July 26 , 2016

by Mark Witton

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.

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.

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

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.

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.

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!

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.

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.

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.

http://markwitton-com.blogspot.ru/2016/07/the-pteranodon-complex-and-dismantling.html