July 28, 2014 | 53
The event you’ve all been waiting for is here: Simbirskiasaurus and Pervushovisaurus have been resurrected, and we’re all wondering what the hell’s going on with their absurd, complex nostrils. Yes, welcome to another instalment in the long-running, slow-burning series of Tet Zoo articles on Cretaceous ichthyosaur diversity. In previous articles we’ve looked at the 2012 description of the ophthalmosaurid Acamptonectes – one of the first specimens to show that numerous ophthalmosaurid lineages survived across the Jurassic-Cretaceous boundary (Fischer et al. 2012) – and at the surprising discovery of Jurassic-style, Ichthyosaurus-like ichthyosaurs into the Early Cretaceous (Fischer et al. 2013a).
Today we look at another part of Cretaceous ichthyosaur research: it involves the dissolution of an ophthalmosaurid taxon that – supposedly – occurred worldwide during the Cretaceous, persisted (supposedly) for over 50 million years, and contained a very high number of species. The ichthyosaur concerned is Platypterygius. Here’s what I said about the status of Platypterygius in a 2012 Tet Zoo article…
“globally distributed, super-speciose Platypterygius … is increasingly being regarded as a ‘taxonomic wastebasket’ that actually contains a higher diversity than previously supposed (e.g., Maxwell & Caldwell 2006, Fischer 2012). In other words, the many species currently included within Platypterygius are almost certainly not all close relatives, and Platypterygius is (I predict) eventually going to be found non-monophyletic. The outcome of such a conclusion will be its dissolution into several discrete ‘genera’ (several authors have already gone down this route, but their efforts to name new genera for the platypterygiine taxa concerned haven’t been widely accepted).”
As hinted at in that quote, the problem with widespread, long-lived, supposedly homogenous genera like Platypterygius is that they become ever larger as people come to assume that new discoveries made elsewhere in the world simply ‘must’ belong to these widespread, long-lived, supposedly homogenous genera. We end up with a situation where distinct animals are referred to said widespread, long-lived, supposedly homogenous genera simply because this becomes the default thing to do, even though the taxon concerned ends up occupying a vast span of time (well beyond that considered normal for other taxa within its group) and a vast geographical range. Ichthyosaurs are particularly affected by this problem: partly, I think, because views on how genera should be distinguished have relied in large part on phenetic notions of overall proportional similarity, not on the recognition of special characters. I previously discussed this issue in terms of how it affects the Lower Jurassic temnodontosaurs.
Because it would be helpful to better understand the relationships between the many species included within Platypterygius, several authors have coded several Platypterygius species within phylogenetic analyses. The results? The species concerned do not form a clade (Druckenmiller & Maxwell 2010, Fischer et al. 2011a, 2014a, Fischer 2012), and the characters previously used to define Platypterygius (these include the presence of large humeral trochanters, quadrangular tooth roots and an extremely reduced extracondylar area on the basioccipital) instead appear to be synapomorphies of Platypterygiinae as a whole (Fischer 2012, Fischer et al. 2012).
In order to test how similar controversial platypterygiine species are to those taxa non-controversially regarded as part of Platypterygius, you need, of course, to go and look at them. Given that they’re known from across Europe, Australia, the USA, Argentina, Colombia, Russia and elsewhere, you have you work cut out if you want to see them all. As part of a larger study of ophthalmosaurid diversity and phylogeny, my colleague Valentin Fischer of the Université de Liège (Liège, Belgium) and the Royal Belgian Institute of Natural Sciences (Brussels, Belgium) has recently been spending time in Russia (and elsewhere), looking at specimens that supposedly belong to Platypterygius.
And, as discussed here, his analysis of two of these animals – the Russian taxa Simbirskiasaurus and Pervushovisaurus – shows that good reasons for including them within Platypterygius are lacking: both are distinct, diagnosable taxa that are not especially closely related to the ‘original’ Platypterygius species. The paper concerned, co-authored with Maxim Arkhangelsky, myself, Ilya Stenshin, Gleb Uspensky and Pascal Godefroit, has just been published in Zoological Journal of the Linnean Society (Fischer et al. 2014a). Let’s look at the (fairly obscure and unfamiliar) ichthyosaurs themselves before discussing some of the more interesting of their peculiarities and ramifications.
Simbirskiasaurus is not a nomen dubium
Simbirskiasaurus birjukovi was named in 1985 for a large, three-dimensional skull that’s about 70 cm long and consists of everything except the jaw tips and postorbital region. It was found in Barremian sediments on the bank of the Volga River in the Ulyanovsk Region, Russia. Its teeth have elongate, pointed crowns and are typical of ichthyosaurs that belong to the ‘generalist’ feeding guild seen in some other ophthalmosaurids (e.g., Fischer et al. 2011b). Since its initial description, ichthyosaur workers haven’t been kind to Simbirskiasaurus. Maisch & Matzke (2000) retained it as a valid species but argued that it should be subsumed into Platypterygius. McGowan & Motani (2003) also regarded it as similar enough to Platypterygius to be subsumed into its synonymy; they further argued that it lacked diagnostic features of its own and hence should be regarded as a nomen dubium.
In fact, Simbirskiasaurus has several weird, unique features of its own (we call unique, diagnostic features autapomorphies). A pillar-like bar of bone splits the nostril opening (properly termed the naris) in two (more on that in a moment) and the junction between the prefrontal and lacrimal bones involves a wavy, interdigitating suture. That last feature is a surprising one for an ophthalmosaurid, since it superficially recalls the condition present in the far more archaic temnodontosaurs (Fischer et al. 2014a). The exact configuration of bones around the nostril opening of Simbirskiasaurus is also unique. Accordingly, Simbirskiasaurus does not deserve to be dismissed as a nomen dubium or regarded as nondescript and undefinable: it is a valid taxon with unique features of its own.
Those who keep up with the ichthyosaur literature (it’s proving increasing hard) might think of another ophthalmosaurid when hearing about Barremian Russia: Sveltonectes, described by Valentin and colleagues in 2011 and notable for its sharply-pointed teeth and tightly packed forelimb bones (Fischer et al. 2011a). Sveltonectes resembles Aegirosaurus and Maiaspondylus in lacking striated crowns and in the weird, semi-divided form of its naris; the most recent runs of the data-set find it to be a platypterygiine (e.g., Fischer et al. 2014a). Anyway, while Sveltonectes is a Barremian ophthalmosaurid, it’s from the upper Barremian, not the lower Barremian like Simbirskiasaurus, so the two weren’t contemporaneous.
The second animal discussed in our new paper (Fischer et al. 2014a) is Pervushovisaurus bannovkensis, named in 1998 for a complete rostrum found in the middle Cenomanian sediments of the Saratov Region in Russia (a skull roof was found as well, but it’s since become lost). Again, it’s pretty big – the rostrum alone is about 1 m long. This is one of the youngest ichthyosaurs known, and its Cenomanian age means that it’s about 20 million years younger than the European type species for Platypterygius (Pl. platydactylus).
Pervushovisaurus was originally named as a ‘subgenus’ of Platypterygius but was eventually dismissed as a nomen dubium by McGowan & Motani (2003). Again, however, it clearly possesses several unusual features that support its reality as a distinct taxon: it has a suboval bony opening on the side of the premaxilla, anteroventral to the external naris, peculiar bony ridges on the side of the maxilla, and the form of the nasal and splenial bones are also distinctive. Again, the arrangement of bones around its naris is unique: new bone from the nasal bone substantially covers the naris, reducing it to a short, deep, semi-circular opening. On the basis of these features we argue that it warrants recognition as a distinct taxon that was never placed in Platypterygius for any especially good reason (Fischer et al. 2014a).
A brief aside about a non-credible source
There’s something I want to say here which isn’t especially relevant to platypterygiine ophthalmosaurids but which requires comment anyway. You’ll see from the figures reproduced throughout this article that we’ve followed a system of ‘colour coding’ individual bones in order to aid the reader’s interpretation. There’s nothing new about this: many authors, past and present, have used similar systems of colouring bones such that they can be more easily distinguished from the surrounding ones.
Some of you will be familiar with the work of David Peters, a highly prolific and idiosyncratic researcher who promotes his novel interpretations of animal morphology and phylogeny via a website and blog. As I’ve explained before, his observations (and hence hypotheses and claims regarding phylogeny, functional morphology and palaeobiology) cannot be considered reliable, since he relies on a magic eye technique whereby vague and ambiguous structures observed on photos of fossils are interpreted as genuine anatomical features that can be identified with precision and confidence. Peters uses a technique that he calls DGS (= Digital Graphic Segregation) in finding these structures: it’s a technique that involves tinkering with contrast and tones and tracing the perceived edges of structures in Photoshop. As is obvious from the reconstructions of animals he produces, this results in the picking up of all kinds of artefacts and misidentifications, and it has led Peters to make all kinds of incredible and flat-out false claims (that he has discovered the entire back end of Longsiquama, to mention just one example).
So, claims that colour-coded images like those you see in this article (and Fischer et al. 2014a, and Valentin’s other papers) represent application of DGS are an outright lie [link here: be warned, it takes you to Pterosaur Heresies]. DGS (or DGS-like) techniques are not used by working palaeontologists because it’s acknowledged in the real world that you can’t reliably identify structures by tracing their outlines on a computer screen. But do working palaeontologists colour-code bones to aid visual representation? Sure, all the time.
Ok, back to the ichthyosaurs…
Those weird, weird nostrils
These Russian ichthyosaurs are interesting for two primary reasons. One is that they’re morphologically weird, specifically with reference to their nostrils. In each nostril of Simbirskiasaurus, a vertical bony pillar descends from the nasal bone to divide the nostril opening into two distinct sections. There’s a small, oval anterior part and (posterior to the pillar) a tall, vertical posterior part. This ‘two-part’ nostril configuration is not unique to Simbirskiasaurus: it’s also present in Platypterygius australis from Australia and probably Pl. sachicarum from Colombia (Fischer et al. 2014a). Furthermore, what seems to be an antecedent condition (where a bony process descends from the nasal to reduce the depth of the naris at mid-length) is present in Ophthalmosaurus, Acamptonectes, Cryopterygius and Pl. americanus at least, with Sveltonectes possessing a ventrally descending bar that very nearly divides the naris in two.
Pervushovisaurus lacks a vertical bar, but its peculiar narial condition (a small, elongate foramen is present anteriorly, and a deep, narrow, subcircular naris is present posteriorly) seems to represent the ‘end point’ of a trend whereby the middle part of the naris was filled in by bone. The anterior foramen can therefore be regarded as a significantly reduced remnant of the original, anterior part of the naris (Fischer et al. 2014a).
Exactly what’s going on here is unknown. Why evolve these weirdly shaped nostrils in the first place, why then go on to evolve bipartate nostrils of the sort seen in Simbirskiasaurus, and why end up with a condition whereby the anterior part of the naris is represented only by foramina, as seen in Pervushovisaurus? We don’t know, of course, and further study would be needed to work out what’s going on internally and what any of this might mean as goes breathing, airflow or olfaction. Asymmetry in ichthyosaur nostrils has been remarked on several times in the past, and teardrop-shaped, comma-shaped and kidney-shaped nostrils are present in assorted ichthyosaurs, including leptonectids, Stenopterygius and ophthalmosaurids.
Maybe different parts of the nostrils had different functions, making division of this sort advantageous. It’s been suggested that ichthyosaurs had a flow-through nasal system like that also hypothesised for plesiosaurs (Cruickshank et al. 1991): supposedly, water entered the internal nostrils on the palate, flowed through the nasal chamber, and left the head via the external nostrils. Bipartate nostrils could mean that one section specifically functioned to release water while the other was used for breathing. Or maybe it was simply advantageous to roof over the anterior part of the naris? Or maybe the ichthyosaur nostril was always partially roofed by a valve or flap of some sort, and platypterygiines simply took to ossifying this flap?
Platypterygius: quo vadis?
The other big-picture thing about these ichthyosaurs concerns their implications for the fate of the name Platypterygius. The Platypterygius species do not group together in our analysis (nor in other versions of the same dataset: Fischer et al. 2012, 2013a, b; see also Druckenmiller & Maxwell 2010): Pl. hercynicus from Germany is some distance from Pl. australis (an extremely well known, well described species), with Caypullisaurus and Athabascasaurus grouping closer to Pl. australis than to Pl. hercynicus. The type species of Platypterygius – Pl. platydactylus – is of uncertain phylogenetic position.
As mentioned in the paper, we’re still not sure what to do about this and there are competing opinions. Our preferred opinion (Fischer et al. 2014a) is to restrict the name Platypterygius to Pl. platydactylus and those species most closely related to it. However, this is not ideal, creates some other problems and – as we’ve found through consultation with colleagues – is unlikely to be accepted by everyone who works on ophthalmosaurids. For now, things are undecided and more work is needed.
And that where we’ll end for now. Since the description of the new platypterygiine Sisteronia seeleyi and the resurrection of Cetarthrosaurus walkeri – both from the Cambridge Greensand of England (Fischer et al. 2014b) – we’re now in a position whereby Cretaceous ichthyosaur diversity has surpassed that of the Jurassic. The old idea that ichthyosaurs had their heyday in the Late Triassic or Early Jurassic and only had the most tenuous, low-diversity existence in Cretaceous times is now officially dead. Actually, they were thriving at reasonably high diversity from Berriasian times all the way until the Cenomanian. Notably, however, the Cretaceous ichthyosaurs are more ‘samey’ than those of the Jurassic – that is, they belong predominantly to two rather homogenous evolutionary radiations (Ophthalmosaurinae and Platypterygiinae). Regardless, these are (here comes that clichéd phrase again!) very interesting times to be interested in ichthyosaurs…
More from the world of ophthalmosaurid ichthyosaurs some time soon!
For previous Tet Zoo articles on ichthyosaurs, see…
Refs – -
Cruickshank, A. R. I., Small, P. G. & Taylor, M. A. 1991. Dorsal nostrils and hydrodynamically driven underwater olfaction in plesiosaurs. Nature 352, 62-64.
Druckenmiller, P. S. & Maxwell, E. E. 2010. A new Lower Cretaceous (lower Albian) ichthyosaur genus from the Clearwater Formation, Alberta, Canada. Canadian Journal of Earth Sciences 47, 1037-1053.
- ., Appleby, R. M., Naish, D., Liston, J., Riding, J. B., Brindley, S. & Godefroit, P. 2013a. A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs. Biology Letters 9 (4) 20130021
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