spinosaurus material, permitted more confident interpretations of Spinosaurus size and form as we approached the new millennium. By the 2010s, Spinosaurus was recognised as a gigantic, derived and perhaps semi-aquatic spinosaurid, adapted for feeding on large aquatic prey (above). Much of this interpretation relied on new Spinosaurus remains from multiple locations in northern Africa, including the famous Moroccan Kem Kem Beds, an expanse of Late Cretaceous rocks roughly contemporaneous with those Egyptian deposits yielding the original, destroyed Spinosaurus remains.

*For succinct overviews of Spinosaurus research prior to 2014, check out posts at Tetrapod Zoology and Laelaps.

Famously, last year saw Spinosaurus reinvented again, this time as a quadrupedal, knuckle-walking, long-bodied, tiny-legged dinosaurian take on a crocodile or early whale (below). The authors of this widely publicised study, Nizar Ibrahim and colleagues (2014), synthesised existing and new data on African spinosaurids to create this reconstruction, synonymising several taxa into S. aegyptiacus and presenting new Spinosaurus remains obtained from the Kem Kem beds. The most significant of these was a set of associated vertebrae, pelvic and hindlimb remains which were proposed as a neotype specimen for Spinosaurus (a specimen to hold the Spinosaurus name now that the original material is lost to science). That this neotype represents Spinosaurus was bolstered by it bearing similar hindlimb and vertebral proportions to 'Spinosaurus B', a collection of Egyptian spinosaurid specimens described by Stromer, considered referable to Sp. aegyptiacus by Ibrahim and colleagues. Spinosaurus B is also now lost, also being destroyed in WWII. The Ibrahim et al. study provided a lot of new data on Spinosaurus and has helped cement the concept of it being a semi-aquatic animal, but several aspects of the paper didn't meet the warmest reception from a number of academics. Specific issues were scaling of the skeletal components, how sensible it was to lump so much north African spinosaurid material into one species, and uncertainty about the provenance of the neotype specimen. Some of these concerns were diffused by the authors, but we await a promised monograph for answers to all the questions raised by their first paper. In the mean time, the 2014 Spinosaurus interpretation remains a debated topic among those interested in dinosaur palaeontology.

The Ibrahim et al. (2014) take on Spinosaurus aegyptiacus. Different colours represent different specimens: red is the neotype; brown is the original Spinosaurus material; yellow is referred, isolated Spinosaurus remains; green bones are borrowed from other spinosaurids, and blue bones are crafted to fit the skeleton based on neighbouring elements. Image borrowed from

One year later...

This week, the Spinosaurus tale has taken another twist with publication of a mammoth (open access) paper penned by a team of European spinosaurid experts, led by Serjoscha Evers. Evers et al. have reappraised the affinities of Moroccan specimens seemingly related to Spinosaurus: Sigilmassasaurus brevicollis and Spinosaurus maroccanus. These animals, known only from vertebrae, were subsumed into Sp. aegyptiacus by Ibrahim et al. (2014) as part of their trans-African Spinosaurus concept, and that decision is a core focus of the Evers et al. paper. Their work contains extensive commentary on the detailed anatomy of Moroccan spinosaur material and what it might mean for recent interpretations of Spinosaurus form and lifestyle. Given the wide interest in Spinosaurus and the 2014 reconstruction, I thought it might be of interest to summarise some of what they outline here.

Firstly, taxonomic revisions proposed by Evers et al. present a very different picture of what fossils we can identify as belonging to Spinosaurus. Their work on Si. brevicollis and Sp. maroccanus suggests these species are probably one and the same (the latter being sunk into the former), and that Sigilmassasaurus should be considered distinct from Sp. aegyptiacus. They go on to suggest that other Kem Kem vertebrae hint at a second spinosaurid species in the Kem Kem fauna, and outline several reasons why the Ibrahim et al. 'neotype' specimen cannot be referred to Spinosaurus. For one, the neotype is anatomically quite different from Stromer's Egyptian 'Spinosaurus B' specimen. Ibrahim et al. considered Spinosaurus B as representing Sp. aegyptiacus, but Evers and colleagues argue that Spinosaurus B is anatomically more similar to Sigilmassasaurus than Spinosaurus. Spinosaurus B therefore might have no use for linking any specimens specifically to Sp. aegyptiacus, including that all-important neotype.

In addition to these morphological objections, Evers et al, also raise palaeobiogeographic issues with the 'neotype' referral. Evidence for Egyptian dinosaur species being present in Morocco is scant at best, most data indicating little mixing of eastern and western African dinosaur species during the Late Cretaceous. It would be unusual, then, to find the Egyptian species Sp. aegyptiacus in Morocco. Palaeobiogeography is not a deal clincher for taxonomy of course - careful examination of the neotype and genuine Spinosaurus remains will be the deciding factor here - but it is another stick in the mud for the neotype proposal. Although the exact identity of the 'neotype' specimen is left in the air by Evers et al. - ongoing descriptive work on the specimen needs to be completed to truly assess this - they reject the proposal of the Kem Kem specimen as a Sp. aegyptiacus neotype, and leave Spinosaurus characterised by features in Stromer's illustrations. This is obviously quite a shake up of the suggestions made last year: Spinosaurus 2014 might be a mix of at least two named species, incorporate material of under-appreciated taxonomic importance, and substantial, newly published material might have little, if anything, to do with Spinosaurus.

The proposed Spinosaurus neotype. Image borrowed from Andrea Cau's excellent Theropoda blog.

Moving on, Evers et al. also raise concerns about interpretations of Spinosaurus in context of Kem Kem fossil collecting practises. Museum exhibitions and PR exercises suggest that the Kem Kem yields complete skeletons of dinosaurs and other fossil vertebrates, but the reality is quite the opposite. Kem Kem vertebrates are typically preserved as isolated, often broken bones in multitaxic bone beds (that is, bone beds comprising many species). Associated skeletons of single individuals do occur, but they're relatively rare and rely on precise collecting documentation to prove their authenticity. Unfortunately, historic and recent records of Spinosaurus occurrences and excavation are often poor. We might chalk a lack of historic documentation to the practises and technological limitations of bygone times, but recent issues are caused primarily by the commercial value of Kem Kem fossils. The greater majority of Kem Kem fossils, including dinosaurs, are collected without extensive documentation and then sold by private dealers. Even if localities are recorded, ambiguity often surrounds association of fossil material prior to excavation. Several alleged associated Spinosaurus specimens are meant to have come from single localities, but being from the same place is really only half the battle if they stemmed from multitaxic assemblages. Concordant size of bones might suggest genuine association, but this is not always certain either: Evers et al. report practises where collectors sort loose material from disparate locations into type and size categories before sale - nefarious individuals making fossil skeletons more substantial with unassociated elements is a real problem the world over. It's sad but true that the monetary value associated with substantial vertebrate fossils makes ascertaining their authenticity crucial for subsequent credible interpretation.

Unfortunately, Evers et al. report these factors as affecting virtually all associated Spinosaurus material, including the 'neotype' and the other specimen key to the 2014 reconstruction, Spinosaurus B. In the case of the latter, all we have to go on to establish association are Stromer's notes, which are not quite as detailed as we might like. For the neotype, we know some of the specimen was directly collected in the field, and that other bits were purchased from dealers by two academic institutions over a two year period - exact documentation of this remains to be presented (hopefully it will in the 'neotype' monograph). Without strict certainty over how many individuals these specimens might represent, Evers et al. suggest some of the odd proportions in recent Spinosaurus reconstructions may reflect the marrying of mismatched bones to one another. That's not a certainty, of course, but it's also something which shouldn't be casually ignored.

Collectively, Evers et al. use these points to provide an alternative take on Spinosaurus to that presented in 2014. Ibrahim et al. argued that their new material helped simplify and integrate different interpretations of African spinosaurid material, but Evers et al. argue the opposite: they emphasise how poorly known Spinosaurus and kin are, and how interpreting fossils of north African spinosaurids is getting increasingly complex. Spinosaurus fossils remain very fragmentary to the point where most cannot be directly compared, they seem to hint at, but don't really crystalise, an apparent high species diversity, and are often of uncertain association or exact origin. At face value, that doesn't leave us with a lot to be confident about, although we'll have to see how this more despondent view goes down with other spinosaurid researchers. More complete and well documented discoveries will soon help smooth out bumps in our knowledge, but it seems likely that a lot of work and discussion remains to sort out what is really going on with north African, Late Cretaceous spinosaurids.

What does this mean for 'the Spinosaurus reboot'?

That's not quite the end of our discussion, however. It might be assumed that the points outlined above sound the death knell for the strangely proportioned 2014 Spinosaurus reconstruction, and that we should go back to our traditional interpretation of this animal. That might not be quite right, for two reasons. Firstly, given how distinctive many 'Spinosaurus' remains now seem to be, it's actually questionable what specimens should be considered Sp. aegyptiacus at all, other than the first specimen described by Stromer. A lot of referred isolated Spinosaurus specimens have been incorporated into our 'traditional' reconstructions in recent years, and we might need to think hard about their role in our interpretations of this animal. What we've become typically used to thinking of as Spinosaurus may not entirely be Spinosaurus!

Secondly, while some aspects of the 2014 interpretation of Spinosaurus have clearly been challenged by the Evers et al. paper, not all proportional aspects of the recent Spinosaurus reinvention are obviously erroneous. Last year, Ibrahim et al. noted that both Spinosaurus B and the 'neotype' have reduced hindlimbs with respect to their associated vertebrae, and used this fact as support for the diminutive legs in their reconstruction. Although arguing that there is no longer evidence for short hindlimbs in Spinosaurus itself, Evers et al. don't completely dismiss the notion of some African spinosaurids being short legged. The hindlimb proportions of those specimens is very similar despite the vagaries surrounding fossilisation and exhumation of ancient animal remains, maybe more similar than you'd expect from chance alone. If it is coincidence, it's certainly a startling one.

Stromer's 'Spinosaurus B' material: proportionally similar to the 'neotype' specimen, but does that tell us anything about spinosaurid proportions? Another image borrowed from Theropoda.
However, Evers et al. also attach some important caveats to this point. Stromer's notes clearly state that he did not consider the 'Spinosaurus B' material to represent one individual, and his testimony is the closest thing we have to a report on the excavation of the material. He specifically comments on the hindlimb being too small and slender to match the vertebrae, and thus interpreted them as representing a second individual. Other workers have agreed that this material must represent multiple animals or even several types of dinosaur (discussions about the possibly chimeric nature of Stromer's spinosaur specimens are not new - e.g. Rauhut 2003; Novas et al. 2005). Interpretation of the Spinosaurus B material as representing one animal is thus against some current thought and, of course, Stromer's original declaration. While the 'neotype' specimen might make a case for Stromer being mistaken, we really need to know more about the collection history to ascertain that. We're left with an intriguing set of measurements hinting at the reduced hindlimbs proposed by Ibrahim et al., but little in the way of objective information to explain their significance. The discovery of new specimens is needed to establish whether some spinosaurids were really short-legged, or if confusion of specimen inventories just made it look that way. In short, and no-doubt to the disdain of people who lose sleep about 'what science has done' to one of their favourite theropods, there's still something to play for with these short-legged spinosaurids.

So that's the latest chapter of research in Spinosaurus, then: I don't doubt that it's going to cause a lot of discussion in popular and academic circles. My personal take-home is that we seem to know less about Spinosaurus than might have been recently suggested, or at least that some issues need to be ironed out before we can develop a clear picture of what Spinosaurus is, and what sort of lifestyle it led. I don't know that any recent proposals about this animal have been shot down entirely yet, although clear gauntlets have been established for some of the more extreme ideas suggested in the last few years. It's going to be very interesting to see how others interpret these latest developments in the ongoing Spinosaurus saga, and where our understanding of this animal moves to next.

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  • Evers, S. W., Rauhut, O. W. M, Milner, A. C, McFeeters, B, & Allain R. (2015) A reappraisal of the morphology and systematic position of the theropod dinosaur Sigilmassasaurus from the “middle” Cretaceous of Morocco. PeerJ 3:e1323
  • Ibrahim, N., Sereno, P. C., Dal Sasso, C., Maganuco, S., Fabbri, M., Martill, D. M., & Zouhri, S., Myhrvold, N. and Iurino, D. A. (2014). Semiaquatic adaptations in a giant predatory dinosaur. Science, 345(6204), 1613-1616.
  • Novas, F., Dalla Vecchia, F., & Pais, D. (2005). Theropod pedal unguals from the Late Cretaceous (Cenomanian) of Morocco, Africa. Revista del Museo Argentino de Ciencias Naturales nueva serie, 7(2), 167-175.
  • Rauhut, O. W. M. (2003). Special Papers in Palaeontology, The Interrelationships and Evolution of Basal Theropod Dinosaurs (No. 69). Blackwell Publishing.

Saturday, 10 October 2015

We just can't quit you, Pterodactylus

A small flock of Pterodactylus antiquus, represented by small juveniles (left) up to big adults (right) scope out foraging options in a Jurassic marsh. The animal on the right is luring prey to the surface through paddling forefeet, a behaviour common to (at least) several modern gull species.
Pterosaur researchers are infamous for their frequent disagreements over flying reptile evolution, lifestyles and even basic anatomical interpretation. I can certainly attest that there is some truth to this: writing about pterosaurs can be frustrating because of continual need for clarification and digressions to ensure all points of view are represented. But if there's one pterosaur we must have reached a consensus over, one species we must all agree about, surely it's the Late Jurassic, Solnhofen species Pterodactylus antiquus. The holotype of Pterodactylus - below - has been known to us longer than any other pterosaur fossil, this being the specimen which kick-started flying reptile studies in 1780. Since then, it's almost a rite of passage for researchers to see this specimen along with some of the other several dozen Pterodactylus fossils in museums around the world. Pterodactylus has been looked at so much that a general agreement over what it is, and the species it is related to, has more or less been established. Characterised by a long, low skull, simple teeth, a long neck and largish feet, we consider Pterodactylus a relatively early form of pterodactyloid, and most likely a member of the ctenochasmatoid/ archaeopterodactyloid* branch of pterosaur evolution. This puts it in the same lineage as several other familiar species, such as comb-toothed Ctenochasma and twisty-jawed weirdo Cycnorhamphus. We're all basically happy with the idea that our dataset for Pterodactylus is fairly good: 30 specimens (a very conservative estimate) provide numerous complete skeletons and a growth series from small juveniles up to very large adults.

*As with many parts of the pterosaur tree, nomenclature for 'Pterodactylus-line' pterodactyloids is confused by the use of several, conflicting names and definitions. I wasn't kidding about those caveats and digressions.

All that said, some areas of Pterodactylus research remain contentious, and new insights into its anatomy and disparity are still being published more than 200 years after it was discovered. Surprisingly, unlike the way we often gain novel appreciation for familiar taxa - new specimens shedding new light on old problems - much of our recent understanding of Pterodactylus relies on the same, well-worn specimens we've been analysing for centuries. It's actually quite sobering to see specimens which have been interrogated so much still providing talking points, and it makes me wonder what we're missing from those briefly described, rarely analysed specimens comprising so much of our vertebrate fossil dataset. Here, I want to cover some of the new insights provided on Pterodactylus in just the last two years.

The specimen which started it all: the Pterodactylus antiquus holotype. The wingspan of this specimen, preserved in a 'falling forward' posture rather atypical for a Solnhofen pterodactyloid, is about 45 cm.

Anatomy and palaeobiology

Until recently, most of us have been used to seeing Pterodactylus depicted as a crestless species. A privately owned specimen described in 2003 showed that, like many other pterosaurs, this animal bore a set of soft-tissue structures associated with the top and back of the skull (Frey et al. 2003). Specifically, it seems Pterodactylus bore a soft-tissue crest along the posterior region of the head and a pointed, posteriorly-projecting 'occipital lappet' at the back of the skull. The latter, for now at least, seems unique to Pterodactylus. This information is well known to 21st century scholars, but it's less appreciated that these soft tissues were first mentioned almost 100 years ago. Pterodactylus crests were first reported in the 1920s, and the lappets in 1970 (see Bennett 2013). I find it bizarre that we didn't start restoring Pterodactylus with these interesting structures until the 2000s: this seems to be an example of artists and scientists not working together as well as they might.

Unlike other pterosaurs, the soft-tissue crest of Pterodactylus did not seem to anchor on a low, striated bony ridge. The absence of this feature, even when preservation was sublime enough to record soft-tissues and detection methods were of late 20th century quality, was likely a key factor in our general consideration of Pterodactylus as a crestless species. I always found the occurrence of soft-tissue crests without corresponding bony structures an alarming prospect, one implication being that we could be ignorant of soft-tissue crests in a huge number of pterosaur species.

It was somewhat relieving, therefore, to see Chris Bennett reporting a crest-anchoring structure for Pterodactylus in 2013. It's small, and often smooth rather than striated, but Pterodactylus definitely does have a midline ridge for crest anchorage - even on the holotype has one when we look close enough. Exactly how extensive these structures were remains unknown thanks to many historic specimens being accidentally damaged during preparation. It's easy to see how this occurred: the crests are low, extremely fragile, and only 0.2 millimetre thick. They'd be hard to detect and avoid damaging even if you were looking for them. Hopefully, preparators working on unprepared specimens can recover intact crests now we know they exist.

The most extensive example of cranial soft tissues known thus far from Pterodactylus. Unfortunately, we're still some way from knowing what shape they took in life, although this specimen indicates that almost half of the skull was covered by the crest and that the lappet was also quite large. Parts of the diagram labelled 'fa' record sediments which fluoresce under UV light - they're likely matrix contaminated by organic seepage from the decaying pterosaur head. They are unlikely to tell us much about the appearance of the animal in life. From Bennett (2013).
Pterodactylus cranial soft tissues are now known to occur in a number of specimens, but it remains unclear how large or what shape the crests were. The lappets seem to be of a consistent size and position, and many curve upwards, but whether they are joined to the rest of the crest (as suggested by Frey et al. 2003) remains to be confirmed (Bennett 2013). At least some aspect of crest and lappet development matches what we see in other pterosaurs, in that we only start picking up evidence of these structures in larger Pterodactylus specimens. There also seems to be a rough correlation between crest proportions and body size. Pterodactylus thus seems to be another pterosaur species where cranial ornament signifies entry into adulthood, suggesting a function of sexual communication (Bennett 2013).

Speaking of adulthood, it was also only recently that we've obtained a true sense of how large Pterodactylus may have grown. We typically imagine this animal as small bodied - maybe with a 50 cm wingspan - but a newly described skull and lower jaw (below) makes the first unambiguous case for Pterodactylus reaching at least 1 m across the wings (Bennett 2013). To put this in a modern context, large Pterodactylus would be of comparable size to smaller heron species, and large individuals would have been conspicuous components of the Solnhofen pterosaur fauna. A trend where skull, neck, and limb proportions increase with body size, first intimated by Peter Wellnhofer (1970), seems to hold up in modern interpretations of Pterodactylus specimens. Realising how variable this pterosaur's proportions might have been throughout life has been very informative to recent considerations of Pterodactylus taxonomy.

The mother of all Pterodactylus skulls. A preserved skull length of 142 mm indicates a skull of around 200 mm long in life, and an animal reaching a 1 m wingspan. From Bennett (2013).

One species, two species, or three genera?

This brings us to some of the more contentious recent developments in Pterodactylus studies: just how many species are represented in the Pterodactylus dataset? Many readers will be aware that the name Pterodactylus was once applied to almost any new pterosaur fossil, and around 80 'Pterodactylus' species have existed in the last 200 years (Ford 2013). The taxonomic history of Solnhofen pterodactyloids has been especially mixed up with the name Pterodactylus and, by the end of the 1800s, their taxonomy was in a real tangle. Work in the mid-20th century, particularly by Peter Wellnhofer (1970), streamlined systematic interpretations of Pterodactylus so that, by the 2000s, only two species were considered valid: P. antiquus and P. kochi. A couple of 'hangers on' were still knocking about ('P'. longicollum and 'P'. micronyx), but researchers universally agreed that these animals were not true members of Pterodactylus, and were simply awaiting new generic names (they now have them: Ardeadactylus and Aurorazhdarcho, respectively).

Distinguishing features between kochi and antiquus were subtle, being primarily aspects of tooth shape, tooth number, and proportions of the skull, neck and torso. This is not a new observation, and suggestions that they may represent the same taxon date back to the 1800s. Eventually, studies of Pterodactylus teeth was used to suggest outright synonymy of these two species (Jouve 2004). Many pterosaurs, as with most reptiles, increase their tooth counts with age and size. Jouve realised that the allegedly distinctive tooth count of P. kochi aligned perfectly with tooth numbers predicted for antiquus of comparable body size. At least in this respect, these two species could not be distinguished. More recently, Bennett (2013) bolstered this synonymy with an assessment of kochi proportions, noting that perceived distinctions in skull and body length were reliant on erroneously recorded measurements. Once corrected, kochi proportions were very similar to comparably sized antiquus individuals (there's a lesson there about the importance, and repetition, of basic data recording in this) and, along with Jouve's work, this study have eroded the foundations of the kochi/antiquus split considerably. Remaining distinguishing features between these species are rather poorly defined, and certainly not divorceable from effects of growth, preservation and preparation. Finally, after 200 years, it was looking like Pterodactylus taxonomy had finally been tidied up: we have one Pterodactylus species, not two, or 80.

Historically considered to represent Pterodactylus antiquus, recent work argues this specimen (along with some referred material) is a wholly distinct species, and distantly related to P. antiquus. It was recently christened Aerodactylus scolopaciceps. Image from Vidovic and Martill 2014 (this particular version from Steve Vidovic's Mesozoic Monsters blog).

Except... the story doesn't end there. Last year, my University of Portsmouth colleagues Steven Vidovic and David Martill suggested that not only were 'cryptic taxa' present in the Solnhofen Pterodactylus dataset, but that the traditional phylogenetic placement of some Pterodactylus-like animals might be erroneous. Using a variety of methods, Steve and Dave proposed that Pterodactylus contained at least three taxa: antiquus (which they considered the only true member of the genus), kochi (a separate genus in their interpretation, and more closely related to other pterodactyloids than antiquus), and a resurrected Pterodactylus species from the 1800s, scolopaciceps (Vidovic and Martill 2014, see image, above). Steve and Dave created the generic name Aerodactylus for this animal, providing a diagnostic combination of over 10 character states relating to skull shape and proportions, orbit shape, tooth count, neck length, humeral curvature and limb bone robustness. Attempting to establish the relationships of these three 'Pterodactylus' taxa saw Ctenochasmatoidea/Archaeopterodactyloidea dissolve into a paraphyletic spread across the base of Pterodactyloidea. In this topology, antiquus and kochi anchor the base of Pterodactyloidea (without forming an exclusive clade themselves) and scolopaciceps is at the other end of the 'ctenochasmatoid' range, in a sister clade to the rest of Pterodactyloidea.

That's quite a shake up, contradicting virtually all other recently published opinions on the taxonomy and evolution of these animals. But although different, at least some of these ideas are not be untenable. For instance, the idea that Ctenochasmatoidea/Archaeopterodactyloidea might be paraphyletic is suggested by the 'Painten pro-pterodactyloid', an unusual pterosaur specimen revealed two years ago (below, Tischlinger and Frey 2013). This taxon, which shows a Pterodactylus-like skull combined with postcranial features somewhat like those of non-pterodactyloid pterosaurs, suggests aspects of 'ctenochasmatoid' anatomy developed outside of Pterodactyloidea proper. It therefore will not be that surprising if this taxon pulled some 'basal' ctenochasmatoids of traditional lore to the root of Pterodactyloidea once it's included in phylogenetic studies. (Those interested in the influence of the 'Painten pro-pterodactyloid' animal on our understanding of pterosaur evolution might find this previous post of interest).

The 'Painten Pro-pterodactyloid' specimen, messing up our nice, neat interpretation of pterodactyloid evolution since 2013. Notice the Pterodactylus-like posterior skull morphology alongside traditional non-pterodactyloid features - a long(ish) tail and big fifth toes. From Tischlinger and Frey (2013).
But do our few dozen Pterodactylus specimens really comprise three, distantly-related species? On this, I'm less certain. We see a lot of variation across Pterodactylus specimens reflecting those aforementioned factors of ontogeny, preservation and preparation - not to mention individual variation. Having played with Pterodactylus data a little myself, and seen a fair share of specimens relevant to these discussions (though I stress not all), I find the arguments for synonymy more compelling than those for splitting Pterodactylus apart. This said, I have no horse in this race and could be persuaded otherwise. What we really need - and a number of folks in pterosaur research have been saying this for a while now - is someone to travel the world exhaustively documenting and illustrating Pterodactylus specimens, ultimately producing a modern synthesis on its anatomy. Such a study would not only be a valuable research aid (the last attempt at this was 50 years ago, which is an age ago in terms of research and publication techniques), but would pack a lot of weight in resolving ongoing, long running disputes in this animal's taxonomy.

Talking about the future of research into Pterodactylus seems like a sensible place to leave off, and I'll summarise in saying that - as with much else in pterosaur research - we're a little while off a complete consensus on Pterodactylus for now. Clearly, although the concept of Pterodactylus is over two centuries old, there's still things learn about it. Who knows what we'll be saying about this most familiar of pterosaurs in years to come?

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  • Bennett, S. C. (2013). New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Paläontologische Zeitschrift, 87(2), 269-289.
  • Ford, T. L. (2013). Is Pterodactylus monophyletic or paraphyletic? Short Communications - International Symposium on Pterosaurs, Rio Ptero 2013. 68-70.
  • Frey, E., Tischlinger, H., Buchy, M. C., & Martill, D. M. (2003). New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. Geological Society, London, Special Publications, 217(1), 233-266.
  • Jouve, S. (2004). Description of the skull of a Ctenochasma (Pterosauria) from the latest Jurassic of eastern France, with a taxonomic revision of European Tithonian Pterodactyloidea. Journal of Vertebrate Paleontology, 24(3), 542-554.
  • Tischlinger H, Frey E. 2014. Ein neuer Pterosaurier mit Mosaikmerkmalen basaler und pterodactyoider Pterosaurier aus dem Ober-Kimmeridgium von Painen (Oberpfalz, Deutschland) [A new pterosaur with moasic characters of basal and pterodactyloid Pterosauria from the Upper Kimmeridgian of Painten (Upper Palatinate, Germany)]. Archaeopteryx 31, 1-13.
  • Vidovic, S. U., & Martill, D. M. (2014). Pterodactylus scolopaciceps Meyer, 1860 (Pterosauria, Pterodactyloidea) from the Upper Jurassic of Bavaria, Germany: the problem of cryptic pterosaur taxa in early ontogeny.
  • Wellnhofer, P. (1970). Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Bayerische Akademie der Wissenschaften, Mathematisch- Wissenschaftlichen Klasse, Abhandlugen, 141, 1-133.

Tuesday, 6 October 2015

New sauropodoramas: Stormy brachiosaurs! Apatosaurine brontosmash!

Realising that Recreating an Age of Reptiles was a bit light on sauropod art, I've been beavering away on two additional sauropodoramas* to pad things out a bit. I thought I'd share them here.

*Sauropods are such special animals that they deserve their own nomenclature for most things, including artwork. See, for another example, 'shards of excellence'.

The first is a reworking of a 2013 image of the Wealden (probable) brachiosaur Pelorosaurus conybeari in hammering wind and rain. We know that Wealden climates were subject to storms and intense downpours on occasion (lightning and floods being, of course, key elements in the production of fossil-rich plant debris horizons in certain Wealden deposits) and it stands to reason that any sauropods around when those rains arrived would have got quite wet indeed. I don't say that just casually: the prospects of being a wild animal the size of a house mean that you're actually pretty exposed to just about everything weather can throw at you. When unexpected meteorological fit hits the shan, your options as a giant are pretty limited. Running away is out, because your legs are pillar-like structures adapted for supporting immense weight, not nimble escape. Seeking shelter is not an option either, because you're bigger than everything else around you. You're just too darned huge to do anything but stand there and take it. The life of a sauropod must've been spent baking in the sun, being battered by wind, and drenched in rain. I find that idea quite romantic and evocative as an artist. When painting sauropods, I often wonder how cracked, weathered and worn their skin must've been through a lifetime of battles with changing weather.

Like masts in a storm, three Pelorosaurus conybeari brave typically English weather, c. 135 million years ago. They're doing their best to look tough next to a couple of rainbows.
Second is an image inspired by a recent SVPCA talk by sauropod expert Mike Taylor and his colleagues Matt Wedel, Darren Naish and Brian Engh. Regular readers of the palaeoblogosphere will probably already know where this is going, given that Mike's talk (and the upcoming Wedel et al. paper) has been given some hefty coverage at SV:POW!. Those familiar with sauropods will know that apatosaurines (Apatosaurus, Brontosaurus and a few other taxa) have atypically proportioned, large and robust neck vertebrae, with their cervical ribs being especially elongated and reinforced. These structures possess peculiar buttresses on their underside which, it seems, are not products of muscle or ligament attachment (if they are, they have no modern analogue). Instead, they might relate to an epidermal feature like a boss or horn, as such structures sometimes leave peculiar swellings on underlying bones. Exactly what these anatomies indicate has long been puzzling, and all the more so because all apatosaurines show neck vertebrae with these features. Some (like Brontosaurus) were more extreme than others in development of these features, but even modest apatosaurines were doing crazy, mysterious stuff with their neck anatomy. Question is, what?

Matt, Mike and others have recently been outlining a first principles approach to this conundrum. They note that the reinforced construction of apatosaurine necks, the additional muscle attachment afforded by vertebral expansion, and those strange vertebral buttresses might render their necks effective clubs or wrestling appendages, particularly well suited to rapid, powerful downward motions. Summarised a little more succinctly: there is reason to think Brontosaurus and kin might've smashed the crap out of each other, or other animals...

...with their necks.

Yowsers. But outlandish as the Brontosmash hypothesis seems, it really isn't just idle speculation: a paper is in the works, the Taylor et al. SVPCA talk abstract is a preprint at PeerJ, and you can see the case explained in Mike's talk slides here. I find it pretty convincing myself: I mean, there had to be some reason apatosaurines had those crazy necks. Evolution is a sloppy craftsman at times, but the energy put into growing and maintaining such massive neck anatomy must've been substantial, and that almost certainly reflects a certain adaptive purpose. Combat might well have been that driving force. We also know from living animals - camels, giraffes and some seals - that necks are used for fighting, and that neck-based combat can promote reinforcement and restructuring of neck anatomy. It certainly sounds provisionally convincing to me, and I'm sure we'll hear a lot more about it in the future as the hypothesis is developed.

We're also sure to see this concept frequently in future palaeoart. Mike has been collecting some of the early artwork of this idea over at SV:POW!, including a wealth of coloured sketches and concepts by Brontosmash coauthor and palaeoartist Brian Engh, palaeoartist Bob Nicholls, #MikeTaylorAwesomeDinoArt (the revolution palaeoart deserves, if not the one it needs) and an alternative interpretation of apatosaurine neck data provided by myself (we secretly know I'm on the money with that one). I also decided to attempt a full on painting:

Multiple tonnes of Brontosaurus excelsus in disagreement.
There're two nods to classic palaeoartists here. There's a Knightian influence to the style (not the first time he's infected my work), as well as, via the very upright postures of the wrestling animals, a hat-tip to Robert Bakker's famous 'boxing Brontosaurus' image. The latter had a big impact on me when I first saw it as a teenager, and it's been on my mind for obvious reasons with all this talk of fighting apatosaurines. I thought it also made for a bit of a contrast to Brian's 'official' depictions as well, these showing the animals in quadrupedal or near-quadrupedal poses (I assume at least some of the postures in those artworks mimic neck combat in elephant seals, a favoured modern behavioural analogue of Team Brontosmash). The setting is meant to be in the wetter, northern parts of the Morrison Formation palaeoenvironment, alongside swollen river margins. Initial plans were to record the progression of the wrestling match in muddy footprints, but adding splashes and visual noise to proceedings was too much fun, especially with those tails whirling around everywhere. Sloshing water provided a means showing specific actions, too, the splashes from colliding brontosaur hide signifying each powerful, multi-tonne impact. This was definitely a fun image to put together, and it's certainly a favourite of my recent work. Brontosmash!

That's all for now. Coming soon (probably): The Triassic! And a boring old pterosaur that we just can't leave alone!

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Friday, 25 September 2015

What pterosaurs tell us about the evolution of feathers

2011 PR image for the 2014 description of Laquintasaura venezuelae, a basal ornithischian from Venezuela. Scales were the requested integument for this reconstruction, but how does that decision hold up today?
For the last two weeks I've been revising an image of the Jurassic ornithischian Laquintasaura venezuelae. The original (above) was produced in 2011, but a request to include it in an upcoming book was impetus to tidy up the art and update the anatomy. One significant question for updating this old piece was whether the animals should stay scaly or receive a coat of filaments. The systematic placement of Laquintasaura isn't certain, but it seems to lack features allying it to major ornithischian clades and, for now, is simply considered a basal member of Ornithischia (Barrett et al. 2014). This puts it in a controversial spot as goes interpretation of dinosaur integument: scales, filaments, or a mix of both?

The origins on filamentous integuments and feathers in reptiles remains an ongoing source of fascination and investigation for palaeontologists. It has been known that filamentous reptilian integuments extend deep into geological time since the 1800s, but research into these structures exploded in the 1990s and 2000s when fossils of many non-avian theropods - seemingly all coelurosaurs - were found adorned with feathers or filamentous feather precursors. Soon after, recovery of quills, filaments and strange, fibrous scales in ornithischians made a reality of once speculative ideas about filaments being widespread across Dinosauria. For years now, palaeontologists have been discussing the possibility that theropod filaments and feathers share ancestry with those of ornithischians. One implication of this is that bodies of dinosaur ancestors would be covered in fuzz instead of, as traditionally supposed, scales. Unravelling this conundrum is of key interest to those attempting to understand ancient reptile evolution and physiology, as well as for artists wanting to know how to credibly restore early dinosaurs. However, integument preservation, and particularly filamentous hide, is rare in the fossil record. Much as we might want to, we currently have insufficient data about the skin of early dinosaurs to address this issue directly.

All is not lost, however: some insight into dinosaur filament evolution can be provided by pterosaurs. Flying reptiles and dinosaurs are largely thought to form a more or less exclusive clade, the Ornithodira, which we now recognise as being characterised by a suite of anatomies - not just hindlimb features, as originally proposed - and commonalities of interpreted anatomy: postcranial pneumaticity, upright postures, elevated metabolisms, and filamentous integument. It's the latter which makes pterosaurs potentially useful to understanding the ancestral state of dinosaur skin. It's a little surprising that it's taken us so long to capitalise on this data, since we've had conclusive evidence of pterosaur filaments (we call them pycnofibres) since the 1970s (Sharov 1971). Suggestions that pycnofibres may have been homologous to dinosaur fuzz arrived much later, in the 2000s, when the evolutionary depth of dinosaurian filaments had become apparent and new discoveries of fuzzy pterosaur fossils were being reported (Czerkas and Ji 2002; Ji and Yuan 2002). Perhaps it was the coincidence of these events, the realisation that filaments were widespread in Pterosauria, and increased confidence in the sister relationship between dinosaurs and pterosaurs which lead to this idea finally being proposed.

Late Jurassic pterosaur Sordes pilosus, described in 1971, was one of the first pterosaurs confirmed to have a filamentous body covering. But are pterosaur filaments tied to those of dinosaurs, or independently evolved?
Studies into pterosaur and dinosaur filament homology remain thin on the ground, and much of what has been said thus far is reliant on gross filament morphology. Earlier this year, a team of researchers (Barrett et al. 2015) tackled the issue of ornithodiran filament evolution quantitatively, estimating the likelihood of homology between theropod, ornithischian and pterosaur integuments via their distribution on the ornithodiran tree. Using 18 different variations in methods, calculations and data values, they predicted the likelihood of ancestral integument states in dinosaurs and ornithodirans: were they scaly, filamentous, or feathered? The result, announced in not only the paper but also a subsequent media release, was that 12 of those 18 assessments suggested scales were ancestral to ornithodirans, and the filaments seen in pterosaurs, ornithischians and theropods were derived independently from a common scaly ancestor.

This conclusion was undoubtedly surprising to some and, indeed, a clear caveat accompanies it: scaly ancestral dinosaurs are "sensitive to the outgroup condition in pterosaurs". Support for ancestrally-scaly ornithodirans relies on the assumption that pterosaur ancestors were also scaly. This condition assumed for 50% of those 18 assessments to account for uncertain ancestral condition for pterosaur integument. In the 9 analyses where pterosaurs were treated as wholly filamentous - and thus consistent with what we see in existing pterosaur fossils - six returned results indicating an ambiguous scaly/filamentous ancestral condition for ornithodirans and dinosaurs, and only 3 supported a wholly scaly interpretation. Of those six 'ambiguous' results, most reported a strong likelihood of ornithodirans being ancestrally filamentous, and many gave dinosaurs a good chance of being ancestrally filamentous too. Moreover, treating pterosaurs as filamentous has knock-on effects through the dinosaur tree: suddenly, there are reasonable, or at least equivocal, chances that ornithichians and saurischians were also ancestrally filamentous. This is a different conclusion to the straighter story of ornithodirans and dinosaurs simply being ancestrally scaly.

What influence do fuzzy pterosaurs have on dinosaur skin evolution? Seemingly, quite a bit. The tree on the left shows integument likelihoods (pie charts) where pterosaurs are considered scaly, tree on the right shows a filamentous analysis.  Modified from Barrett et al. (2015).

Clearly, the crux of all this is the assumption that pterosaur ancestors were scaly: just how defendable is this? Because we know little about pterosaur origins, it's hard to say anything conclusive about the evolution of pterosaur integument with our current fossil record. The stratigraphically oldest pterosaur fossil with pycnofibres is from Middle/Late Jurassic deposits, and thus about 50-60 million years younger than the oldest pterosaur fossils - little help in determining if the first pterosaurs were fuzzy. Ongoing disagreements over pterosaur phylogeny complicate attempts to estimate the appearance of lineages with confirmed pycnofibres. Some schemes (those derived from Kellner 2003 and Unwin 2003) suggest pycnofibres must have appeared by the Triassic, close to or at the base of pterosaur ancestry, but others (e.g. Andres et al. 2010) indicate pycnofibres reliably extend no further than the Lower Jurassic. Of course, such assessments of filament distribution might not even be meaningful at this stage, given that pycnofibres are very rare components of pterosaur fossils. They are nowhere near as common as other soft-tissues, such as wing membranes, and we should probably be cautious about any assessment of their evolutionary pathways until we have more data. Perhaps the only significant observation we can make from our current, limited dataset is that, to date, no pterosaur is known with a scaly body covering, even when regionalised scalation - foot pads - preserves in their fossils (Frey et al. 2003).

A possible pterosaur relative with scaly hide is known: the Triassic archosaur Scleromochlus taylori. Benton (1999) described structures interpreted as thin, transversely orientated scales across the back of multiple specimens of this animal. This might provide vindication of the scaled pterosaur ancestor model, but, again, there are some caveats with this idea. For one, Scleromochlus fossils are not well preserved. The scales are feint sediment impressions, visible only in strong, low angle light, such that that they are only considered 'probable' integument impressions by Benton (1999). Previous workers have interpreted them in a different way (as gastralia). Clearly, the evidence for them being scales could be more compelling, and there's certainly not much to work with if we want to test their identification. Secondly, exactly how Scleromochlus is related to pterosaurs is not precisely agreed. Some workers consider it the sister taxon of Ornithodira, others as a member of the pterosaur branch, and others see it as more closely related to dinosaurs than pterosaurs. That might seem a minor issue, but we've already seen how sensitive models of ornithodiran integument are to changes of single variables at the base of the tree. We would probably need to run many variants of the integument probability calculations to account for all the uncertainty surrounding Scleromochlus. This might give more idea of the range of possible integuments at the base of ornithodiran evolution, but that's not much of an improvement on our current situation.

Was Scleromochlus taylori scaly? Maybe - weakly preserved structures on several specimens seem to suggest so. On this diagram, from Benton (1999), possible transverse scales can be seen on the left and middle specimen.
In all, I feel like we're hitting a bit of a wall here. It seems we just don't know enough, and have too many caveats with the limited data we have, to make even a half convincing best guess on this. Thus, how much weight we put on models of ornithodiran integument using scaly pterosaurs is almost a philosophical issue. From my end, I don't think they should be used to argue for scaly ornithodiran and dinosaurian ancestors, at least not with the same weight as tests made using a filametnous pterosaur lineage. When reconstructing ancestral states, characters objectively observed in fossils have to trump assumed character states, even if we know that our dataset is full of holes. After all, the whole point of attempting to figure out an ancestral state is establishing links between character data we have, so introducing opposing character states seems a little contrary to that objective. To be clear, I'm not saying that running models with scaly pterosaur ancestors is a waste of time. To the contrary, it's a good test of model robustness, and Barrett et al. (2015) certainly demonstrate how sensitive our models of ornithodiran integument evolution are by using this approach. Their hypothetical scaly pterosaurs demonstrate that we really do need more early ornithodiran fossils to understand ornithodiran skin evolution. However, I do not think that results of the scaled pterosaur analyses are as informative as their other assessments, as we have to overlook existing data to consider them equally valid.

With all that said, do pterosaur fossils really help us understand the evolution of dinosaur filaments? Playing the conservative card here, it seems they do not provide super strong evidence for an all-fuzzy Dinosauria, but they certainly make it difficult to defend ideas of entirely scaly dinosaur ancestors. Forcibly arguing for either scales or filaments at the base of Dinosauria seems premature at this stage, and, whatever our personal hunches are, it seems sensible to accept some ambiguity in this situation for now.

I began this article with my Laquintasaura conumdrum: how did that play out when, apparently, I can't make up my mind about this scales and filaments debate? Well, I've argued elsewhere that palaeoart can do no better than illustrate credible interpretations of the past and that, so long as the hypotheses they depict are sound, they're doing OK. When we have conflicting or ambiguous hypotheses, we just have to make a judgement call based on our own opinions, gut feelings and interpretations of existing arguments. With my own leaning being towards data showing that scales may not be ancestral to ornithodirans, but also knowing that some dinosaurs are mosaics of filaments and scales, I decided to partially enfluffen my Laquintasaura, while leaving their snouts, tails and limbs scaly. I'll leave you with the revised image.

Laquintasaura venezuelae 2015 edition: basically the same picture, but a bit fluffier, and a bit greener.

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  • Barrett, P. M., Butler, R. J., Mundil, R., Scheyer, T. M., Irmis, R. B., & Sánchez-Villagra, M. R. (2014). A palaeoequatorial ornithischian and new constraints on early dinosaur diversification. Proceedings of the Royal Society of London B: Biological Sciences, 281(1791), 20141147.
  • Barrett, P. M., Evans, D. C., & Campione, N. E. (2015). Evolution of dinosaur epidermal structures. Biology letters, 11(6), 20150229.
  • Czerkas, S. A., & Ji, Q. I. A. N. G. (2002). A new rhamphorhynchoid with a headcrest and complex integumentary structures. Feathered Dinosaurs and the origin of flight, 1, 15-41.
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Friday, 18 September 2015

Humps, lumps and fatty tissues in dinosaurs, starring Camarasaurus

I like to see fossil animals restored as if they belong in the world they're depicted in. That is, not just as basic, conservative reconstructions of ancient species in an certain landscape, but instead with colours, integument and soft-tissue adaptations suited for their possible lifestyles and the environments they frequented. To this end, last year I published an illustration of the Late Jurassic, North American sauropod Camarasaurus supremus as an species well adapted for life in arid settings. As a common part of the famous Morrison Formation dinosaur fauna, dry conditions would be familiar to Camarasaurus, and especially because it occupied the drier, desert-like southern extent of the Morrison palaeoenvironment. I rendered Camarasaurus as a dinosaurian camel, complete with several common cranial adaptations to resisting dry conditions and, most obviously, a fat hump on its back.

2014 restoration of Camarasaurus supremus, published in Witton (2014). Painted to make a point about palaeoart (as well as plugging the awesomeness of All Yesterdays), here's what the caption read. "Reasoned speculation in palaeoart. The sauropod Camarasaurus supremus depicted with adaptations for living in a very dry environment: enlarged nasal cavities to aid resorption of moisture, sealable nostrils to reduce evaporation, wrinkled skin to enhance heat dissipation, white and tan colouring to resist heat soaking, and a fat hump to store energy. Such features are speculative, but do not contradict any data we have for this taxon, and are consistent with the adaptations of modern desert-dwellers."

I decided to revisit this image this week to boost the sauropod content of Recreating an Age of Reptiles (coming soon, I swear!). In doing so, I decided to conduct some more research into the likely nature of non-avian dinosaur fatty tissues. I wanted to keep the fat store on Camarasaurus, as equivalent structures provide energy and water reserves for many modern desert species, and there's no reason to think that extinct dinosaurs would not have developed fat stores for similar purposes. However, is a camel-like hump really likely in a dinosaur? Can we credibly restore any details of dinosaur fats? These were questions I sought to investigate more thoroughly before jumping into my revisions.

Yo extant diapsids so fat

If we're thinking about how to restore dinosaur fats, we need to investigate what the reptile lineage is capable of when it comes to producing and storing fatty tissues. The composition of diapsid fats is a little different to our mammalian ones, although we share functionally comparable approaches to fatty tissue makeup in many respects, including responses to endothermic demands (Goff and Stenson 1988; Saarela et al. 1991; Azeez et al. 2014). Amniotes, as a whole, have fairly similar approaches and uses for fatty tissues, which is great, because that allows us to make some reasonable inferences about fossil species.

Modern reptiles generally have lower fatty tissue fractions than mammals because of their lower energy requirements (Birsoy et al. 2013; Azeez et al. 2014). However, this is not to say that they are incapable of storing large quantities of fat, or even putting on weight rapidly. Some reptiles are indeed lean species, but some - most famously certain geckos, but also some iguanas, skinks and snakes - periodically or permanently hold large stores of fat in case of hard times, or to prepare themselves for energy-intensive feats (e.g. reproduction or long distance travel). Reptiles generally sequester fatty deposits within their torsos or in their tails, but some species also store them in their armpits and in fat pockets located at the back of the head. Individuals of many lizard species are considered healthy when these regions are literally bulging with fatty mass. To my knowledge, these masses are not directly supported by the skeleton or other tissues: it is simply the cohesive nature of fatty tissues and dermis which keeps them in place. It is known that some lizards can pack their tissues with fat rapidly when necessary, some experiments finding geckos can increase their body mass by 50% in four days (enough fuel to sustain them for over half a year!) (Mayhew 2013). Indeed, reptiles are so good at packing on fat, and maintaining it, that owners pet reptiles will know that obesity can be a real issue for captive lizards.

What about living dinosaurs? As with other diapsids, birds can rapidly generate fatty tissues in anticipation of stressful periods, and frequently do so before, for instance, migrating (Lindström and Piersma 1993). 10-15% body fat is considered low for a migrating bird, with the bodies of some species comprising 50% fatty tissues before embarking on their travels - seasoned ornithologists recognise birds as positively emaciated when they finish their journeys (Alerstam and Christie 1993). However, birds are not fully reliant on fatty tissues as energy stores, some species routinely using their muscles and organs as fuel sources during long migrations. It seems only their lungs and brains are safeguarded against being turned into energy (Battley et al. 2000): everything is fair game for fuel or other components needed to maintain a functioning body. Avian fatty tissues are, like those of lizards and crocs, deposited within their torsos but, in lieu of large tails, they also store them across the surface of the chest and abdomen. Bird skin has some transparency, and field ornithologists interested in avian fat tissue fractions can determine their extent by simply checking the amount of yellowish fat tissue visible underneath bird feathers (e.g. Rogers 1991).

The dinosaur hump controversy

Is there any direct indication of fatty tissues in Mesozoic dinosaurs? The answer is probably 'no', except for the controversial idea that the elongate dorsal neural spines if some dinosaurs are indicative of a camel-like 'hump' morphology. Spinosaurus, Ouranosaurus and Deinocheirus are key species here, these animals being depicted sometimes as humpbacked creatures. These interpretations are not the sole remit of artists, either: Bailey (1997) proposed that the tall neural spines of certain dinosaurs supported masses of tissue acting as energy stores or heat buffers - in other words, a heap of fat.

I must admit to being very sceptical that neural spine anatomy is linked to fat humps. For one,it seemingly violates what we see in the extant phylogenetic bracket for dinosaurs, where no species (to my knowledge) have substantial fat deposits on their backs. Of course, it might be queried how meaningful phylogenetic bracketing is for this issue. Fatty tissues seem quite pliable in an evolutionary sense, being chucked around animal bodies with ease as lineages adapt to new conditions (Birsoy et al. 2013). It isn't crazy to think that dinosaur bodies are different enough from those of modern diapsids that they could not have their own take on fat distribution, and there are certainly functional constraints on extant diapsid fatty tissues which are unlikely to apply to non-avian dinosaurs. However, that's only speculation, and one which conflicts with a big pool of direct data on this issue.

Another approach might be to look at animals which do have fatty humps on their backs - several types of mammal - to see if their composition is analogous to anything we see in non-avian dinosaurs. What do their humps look like internally?

A collection of animals with humpbacks and sails. Fatty humps are not directly supported by skeletons in modern species including (B) lowland gorillas (Gorilla gorilla), (C) dromedaries (Camelus dromedaries) and (D) white rhinoceros (Ceratotherium simum). Vertebral spines anchor sails in some modern lizards, such as crested chameleons (Trioceros cristatus; E), and withers anchor powerful neck muscles as in American bison (Bison bison; F). Cropped figure from Witton (2014); B–D and F from Goldfinger (2004); E historic x-ray (1896) by Josef Maria Eder.

Turns out that most mammalian humps are akin to those bulging reptile fat masses mentioned above: they tend to exist without internal support or even osteological correlates. Where humps do correlate with bone, they are comprised of powerful musculature, not fat: the shoulder humps of rhinos and bison show this well. These structures might have subcutaneous fat on them, but this is not their primary composition, nor does fat storage seem to be a principle adaptive purpose. In several species, like camels and rhinos, the longest neural spines do not align with soft-tissue humps at all, these actually being located over dorsal vertebrae with smaller neural spines (camels) or short-spined cervical vertebrae (rhinos). Taking our attention away from mammals, and turning to reptiles, we see that elongate neural spines anchor laterally compressed sail-like structures, not masses of fat. It thus seems that we have no modern correlation between fatty humps and skeletons at all, and that there is no link between elongate neural spines and fatty deposits - quite the opposite actually seems true. It was this suite of observations which led to my 2014 humped Camarasaurus image: bizarrely, it is more consistent with modern data (though still extremely speculative) to put a camel-like hump on something without long neural spines, like Camarasaurus, than it is to put one on Spinosaurus, Ouranosaurus or Deinocheirus. Sail-like structures or (at least for the lower regions of the spines) muscle attachment seem more parsimonious interpretations of their strange vertebrae - if we're being scientific (as we should be in palaeoart), we really shouldn't be looking at those tall neural spines and thinking 'fat hump correlate'.

Tying all this together

Although we may lack direct evidence of them from fossils, data from extant animals suggests it is sensible to restore dinosaurs with noticeable, prominent fatty tissues, especially if we're reconstructing animals associated with extremes of behaviour, climate or environment. Animals about to undertake migration should look well fed and bulky, and those at the other end might look leaner and less nourished. We certainly have good precedent for restoring desert-dwelling Mesozoic dinosaurs - of which there are many - with energy and water reserves, given that even energy-limited ectothermic diapsids take such precautions, as do some endotherms. We should probably not limit fatty tissues to bulky energy stores, either: as in modern lizards, some extinct reptiles may have housed pockets of fat in prominent places to serve as advertisements of health and virility.

Where should we locate those big energy stores? With no direct indication from fossils, I suggest we err on the side of caution and follow the diapsid condition, principally locating them around the tail base and abdomen. Most Mesozoic dinosaurs had well-developed, powerfully muscled tails, and were thus likely capable of supporting a wad of adipose tissue at the tail base. We could start restoring humps in other places, but it seems sensible to keep speculative anatomy grounded somewhere. Besides, it's not like a fat-tailed dinosaur is boring concept!

Combining all this together, I'll leave you with the completed, revised version of my desert-adapted Camarasaurus image, now with fatty tissues fully consistent to those of modern diapsids. This meant chopping off the back hump (I'm not going to pretend I wasn't disappointed to do that), but it's worth it for a more defensible image. Note that the adult is sporting not only a fat tail, which is meant to represent sustenance for wandering through harsh desert settings, but also a pair of natty fat pockets behind the skull. It looks fairly happy with them.

Camarsaurus supremus, queen of the desert, not a member of Weight Watchers.

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