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Monday, 25 July 2016

The 'Pteranodon complex' and dismantling our understanding of the most famous flying reptile

Pteranodon longiceps, Pteranodon sp. or something else entirely? In recent years one of our most famous and abundant pterosaurs has been carved up into multiple species, but is this overzealous taxonomic splitting, or is there more to it than that?
Writing about pterosaurs can be difficult because so much of their classification is disputed. The number of pterosaur species, their assignment to different groups, appropriate clade nomenclature and the arrangement of branches in the pterosaur tree are all contested, sometimes to polarising extents.
A bastion of taxonomic stability in all this is Pteranodon, everyone's favourite giant, toothless Late Cretceous ornithocheiroid (or pteranodontoid) from interior regions of the United States. Known since the late 1860s, Pteranodon is one of the most substantially sampled of all pterosaurs and we now have well over 1100 specimens in museums around the world. This record stems from a relatively limited geographical area and is constrained stratigraphically to the Smoky Hill Chalk Member of the Niobrara Formation, with a smattering of fossils from the overlying Pierre Shale Group.

A series of papers documenting Pteranodon anatomy, variation and stratigraphy, all penned by pterosaur expert S. Christopher Bennett during the 1980s-2000s, have made this pterosaur one of the best understood of all flying reptiles (perhaps the most important entries in this series are Bennett 1992, 1993, 1994, 2001a, 2001b). These publications are the result of examining several hundred Pteranodon specimens and are among the most significant and comprehensive contributions to pterosaur literature in modern times. I recommend them to any students of vertebrate palaeontology: even if you don't agree with their conclusions, they're great examples of clear writing, of hypotheses being established and tested, and of large amounts of data being presented clearly and logically.

Skeletal restorations of P. longiceps male (the larger animal) and female morphs, based on Bennett (1992). Illustration from Witton (2013).
For pterosaur workers, one of the most important outcomes of Bennett's work was a robust taxonomy for Pteranodon. This genus was once a polyspecific monster composed of 13 species, but Bennett (1994) whittled it down to two, stratigraphically segregated forms: the geologically older Pteranodon sternbergi and its direct descendent, Pteranodon longiceps (Bennett 1994). Measurements and observations of hundreds of Pteranodon fossils and detailed analysis of its growth regime suggested that most variation seen in Pteranodon samples resulted from sexual dimorphism (above), where (presumed) males are identical to females except for being 50% larger, bearing bigger headcrests and narrower pelves (Bennett 1992). We can recognise osteologically mature Pteranodon by details of skeletal fusion, bone texture and histological structure (Bennett 1993), thus allowing us to determine that the small, 'female' individuals were not just juveniles but, in fact, relatively small adults. Although sexual dimorphism had been proposed for pterosaurs previously, few studies went to such detail in making their case and Bennett's 1992 work stands as one of the better cases made for sexual dimorphism in a fossil reptile. This complex consideration of Pteranodon diversity can be viewed as a milestone in our modernisation of pterosaur research, it being a clear sign that pterosaur studies were maturing to the level attained by dinosaur or mammal vertebrate palaeontology in the 1980s and 1990s. This work has been uncontested for over a decade and subsequent studies have since found evidence for similar morphological trends in other pterosaur species. Hurrah, hooray and huzzah for Pteranodon, then, the pterosaur worker's faithful friend and our securest mast in a taxonomic storm.

But then things got a complex

Given the established status of Pteranodon taxonomy it came as something of surprise when, in 2010, a counterargument to Bennett's interpretation of Pteranodon was published. Another big name in modern pterosaur research, Alexander Kellner, proposed that Bennett's Pteranodon was in fact a 'complex' of at least four species (perhaps five) in three genera (Kellner 2010). Kellner's alternative scheme suggested that the giant, swollen-crested sternbergi was different enough from longiceps to warrant a separate genus, and resurrected the 'subgenus' Geosternbergia for this purpose (giving us the rather daft name Geosternbergia sternbergi). A second Geosternbergia species was proposed for a partial skull referred to P. longiceps by Bennett (1994), which Kellner named G. maiseyi. Another skull, this one with a broken crest but the best preserved rostrum of any giant Pteranodon specimen, was said to represent a third pteranodontid genus, the deep-snouted Dawndraco kanzai. Finally, although not naming a new taxon, Kellner (2010) singled out another P. longiceps specimen as being distinct from this species, arguing that this long-crested specimen has a crest which is too upright to be referred to longiceps: he referred this simply to Pteranodon sp. for now. You can see these skulls, and how they contrast with Bennett's older scheme, below.
Differing interpretations of some important Pteranodon skulls. Blue text and panelling reflects the Bennett (1994) interpretation of Pteranodon skull taxonomy, green text shows where Kellner (2010) differs. Skull images borrowed from Bennett (1994).
This might not seem like a big deal - after all, famous fossil species are carved up all the time - but this has implications beyond just having to learn a few new binomials. The presence of multiple genera in our 'Pteranodon' sample makes it difficult to classify the majority of Smoky Hill pterosaur material, and thus our thousand-strong Pteranodon catalogue mostly becomes Pteranodontidae incertae sedis, with a few named skulls. With that, the statistical support for our hypotheses of Pteranodon variation, growth and sexual dimorphism require reevaluation, because we've lost our grip on what animals those hundreds of measurements actually pertain to. For pterosaur workers, this is something to pay attention to: one of our 'cornerstone' taxa might not be the dependable, go-to reference pterosaur that we thought it was, and its palaeobiology may not be as well understood as previously considered.

I've been asked about the 'Pteranodon complex' several times and thought it was time to share my thoughts here. I normally avoid talking about detailed taxonomy because I'm aware how dry it can be, but the Pteranodon controversy is pretty interesting. There are lot of strands of data to consider, some philosophising about palaeontology itself, and - if nothing else - the reality about the fossils behind Pteranodon might be of interest. This is only a summary of course - if you're interested, you really need to check out the papers cited below for the full details.


How understanding hundreds of Pteranodon specimens hinges on a handful of important ones

The holotype skull of Pteranodon longiceps, the only Pteranodon specimen which can be objectively referred to the genus. This skull is from a small (presumed female) morph. From Eaton 1910.
Since at least Eaton (1910) it's been recognised that the majority of Pteranodon specimens are not diagnostic to specific level. Most Pteranodon fossils are bits of limb or scraps of bodies that can be identified as Pteranodon (or pteranodontid, if you prefer) but not much further. To know what species we're looking at we need the back of a skull, and ideally, a big one with a good amount of crest. One of the key points to stem from both Bennett's (1994) taxonomic review and Kellner's (2010) paper is that Pteranodon species are best differentiated by the orientation and shape of their headcrests. Bennett (1994) considered this in a fairly simple way: sternbergi has an upright and distally swollen crest, while longiceps has a more posteriorly directed, distally tapering one. These distinctions can be seen in smaller skulls, but are most obvious in the bigger ones. sternbergi and longiceps might also be distinguished by the orientation of the posterior skull margin (sternbergi being more upright than longiceps) and slenderness of the mandible (sternbergi being a touch shallower) but the crest shape and angle is the best way to tell these taxa apart.

Bennett's characterisation may seem quite broad, maybe even simplistic, but there's a reason for that: no two Pteranodon crest specimens are entirely alike and none of our better, larger skull specimens are complete (below). We have some excellent and complete smaller skulls (above), and several incomplete large specimens, but any visage you see of a long skulled, long-crested Pteranodon fossil is an interpretation of fragmentary specimens. Bennett's (1994) taxonomy reflects this, using relatively broad characters to separate the species because the material ultimately offers limited scope for detailed comparison or augmentation with other characters. The fact that the crests differ somewhat within Bennett's species is explained by their likely role in visual communication rather than biomechanics (Bennett 1992; Tomkins et al. 2010): such structures are often far more variable in appearance, and sensitive to factors like ontogeny, than strictly 'functional' anatomies.
Line drawings of important Pteranodon/pteranodontid skulls from Witton (2013). A, skull still referred to P. longiceps; B, isolated crest and part of the braincase region referred to either longiceps (Bennett 1994) or Pteranodon sp. (Kellner 2010); C, holotype of longiceps; D, holotype of Pteranodon (or Geosternbergia) sternbergi. Note the twisted posterior skull face in B and how little of the skull remains in D.
Kellner (2010) argues that Bennett's interpretation accommodates too much morphological variation however, picking out several skull characters as sufficiently distinctive to warrant erecting new genera and species. The diagnoses for these new taxa are much more specific than those offered by Bennett, pertaining not only to crest shape and angle, but also size and shapes of skull bones, skull openings and rostrum morphology. Partly because these criteria are quite specific, these novel pteranodontids are currently represented by single specimens. And it's here that I think we hit a bump with the 'Pteranodon complex' hypothesis. The diagnoses are quite specific, and we have good reason to think a lot of the variation apparent in Pteranodon fossils is not taxonomic in origin. For instance, taphonomic damage and the significant crushing that affects all Pteranodon bones (most Pteranodon bones are reduced to thicknesses of mere millimetres) means no two Pteranodon skulls are identical, and many diagnostic characters suggested by Kellner (2010) - specifically those pertaining to bone lengths, fenestra sizes and so on - have yet to be demonstrated through illustrative or quantified means. We've yet to see the measurements, data tables or an illustrated series of Pteranodon skulls which show these features are atypical against a range of specimens, and thus suitable to base new taxa on.

It's not just taphonomic and diagenetic effects which are of concern: there are palaeobiological trends to consider, too. For example, Kellner (2010) uses the breadth of the crest base as a diagnostic feature for both Dawndraco and G. maiseyi, noting that the former has a crest base located largely behind the eye socket, while the latter is expanded to erupt well in front of the orbital region. But Bennett (1994) gives reason to think that crest base size is linked to growth and size, not taxonomy. As can be seen above, there's a steady correlation between crest base size and skull size: larger skulls have much thicker crest bases extending far in front of the orbit than smaller skulls (Bennett 1994, 2001a). Although Kellner (2010) mentions that Dawndraco is a relatively mature specimen, and thus maybe unlikely to change its crest size, there's no discussion of the fact that the Dawndraco skull is quite a bit smaller than some other 'large' Pteranodon skulls (below). The fact this small skull has a smaller crest is, of course, consistent with Bennett's crest scaling hypothesis. Similarly, the wide-crested maiseyi skull meets Bennett's predictions that it should - as a big individual - also have a relatively large crest base.

Dawndraco (red) is a bit of a wimp compared to the largest Pteranodon skulls. Black is the sternbergi holotype, blue is the maiseyi holotype. Note how the crest bases of the black and blue skulls are much broader than that of Dawndraco. Illustrations adapted from Bennett (1994).
Some parts of the 'Pteranodon complex' hypothesis also face issues with specimen comparability. Some allegedly diagnostic features are based on very poorly understood aspects of Pteranodon anatomy, such as the relatively deep jaw of the Dawndraco skull. According to Kellner (2010) this rostrum is diagnostically deep and peculiarly shaped: this is certainly true when compared to complete smaller Pteranodon skulls, but no large Pteranodon has well-preserved jaws and we can't compare like-with-like. The best we can do is look at fragmentary remains, all of which suggest large Pteranodon also had deep, subparallel-sided jaws (below; Bennett 1994, 2001a). However, because none of these are associated with posterior skull remains, we can't gauge their depth in any context. This being the case, the fact that Dawndraco has the deepest rostrum known from a pteranodontid is of questionable significance: similar morphologies clearly existed in other Pteranodon, we just can tell if they're identical to Dawndraco or not. Similar issues occur when trying to fathom the significance of cranial crest shape and orientation for some unusually crested specimens. Many of these crests are only partly preserved, or not associated with substantial skull remains. As noted above, we have reason to think the context of the wider skull anatomy is important for interpreting crest anatomy, and this is reason for caution when it comes to erecting new pteranodontid taxa based on these specimens. Clearly, the issue here is that we have a huge amount of data for Pteranodon, but only a tiny part of it is taxonomically relevant, and only a fraction of that portion can be compared to a meaningful degree across a good number of specimens. Big sample sizes are meant to make things clearer in science, but for Pteranodon they seem to make things more complicated!

The Dawndraco skull compared to fragmentary Pteranodon sp. jaw tips. Note how the subparallel dorsal and ventral margins and (predicted) Dawndracro overbite are present in other Pteranodon fossils. Note that some small Pteranodon have overbites too. Drawings after Bennett (1994).

Pteranodon stratigraphy and the significance (or not) of geological boundaries

Both Bennett's and Kellner's taxonomies consider Pteranodon distribution through the Niobrara Formation and neighbouring rock units, but there are fundamental differences in how they treat this data. Bennett's (1994) approach sees morphology trump stratigraphy in that the ranges of his species are dictated wholly by specimen anatomy. This is essentially the approach typically taken by biostratigraphers, where it is considered (and relied upon) that species distribution is not linked to our designation of rock units. In this scheme, it doesn't matter where the specimen occurs, but what it looks like that matters. The fact that all the 'sternbergi morphs' occur at the base of the Smoky Hill Chalk Member, and all the 'longiceps morphs' occur at the top (and somewhat beyond - see below) is the basis for Bennett's (1994) idea that our Pteranodon sample is a single, evolving population which entered the fossil record as sternbergi, and left as longiceps. The fact that these species do not overlap can be viewed as helping the verify the Pteranodon chronospecies hypothesis.

Kellner (2010) takes a different approach to stratigraphy, where provenance is a factor in the likelihood of a specimen being assigned distinct taxonomic status. A good chunk of Kellner (2010) is devoted to discussing the role of stratigraphy in taxonomy, it being argued that Pteranodon skulls found several levels away from each other were not contemporaries and thus cannot be reliably assessed for intraspecific variation. When this happens, taxonomic significance takes over as the most likely (or perhaps default) interpretation of morphological differences.

Kellner (2010) makes specific mention of the fact that neither the Dawndraco or maiseyi skulls are from the same horizons as other Pteranodon type material (below). Particular attention is drawn to maiseyi, which comes from the Sharon Springs Formation: a unit two formations above the Niobrara Formation and its glut of Pteranodon material. Of this, Kellner states: "One could argue that the morphological differences of Geosternbergia maiseyi might be due to ontogeny, individual variation or even sexual dimorphism, but there is a considerable time gap between these [pteranodontid] species that never co-existed." (Kellner 2010, p. 1078). The implication here is that there is a stratigraphic limit to when similar-looking animals might be considered conspecific, and that morphological similarity is eventually overruled by provenance.

Pteranodons in time - click to embiggen and see full details. Grey lines show distribution of key Pteranodon
specimens, black lines show those associated with skull illustrations. Skull diagrams from Bennett (1994), data from Bennett (1994); Hargrave (2007) and Kellner (2010). These discussions touch on almost philosophical elements of palaeontological science, and I expect readers will differ as to which approach they think is most useful. Personally, I don't agree with the use of stratigraphy in taxonomic considerations. It's generally accepted that paleontology uses a morphology-based species concept (morphospecies) and, if that's the case, we have to stick by it. This means letting morphology dictate the ranges of fossil species and not deciding a priori that a span of time/extent of rock exceeds an acceptable 'species range'. For abundant, well-documented groups we may be able to bolster such concepts with a sense of their speciation frequency but, with rare fossils like pterosaurs, we know next to nothing about their evolutionary rates. And as unusual as it may seem for a pterosaur to span several formations, there are taxa that seem to do this (Anhanguera, Istiodactylus, Quetzalcoatlus, Rhamphorhynchus are well known examples). Moreover, plenty of other groups pay little attention to the stratigraphic boundaries that we set. Indeed, the whole science of biostratigraphy is is more or less founded on this fact: we can date the rock record using fossils because so many species do transcend stratigraphic boundaries. Stating that a fossil cannot be conspecific with another just because it occurs in younger or older rocks seems presumptuous and at odds with trying to understand evolutionary history.

More specific concerns with the 'Pteranodon complex' approach to stratigraphy is that its perceived issue with Pteranodon ranges are not mirrored by those who work on other Niobrara Formation vertebrates. From fish to marine reptiles, it's widely thought that many Niobrara species persisted through big chunks of the three million year period recorded by the Smoky Hill Chalk Member and Pierre Shale Group (e.g. Everhart 2005; Carpenter 2008). If large swathes of the Smoky Hill Chalk fauna can survive over long periods of time, why can't Pteranodon species? It is noteworthy here that Hargrave (2007) identified new, potentially diagnostic Pteranodon longiceps bones from the Pierre Shale. If so, this bolsters older suggestions that longiceps occurs above the Niobrara Chalk (Kellner (2010) was unconvinced of their referral to longiceps, however). We might also note that the 'Pteranodon complex' taxa accord less with stratigraphy than alternatives, in that Geosternbergia disappears during the interval represented by the upper Smoky Hill Chalk and Gammon Ferruginous Formation, only to reappear in Sharon Springs beds. This is despite there being a higher number of skulls the upper Smoky Hill than any other Pteranodon bearing interval (Bennett 1994). This isn't an insurmountably complex distribution of course, but in terms of parsimony, Bennett's (1994) scheme must be seen as simpler and more congruent with stratigraphic data.

'Pteranodon complex', or Pteranodon simple?

Tying this all together, I hope it's clear that the 'Pteranodon complex' is quite a complicated issue, and one that will take some work to resolve one way or the other. I've had to skim over many of the details here, so be sure to read the papers cited herein if you'd like to read the full story. Many are available online.

It would perhaps be remiss to outline all this without giving my own take on this shake up of Pteranodon taxonomy. In my 2013 book I said I preferred Bennett's (1994) scheme and followed it accordingly and, revisiting this debate several years later has not changed my mind. I stress that I'm not 'against' the idea of more Pteranodon species, just that - in my opinion - the evidence points to Pteranodon containing longiceps and sternbergi, and that these species are each others closest relatives and might as well stay congeneric in Pteranodon. For reasons outlined above I find the stratigraphic arguments about separating these taxa unconvincing, and I don't think the morphological arguments are developed enough yet to overturn those for synonymy.

Concerning the specific taxa, the Dawndraco skull seems to be about right for a small 'male morph' P. sternbergi, and probably mostly seems atypical because of it's relatively completeness. Most large Pteranodon probably have those big rostra (you'll note that all my paintings of large Pteranodon, like that above and here, have this feature). What I've seen of its postcrania is extremely Pteranodon-like too, right down to its peculiar, highly characteristic tail (see Kellner 2010, p. 1074). I can appreciate why some folks might consider the maiseyi specimen a different taxon because of its seemingly unusual crest. However, the fact the leading crest edge is relatively complete but does not swell forwards means it is not particularly sternbergi-like, despite Kellner's (2010) suggestion that the maiseyi specimen is more closely related to sternbergi than anything else (Kellner 2010). Indeed, as preserved, the maiseyi crest meets the criteria of longiceps provided by Bennett (1994) as well as his predictions that it should have a huge crest base because of its large overall skull size. Moreover, the posterior and dorsal crest margins are broken: there is greater potential for the complete maiseyi crest to be more longiceps-like (longer, posteriorly directed) than sternbergi-like (tall, expanded forwards).

As for the large longiceps crest referred to Pteranodonsp., the specimen is not only (and obviously) very incomplete but the crest base is badly deformed, and I find it difficult to orientate the specimen against other skulls to determine the crest angle. There are suggestions that the crest base is too tall over the orbit to be longiceps (Kellner 2010) but, again, this region seems to change a lot with size and this specimen seems to have belonged to a big skull (judging by the orbit proportions): this needs to be considered carefully. The crest shape itself is generally longiceps-like, of course, and I suspect this specimen is just a big, mature version of this species.

So cheer up matey, you might not be a 'sp.' after all.
Of course, all this is subject to change should new ideas and data on Pteranodon be published in future. I should close by saying that the 'Pteranodon complex hypothesis' will soon become the 'Pteranodon complex debate': several authors are working on technical follow ups to Kellner's (2010) paper and describing relevant specimens that have bearing on this topic. This matter, then, is far from closed, and it's going to be interesting to see how it pans out. Now that we have a 'primer' article, if and when new papers are published, perhaps we'll cover them here.

This blog post on the 'Pteranodon complex' was made less complex because of support from Patreon

The paintings and words featured here are sponsored by a most excellent group of people, my Patreon backers. Supporting my blog from $1 a month helps me produce researched and detailed articles with paintings to accompany them, and in return you get access to bonus blog content: additional commentary, in-progress sneak-previews of paintings, high-resolution artwork, and even free prints. For this post, we'll be taking a look at one of the most interesting, and barely ever mentioned parts of Pteranodon anatomy. If you want to know what it is, head over to Patreon to get access!

References

  • Bennett, S. C. (1992). Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology, 12(4), 422-434.
  • Bennett, S. C. (1993). The ontogeny of Pteranodon and other pterosaurs. Paleobiology, 19, 92-106.
  • Bennett, C. S. (1994). Taxonomy and systematics of the late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occasional papers of the Natural History Museum. 169, 1-70
  • Bennett, S. C. (2001a). The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon Part I. General description of osteology. Palaeontographica Abteilung A, 1-112.
  • Bennett, S. C. (2001b). The Osteology and Functional Morphology of the Late Cretaceous Pterosaur Pteranodon Part II. Size and Functional Morphology. Palaeontographica Abteilung A, 113-153.
  • Carpenter, K. (2008). Vertebrate biostratigraphy of the Smoky Hill Chalk (Niobrara Formation) and the Sharon Springs Member (Pierre Shale). In High-Resolution Approaches in Stratigraphic Paleontology (pp. 421-437). Springer Netherlands.
  • Eaton, G. F. (1910). Osteology of Pteranodon. Connecticut Academy of Arts and Sciences, Memoirs.
  • Everhart, M. J. (2005). Oceans of Kansas. Indiana University Press.
  • Hargrave, J. E. (2007). Pteranodon (Reptilia: Pterosauria): stratigraphic distribution and taphonomy in the lower Pierre Shale Group (Campanian), western South Dakota and eastern Wyoming. Geological Society of America Special Papers, 427, 215-225.
  • Kellner, A. W. (2010). Comments on the Pteranodontidae (Pterosauria, Pterodactyloidea) with the description of two new species. Anais da Academia Brasileira de Ciências, 82(4), 1063-1084.
  • Tomkins, J. L., LeBas, N. R., Witton, M. P., Martill, D. M., & Humphries, S. (2010). Positive allometry and the prehistory of sexual selection. The American Naturalist, 176(2), 141-148.
  • Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.

Wednesday, 6 July 2016

And drepanosaurs might fly... wait, really?

Minor update (06/07/16): Thanks to Andrea Cau, a few additional citations and points of discussion have been made below - the thrust and arguments of the post are the same, but the context is improved. Thanks Andrea!

Hypuronector limnaios restored as a glider. Have palaeontologists been smoking something of variable legality, or is there some basis to this?
Assuming you've reached level 5 of palaeontological geekdom you can't fail to know of the exceptionally weird Triassic clade Drepanosauromorpha. These generally small, long-bodied reptiles are largely, but not incontrovertibly, thought to nest at the base of Archosauromorpha (so between lizards and crocs in the landscape of modern animals) and are famous for their highly aberrant anatomy. Gracile, bird-like heads and necks sit atop long, robust and tubular bodies with deepened tails and stout limbs. The hands and feet are highly modified in each species, some bearing powerful claws, others having chameleon-like opposable digits. The end of their tails are modified into either grasping, prehensile organs or sharp hooks, these being interpreted as adaptions for anchoring the tail to vegetation or substrata. Exactly what drepanosaurs did for a living has long been a subject of discussion among academics, and they are nowadays generally considered arboreal or fossorial - or a blend of both. They're pretty awesome animals.

Because the Triassic was evolution's drug-fuelled, rebellious college days, it can't be considered shocking to learn that there's a drepanosaur species which is to drepanosaurs what they are to everything else. This distinctive, strange, and controversial species is Hypuronector limnaios (above). Reasonably good fossils of this small (c. 12 cm long) animal have been known for decades from upper Triassic deposits of New Jersey, but it received its name only relatively recently (Colbert and Olsen 2001). Hypuronector is often regarded as a swimming creature because of its dorsoventrally expanded, 'leaf-shaped' tail which lacks a hooked or prehensile termination (Colbert and Olsen 2001). Its tail is remarkable for the enormous chevrons (prongs of bone projecting downwards from the underside of tail vertebrae) which extend far below and behind their vertebra of origin to create the majority of the tail depth and its 'leaf-like' profile. Some authors have likened the outline of the tail skeleton to the body shapes of gymntoid or gymnarchid fish and suggested that it propelled Hypuronector through the deep, freshwater lakes its fossils were buried in, perhaps in a newt- or crocodile-like fashion (Colbert and Olsen 2001). Although possessing unusually long legs relative to other drepanosaurs and swimming animals, it's been argued that these were also related to an aquatic lifestyle. Specifically, it's suggested that they held the long, deep tail off the ground during terrestrial bouts, the tail apparently being incapable of elevation at its base (Colbert and Olsen 2001). This aquatic Hypuronector hypothesis has been around for some time. The animal was informally known as the 'deep tailed swimmer' in the 1980s (Fraser and Renesto 2005) and this moniker was transferred more or less entirely to its scientific name in 2001: loosely translated, Hypuronector means 'deep-tailed lake swimmer'.

Hypuronector limnaios skeletal reconstruction, from Renesto et al. 2010. Scale represents 10 mm.
At first glance at least, none of this sounds too outlandish: the tail of Hypuronector certainly has an oar-like shape, and we all know that lateral undulation of a tail is the commonest means of water propulsion for vertebrates. But there are other interpretations of Hypuronector which suggest it may not have been a swimmer at all. These alternative views suggest it was more like other drepanosaurs in being suited to climbing but, more remarkably, possibly a glider (Renesto et al. 2010). Sharing early versions of my gliding drepanosaur art (above) suggests that the latter hypothesis is not well known, even among experts. However, I want to stress from the outset that this is not All Yesterdays-style artistic speculation or the bizarre opinion of a 'fringe' worker. Challenges to the aquatic Hypuronector concept and suggestions that Hypurnoector was a more 'typical' arboreal form have been made by several authors (e.g. Senter 2004; Spielmann et al. 2006; Renesto et al. 2010; Castielloa et al. 2015), and the notion that it may have been a glider has been raised on reasonable (if perhaps not yet conclusive) evidence (Renesto et al. 2010). It follows older suggestions that some drepanosaurids - Megalancosaurus specifically - were gliders (see below; Ruben 1998; Renesto 2000) and, though this all might seem bizarre, there is some genuine scientific basis to it.

The aquatic Hypuronector hypothesis under scrutiny

Aquatic drepanosaurs are were first proposed in the early 90s (Berman and Reisz 1992) and quickly received criticism from drepanosaur workers (see Renesto 2010 for history). Hypuronector perhaps remains the best candidate for an aquatic, or at least amphibious species because of its unusual tail, but somewhat ironically, it's actually this paddle-shaped organ which seems to be the main problem for this hypothesis.

Holotype of Hypuronector limnaios, a partial skeleton with the 'paddle tail' (left), disarticulated torso and bits of limb and limb girdle. From Colbert and Olsen (2001).
One thing we should address straight out is that the resemblance of the Hypuronector tail to the body of certain fishes is not a the best endorsement for swimming habits. Fish do not swim using their whole bodies (the front end of any undulating swimmers needs to be stiff), and the gymntoid or gymnarchid fish likened to the Hypuronector tail don't really move their bodies at all when swimming. Rather, they propel themselves with oscillations of long, low fins along the top of bottom of their bodies. Thus, they may be a poor shape analogue for a sculling organ, and we're better off looking at the fins and paddles of swimming animals, not their entire bodies, for clues about the aquatic potential of the Hypuronector tail.

It stands to reason that Hypuronector would have swum like a crocodylian, newt or swimming lizard, where waves of lateral undulation in the tail generate forward thrust (Colbert and Olsen 2001). This requires tail anatomy which can accommodate a lot of lateral motion, and it's here that Renesto et al. (2010) suggest we hit a major issue. The caudal vertebrae of Hypuronector seem to permit some movement at the base and tip of the tail, but the mid-tail was pretty stiff. This is because the zygaopophyeses - processes of bone that overlap neighbouring vertebrae to guide their motion - are very long and have steep articular surfaces (below). In simple terms, they seem to have 'clamped' their adjacent vertebrae rather than - as expected for an undulatory tail swimmer - provided flat, horizontal surfaces for the vertebrae to slide over.

Further rigidity is provided by those amazing chevrons (Renesto et al. 2010). These rearward-projecting bones underlie the articulations of the adjacent 7-8 vertebrae, meaning any lateral motion at the vertebral joints had to overcome the stiffness of the 7-8 bony rods hanging beneath them. Although thin bones are somewhat compliant and the Hypuronector chevrons may have been flexible to a degree, it's difficult to see their arrangement as optimised for sculling habits: they may made the tail more paddle shaped, but to obvious detriment of tail flexibility and sculling potential. Indeed, we have to note that this configuration is very similar to biological structures adapted to resist bending. Tetrapod wings are a good example: the arrangement of bat fingers, pterosaur structural fibres and bird feather shafts with respect to the wing bones echoes the chevron distribution in Hypuronector. By contrast, deep-tailed swimmers, like crocodylians and newts, have chevrons which are short, robust, and do not significantly underlie neighbouring vertebrae. They are ideal structures for anchoring tail musculature, increasing tail depth and not interfering with tail motion. I have to agree with Renesto et al. (2010) that the potential of the Hypuronector tail as a swimming organ seems limited.

Hypuronector limnaios posterior trunk (left) and tail base (right) - note elevation of the latter with respect to the former, and the significant overlap of the zygapophyses. From Renesto et al. 2010, scale represents 10 mm.
Of further relevance here are the limbs of Hypuronector, which do not have obvious aquatic signatures. Aquatic, or even semi-aquatic animals tend to have proportionally short, squat limbs, often with expanded, paddle-like bones. But the limbs of Hypuronector are elongate, gracile and hollow (Renesto et al. 2010). Its hands and feet are not well known and variably interpreted, but the elements we have suggest that they were not paddle-like. Colbert and Olsen (2001) proposed that the limbs of Hypuronector were long to lift the tail from the ground when it left the water, their work suggesting that the vertebral column was too stiff to lift the tail on its own. But this can be seen as problematic for three reasons. Firstly, as pointed out by Renesto et al. (2010), articulated fossils of Hypuronector show the tail arcing upwards with respect to the trunk vertebrae (above): this is not thought to be taphonomic or diagenetic distortion. Secondly, the forelimbs of Hypuronector are somewhat longer than the hindlimbs, which is perhaps the opposite of what we would expect if dragging the tail was a concern - surely the body would tilt backwards with this arrangement? Thirdly, since when did reptiles, aquatic or otherwise, care about dragging tails? We need to be careful that we're not providing 'empty support' for hypotheses by inventing problems for our fossil animals to solve.

Maybe Hyperonector isn't 'the weirdo drepanosaur 'after all?

Taken collectively, these points about tail shape, tail arthrology and limb size must be viewed as problematic for the aquatic Hypuronector hypothesis, and maybe we should see if there are other interpretations of Hypuronector lifestyle which are more in tune with its anatomy. A good strategy for understanding strange fossil animals is putting the controversial, weird bits of anatomy to the side and first focusing on the more reliably interpreted components. With that said, let's ignore the controversial tail of Hypuronector for a moment and look at its limbs, limb girdles and trunk anatomy. As with all drepanosaurs, the shoulder and hip bones of Hypuronector are very tall and somewhat reminiscent of the limb girdles of chameleons (Renesto et al. 2010). It is thought both limb sets were highly mobile, although the drepanosauromorph fusion of the pectoral girdle into one solid structure, as opposed to having two separate halves like chameleons, would limit forelimb reach somewhat. The limbs were likely held in a sprawling pose and, because the femora and humeri are greatly elongated, Hypuronector likely had a wide, stable base to walk and stand on.

Bits and pieces of AMNH Hypuronctor specimens, including the only known cranial material (mandible, A-C) and the ventral view of a trunk and pectoral skeleton. Note the huge, curving ribs. From Renesto et al. 2010.
Hypuronector lacks the large, fused vertebrae over the pectoral region that we see in other drepanosauromorphs, but given that these likely reflect increased forelimb muscle mass and a reinforced pectoral region for digging and prey-capture (Castielloa et al. 2015), this may not impact locomotor mechanics too much. The trunk of Hypuronector was evidently powerfully muscled all the same, the tall neural spines of the dorsal vertebrae and the presence of large, curving ribs along the entire torso suggesting large muscles enveloped most of the body.

It can be seen that Hypuronector trunk and limb anatomy matches pretty well with what we see in other drepanosaurs: powerful torsos and mobile limbs that seem well suited to walking and climbing. We might view its limb elongation as an adaptation to climbing, the increased length of the upper limb segments simultaneously increasing stability and enhancing reach while also keeping the centre of mass close to the substrate. Perhaps more surprisingly, Hypuronector is also similar to other drepanosaurs in certain aspects of tail anatomy. Although its tail has a different overall shape and lacks the derived tail-tips of true drepanosaurids, it shares the specifics of drepanosaur tail motion - flexible base and tip, rigid mid-length - with the rest of the group (Renesto et al. 2010). So perhaps the tail of Hypuronector was just a simpler, oddly-shaped variant on the drepanosauromorph tail and used for similar purposes: stability when climbing (a simple prop can aid traction, balance and recovery from accident), a brace when rearing to dig and feed, or simply for showing off (Renesto et al. 2010).

Putting these lines of evidence together, several authors have started to interpret Hypuronector as a more 'typical' drepanosaur, albeit a less-specialised species that lived like a modern arboreal lizard rather than a reptilian tree pangolin or pygmy anteater (Spielmann et al. 2006; Renesto et al. 2010). If this is true, we might view the shape of its tail as a mechanical red-herring, something which seems more important to Hypuronector behaviour than it actually was. Perhaps it had no more significance to locomotion and behaviour than do the cranial ornaments of dinosaurs and pterosaurs, structures which most now agree were more to do with communication and display than the mechanics of day-to-day life.

Yes yes yes, but we're here for the gliding stuff

Taking this idea of a climbing, generalist Hypuronector a step further, Renesto et al. (2010) note that there are several features of Hypuronector which might indicate it was a patagial glider - that is, an animal with membranes extending between its limbs to facilitate slower falls from elevated positions or glide between perches. The chief features of interest here are the the elongate limbs and, in particular, the forelimbs being as long, if not slightly longer, than the hindlimbs. This configuration is uncommon among reptiles. Well known reptiles with disproportionately long arms include canopy-browsing herbivorous dinosaurs, completely aquatic lineages like ichthyosaurs, derived sauropterygians and turtles, and flying animals like pterosaurs. It's clear that the former animals are playing an entirely different game to drepanosaurs, but the basic similarity between pterosaurs - small, gracile boned creatures which probably had climbing and gliding ancestors - and Hypuronector might be a little more intriguing. Forelimb elongation occurs again and again in patagially gliding tetrapods - pterosaurs, cologus, scaly tailed gliders etc. - and it's not unreasonable to wonder if the same phenomenon in Hypuronector betrays the presence of gliding membranes. The limb proportions of this species are not so extreme as to think it was an exemplar glider and able to travel long distances from vertical starts, but they may have housed membranes of sufficient size to cushion the fall of these small animals if they jumped or fell from high places. The deep, rounded shape of the tail becomes something to pay attention to here as well, it perhaps being well-shaped to help 'correct' a tumbling Hypuronector into the right posture for a steady glide.

Which might have been handy if the initial glide trajectory was what glider pilots call 'less than ideal'
As noted above, at least Megalancosaurus has been also posited as a potential glider in the past (Ruben 1998; Renesto 2000). These conversations were inspired (at least in part) by long-defunct (if you could ever really consider them credible!) ideas that birds may have had shared, close ancestry drepanosaurs or drepanosaur-like animals - let's quickly duck aqay further discussion of that. But why has the idea of gliding Megalancosaurus not caught on? Although not ruled out entirely (Renesto 2000), gliding doesn't seem to have stuck with this species because it its spiked tail, highly mobile wrists and ankles, and grasping appendages suggest it was quite highly adapted to climbing. While climbing and gliding are not incompatible, it also lacks features like the long, gracile limbs we would expect from flighted animals. The anatomy of Hypuronector, by contrast, is a little more generalised and ticks enough boxes in the glider column to think it could be possible.

Of course, it's worth stressing that any gliding drepanosaur is hypothetical at this stage, but we should not take this as reason to dismiss the idea out of hand. In addition to the evidence mentioned above, consider that many, perhaps all drepanosauromorphs seem to have been climbers of one kind or another, and we know from extant faunas that the step from climbing to gliding is often a short one (Renesto 2000). It's really not crazy to think extinct lineages were any less able to develop gliding forms than our modern ones, and drepanosaurs were exapted for gliding flight in many ways. Their skulls had large brains and overlapping visual fields (Renesto and Dalla Vecchia 2005) (ideal for judging distance and processing flight data); they were generally small animals with hollow limb bones (lightweight); their torsos were stiffened and reinforced (aids stability); their limbs were powerfully muscled and highly mobile (control of aerofoils) and their deep, strong tails might be ideal rudders and stabilisers. And as bizarre as it may seem to be discussing the possibility of gliding in an animal only known from bones, recall that pterosaurs were identified as flying animals in the early 1800s long before we discovered fossil remains of their wing membranes: we can identify flying animals if we look carefully enough at their bones. The challenge now is to see if we can test these ideas, perhaps carefully comparing the limb anatomy and myological signatures of Hypuronector with other drepanosaurs, modelling the effects that crazy tail has on a falling animal and so on. We can also look for Renesto et al.'s membranes on Hypuronector fossils, examining them with UV light and being extra-careful when preparing future Hypuronector specimens: experience with other delicate reptile specimens shows that it helps to know where to expect soft tissue when removing matrix.

So there we go, then: the Triassic, and drepanosaurs, might have just got even weirder/cooler/complicateder/more frustratinger than we all knew. I'm thinking we need to hang out in the Triassic even more in future blog posts - check out this label for previous conversations on Triassic topics. And note that my new art book, Recreating an Age of Reptiles, has several pages dedicated to Triassic animals - including Drepanosaurus.

This blog glides on the gentle, supportive updrafts of Patreon

The paintings and words featured here are sponsored by the organisms almost as awesome as Hypuronector: my Patreon backers. Supporting my blog from $1 a month helps me produce researched and detailed articles with paintings to accompany them, and in return you get access to bonus blog content: additional commentary, in-progress sneak-previews of paintings, high-resolution artwork, and even free prints. For this post, we'll be taking a look at a (currently unpublished) painting of a more familiar drepanosaurid.

References

  • Berman, D. S., & Reisz, R. R. (1992). Dolabrosaurus aquatilis, a small lepidosauromorph reptile from the Upper Triassic Chinle Formation of north-central New Mexico. Journal of Paleontology, 66(06), 1001-1009.
  • Castiello, M., Renesto, S., & Bennett, S. C. (2015). The role of the forelimb in prey capture in the Late Triassic reptile Megalancosaurus (Diapsida, Drepanosauromorpha). Historical Biology, 1-11.
  • Colbert, E. H., & Olsen, P. E. (2001). A new and unusual aquatic reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup). American Museum Novitates, 1-24.
  • Fraser, Nicholas C., and Silvio Renesto. Additional drepanosaur elements from the Triassic fissure infills of Cromhall Quarry, England. Virginia Museum of Natural History, 2005.
  • Renesto, S. (2000). Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia (Research In Paleontology and Stratigraphy), 106(2).
  • Renesto, S., & Dalla Vecchia, F. M. (2005). The skull and lower jaw of the holotype of Megalancosaurus preonensis (Diapsida, Drepanosauridae) from the Upper Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia (Research In Paleontology and Stratigraphy), 111(2).
  • Renesto, S., Spielmann, J. A., Lucas, S. G., & Spagnoli, G. T. (2010). The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepnosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha): Bulletin 46 (Vol. 46). New Mexico Museum of Natural History and Science.
  • Ruben, R. R. (1998). Gliding adaptations in the Triassic archosaur Megalancosaurus. Journal of Vertebrate Paleontology, 18 (3), 73A.
  • Senter, P. (2004). Phylogeny of Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology, 2(3), 257-268.
  • Spielmann J. A., Renesto S. and Lucas S. G. (2006). The utility of claw curvature in assessing the arboreality of fossil reptiles.Bulletin of the New Mexico Museum of Natural History and Science 37: 365-368.

Monday, 4 July 2016

Recreating an Age of Reptiles: the motion picture (and other promotional material)

Last week I put my new palaeoart book Recreating an Age of Reptiles on sale: you can see previews and buy copies here, and check out this post for some basic details.

If you'd like a more in-depth introduction to the project and enjoy the experience of disembodied voices narrating over slides, why not make a cup of tea and watch this 24 minute book launch video? It features some of the new art, explains why the book has such an old-timey title, and outlines some of the palaeoart philosophising that takes place therein.


To whet your appetite further, here's the full set of adverts that I put out for the book on Twitter. They set the tone for the book pretty well.





Wednesday, 29 June 2016

New palaeoart book, Recreating an Age of Reptiles, out now!


Finally, my long promised palaeoart book Recreating an Age of Reptiles is available from online retailers! Conceived as a short, 'how long can it take to publish a print-on-demand book where I have full control?' sort of project, today marks the end of the year of design, illustration and writing work it actually took to take this broad, palaeoart-led look at various parts of the Mesozoic. The result is a Letter page-sized (that's 21.59 x 27.94 cm) full-colour paperback with 108 pages of text and imagery, and over 90 bits of artwork. About 20% of the artwork has not been published anywhere before, at least not in entirety, and virtually none of the pictures have been featured in other publications. So if you're after some new entries on your palaeoart bookshelf, or hard copies of images of mine that you've seen around the internet, this might be the book for you. You can access a preview of the book interior via its page at Lulu.com.

The opening spread of the azhdarchid pterosaur section. This is one of three sections featuring flying reptiles.
The book is divided into a number of thematic sections based around animal clades, specifics of behaviour, or types of habitat. In selecting the art and generating new pieces for this project I tried to keep things varied and interesting. This is not a book dominated by any one particular type of animal, nor a tome where every picture shows prehistoric animals ripping each other's throats out (if, indeed, that can be said to feature at all). Dinosaur groups account for 50% of the book's content, with the rest taken up by mammal-like creatures, Crocodyliformes, Triassic archosauromorphs, pterosaurs and others. Many of the pictures show atypical behaviours such as burrowing, swimming, sleeping, falling over, shyness and nocturnality, and weather - rather than just variably coloured skies - plays an active role in a good number of illustrations. I'm not going to boast that "you've never seen the likes of this before!" but, presented as a collective, I hope it presents a nuanced take on Mesozoic palaeoartworks.

Brontosmash! needs a double page spread.
Although primarily an art book, I've tried to make this something worth reading too. Each picture is accompanied with details about the research, artistic decisions and researcher collaborations that informed their production. The book is bracketed by essays musing on aspects of the palaeoartistic process: how many ways we can reconstruct extinct animals without leaving the realm of scientific credibility; the role of artistic personality and biases in palaeoart; whether we constrain our art by adhering too tightly to familiar parts of science, and whether we should view the inevitable outdating of our work as positive or negative. While (hopefully) avoiding naval gazing, I've tried to outline some of my own inspirations and philosophy concerning palaeoart production throughout this text. We don't really discuss our individuality as palaeoartists very much - why we prefer certain colours or animal behaviour, why we choose certain compositions - but it's something I'm curious to hear more about from the palaeoart community, so I've shared some of my views in this book. It seems that discussing palaeoart as 'art' rather than a strictly illustrative or scientific endeavour seems like an important step to improving its standing and perceived value among its patrons.

So, where can you buy it from, and how much is it?

The cover price for Recreating an Age of Reptiles is £26, and it's available now, direct from Lulu.com (below). You can also buy it at all major online book stores (e.g. Amazon, Barnes and Noble etc.). But before you click the Amazon link, note that Lulu.com is, and will always be, the cheapest place to buy Recreating an Age of Reptiles. I've set a 5% discount at their store which means it's retailing for £24.70, not £26. I'll be honest about why I've set this incentive: major retailers take 50% of sale profits before the rest can be divided up among printers, publishers and authors, which means book authors are not left with much from their sales. Lulu offers the same shop service as anywhere else online (and you can pay with Paypal, too) and their service, in my experience, is swift and efficient - you should have the book within a week from ordering.

There are other ways you can get a copy. One way is to support me on Patreon, a copy of the book being a reward for the highest support tier. This copy will be signed and doodled on if requested. If you have any requests for a small sketch in the front pages, please let me know when you place your order!

The final way is to buy a signed and doodled copy through my website store. These are a bit more expensive than the unsigned copy, because there's two sets of shipping to factor (once to me, and then again to you) but hopefully not too steep at £30. These will be on sale any day now. As above, if you have any requests for a small sketch along with my signature please let me know when you place your order!

I'll have more info and promotional material for the book here in a few days - in the meantime, if you have any comments or questions, be sure to ask them in the comment field below, on Facebook or Twitter (#RecARep is the Recreating an Age of Reptiles hashtag). And for those who buy the book, I hope you enjoy it!

Thursday, 23 June 2016

Why the giant azhdarchid Arambourgiania philadelphiae needs a fanclub

Two giant azhdarchids, Arambourgiania philadelphiae, attempt to portion a troodontid. The troodontid objects.
When people talk about giant azhdarchid pterosaurs (odds are most readers of this blog don't need an introduction to azhdarchids, but if you do, click here) they typically mention two taxa. The first is Quetzalcoatlus northropi, a giant Texan pterosaur discovered in the 1970s and now one of the most famous pterosaurs of all (Lawson 1975, Langston 1981). The second is Hatzegopteryx thambema, a relatively robust giant discovered in the 1990s and initially - because of its size and reinforced bone construction - thought to be a giant predatory dinosaur (see Buffetaut et al. 2003). From internet forums to TV show producers, if you want to chat about giant pterosaurs, these species are your most likely subjects.

Many readers will be aware that these aren't the only giant azhdarchids, however. The record of these animals cannot be described as extensive, but it is sufficient to indicate that they were present across most of the world and probably not particularly rare in Late Cretaceous ecosystems. But most fossils of giant azhdarchids are unnameable on account of being too fragmentary, being represented by parts of undiagnostic anatomy, or being too poorly preserved. This makes it all the more surprising that the third named giant azhdarchid doesn't get much attention: the Maastrichtian species Arambourgiania philadelphiae, known from several bones from phosphate mines in Jordan.

I'm not sure why we generally overlook this giant. Perhaps it's because Arambourgiania - 'Arambourg's giant' - is one of those old-fashioned names which works better in translation than the original Greek. It certainly doesn't sound as evocative or exotic as Quetzalcoatlus or Hatzegopteryx. Moreover, it's the least known of the three named giants, being primarily represented by a long - 620 mm - cylindrical neck vertebra, and not much else. The other named giants are not well represented either, but we have more than a handful of bones for them, and they're represented by intuitively intriguing anatomies: giant wing skeletons, bits of skull and jaw and so on. But whatever the cause, there are reasons to consider our relative neglect of Arambourgiania as unwarranted. It may not be as well-known as Quetzalcoatlus, or as immediately intriguing as Hatzegopteryx, but if you're interested in giant azhdarchids (and, hey, who isn't?) you this animal deserves your attention just as much as the other species. Here are just three reasons why.


History has been unkind to Arambourgiania

We typically start the story of giant azhdarchid studies in the early 1970s and the discovery of Quetzalcoatlus, but Arambourgiania was found and described long before then. Indeed, it's among the first accounts of an azhdarchid in scientific literature. When exactly the first Arambourgiania material was unearthed remains mysterious - it was likely the late 1930s or early 1940s - but the holotype cervical vertebra emerged in a scientific paper in 1954 thanks to French palaeontologist Camille Arambourg. Five years later, he would name this bone Titanopteryx philadelphiae (Arambourg 1959), a title which would be modified to Arambourgiania in the 1980s once the preoccupation of Titanopteryx by a black fly became apparent.

Aramboug misidentified this vertebra as being wing metacarpal of a large pterosaur (below). This might seem surprising - how do you confuse a vertebra for a wing bone? - but this tubular bone must have been a bizarre object to him. Consider that no-one in the 1950s had a clue what an azhdarchid was; that no-one imagined pterosaurs could have the incredibly long necks now known for azhdarchids; and that there weren't any pterosaur specialists at this time (pterosaur researchers collectively took a breather in the early-mid part of the 20th century, only really returning to work from the 1970s onwards). The vertebra itself is near-devoid of features we would expect from an axial element, with only the lightest development of typical vertebral processes, and it has a near circular cross section, a condition at odds with a typical pterosaur vertebra but pretty typical of limb bones. In the context of the time, wing metacarpal was not a silly suggestion.

Despite his misidentification, Arambourg made one thing very clear in his reports: his animal was big. In both his 1954 and 1959 works he wrote that this bone, fragmentary as it was, clearly indicated an animal vastly superior in size to the 7 m wingspan Pteranodon, then considered the largest flying animal of all time. This is important: as early as the 1950s Arambourgiania was being interpreted as evidence that pterosaurs with wingspans rivalling small planes once existed.
Arambourg's (1954) illustration of the Arambourgiania vertebra as a wing metacarpal.
What Arambourg didn't do was elaborate on this point further: he made no fanfare about 'largest flying animal of all time' or whatever, though he might have been justified in doing so. I quite admire Arambourg's restraint in not running too far with the size of his giant: sometimes it's good to admit we don't have enough data to provide a full answer to certain questions, and given how bizarre this bone must have seemed he probably made the right call in being conservative. But his lack of excitement about his gigantic animal might explain why little fuss was made over Arambourgiania after the 1950s. The discovery of Quetzalcoatlus in the 1970s made the vertebral identification of the Arambourgiania holotype clear (Lawson 1975; Wellnhofer 1978), but no mention was made of its significant size compared to the then newly discovered Quetzalcoatlus vertebrae, nor its implication that giant azhdarchids were not only gigantic in wingspan, but must be enormous in neck proportions too.

Other authors missed the significance of Arambourgiania too. For instance, when writing about giant pterosaur flight in 1974, Cherrie Bramwell and G.R. Whitfield stated that Pteranodon was the largest flier ever. Ross Stein's (1975) work on a similar topic provided the same fact, and Wellnhofer's (1978) review of Pterosauria made no mention of the size of Arambourgiania. It wasn't until the 1980s and 1990s that Arambourg's interpretations finally penetrated the pterosaur research zeitgeist, but by this time a flurry of media and scientific attention had made Quetzalcoatlus 'the' giant pterosaur. Arambourgiania would eventually get more dedicated scientific treatment - including wingspan estimates - in the mid 1990s (Frey and Martill 1996; Steel 1997; Martill et al. 1998), but this did little to elevate the status of Arambourg's work and his giant in the story of giant azhdarchid research.

I have to admit that I'm as guilty as anyone in not been kind to Arambourgiania. In Witton (2010), a paper on the history of giant pterosaur discoveries, I didn't even feature it in this figure of 'world record' claims of pterosaur wingspans and equivalent standing heights. A, a 3 m span Andean condor (Vultur gryphus); B, 3 m span wandering albatross (Diomedea exulans); C, Marsh’s 1876 7.6 m span Pteranodon longiceps; D, Stoyanow’s 1936 (apocryphal, and never published in a peer reviewed journal) 10 m span Jurassic pterosaur; E, Harksen’s 1966 9.1 m span Pteranodon sternbergi (now considered too big - 6-7 m max is likely for Pteranodon); F, Lawson’s 1975 11 m span Quetzalcoatlus northropi; G, Buffetaut et al. (2002) 12 m span Hatzegopteryx thambema (probably a smidgen too large); H, another apocryphal giant, a 20 m wingspan form announced at the BA Festival of Science. I want to stress that this animal really, really doesn't exist. Humans used for scale are 1.75 m tall.
Of course, it's easy to see why the 1970s discovery ofQuetzalcoatlus had the impact it did: the fossil material was better, it was announced in Science, and the Texan team did a lot of work to promote their discovery (indeed, there might be more information about Quetzalcoatlus in popular articles than in scientific papers...). By contrast, Arambourg presented Arambourgiania in a couple of very dry articles, published all his work on this animal in French*, and without fanfare. Needless to say, history is more likely to record the bigger splashes than the ripples on the pond, and Quetzalcoatlus made a big splash. But with hindsight, I think we can say that the sidelining of Arambourg's work in historic accounts and our frequent omission of Arambourgiania in discussions of these animals is something we should address. Arambourg was saying decades before anyone else that Arambourgiania was significantly bigger than Pteranodon, and we have to recognise the concept of 'truly' giant pterosaurs as his creation. We might have put numbers to his animals with our 10 m wingspan estimates and 200-250 kg mass predictions, but he put the concept on paper first. The fact he did this from such scant material, and at a time when our knowledge of pterosaur palaeontology was rusty, is impressive, and it really doesn't matter that he got a few things wrong. So yeah, from now on I'm saying that Arambourgiania - not Quetzalcoatlus - was, and always has been, the original giant azhdarchid, and that Arambourg knew this decades before anyone else.

Predicted size and neckage of Arambourgiania next to a Masai giraffe and a human wife. C. Arambourg predicted this 20 years before anyone else, yet we rarely give him any credit for his insight.

Arambourgiania is more than just a neck bone

It's rarely mentioned that Arambourgiania is known from material other than just a gigantic neck bone: a smattering of other bones from the same Phosphate mines might - probably- pertain to the same species. These were re-discovered and outlined by Frey and Martill (1996), and comprise the proximal and distal end of first wing phalanx (below), and a heavily eroded bone interpreted as a second cervical vertebra. Given the uncertainty about their association with the holotype - remember that the circumstance of its collection are lost to history - Frey and Martill classified these as cf. Arambourgiania.

Line drawing and reconstruction of the lesser seen cf. Arambourgiania first wing phalanx fragment (a, c-d). That's the wing phalanx of Quetzalcoatlus sp. in panel b. Scale bars equal 20 mm, which shows the cf. Arambourgiania bone as pretty darned big. From Frey and Martill (1996).
There isn't that much which can be said about the additional cervical - it has some identifiable features, but it's a few flecks of broken bone and bumps of internal mould away from being a featureless tube. It's a little smaller in diameter than the big holotype vertebra, and much shorter. I'm not sure it should be considered as belonging to an animal of the same size as the holotype individual.

The wing phalanx elements however, are more interesting. For one, they're enormous, and look proportionate to the holotype vertebra when juxtaposed in a skeletal reconstruction (below). If they're not from the same individual, they must be from a very similarly sized one. Frustratingly, the wing phalanx ends are broken in a way that hints at the bone shaft bone surviving to the modern day as well, but being lost in recent times.
Arambourgiania (known elements in white, restored, hypothetical neck length of 2.6 m indicated by grey vertebrae) compared to Quetzalcoatlus sp. Note the chunky wing finger bones.

It might be difficult to understand why these scraps of a wing bone are exciting, but they inform us of some fundamental aspects of giant azhdarchid anatomy and wing structure. There aren't many giant pterosaurs where we have recognisable wing and neck material from the same species so, however scrappy it might be, this is already useful material for building a picture of their proportions and appearance. From a functional perspective, they are interesting in showing that wing finger of Arambourgiania articulated with the metacarpal in exactly the same way as it did in smaller pterosaurs. This is good to know, as it confirms the notion that understanding the smaller azhdarchid species is our best route to fathoming the bigger ones. And of further mechanical note is that these elements show the wing finger as proportionally robust, with a big articular surface for the metacarpal/phalanx joint and a wide space for insertion of ligaments pulling the wing open in flight. Increased robustness is a sign of greater resistance to stresses and strains, and a good indication that Arambourgiania had scaled its wing bones to be flightworthy. This is an important counterpoint to proposals from some researchers that the extreme size of giant azhdarchids rendered them flightless. Of course, these scraps of wing bone don't tell us much about flight performance or style, but they are a good indication that flight of some kind was happening in these forms.

The neck of Arambourgiania was a high point of tetrapod evolution, and we need to learn more about it

Of course, we can't talk about Arambourgiania without mentioning its long, tubular neck skeleton. To appreciate it fully, we should outline some generalities of azhdarchid neck anatomy. Proportionally speaking, azhdarchids have some of the longest necks of any tetrapod, a feat all the more remarkable given several aspects of their head and neck skeleton. While the idea of their necks being made of nothing more than simple, near-featureless tubes is overstated, we can't escape the fact that the majority of the azhdarchid neck skeleton had highly reduced features: no big processes, no elongate cervical ribs, no complicated corporeal geometry. This means they had atypically reduced opportunities for muscle attachment and soft-tissue neck support, and they must have been doing something clever to keep their necks aloft - exactly what that was remains a mystery. Like all pterosaurs, azhdarchids also only had seven 'true' cervicals (cervicals eight and nine are 'dorsalised') so that their neck length largely had to stem from just a few bones. This can be seen as peculiar as other long necked reptiles tend to increase their cervical counts to aid elongating their necks, but azhdarchids made do with their ancestral condition. The job of the azhdarchid neck was a significant one: most long necked animals have proportionally small heads, but azhdarchid heads were enormous (see Quetzalcoatlus skeletal restoration, above) and, even allowing for pneumaticity, they probably represented a good chunk of their body mass. Indeed, azhdarchid skulls are big for any tetrapod, their jaws being about about three times longer than their bodies, and those of the giants are predicted as being among the longest of any terrestrial animals, ever. The fact these huge heads were atop these long, skinny neck skeletons is pretty remarkable. In my view we should consider the azhdarchid neck as a real marvel of evolution: these animals did some pretty amazing things with an outwardly simple approach, and achieved some pretty extreme anatomy using a seemingly maladapted to enlarging neck tissues.

The 620 mm long holotype of Arambourgiania philadelphiae as illustrated by Martill et al. 1998. Top is ventral view, bottom is left lateral. Anterior is to the left of the image, scale bar is 100 mm. This bone is predicted to reach 770 mm when complete.
Taking all these points and multiplying them across the Arambourgiania holotype cervical suggests this tubular bone is a pretty fantastic piece of anatomy. We can reconstruct the length of the holotype cervical (presumed to be a fifth, the longest bone in the neck) as 770 mm, and this translates to a neck length estimates of 3 m using scaling based on Quetzalcoatlus (Frey and Martill 1996), or 2.6 m using a range of azhdarchid necks (specifically lengths of cervicals III-VII - this from my an unpublished dataset). However you want to cut it, it's clear this was a very long-necked animal, perhaps up there with the longest necked of all non-sauropodan terrestrial animals (below). On top of this we have to put a big azhdarchid skull, which is going to be about 2-3 m long for a giant. If these estimates are correct, Arambourgiania would be loaded with 5 m of neck and head, and was supporting the whole lot with a small number of bones resembling packing tubes. It has to be regarded as one of the most 'extreme' tetrapod bodyplans known.

Mike Taylor and Matt Wedel's (2013) take on the non-sauropod contest for longest tetrapod neck. It's a close call in my mind as to who wins out of Arambourgiania and a large Tanystropheus, but the important point is that Arambourgiania has an extremely long neck.
So how did the neck of Arambourgiania work? How did a series of bony tubes support a 2-3 m long head? Where did the muscles attach to on the simple structure of cervical V? Full answers to these questions remain part of a broader mystery about the functionality of azhdarchid necks, and this is something that researchers are only just starting to address. But what we know of Arambourgiania is sufficient to give some provisional, partial insight here. The basic construction of the Arambourgiania cervical is basically similar to what we see in smaller azhdarchids, where large, stiffened joints between the neck bones helped support and reinforce the neck (artists: please stop drawing azhdarchids with S-shaped necks in flight!). But subtle modifications to its vertebrae likely enabled each element to grow to much greater lengths without failing. Most azhdarchid cervicals are dorsoventrally flattened, which makes them weakest against vertical loads. Most of the time, vertical loading is created by the weight of the neck and head, but it will also include any food being picked up. The Arambourgiania cervicals are expanded dorsoventrally to the extent that they are slightly taller than wide (Frey and Martill 1996), reinforcing them against vertical bending, and thus potentially able to support greater weights than their smaller cousins. Furthermore, in expanding the bone dimensions to a near circular cross section, and all the while retaining a characteristically thin pterosaurian bone wall, Arambourgiania likely had vertebrae more resistant to torsion and bending than those of the smaller forms.

So counter-intuitive as it seems, making a neck out of tubes is a good way to produce a strong, long, lightweight skeleton, especially if it has to support heavy loads like a huge head. Pterosaurs used the same tactic to enhance their wings, and it seems azhdarchids - especially Arambourgiania - transferred some of these mechanical properties to their vertebral column. We can only guess at the exact proportions of the Arambourgiania head, but adaptations of its neck bones indicate it might have been just as large as those of its smaller cousins. While assessments like this are very basic and clearly only the tip of the iceberg as goes azhdarchid neck mechanics, they demonstrate that Arambourgiania is, and will continue to be, a critical species for understanding the neck proportions, mechanics and scaling of giant azhdarchids.

So, what I'm saying is...

These are just three reasons why we shouldn't be overlooking Arambourgiania when considering the largest pterosaurs. It might not have the sexiest name, and it might not be known from as many elements as the other named giants, but it has historic and anatomical significance that cannot, or should not, be eclipsed from other species. It's clearly an animal that needs to be brought back into the fold of popular science so, the next time giant azhdarchid pterosaurs come up in conversation, remember that there are three named giant species, not just those other two, and that forgotten, old-timer Arambourgiania still has plenty of things to tell us about giant azhdarchid palaeobiology.

Coming really, really soon: you guys like pterosaurs, right?



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References

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