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Friday, 22 January 2016

Deinosuchus: the Dalek-backed alligatoroid that (sometimes) made chew toys of dinosaurs

Deinosuchus rugosus swallows the remains of a large Cretaceous sea turtle. Other archosaurs notice, decide to interrupt.
Any palaeontological geek worth their salt knows that several gigantic crocodyliform species - colloquially called 'supercrocs' - have appeared in the last 100 million years. They include the Moroccan, Cretaceous pholidosaurid Sarcosuchus imperator, several species of the South American Miocene caimanine Purusaurus, and the grandfather of them all, Deinosuchus, from the Late Cretaceous (Campanian) of North America. Deinosuchus is not the most 'extreme' of these giant crocodyliforms in terms of anatomy or size (but see below), but discovery of a partial skeleton in 1903 (and description six years later) was the first evidence for some croc-line archosaurs being very, very large. It is also perhaps the best publicly known giant crocodyliform, various reconstructions of its skull and skeleton appearing in museums all over the world, and being a semi-regular component of palaeoart.

Perhaps because Deinosuchus is 'only' a giant crocodylian and not a member of a completely extinct, weirdo lineage, coverage of its palaeobiology is often limited to factoids on its immense size and probable habits of eating dinosaurs. Our short attention span for this animal is not new: one of the key players in its discovery, John Bell Hatcher, lost interest in describing the first significant Deinosuchus remains once its crocodylian identity (rather than dinosaurian, as originally supposed) became apparent in 1903 (Schwimmer 2002). It took further persuasion and a number of years for palaeontologists to actually publish, name and describe this animal after Hatcher died in 1904 (Holland 1909). As we'll discover in this article, the scant attention paid to this animal is rather criminal: over the last century a detailed and fascinating picture of Deinosuchus has developed.

The Colbert and Bird (1954) reconstruction of Deinosuchus riograndensis, based on partial skull material collected from the late Campanian Aguja Formation, Texas. We now appreciate this reconstruction as being erroneous in a number of crucial ways, and it should not be considered representative of the appearance of this animal. Note the apertures in the snout tip alongside the actual nares. Images from Colbert and Bird (1954).
I'm going to start by sidestepping the confused history surrounding the discovery and naming of Deinosuchus material - interested parties can find a full summary in David Schwimmer's (2002) book Deinosuchus: King of the Crocodylians. It will suffice to state that fossils of this animal have been known since at least the mid-1800s and repeatedly hopped between species and genera during the last 150 years. Only one or two species are recognised nowadays. D. rugosus is the type species of the genus, first identified from two characteristically large, blunt teeth with wrinkled, thick enamel collected from the eastern US in the 1858 (examples below). Over time, these teeth were found to be linked with other Deinosuchus material including extremely thick, massive and deeply pitted osteoderms. These elements are highly characteristic even today, and permit even isolated teeth and osteoderms to be referred to this species (Schwimmer 2002). A possible second species is D. riograndensis, based on very large Deinosuchus fossils from Texas recovered in the 1940s (Colbert and Bird 1954). It's this riograndensis material that most of us think of when Deinosuchus is mentioned, it providing the basis of a famous, 2 m long and largely artistic Deinosuchus skull reconstruction unveiled at the AMNH in 1954 (above). The riograndensis skull was once thought characteristic because of unusual openings in the side of the snout tip (Colbert and Bird 1954), but these are now thought to be damage caused during preparation (Schwimmer 2002). As more and better Deinosuchus remains have been recovered in recent decades, some have argued riograndensis and rugosus are one and the same animal, the latter holding nomenclatural priority. Full agreement on this does not seem apparent from current literature, but on-going work on relatively complete Deinosuchus material will help clarify the taxonomy of this animal in future.

Deinosuchus is recognisable from its teeth alone. These specimens are from the posterior end of the jaws, and show the wrinkled enamel typical of the genus. Note the heavily worn and broken the tips. From Schwimmer (2010).
We now appreciate that Deinosuchus is one of the oldest members of Crocodylia, the crown group crocodile-line archosaurs. Features of its skull and jaws indicate it was specifically a member of Alligatoroidea, the lineage of Crocodylia represented today by alligators and caimen. It's not uncommon to see Deinosuchus referred to as a 'giant alligator', but this is not really accurate and I recommend avoiding such terminology, even for lay audiences. Alligators and Alligatoridae are a distinctive group of alligatoroids with particular habits and anatomy, and they are no more closely related to Deinosuchus than caimen. It must also be stressed that neither of our modern alligatoroid clades are especially closely related to Deinosuchus. I figure most people will intuitively grasp the rough meaning of 'giant alligatoroid', and suggest this term is used in preference of 'alligator' in outreach media about this animal.

The Deinosuchus fossil record is something of a mixed bag. There are hundreds of fossils of it, but most of them are isolated postcranial bones, broken bits of skull and, especially, those massive teeth and osteoderms. The state of many Deinosuchus fossils can be ascribed to its remains being reworked by storms after their initial burial. Some partial skeletons and more complete skulls escaped this treatment but are not yet described in detail. A silver lining to not having much in the way of complete material is that isolated Deinosuchus bones are distinctive enough to map its range across Campanian North America. Many of us might think of Deinosuchus as a Texan animal, but it actually enjoyed a wide distribution, and being most abundant in the southeastern United States. A clear palaeobiogeographical pattern can be gleaned from Deinosuchus fossils, a divide separating occurrences in Montana, Wyoming, Utah, Colorado, New Mexico, Texas, and Northern Mexico from remains on the eastern side of the United States - Mississippi, Alabama, Georgia, North Carolina, and New Jersey (Titus et al. 2008). This east-west distribution is no fluke of preservation but reflection of Deinosuchus populations being separated by the Western Interior Seaway, the continental sea which divided North America during the Cretaceous. Although apparently not a fully marine creature, it is thought Deinosuchus lived in the coastal waters and estuaries of this seaway as, to date, its fossils have not occurred in fully freshwater or terrestrial deposits. Further evidence of its preference for coastal waters is the recovery of more complete and associated remains from wholly marine deposits.

A modern depiction of the Deinosuchus rugosus skull and mandible. From Schwimmer (2002).
Our incomplete knowledge of Deinosuchus anatomy means we can only form a partial picture of what it looked like in life. We can say, however, that the famous 'Colbert and Bird' riograndensis skull sculpture from the 1950s is erroneous in several regards. Manufactured as a display for the American Museum of Natural History (and then reproduced for museums around the world), the Colbert and Bird skull is too large, the snout too narrow, the tooth morphology inexact and, as noted above, the openings in the snout tip are likely erroneous (Schwimmer 2002). This is not a dig at AMNH artists course: they did the best they had with material available to them, and it's only with the discovery of better fossils that we can now spot errors. Perhaps unfortunately, museums continue to display this reconstruction and artists continue to use it as a reference. A modern picture of Deinosuchus is rather different: a broad- and deep-snouted crocodylian with a skull known to be at least 1.3 m in length, and perhaps a little longer if very fragmentary remains are being correctly interpreted (Schwimmer 2002). Most of its cranial features are typically crocodylian, including the presence of huge spaces for jaw muscle attachment, development of a secondary palate and dorsally situated orbits and nares. Befittingly for such an enormous animal, the teeth are huge and consistently robust along the jaw, those at the front being conical and pointed, and those at the back being increasingly stunted and shortened. The toothrow is very long in stretching from the jaw tip to just behind the eye. All teeth have the distinctively thickened, wrinkled enamel mentioned above. Unlike modern alligatoroids, a notch in the side of the upper jaw acts as a receptacle for the fourth tooth of the lower jaw and presumably rendering it visible even when the jaw was closed. This is the 'primitive' condition for crocodylians, and means that although Deinosuchus is an alligatoroid, it likely had a crocodile's smile.

Less can be said about the body and limbs of Deinosuchus. Schwimmer (2002) reports that partial skeletons hint at a general form and proportion not unlike a modern alligator. However, some authors have noted discrepancies in scaling of Deinosuchus limb bones which might indicate reduced limbs in at least the largest specimens (Farlow et al. 2005, see below). One thing we can be sure of is that the body of Deinosuchus was covered in those aforementioned large, thickened osteoderms (below). The exact arrangement of these elements remains unknown, but we can predict that at least four rows of osteoderms extended along the body of Deinosuchus because of its affinities to modern crocodylians. These osteoderms become disproportionately massive and robust with growth, so that those of the largest individuals are distinctively chunky and have lost some definition of a keel found in smaller examples. Artists should take note of this: the dorsum of a big Deinosuchus would have looked more like a gnarly Dalek chassis than the back of any modern crocodylian. As is typical for crocodyliforms, these dermal bones might have reinforced the trunk skeleton as well as providing armour plating, forming a network of muscle, ligaments and bone which bound the torso together (Salisbury and Frey 2000). It is speculated that the presence of very large, robust osteoderms in the biggest Deinosuchus indicates the presence of a torso strong enough for terrestrial locomotion (Schwimmer 2002).


The huge, deeply pitted and bulbous scutes which characterise Deinosuchus, as illustrated by Holland (1909). This image is a composite of two scutes from Holland's work, put together by FanCollector for Wikipedia. Both show cervical osteoderms, the left being a particularly big one
The maximum size of Deinosuchus is the source of much fascination and discussion. The largest estimates based on reliably measured remains suggest body lengths of around 12 m (Schwimmer 2002), but - as usual with giant fossil animals - there are a number of factors and caveats worth considering here. The scant nature of Deinosuchus fossils dictates that we must extrapolate the size of large Deinosuchus from much smaller, better known individuals and modern crocodylians. The 12 m figure stems from scaling the largest Deinosuchus vertebrae to total body length estimates of smaller individuals (Schwimmer 2002). Skulls and mandible lengths, when compared to a dataset of modern alligator proportions, indicate the largest animals achieved 10 m in length (Schwimmer 2002; Farlow et al. 2005). Another approach, using femoral measurements, results in a maximum body length estimate of 6-8 m, this being estimated from big femora typically thought to indicate 10 m+ animals. The explanation for these differences will be familiar to anyone who's estimated the size of a big, extinct animal: uncertainty about the proportions of the animal in question, the need to extrapolate well beyond the size boundaries of modern analogues, and the lack of associated remains of the biggest individuals. Most workers seem happy with a 10 m length estimate for a big Deinosuchus, those lower estimates based on femoral size being explained as possible evidence of reduced limb proportions in the biggest Deinosuchus individuals (Farlow et al. 2005). As usual, we await the discovery of more complete and informative remains to tell us the full story here. It should be stressed that similar caveats apply to size estimates of other 'supercrocs': despite the media hype associated with some discoveries, it's quite difficult to know which crocodyliform species was largest based on our current material.

Regardless of what the actual maximum size of Deinosuchus was, we have good reason to think that many individuals were not true giants. No specimens indicative of 10 m body length have been found among the many hundreds of Deinosuchus remains from the eastern side of the US, it being instead thought that eastern Deinosuchus didn't grow longer than 8 m. That's still pretty big of course, but not too far off crocodylians that we're familiar with today (the biggest saltwater and Orinoco crocodiles on record are a little over 6.5 m - Grigg and Krishner 2015). The true giants only occur in the west, and are much rarer fossils than their eastern counterparts. These fossils are also slightly younger than the eastern specimens, perhaps indicating changes across time and geography were responsible for Deinosuchus becoming exceptionally large. Research into the growth rates of Deinosuchus indicate that there might be nothing unusual about it's growth trajectory despite its size. It seems to have grown with a similar strategy to other crocodylians - relatively fast at first, and progressively slower over time - but simply stretched out the growth duration to many decades (Erickson and Brochu 1999). Growth rings in osteoderms indicate that the largest animals were about around 50 years old (below).

Growth rates in living and extinct crocodylians (a) and growth rings in a Deinosuchus osteoderm (b). Note how the Deinosuchus growth trajectory is essentially a scaled up version of its smaller relatives. From Erickson and Brochu (1999).
For many, the main discussion to be had about Deinosuchus is the impact it had on local dinosaur populations: was this animal a dinosaur predator? Attempts to answer this question stem from two sources: biomechanics and fossil evidence of ancient faunal interactions. The first, biomechanics, includes a recent study of 'death rolling' (the crocodylian habit of rotating around the long axis of the body while gripping prey with their jaws, literally twisting it apart) and whether Deinosuchus could use this strategy to dismember large prey (Blanco et al. 2015). Snout strength can be correlated quite accurately to death rolling capabilities in modern crocodylians (Blanco et al. 2015) and, seeing as this can be inferred from upper jaw skeletons alone, we can obtain some insight into the death rolling capabilities of extinct crocodyliforms. Perhaps surprisingly, estimates of Deinosuchus jaw strength were approximately one third of the strength required for this behaviour (Blanco et al. 2015). Scaling factors have been used to explain this unexpected result and, despite the outcome of their experiments, Blanco et al. suggest that death rolling was possible in Deinosuchus. I must admit to thinking additional experimentation is needed to quantify those scaling factors before considering this matter closed. Moreover, in checking the Blanco et al. data for this article I noted that their study modelled the Deinosuchus skull as 1.8 m long, a figure noted as speculative by Schwimmer (2002) and almost 40% longer than the largest measured skull length reported by the same author (1.31 m). A 40% shorter skull would be less prone to the scaling effects outlined by Blanco et al. and may result in a jaw strength more suited to death rolling - it would be great to see this checked out in future.

Partial theropod hindlimb bone (tibia or metatarsal) post a one-on-one session with Deinosuchus jaws. This bone is meant to be subrounded in cross section. From Schwimmer (2010).
More positive and definitive answer about Deinosuchus dinosaur predation stems from fossil evidence. It seems that, yes, Deinosuchus did dine on dinosaurs, but not exclusively or maybe even often (Schwimmer 2010). A handful of Campanian dinosaur bones - including a theropod hindlimb element from Georgia (above) and hadrosaur vertebrae from Texas - possess bitemarks characteristic of crocodylians, albeit on a scale unseen in the modern day (Schwimmer 2002, 2010). The theropod bone can only be described as exceptionally chewed: numerous, overlapping circular potmarks show where the bone was repeatedly bitten and crushed by a very powerfully jawed animal. So pulverized is this bone that its once subrounded cross section has become quadrangular - Schwimmer (2002) summarises the state of this specimen as 'resembling a dog's worn chew toy' (p. 186). The crocodylian signature, bite mark size and provenance of both specimens point to Deinosuchus as a possible perpetrator, and evidence that it did eat dinosaurs on occasion. Of course, it cannot be easily established whether these animals were killed by Deinosuchus or merely scavenged by them - some reasoning for the former is discussed by Schwimmer (2010).

Deinosuchus bite marks in fragments of a turtle (Chedighaii barberi) plastron. The tooth marks are about 4-5 times larger than those made by 4 m long nile crocodiles. From Schwimmer (2010).

There are reasons to think dinosaur meat was not a mainstay of Deinosuchus diet, however. Whereas a few dinosaur bones have been linked to the jaws of this crocodylian, more than a dozen Campanian sea turtle specimens have been found with bite marks made by a giant crocodylian (Schwimmer 2010). A diet of turtles is not surprising when we consider that the skull and dentition of Deinosuchus is more adapted to crushing bone than piercing skin and flesh (Schwimmer 2002): those robust posterior teeth are especially reminiscent of teeth in modern, turtle-eating crocodylians. Many Deinosuchus teeth are considerably worn and broken too, a likely consequence of being smashed into hard, bony prey rather than soft, spongy dinosaur limbs. Healed turtle shells suggest that these animals were predated by Deinosuchus rather than just scavenged, and raking bites across some specimens may record less fortunate turtles being juggled about Deinosuchus jaws when being eaten (Schwimmer 2010). The coastal and estuarine environmental bias of Deinosuchus fossils is consistent with it being a serial turtle predator, this being ideal habitat to find sea-going prey. Curiously, other marine inhabitants of the Western Interior Seaway have yet to be associated with Deinosuchus bite marks: perhaps it really did have a preference for turtles, or perhaps other skeletons were simply pulverised beyond recognition by those massive jaws. Either way, our discussion of this animal's feeding habits would not be complete without mentioning the numerous, possible Deinosuchus coprolites which have recently been identified (Harrell and Schwimmer 2010). Sadly, these do not reveal much about diet or digestive anatomy, other than the obvious fact that Deinosuchus poop was on average a lot larger than that produced by other crocs. Several anomalous features of these coprolites have led some authors (e.g. Hunt and Lucas 2010) to be sceptical of their organic origins however, their alternative being that they are simply calcareous nodules.

The foraging habits of Deinosuchus brings us to new perspectives on where it fits into Mesozoic ecology. New evidence is eroding the uniqueness of Deinosuchus in Campanian North America, it no longer being the only very large or even giant crocodyliform species in some localities. These new finds include a currently poorly known, but obviously giant neosuchian from the Williams Fork Formation of Colorado (Foster and Hunt-Foster 2015) and a more completely known, 7 m long, undescribed neosuchian from Woodbine, Texas (Main 2012). The latter is currently being worked on, and early indications are that it might represent a late surviving goniopholidid - a much older branch of the crocodyliform lineage. Whatever they turn out to be, the Woodbine and Williams Fork animals suggest that very large crocodyliforms might not have been unusual in Campanian North America. Their presence in a timeframe deficient of large theropods has not gone unnoticed, it being speculated that these large crocodyliforms may have been doing work normally reserved for big predatory dinosaurs (e.g. Schwimmer 2002). Similar proposals have been made about other large bodied Late Cretaceous carnivores taking over typically theropodan roles (e.g. Witton and Naish 2015) - the notion of the Mesozoic as an all-dinosaur show is looking increasingly out of date.

As a closing thought, I find it interesting that we tend to portray Deinosuchus as something of a freak species, one of those rare forays of crocodylian evolution into gigantic size which never really seemed to last that long or lead anywhere. As might be apparent from this article, this view is somewhat misleading. Deinosuchus certainly represents an 'extreme' of crocodylian evolution, but it's at the end of a spectrum, not a weird outlier from the rest of the group. Much of what it did, how it did it, and what makes it a fascinating animal, is mirrored in its modern and fossil relatives. Contrary to some perspectives on this animal, the fact it represents an ancient member of a modern group does not make it tedious or dull. Quite the opposite is true: Deinosuchus reminds us that animals from Deep Time are part of a continuum with our own fauna, revealing the awesome things modern lineages have been capable of, the potential their anatomies have in the present, and what they might be up to in future. How anyone can find pondering an animal that gives such a raw perspective on evolution and adaptation boring or uninteresting is beyond me.

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References

  • Blanco, R. E., Jones, W. W., & Villamil, J. (2015). The ‘death roll’of giant fossil crocodyliforms (Crocodylomorpha: Neosuchia): allometric and skull strength analysis. Historical Biology, 27(5), 514-524.
  • Colbert, E. H., Bird, R. T., & Brown, B. (1954). A gigantic crocodile from the Upper Cretaceous beds of Texas. American Museum Novitates; no. 1688.
  • Erickson, G. M., & Brochu, C. A. (1999). How the ‘terror crocodile’ grew so big. Nature, 398, 205-206.
  • Farlow, J. O., Hurlburt, G. R., Elsey, R. M., Britton, A. R., & Langston Jr, W. (2005). Femoral dimensions and body size of Alligator mississippiensis: estimating the size of extinct mesoeucrocodylians. Journal of Vertebrate Paleontology, 25(2), 354-369.
  • Foster, J. R., & Hunt-Foster, R. K. (2015). First report of a giant neosuchian (Crocodyliformes) in the Williams Fork Formation (Upper Cretaceous: Campanian) of Colorado. Cretaceous Research, 55, 66-73.
  • Grigg, G., & Kirshner, D. (2015). Biology and Evolution of Crocodylians. Csiro Publishing.
  • Harrell, S. D., & Schwimmer, D. R. (2010). Coprolites of Deinosuchus and other crocodylians from the Upper Cretaceous of western Georgia, USA. New Mexico Museum of Natural History and Science, Bulletin, 51, 209-213.
  • Holland, W. J. (1909) Deinosuchus hatcheri, a new genus and species of crocodile from the Judith River beds of Montana. Annals of the Carnegie Museum, 6, 281–294.
  • Main, D. J. (2012). Crocodiles of the Texas Cretaceous; the Campanian of Big Bend to the Cenomanian of North Texas, a comparison of great size, feeding behaviour and paleoecology. Geological Society of America Abstracts with Programs, 44, 3.
  • Schwimmer, D. R. (2002). King of the crocodylians: the paleobiology of Deinosuchus. Indiana University Press.
  • Schwimmer, D. R. (2010). Bite marks of the giant crocodylian Deinosuchus on Late Cretaceous (Campanian) bones. New Mexico Museum of Natural History and Science Bulletin, 51, 183-190.
  • Titus, A.L., Knell, M.J., Wiersma, J.P., Getty, M.A. (2008). First report of the hyper-giant Cretaceous crocodylian Deinosuchus from Utah. Geological Society of America Abstracts with Programs, 40, 58.
  • Witton, M. P. and Naish, D. (2015) Azhdarchid pterosaurs: water-trawling pelican mimics or "terrestrial stalkers"? Acta Palaeontologica Polonica 60, 651-660.

Thursday, 24 December 2015

Dinosaur scales: some thoughts for artists

Turns out that Triceratops horridus had some of the coolest scales of any dinosaur: huge, interlocking tubercles with low bosses and spikes. No other dinosaur has skin like this - at least, not without supporting osteoderms. But what are dinosaur scales actually like, and are we depicting them accurately in our art?

The discovery that many Mesozoic dinosaurs were superfuzzyfilamentouspinyalidocious has been an major influence on contemporary Mesozoic palaeoart. This has affected more than just how we depict the gross appearance of dinosaurian subjects, but also our attitudes to their behaviour, demeanour and place in the Mesozoic world. I've written a fair bit about scientific and artistic attitudes to filamentous dinosaurs and joined choruses arguing that it's important to get these new depictions 'right': we want to see filaments of appropriate morphology, size and distribution in reconstructions of these animals.

In light of this, it's a little peculiar that we have slightly more lax attitudes to how we reconstruct scaly integuments in these animals. We have some truly spectacular skin impressions from scaly dinosaurs which provide a wealth of information about their detailed appearance, and yet many of our reconstructions incorporate little of this data. Instead, we often create 'generically' scaly or wholly speculative integuments. Common issues include rendering of scales of homogenous size and shape across an entire animal, showing little difference in scalation between species, and issues with the size, proportions and shape of individual tubercles. Other times, and most egregiously, some individuals understate just how good the records for scales in certain species are, this seemingly giving license to render a more speculative, but flamboyant body covering. It's not just amateurs making these mistakes and, in the interests of not being a hypocrite, I'll state early on that I'm guilty of some of these issues in my own work.

With this in mind, I want to see out 2015 with a fresh look at four exceptionally interesting samples of dinosaur scales, providing something of a refresher for myself and other about scaly dinosaur integument and food for thought on restoring these animals. The amount of scaly skin we have from dinosaurs means this list could easily comprise 10 or even 20 examples, but for the sake of brevity and detail I'm keeping the count low. The specimens here may be familiar to veterans of dinosaur literature, but I hope to cover them in sufficient detail that much of this information will be new to many readers.

The Carnotaurus holotype skin impressions

Outside of the feathered coelurosaurs, substantial remains of theropod dinosaur skin are pretty rare. There are lots of scraps, many of which are only cautiously referred to Theropoda, but large pieces of skin associated with specific skeletons are very thin on the ground. These circumstances make the extensive scaly skin impressions known from the Late Cretaceous Carnotaurus sasteri type specimen quite special. This specimen is already impressive: described in detail by Bonaparte et al. 1990, it comprises a near complete skeleton missing only parts of the legs and end of the tail. The fact this specimen also preserves a host of skin remains means Carnotaurus is an especially well represented large theropod. Many readers will know the skin remains associated with this specimen makes it quite integral to debates over the ancestral state of dinosaur and theropod skin. As one of the few relatively 'basal' theropods known with decent skin remains, Carnotaurus has quite a bit of sway in discussions about filament development in theropods.

Illustration of the tail base Carnotaurus skin impressions from Bonaparte et al. (1990). The deep grooves in the specimen represent topography of the associated axial skeleton, in this case the haemal arches. Scale bars represent 10 cm.
The skin remains of Carnotaurus are a little patchy, but represent many different parts of the body: the anterior neck, shoulder girdle, mid-torso, and the base of the tail. The skull also bore skin impressions before they were accidentally prepared away. The largest piece of skin covers the tail base, and is figured above. A huge amount of detail can be seen across the various skin pieces. They have a relatively uniform texture, each piece showing a mix of two scale types. The most obvious are the large, 4-5 cm diameter tubercles which protrude slightly from the rest of the skin. Instead of being randomly arranged, these are spaced regularly from each other at roughly 10 cm intervals, separated by large numbers of relatively tiny, 5 mm wide scales. The larger tubercles bear something of a keel, but the smaller structures are quite featureless. Parallel furrows with vertical orientation, perhaps representing creases, are impressed into the mosaic of smaller tubercles, but do not seem to leave an impact on the larger structures. Figures in Bonaparte et al.'s (1990) description suggest that this general skin texture extends right the way around the tail - the reduction in tubercle size and density on the ventral surface commonly seen in artwork is erroneous in this respect.

For artists, the Carnotaurus skin impressions enable us to 'connect the dots' as goes the appearance of this dinosaur's hide. It seems scales were present from skull to tail base, and it doesn't seem much of a stretch to assume most or all of the animal was scaly. There are a few reconstructions of extensively filamentous Carnotaurus out there but, sorry guys, this just doesn't jive with what we know of the skin of this animal. It also seems we shouldn't be drawing Carnotaurus with obvious differences in skin texture across the body - it looks pretty homogenous in the fossils. Also noteworthy is the size of most of the scales. It seems we'd only notice the larger, keeled tubercles and furrows on this animal unless we were standing very close. Those 5 mm tubercles might perhaps register as mottled colouration, but I doubt anyone without superhuman vision could distinguish each scale from afar. Note that Carnotaurus is not unusual in this respect - a lot of dinosaurs had much smaller scales than we show in our illustrations.

The Howe Quarry diplodocids

One of the most striking components of the 1999 Walking with Dinosaurs Diplodocus reconstruction was the tall dermal spines adorning the midline of the animal. These structures were not the idle fantasy of sculptors and artists, but actually based sauropod skin fossils from Howe Quarry, a famous Wyoming Jurassic locality. Described by the late palaeoartist Stephen Czerkas in 1992, these finds are frequently discussed by palaeoartists because sauropod skin impressions are extremely rare. The impressions are associated with incomplete skeletons representing animals from 2-3 to 14 m in length, with some skin pieces being exceptionally large at 25 x 75 cm. Unfortunately, Czerkas (1992) did not identify the remains of these animals. Howe Quarry only yields at least one named diplodocid, the recently named Kaatedocus siberi, but it remains to be established if these scaled remains represent the same taxon.

The Howe Quarry diplodocid skin can be described as tessellating hexagonal scales with a rough surface, each about 3 cm across. There is no sign of these scales being divided by differently sized scales to form a pattern like those seen in Carnotaurus. The roughened texture of each scale is formed by small (2-3 mm) tubercles dotted across each large scale. As noted by several authors, this morphology is reminiscent of other examples of sauropod hide and seems common to at least Neosauropoda (e.g. Foster and Hunt-Foster 2011; Upchurch et al. 2015). As a rule, sauropods must've been quite rough to the touch.

Illustrations of the Howe Quarry diplodocid spines from Czerkas (1992). Top row, illustrations of specimens as preserved; bottom, interpretative drawings and reconstructed outlines. Scale bars equal 5 cm.
The truly exceptional part of the Howe Quarry diplodocid skin remains are the 14 subconical structures found dotted amongst the sauropod skeletons (above). Some were isolated, but several of these structures were found in connected rows. Perhaps the most significant of these were associated with a skin impressions wrapped around the tail base of one individual. It's from these remains that we can deduce that they were arranged in a row along back of the animal. This might seem like a minor feat, but - as anyone who's attempted to reconstruct stegosaur or titanosaur osteoderm arrangements might attest - being confident about the arrangement of extraneous pieces of dinosaur integument is nothing to be sniffed at. These cones vary quite a bit in size and shape. The largest, estimated at 18 cm tall when complete, seem to stem from the proximal end of the tail, but those of the distal end are smaller. Some cones are quite tall and straight, others blunter and recurved. The tips of all the cones are flattened laterally, but the bottoms more or less round in cross section. As with hexagonal scales on the body, these spines bear small tubercles across their surface. That these were purely comprised of the dermal tissues, and not osteoderms, is confirmed by the total absence of bone from any of the cones. Quite how far these conical structures extended across their owner's bodies cannot be said from the known remains, nor should we feel confident that we have the full spectrum of size or morphological variation of the spines (Czerkas 1992).

The detail and specificity of the Howe Quarry specimens give artists an atypically good insight into the appearance of these sauropods, and remain significant specimens or this reason. But as cool as this all is, the Howe Quarry skin specimens could be more useful. For instance, it is not clear how large each sauropod individual with associated skin remains was, and it's thus not clear how large those spines or scales were in comparison to each specific animal. The range of body lengths for the Howe Quarry specimens (2-3 -14 m) perhaps indicates that the scales of these animals (3 cm across) might be larger against body size than those of most other dinosaurs, but how visible they might be to observers is really dependent on knowing the sizes of the animals concerned. Likewise, the only published illustrations of these unique, interesting remains are pretty basic: it would be neat to get these specimens figured and described in a lot more detail. Hopefully, these details will be forthcoming soon.

The Sternberg/Osborn Edmontosaurus mummy

You can't discuss scaly dinosaurs without mentioning hadrosaurs. Research into hadrosaur skin is only second to that going into the fuzzballs at the other end of the dinosaur tree, there being so many skin impressions from these dinosaurs that we can gauge variation between species, see pathological skin tissues, and reconstruct virtually complete integuments for some taxa. This relative glut of data has spurned investigation into just why hadrosaur skin crops up so often. The exact cause remains elusive (it's seemingly unrelated to the rocks they occur in, nor their palaeoenvironmental or palaeoclimatic preferences), and it is suspected that there is something intrinsic to their skin anatomy which makes it more preservable (Davies 2012).

The amount of data we have for hadrosaur skin is really impressive. Here, in grey, you can see the skin impressions known for several hadrosaurid taxa: A, Brachylophosaurus canadensis; B, Edmontosaurus annectens; C, Gryposaurus notabilis; D, Maiasaura peeblesorum; E, Saurolophus angustirostris; F, Saurolophus osborni; G, Corythosaurus casuarius; H, Lambeosaurus lambei; I, Lambeosaurus magnicristatus; J, Parasaurolophus walkeri. From Bell (2014).
Even among hadrosaurids, Edmontosaurus annectens stands out as having particularly exemplar skin remains. Collectively, we have skin impressions from virtually its entire body (above). One of the most spectacular Edmontosaurus fossils with scaly remains has to be the "Trachodon mummy", discovered by George Sternberg (Charles Sternberg's son) in 1908 and described by Henry Fairfield Osborn in 1912. Osborn lavished attention on the integument of this near complete, fully articulated specimen, of which skin impressions covered the posterior jaws, neck, shoulders, chest, belly and forelimb. This specimen also revealed the presence of a low frill along at least the posterior part of the neck. Osborn's work on this animal stands out as a landmark document on extinct reptile integument, and interested parties really should download this article from the American Museum of Natural History here (NB. this is a 75 Mb download, it coming bundled with historic descriptions of the skulls of Tyrannosaurus and Allosaurus, whatever they are).

Pectoral (lower) and manual (upper) skin remains from the "Trachodon mummy" specimen. Notice the scales extending onto the unguals - these animals did not have nails or claws on their hands. From Osborn (1912).

Osborn's description revealed details of dinosaur skin which were, at that time, poorly known from other animals. He remarked on how thin the skin layer was and the remarkably small size of the scaly tubercles covering the body (1-5 mm). The fineness of the skin resulted in perhaps a third of it being accidentally destroyed during collection - 'dinosaur mummies' were an unknown quantity before this specimen, and collectors had no idea such data was at risk when skeletons were being uncovered. Edmontosaurus skin was a mosaic of larger and smaller tubercles, but their size variation is more continuous the obviously bimodal configurations of other species. The smaller (1-3 mm) tubercles were rounded structures located between larger (5-10 mm) hexagonal ones. Osborn called these 'pavement scales', and noted that they occurred in small (5-10 cm wide) clusters in some areas, such as the neck, inner surface of the arm and belly, but covered entire other parts of the body, such as the side of the chest, lateral surface of the arm and above the hips. The largest pavement scales, about 10 mm wide, occur on the lateral surface of the arm and tail. Both large and small scales occur on the frill (below). Folds, creases and smaller tubercles seem to correspond with intervertebral spaces, likely reflecting where these tissues flexed and creased with neck movement. The actual height of the frill is unknown from this specimen, the free margin being damaged during collection.

Osborn's illustration of the frill of Edmontosaurus. From Osborn (1912).
We could go on as there's so much detail on this specimen, but you're better off just checking out Osborn's description. He certainly provided lots of interesting details for artists: a visual summary of the distribution of larger and smaller scales in a cartoon hadrosaur (below), comments on his collaboration with Charles Knight to produce a 'trachodont' reconstruction in line with his new information on hadrosaur skin (also below), and even speculation on how pigmentation may pertain to the scale pattern. Of further interest is Osborn's comparison of the skin of Edmontosaurus with other hadrosaurs, this noting that the scales of his mummy specimen were a lot smaller than those of other, closely related animals. Other differences in hadrosaur skin texture has become even more apparent in subsequent years.

Left, Osborn's illustration of Edmontosaurus outlining the distribution of large scale clusters, with their size much enhanced for visibility; right, Charles Knight's iconic 1912 painting of the same taxon, an artwork produced in collaboration with Osborn and data from the "Trachodon mummy". From Osborn (1912) and The World of Charles R. Knight.

So, other than the obvious take-home - that we know a heck of a lot about the skin of Edmontosaurus -are there any obvious pointers for artists here? As noted for Carnotaurus above, it's doubtful that we'd be able to define individual scales or the patchy distribution of pavement scales on this large bodied (12-13 m long) species unless we were right next to it. Secondly, of all dinosaurs, surely this is one species to consider off limits to extensive filamentation. I suppose you could argue that filaments filled the few parts of this animal's hide left unrepresented in the fossil record, but that fuzz is going to look like weeds growing through a pavement if you're paying attention to where we know scales were. I also think it's worth paying attention to what Osborn meant by 'frill' along the back of this species: it does not appear to be a narrow, fibrous structure as commonly depicted, but a scaly continuation of adjacent dermal tissues.

The (unpublished) Triceratops superscales

I've saved what I consider to be one of the most interesting and impressive set of scale impressions for last, even though they are represented by specimens which have only currently received only very superficial publication through online news articles. These specimens belong to one of the most familiar and famous dinosaurs of all, the ceratopsid Triceratops horridus, and yet they demonstrate a scale topography completely unlike that of any other dinosaur. Their discovery is a particularly fun curve-ball because we have skin samples from a number of other ceratopsians, none of which are particularly like those now known for Triceratops. I'm reminded about earlier discussions of 'one skin fitting all': it seems ancient dinosaurs really could be just as varied in skin morphology as modern animals.

Huge patch of Triceratops skin, preserved as an internal mould - look at the size of the individual scales! Borrowed from the Rapid City Journal.

These extensive skin impressions were associated with one of the most complete Triceratops specimens ever found, a Wyoming individual known as 'Lane'. This specimen, including its skin, is now on display in the Houston Museum of Natural Science. Without a full description it's a little difficult to give much in the way of specifics about the skin, but published photographs reveal a network of very large (I'm estimating 50-60 mm wide based on the adjacent images) hexagonal tubercles dividing larger tubercles (perhaps c. 100 mm) with central, conical projections. These large scales are sometimes described being as 'nipple-like', for obvious reasons. Divisions between these tightly interlocked scales are marked, and we might have been able to distinguish individual scales on these animals from some distance away. The function of the larger tubercles with their prominences has been the source of much speculation in art - do these structures represent bosses and low spikes, or tubular supports for large, coarse filaments? I must admit to considering the latter unlikely as neither hair or scales in modern animals grow through scales, but instead around them. I'm happy to be wrong on this, though, and both interpretations could be easily tested by looking for apertures at the tip of each prominence. Hopefully these specimens will get a full write up soon, which might provide such details.

Detail of the large tubercles adorning the outside of Triceratops. Also borrowed from the Rapid City Journal.
Lane's skin impressions suggest that the scales of Triceratops were characteristically coarser, certainly a lot larger and perhaps more sculpted than those of most other dinosaurs. Their overall appearance is very different to the hadrosaur and theropod skin mentioned here, contrasts markedly from the scales known from other ceratopsians, and is rather unexpectedly most similar to the scales of sauropods. It's difficult not to intuitively equate Triceratops skin with that rhinos and armadillos: there's something almost armour-like about those heavy scales and low, projecting bosses. Perhaps this chimes with the unusually solid, reinforced cranial frill we find in this species - was Triceratops something of a horned dinosaur tank? I reckon there's a lot of fun to be had with depicting this animal as looking particularly tough and grizzled, with big skin creases and heavy folds - such a depiction can be seen at the top of this article. It's perhaps worth noting that the actual appearance of Triceratops is not a million miles off the Charles Knight's famous painting of 'Agathaumas' (probably = Triceratops) with its speculative heavy scaling.

Summary time

I hope what's becoming clear here is that we can obtain quite a lot of information from dinosaur skin impressions, and that they show scaly dinosaur species have their own characteristic integuments in the same way that filamentous ones do. There really doesn't seem to be a 'standard' type of dinosaur scale, and even closely related species show some significant variation between them. We have to conclude that those of us hoping to restore these animals accurately really need to pay close attention to these data, considering variation in tubercle size, texture and distribution. I particularly emphasise this for artists who draw every scale: if that's the route you're taking, make sure you're drawing them correctly! Moreover, the specimens outlined here are good reasons to be inventive when skin impressions are lacking. It seems most relatively extensive skin impressions of scaly dinosaurs reveal things like spines, keeled scales, armour-like structures, frilled projections and so on. Mesozoic dinosaur skin must've been as interesting as that of modern reptiles, and we might expect many species to have elaborate structures of some kind.

And that's it for 2015

OK folks, we're done here for this year, but there's plenty more to come in 2016. Weird archosauromorphs, stem mammals, some retropalaeoart and the publication of Recreating an Age of Reptiles will be covered early on. Huge thanks to everyone who's been reading and supporting this blog throughout 2015 - I hope you've enjoyed what I considered to be one of my best blogging years so far. All the best to you all for the festive period, and see you all in 2016!

References

  • Bonaparte, J. F., Novas, F. E., & Coria, R. A. (1990). Carnotaurus sastrei Bonaparte, the horned, lightly built carnosaur from the Middle Cretaceous of Patagonia. Contributions in Science. Natural History Museum of Los Angeles County, 416, 1-42.
  • Czerkas, S. A. (1992). Discovery of dermal spines reveals a new look for sauropod dinosaurs. Geology, 20(12), 1068-1070.
  • Davis, M. (2012). Census of dinosaur skin reveals lithology may not be the most important factor in increased preservation of hadrosaurid skin. Acta Palaeontologica Polonica, 59(3), 601-605.
  • Osborn, H. F. (1912). Integument of the iguanodont dinosaur Trachodon. Memoirs of the American Museum of Natural History v. 1

Friday, 11 December 2015

The lifestyle of Tanystropheus, part 2: coastal fisher or first-day-on-the-job aquatic predator?

The new Tanystropheus cf. longobardicus skeletal reconstruction I presented in my last post. What the dickens did this crazy animal do? That's what we're discussing today.
What sort of animal was the Triassic, long-necked Eurasian protorosaur Tanystropheus? As we discovered in the last post, the lifestyle of Tanystropheus remains controversial over a century after it was first discovered. There is near universal agreement that it ate swimming prey such as fish and squid, but opinion is divided over whether it was obligated to aquatic, swimming lifestyles because of the burden of its long neck, or whether it was a water margin specialist that plundered small prey from shorelines. Previously, we discussed a core argument for the aquatic hypothesis, that the Tanystropheus neck would over-balance the animal. Calculations presented in the last post suggested that the mass distribution of Tanystropheus is not as weird as we might think, and certainly less so than than that of another group of long necked reptiles we are confident lived out of water, the azhdarchid pterosaurs. Based on this very basic test, I expressed some skepticism about the neck being simply too heavy to permit a terrestrial existence.

In the second discussion, I want to look at some finer aspects of Tanystropheus anatomy and palaeontology, how they've been interpreted, and what they might mean for its lifestyle. There are several areas which are relevant here: what we know of Tanystropheus diet, the palaeoenvironmental context of Tanystropheus fossils, aspects of tail and limb anatomy, and of course, the functionality of its neck. There's a lot to get through here, so let's not waste any more time on preamble.

Fossil record

An obvious line of inquiry about ancient animal habits is the palaeoenvironmental bias of its fossil remains, and the fossil organisms it is found with. We mentioned last time that Tanystropheus was a wide-ranging taxon, occurring across Europe, Israel and China in locations representing the coasts and shallow waters around the ancient Tethys ocean. About half of Tanystropheus fossils come from shallow marine settings, the rest being derived from more coastal environments: river and estuarine environments, lagoons, intertidal settings and so forth (for a brief overview, check out the Fossilworks entry on this animal: there's a few localities missing, and the 'terrestrial' occurrence of Tanystropheus there is erroneous, but it gives a flavour of its depositional context). We often find marine fish and seagoing reptiles in the same beds as Tanystropheus, but it also occurs alongside terrestrial or freshwater species such as temnospondyls, terrestrial reptiles, stem mammals and plants in a number of locations. The link of Tanystropheus to these faunas seems complex: in at least one locality with fluctuating marine and terrigenous influences, Tanystropheus fossils only occur in horizons containing a mix of highly terrestrial and highly marine reptiles, without many 'intermediate' semi-aquatic species (Renesto 2005). Because Tanystropheus was likely not adapted for a truly seagoing lifestyle, this has been argued as evidence of it being part of a terrestrial community rather than a marine one (Renesto 2005).

Collectively, it seems difficult to argue a strong terrestrial or marine bias in this record. Tanystropheus seems to have lived in or around aquatic environments, maybe with a bias to those under marine influences, but it does not seem a stranger to brackish or freshwater settings. There is perhaps something of a skewed association with marine animals, but it occurs with enough 'terrestrial' forms to keep the idea of a coastal fishing lifestyle buoyant. It would be interesting to put some actual numbers on this and see how commonly associated with terrestrial influences Tanystropheus is, or whether a couple of sites are skewing our perception of data. Maybe that's a job for another blog post - until then, we probably need to look at other sources of information for clearer lifestyle indications.

Gut content

The idea that Tanystropheus ate swimming prey is verified by the association of digested fish remains and cephalopod hooks in the gut regions of articulated specimens (Wild 1973; Li 2007). The latter is sometimes considered smoking gun evidence for the swimming Tanystropheus lifestyle hypothesis, it being reasoned that cephalopods are exclusively marine animals, mostly found far out to sea, and unlikely to be eaten from land (e.g. Nosotti 2007).

A number of heron species, including the globally distributed black-crowned night heron (Nycticorax nycticorax), are known squid-eaters. Image from Wikimedia (CC ), by Kuribo.
Squiddy gut content certainly matches the idea of a marine-influenced lifestyle for Tanystropheus, but several non-marine, and sometimes non-aquatic, birds and mammals challenge the idea that it had to be a swimming animal to have ingested them. Examples include night herons (Hall and Cress 2008) and several types of mustelid (e.g. Hartwick 1983; Beja 1991). Exactly how night herons obtain squid is not documented in detail, but photographs of two other heron species demonstrate squid can be apprehended without venturing out to sea, or even into deep water. As might be expected, cephalopods also frequently wash up on beaches (sometimes still alive, and in huge numbers) allowing animals such as bears and wolves to also access cephalopod meat. Humans are also adept predators of squid in coastal settings. Shore-based squid angling is reportedly a growing hobby around the world (and apparently requires only very basic fishing equipment) and we routinely collect cephalopods from intertidal environments for use as bait or cooking ingredients (Denny and Gains 2007). Contrary to expectations, accessing cephalopod prey from shore environments appears quite possible for a number of differently adapted species. It seems premature to rule out a coastal fishing lifestyle for Tanystropheus just because it sometimes ate squid-like animals.

Anatomy

One of the most famous and complete Tanystropheus longobardicus specimens known, MSNM BES SC 1018. This illustration is from Nosotti's huge (2007) monograph.
With the fossil record and gut content providing slightly ambiguous insight into Tanystropheus habits, its functional anatomy is probably going to be a deciding card here. A lot has been said about the functional morphology of Tanystropheus, and there is a lack of consensus on many issues. For instance, its neck flexion has been described as almost 'swan-like' (Wild 1973); broom handle-stiff (Tschanz 1988), or somewhere inbetween (Renesto 2005). Its tail has been considered lousy for aquatic propulsion by some (Wild 1973; Renesto 2005) but well suited for the job by others (Tschanz 1988; Nosotti 2007). Clearly, some of these ideas must be erroneous, them being too polarised for all contributing parties to be correct. Such confused functional interpretations are not without precedent: Darren Naish and I noted a similar situation with azhdarchid pterosaurs in our 2008 paper: maybe this is simply what happens when we try to understand weird fossil species.

The main points of contention about Tanystropheus functional anatomy concern its tail, limbs and neck. We might link these attributes to two principle functions: locomotion and foraging. Let's start with the former. Proponents of the aquatic Tanystropheus hypothesis suggest the tail was the likely propulsive organ, it being considered that the limbs are too long and gracile to function as effective paddles (Tschanz 1988; Nosotti 2007), even if the foot might have some aquatic adaptations (below; Kuhn-Schnyder 1959; Wild 1973). Near 'horizontal' articulations between the posterior trunk and tail vertebrae appear to have permitted this part of the body to undulate laterally, permitting a crocodile-like sculling approach to swimming.

Soft-tissue preservation around the tail of Tanystropheus cf. longobardicus specimen MCSN 4451. We're looking at the underside of the tail in the left of the image here - note the width of the soft-tissue (the big grey mass). The verts on the right are shown in left lateral view. From Renesto (2005).
A fly in the ointment here is the gross tail anatomy of Tanystropheus. Rather than being long, and comprised of the robust, tall vertebrae expected of a tail-propelled aquatic reptile, its tail is slender, relatively short and actually broader than tall - hardly an ideal sculling organ (Renesto 2005). This fact has been noted by proponents of the swimming lifestyle hypothesis, and it has been proposed that the tail sported some sort of fin to modify it into a swimming organ (Nosotti 2007). Well, maybe, but this idea is entirely without support from fossil data. Readers may recall that marine reptile workers have been quite ingenious in their ability to detect fins and flukes from osteological correlates, none of which are obvious in the tail of Tanystropheus. Moreover, preserved soft-tissues from the anterior Tanystropheus tail region (above) show no signs of fins but instead a broad tail base unconducive to aquatic propulsion (Renesto 2005). Also worth mentioning is recent work on the relationship between vertebral articulation and swimming capability in crocodyliforms. They can reflect sculling behaviour, but articulations like those seen in Tanystropheus can also be linked to preventing trunk collapse during non-aquatic locomotion (Molnar et al. 2014). We could go on, but I think the point has been made that arguments for the Tanystropheus tail being a swimming organ are, at best, not without complication, and perhaps better described as uncompelling.

Turning our attention to the limbs, I mentioned in the last post that I was surprised how 'leggy' Tanystropheus was when restored as walking rather than, as we're used to seeing it, squatting. The limb proportions and girdle sizes of Tanystropheus compare well with non-aquatic protorosaurs such as Macrocnemus and Langobardisaurus (e.g. Renesto 2005; Nosotti 2007) and, as alluded to above, it is immediately clear that these limbs are not flippers. Not only are they too long and gracile for effective use as hydrofoils, but their long bones are hollow - unexpected features of an aquatic animal. Another protorosaur - Dinocephalosaurus - gives an insight into how these reptiles could modify their limbs into efficient flippers (below), and, without going into detail, they're nothing like the limbs of Tanystropheus (see Renesto 2005 for a long discussion of this). Tanystropheus limb joints are mostly robust and well-defined (but see below), and its hands and feet are strongly built and compactly structured. Some differences between hand and foot proportions can be seen: the hands are short, the feet rather long, and the latter characterised by a peculiarly long first bone in the fifth toe. The limb girdles are well developed, looking proportionally comparable (speaking from pure eyeballing here, not precise measurements) to those of large monitor lizards and crocs. I find the shoulder blade of particular interest, as it is rather large and broad, subequal in proportions to the coracoid (the lower portion of the shoulder girdle). This contrasts with many aquatic animals, which tend to maximise the size of the coracoids while reducing the scapula.

Variations in protorosaur limb anatomy, demonstrated by the aquatic Dinocepahlosaurus (A-B) and Tanystropheus (C-D). Note how both the arm (A) and leg (B) of Dinocephalosaurus are short and wide compared to their equivalents in Tanystropheus (forelimb = C, hindlimb = D), making them much more effective flippers. You can also see the reduced mineralisation in the Tanystropheus wrist here. From Renesto (2005).
I have to agree that Tanystropheus limbs were probably unchallenged by non-aquatic habits (Renesto 2005) and, if this were any other species, I don't think we'd be disputing the fact that its limbs were likely capable of terrestrial locomotion. That said, there are undeniably some hints that Tanystropheus was not always walking on land. Several authors have noted that the wrist and ankle bones of Tanystropheus are not as well ossified as those of other protorosaurs (e.g. Rieppel 1989; Nosotti 2007), and some have suggested that the pelvic bones may also be somewhat less defined (Rieppel 1989). Moreover, the elongation of the fifth toe is atypical for a purely terrestrial reptile, but common among aquatic creatures (see Kuhn-Schnyder 1959 for a good illustration of this point). Proposals that this made the foot somewhat paddle-like, or supported Tanystropheus on soft, saturated substrates do not seem unreasonable. These are fairly minor modifications to the skeleton when viewed overall however: the reduced ossification in the wrist, ankle and pelvis is pretty minor - especially when we consider how cartilage-filled the joints of many giant terrestrial archosauromorphs can be (Holliday et al. 2010) - and the reconfiguration of foot bones do not override the otherwise elongate, gracile structure of the hindlimb. My overall interpretation of the limb configuration broadly agrees with that proposed by Renesto (2005): a bauplan suited to terrestrial locomotion with some aquatic leanings, rather than sustained aquatic propulsion.

Finally, we come to the neck. I've saved discussion of this for last because I consider much of its anatomy significant in terms of where Tanystropheus lived and how it accessed food. Discussing it earlier might have rendered other points a bit superfluous. We make a lot of noise about how strange the neck of this animal is, but Tanystropheus neck anatomy frequently converges with those of other long necked reptiles - pterosaurs and sauropods - and even some long-necked mammals. That doesn't necessarily make it less weird - it's definitely still an 'extreme' biological structure - but does help us put its neck anatomy in perspective with other animals, as well as highlighting significant adaptive differences to neck elongation in aquatic and non-aquatic species.

As with pterosaurs and sauropods, Tanystropheus went to great lengths to lighten its neck. Firstly, its neck is comprised of relatively few (13), slender vertebrae rather than dozens of short ones (see Rieppel et al. 2010 for discussion of cervical vertebra counts in this animal). This is about half as many as some other protorosaurs had (Reippel et al. 2008), and a far cry from the vertebral counts of some dinosaurs (including birds). A low vertebral count reduces the number of heavy joints and muscle attachments in any part of the axial column, so this is a good basis to having a lightweight neck. More weight was lost through hollowing the bony core of each vertebra, a condition Tanystropheus took so far as to need bony struts supporting the interior cavities of each vertebra. Note that there is no evidence that these bones were pneumatised, seemingly lacking openings through which airsacs could penetrate the bone walls. However, simply removing bone - one of the densest, heaviest materials in our bodies - would still throw out a lot of weight. The neck was likely lightly muscled, the mid-series vertebrae being long tubes with highly reduced processes for muscle anchorage (below) - in many respects, the vertebral bodies are similar to those of azhdarchid pterosaurs. The role of these tubular, slender mid-series neck vertebrae is confusing at first, but they make a bit more sense once we realise that most terrestrial animals control their necks via musculature anchoring to the top and base of the neck. This was likely true for Tanystropheus and azhdarchids because anterior and posteriormost neck vertebrae are the most complex parts of the neck skeleton, presumably reflecting attachment of more muscles in these regions. We might therefore assume their necks worked in a broadly similar to those of modern animals, weird as they are.

Three dimensionally preserved mid-series Tanystropheus vertebra described by Dalla Vecchia (2005).
The seemingly lessened set of neck muscles on the Tanystropheus neck would likely limit neck performance (i.e. the size of prey that could be lifted into the air) but, again, would facilitate weight reduction. Strong, restricting joints between the majority of the neck bones and bundles of elongate cervical ribs aided reduction of musculature further, passively resisting inter-vertebral movements which otherwise require muscle action or thick ligaments to control. Elongation of cervical ribs provides another bonus for mass reduction, this trait being linked to shifting muscles down the neck in sauropods and thus lightening the neck anterior (Taylor and Wedel 2013). With passive support structures in place, muscles operating around the neck base may have been able to support and move the neck quite easily. Indeed, areas where neck elevator muscles (such as levator scapulae and the trapezius) anchor on the shoulder blade are unusually broad and well developed in Tanystropheus compared to other protorosaurs, and certainly a lot larger than those of long-necked aquatic animals (Araújo and Correia 2015). These are useful muscles to emphasise if you're looking to economise neck mass, being able to both lift and turn the neck by simply varying the symmetry of their activation. We also see a good set of short, robust cervical ribs and broad coracoids at the base of the neck, anchoring muscles related to strong downward neck motion (unless Tanystropheus differed from all other tetrapods). As Mark Robinson preemptively commented on my last post, this is starting to sound a lot like works like a mechanical crane: a lightweight, strong beam operated by long muscles and ligaments (cables and pulleys in our analogy) from a powerful, mobile base. Quite how much motion was possible at the neck base is debated, but the fact that a number of articulated Tanystropheus specimens are preserved with distinctly elevated neck bases suggests it was more flexible than the rest of the neck, and perhaps capable of a large range of motion (Renesto 2005). This, of course, has implications for balance: if the neck could be drawn up as in fossil specimens the centre of mass would be quite far back in the body (see the last post for more on Tanystropheus mass distribution).

To me, this is all sounding quite sauropod- and azhdarchid-like: an economically constructed neck capable of somewhat limited, but sufficient motion to procure food in terrestrial habits, albeit food that doesn't put up too much of a fight. By contrast, the Tanystropheus neck compares quite poorly to those of long necked aquatic animals. For one, we expect a large number of short vertebrae in long-necked aquatic animals, this permitting greater numbers of muscles working on the neck skeleton. Aquatic animal neck bones are frequently expanded to enlarge the size of muscles attaching to them, these being required to move long appendages through viscous aquatic media. This makes for a heavy neck, but perennial support provided by water renders this a moot issue. Indeed, weight is often a commodity in water rather than a problem, it providing ballast against air-filled lungs or positively buoyant tissues - it's widely known that swimming tetrapods often have entirely solid bones to increase their mass further. The neck of Tanystropheus doesn't really match any of these features. While the number of neck bones is somewhat increased compared to other protorosaurs, the aquatic Dinocephalosaurus has almost twice as many more - 25 - in a neck of similar proportions. Tanystropheus neck length is mainly achieved by stretching each vertebra tremendously, the addition of another three vertebrae perhaps merely being a supportive measure to boost neck length overall (birds and sauropods do the same thing - adding more neck vertebrae is not strictly an aquatic adaptation). Reduction of neck mass in Tanystropheus neck (and limb) bones is also at odds with expectations for an aquatic animal, the hollow cores, stiffened joints and posterior displacement of musculature being unnecessary and even disadvantageous in an aquatic setting. It's actually hard to imagine the neck of Tanystropheus being pulled through water efficiently at all, the reduced muscle profile and long vertebrae being quite problematic and under-powered for this task. It certainly does not seem well suited to chasing and grabbing fast moving aquatic prey such as squid and fish. To me, Tanystropheus neck anatomy just seems to make a lot more sense out of water and, given how much emphasis Tanystropheus put on its neck tissues, I think this is a pivotal consideration when attempting to understanding its lifestyle.

Summary time: a twist in the tale?

Let's sum up these three lines of discussion. The fossil record of Tanystropheus suggests we could find it in a variety of aquatic settings - we might average these out to say it was a denizen of coastal and nearshore environments. It clearly had a taste for seafood, although we need to be careful not to over-state what this might mean about its lifestyle. Anatomically, it seems its propulsor apparatus is best suited to non-aquatic settings and that strange neck finds overwhelmingly superior comparison to terrestrial tetrapods than it does aquatic ones. I therefore have to agree with pretty much everything said about Tanystreopheus anatomy reflecting a 'coastal fishing' habit rather than a strictly aquatic one (e.g. Renesto 2005). I actually struggle to understand how this animal would function as a swimming predator given that its anatomy seems poorly suited to an aquatic lifestyle. Indeed, even proponents of this lifestyle acknowledge that Tanystropheus must have been a sluggish, ineffectual aquatic predator, limited to ambushing prey from darkness (e.g. Nosotti 2007). This brings us to a twist to our Tanystropheus story: acknowledging some big issues with the Tanystropheus swimming hypothesis, Nosotti (2007) proposed that it was a newcomer to the aquatic realm, still carrying a lot of anatomical baggage from terrestrial ancestors. It doesn't look much like an aquatic animal because, in evolutionary terms, it's Tanystropheus first day on the job and it's still learning the adaptive ropes for being a successful marine predator.

My preferred lifestyle interpretation for Tanystropheus: a Triassic croc-o-heron which snatched prey from shorelines and promontories around coastal waterways. Note the animals perched on rocks out to sea - I have no problem with this animal swimming per se (as noted above, there is reason to think it was somewhat aquatically adapted), I just don't think it lived in water.
Personally, I find this sort of argumentation weak. It implies Tanystropheus is somehow exempt from relationships between morphology and function well established in other animals, and seems like an excuse to dismiss contrary evidence more than it does a robust hypothesis. Above all else, this proposal suffers from elevating the proposal of aquatic Tanystropheus to a foregone truth of its palaeobiology, and structuring other lines of evidence around that - I do not think this is not a positive approach to these sort of palaeontological investigations. I would argue contrarily that, when viewed in context of other tetrapods, the weight of evidence is against an aquatic lifestyle, but quite consistent with a more terrestrially-based habit, and that this forms a better starting point for considering its lifestyle. To my mind, Tanystropheus taphonomy, gut content and functional anatomy are fully consistent with it being a Triassic variant on a heron, an animal which struck at swimming prey while supported on land or in bodies of shallow water. Its smattering of minor aquatic adaptations might have been useful to cross small bodies of water, support itself on wet, soft substrates and access better fishing sites. However, the morphological onus seems to be on movement unsupported by deep water, and it might be assumed these formed a minority of adaptive pressures on Tanystropheus anatomy. Although it is difficult to think of a perfect modern analogue for this, we might find comparable functionality and behaviours in a variety of birds, crocodylians and lizards.

OK, time to call it a day with Tanystropheus for now, although we're not done with weird Triassic taxa yet. I've definitely caught their bug, and I'm sure we'll be spending time with several more of these fascinating oddballs in the near future. Before then, the last post I have planned this year returns us to familiar dinosaur territory, featuring an especially obscure species none of you will be familiar with. I can barely remember what it's called... Threecerasaurus? Trihornedabottoms? Dang - I'm sure I'll remember by next time.

This overly-long article and its artwork are made possible by Patreon

Regular readers will know that this blog and artwork is sponsored by patrons who pledge support at my Patreon page. For as little as $1 a month you can help keep this blog going and, as a reward, you get to see a bunch of exclusive content such as prints, a discount at my art store, and bonus posts not seen anywhere else. Articles posted here also typically get some 'bonus content'. For this post, I'll be discussing the scientific and palaeoartistic reasoning behind the two Tanystropheus paintings seen accompanying my two articles on this animal. As always, I'm very grateful to everyone who signs up!

References

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