This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Hey, Darren, how’s it going with that plan to discuss all the fossil crocodylomorph groups? Huh? Well, ha ha, it ain’t going so well... goddam life getting in the way of my blogging. But the publication of a new technical paper, co-authored by myself and colleagues and led by marine crocodylomorph guru Mark Young, gives me an excuse to talk about some fossil crocodylomorph groups that I haven’t much covered before. The new paper appears in a special issue of the Biological Journal of the Linnean Society devoted to the proceedings of the Lower Cretaceous symposium held at the University of Southampton back in September 2013 (and discussed briefly at Tet Zoo). The issue includes papers on Cretaceous birds, dinosaur trackways, the aerodynamics of Microraptor, and several others. And then there’s Young et al. (2014), the paper I want to talk about here.
This paper describes yet another of those frustrating fossils that combines anatomical novelty and possible phylogenetic significance with a most unsatisfactory degree of completeness. The fossil concerned is an ugly chunk of bone (going by the catchy accession number NHMUK PV OR36173), about 13 cm long, that represents the anterior part of the incomplete right dentary bone of a reasonably large, long-snouted Cretaceous crocodylomorph. It comes from Shanklin on the south-east coast of the Isle of Wight, England. While my ever-innovative co-author Lorna Steel proposed the nickname Shanklin Shocker for the creature, I’ll simply refer to it from hereon as the ‘Shanklin croc’.
The specimen isn’t completely new to the literature. We know that it was purchased by the then British Museum (Natural History) (now the Natural History Museum, or NHM) in 1861 from a Mr Simmons. Uber-prolific palaeontologist, zoologist and famed ‘lightning cataloguer’ Richard Lydekker then wrote about it in 1889 but (mis)identified it as the premaxillary bone of the plesiosaur Polyptychodon. Fast forward about a century, and Leslie Noé – known to marine reptile fans for his (still predominantly unpublished) PhD work on pliosaurids – left a note in the NHM collections saying that it isn’t a plesiosaur at all, but more likely a crocodylomorph of some sort. It’s this that bought it to the attention of NHM croc-worker Lorna Steel; Lorna has been working for some years with Mark Young on new (and/or previously overlooked or under-appreciated) crocodylomorphs. Mark and Lorna then assembled a team of people who work on relevant crocodylomorph groups. I became involved because we needed to determine whether the specimen was anything to do with the goniopholidids, a group of long-snouted Jurassic and Cretaceous crocodyliform crocodylomorphs well represented in the fossil record of the Isle of Wight. Regular readers will perhaps recall that Steve Salisbury and I reviewed the Wealden crocodyliforms back in 2011 and therein named or renamed several new species (Salisbury & Naish 2011). This isn’t the first time crocodylomorph remains have been identified as those of a plesiosaur, by the way (e.g., Buchy 2008).
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The precise provenance of the Shanklin croc is somewhat uncertain and there are several different sedimentary strata it could have come from. Based on the fossil’s colour, it seems most likely that it came from the ‘Malm rock’ of the Upper Greensand Formation, a geological unit that was deposited during the late Albian part of the Early Cretaceous. A few other units that crop out in the same area and are Aptian or Albian in age are also possible sources for the specimen, meaning that we’re giving its age as ‘?Aptian-Albian’ (Young et al. 2014). Upper Albian seems most likely, however.
Incidentally, if you’re confused as to why I’m seemingly inconsistent in my use of 'lower'/‘upper’ and 'early'/‘late’, it’s because ‘lower’ and ‘upper’ refer to the position of rock layers while ‘early' and ‘late’ refer to geological time. So, the Shanklin croc lived during the late Albian of the Early Cretaceous, but its fossil was found in the upper Albian of the Lower Cretaceous.
What sort of crocodylomorph might the Shanklin croc be? Could it belong to any of the groups already reported from the Isle of Wight Cretaceous? (these being atoposaurids, goniopholidids, bernisartiids, and hylaeochampsids). Short answer: no. Long answer: read the paper [contact any of the authors if you want a pdf]. Intermediate-length answer: as revealed by comparisons made between the Shanklin croc and the dentaries of those other groups, none possess the particular alveolar configuration or mandible shape that the Shanklin croc does, and all have peculiarities not present in the Shanklin croc (Young et al. 2014). We could also exclude several other Cretaceous crocodylomorph groups, including gavialoids, teleosaurids, metriorhynchids and the several lineages often grouped together in Pholidosauridae (Young et al. 2014). So... what is it?
Based on the presence of enlarged anterior alveoli, the shape of the bone’s outer margin, the large and widely spaced foramina on the lateral and ventral surfaces of the bone and other features, it seems most similar to species included within Dyrosauridae, a group of long-snouted crocodyliforms of the Cretaceous and Paleogene. However, it remains unusual enough compared to undoubted dyrosaurids that we have some reservations about referring it to Dyrosauridae outright. Our conservative conclusion is that the Shanklin croc should be identified as Tethysuchia incertae sedis, Tethysuchia being the crocodyliform clade that includes dyrosaurids, pholidosaurs and allied taxa.
Meet the tethysuchians
Tethysuchians haven’t been much (if at all) discussed on Tet Zoo before, so this is a good time to say some things about them. Experts have long recognised several Jurassic, Cretaceous and Paleogene crocodylomorph lineages that all look roughly alike in being long-jawed, strongly aquatic predators that have a substantially ‘archaic’ osteoderm configuration compared to living crocodylomorphs (the crocodylians). At the core of the assemblage are the dyrosaurids of the latest Cretaceous and Paleogene, and the pholidosaurs of the Early and Late Cretaceous. Benton & Clark (1988) and Clark (1994) grouped these together with the sea-going thalattosuchians into a ‘longirostrine clade’ (they didn’t give it an official name). Goniopholidids have also been allied with the ‘longirostrine clade’ in some studies (Andrade et al. 2012).
Do all of these longirostrine crocodylomorphs really go together? Well, this is controversial. If thalattosuchians really are close relatives of dyrosaurids and pholidosaurs, this makes thalattosuchians ‘advanced’ crocodyliforms, not all that far in phylogenetic distance from crown-crocs. But other studies don’t support a close relationship between thalattosuchians, goniopholidids and the other longirostrine groups, instead finding thalattosuchians to be far, far away from crown-crocs (Pol & Gasparini 2009, Sereno & Larsson 2009, Young et al. 2012, Montefeltro et al. 2013), and perhaps not even in Crocodyliformes at all. This story was summarised a while back at Tet Zoo. Most character evidence puts goniopholidids closer to crown-crocs than are dyrosaurids and pholidosaurs (e.g., Salisbury et al. 2006, Montefeltro et al. 2013), but not everyone agrees with this. Anyway, we are left with a longirostrine ‘core’ that consists only of dyrosaurids, pholidosaurs and close kin.
Buffetaut (1982) suggested use of the name Tethysuchia for this very group, and use of this name is becoming more popular (e.g., Andrade et al. 2012). It seems that ‘pholidosaurs’ are a bit of a mess, perhaps representing a paraphyletic series of outgroups to dyrosaurids. For that reason, I’ll avoid saying more about them here. At least dyrosaurids seem to be a neat, tidy group. This group is predominantly associated with Africa (and almost certainly originated there) but species also inhabited southern Asia and parts of both North and South America. Dyrosaurids were generally slender-snouted animals that probably grabbed fish but some (like giant Phosphatosaurus from the Eocene of Tunisia) have robust jaws and blunt teeth and look capable of handling robust prey items. Others (like Cerrejonisuchus from the Paleocene of Colombia) are especially robust-snouted and yet others (like Anthracosuchus, also from the Paleocene of Colombia) are comically short-snouted. [Dyrosaurus image below by incidencematrix.]
Dyrosaurid skulls are reasonably easy to recognise. The eye sockets are dorsally placed, and the supratemporal openings are about twice as long as they are wide. Dyrosaurids represented by good postcranial remains reveal especially unusual proportions. The forelimbs are long, slender and sometimes longer than the hindlimbs, tall neural spines on the dorsal vertebrae must have created a tall chest region and somewhat hump-backed overall appearance, and the tall is also especially deep and narrow. Schwarz-Wings et al. (2009) reconstructed dyrosaurid musculature and reported evidence for unusually large and strong limb, body and tail musculature. They suggested that dyrosaurids below a certain body size might therefore have been especially strong walkers, that their ability to generate thrust by the tail exceeded that of crocodylians, and that their powerful body and tail allowed them to better move and forage in strong currents. These features could conceivably have been specialisations for life in tidal, coastal habitats.
We’ll look at tethysuchians in more detail some other time. Oh, you like crocodylomorphs? Well, then, maybe you should buy the Tet Zoo Crocodylomorph Empire t-shirt...
What, then, to make of the Shanklin croc?
Our identification of the Shanklin croc as some sort of dyrosaurid-like tethysuchian makes it especially interesting, because it’s geologically older than expected based on what we know about the history of tethysuchians in general. Dyrosaurids are well known from the Maastrichtian onwards but pre-Maastrichtian specimens – some have been reported from the Cenomanian and Campanian (Buffetaut et al. 1990, Churcher & Russell 1992, Churcher 1995) – are fragmentary. Furthermore, the skull fragments concerned resemble those of some ‘pholidosaurs’ and thus might not be from dyrosaurids after all; Buffetaut et al. (1990) did report some vertebrae that look very dyrosaurid-like, however. We conclude for now that there are no definite records of dyrosaurids in the ‘middle’ Cretaceous, the implication from the fragmentary remains reported so far – the Shanklin croc included – being that there might be several ‘proto-dyrosaurid’-type taxa around from the Aptian/Albian onwards, but that better remains are needed before we can say anything with confidence.
As for which specific tethysuchian taxon the Shanklin croc represents, it’s something new: a new species and genus that needs a name. You might argue that we should have named it in the paper (part of me thinks that we should have). However, people are generally discouraged from attaching names to fragmentary specimens such as this, so we opted not to name it. As ever, we hope that newer and substantially better material will turn up in time, and that better remains from this part of the Cretaceous will help us pin down the origins and biogeographical history of one of the most important of marine crcodylomorph radiations.
Crocodylomorphs of several sorts have now been covered on Tet Zoo on a reasonable number of occasions. There’s still a lot more to do. But at least now there’s something about tethysuchians. For previous croc-themed articles, see...
Crocodylomorphs in general
Thalattosuchians
Notosuchians
Archaic neosuchians
Crocodylians
The Saltwater crocodile, and all that it implies (crocodiles part III)
Crocodiles of New Guinea, crocodiles of the Philippines (crocodiles part IV)
The Freshie: Australian crocodile, seemingly from the north (crocodiles part V)
Tool use in crocodylians: crocodiles and alligators use sticks as lures to attract waterbirds
Refs - -
Benton, M. J. & Clark, J. M. 1988. Archosaur phylogeny and the relationships of the Crocodylia. In Benton, M. J. (eds) The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Clarendon Press (Oxford), pp. 295-338.
Buffetaut, E. 1982. Radiation évolutive, paléoécologie et biogéographie des crocodiliens mésosuchiens. Mémoires de la Société Géologique de France 60, 1-85.
- ., Bussert, R. & Brinkmann, W. 1990. A new nonmarine vertebrate fauna in the Upper Cretaceous of northern Sudan. Berliner Geowissenschaftliche Abhandlungen A 120, 183-202.
Churcher, C. S. 1995. Giant Cretaceous lungfish Neoceratodus tuberculatus from a deltaic environment in the Quseir (=Baris) Formation of Kharga Oasis, Western Desert of Egypt. Journal of Vertebrate Paleontology 15, 845-849.
Churcher, C. S. & Russell, D. A. 1992. Terrestrial vertebrates from Campanian strata in Wadi el-Gedid (Kharga and Dakhleh Oases), Western Desert of Egypt. Journal of Vertebrate Paleontology 12 (Supplement), 23A.
Clark, J. M. 1994. Patterns of evolution in Mesozoic Crocodyliformes. In Fraser, N. C. & Sues, H.-D. (eds) In the Shadow of the Dinosaurs – Early Mesozoic Tetrapods. Cambridge University Press (Cambridge, NY, Melbourne), pp. 84-97.
Pol, D. & Gasparini, Z. 2009. Skull anatomy of Dakosaurus andinensis(Thalattosuchia: Crocodylomorpha) and the phylogenetic position of Thalattosuchia. Journal of Systematic Palaeontology 7, 163-197.
Schwarz-Wings, D., Eberhard Frey, E. & Martin, T. 2009. Reconstruction of the bracing system of the trunk and tail in hyposaurine dyrosaurids (Crocodylomorpha; Mesoeucrocodylia). Journal of Vertebrate Paleontology 29, 453-472.
- ., Steel, L., Foffa, D., Price, T., Naish, D. & Tennant, J. P. 2014. Marine tethysuchian crocodyliform from the ?Aptian-Albian (Lower Cretaceous) of the Isle of Wight, UK. Biological Journal of the Linnean Society 113, 854-871.