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Wednesday, 15 January 2014

Baryonyx Begins

Baryonyx walkeri, the famous, much discussed spinosaur from Lower Cretaceous Britain. A number of goniopholidids skulk in the foreground. Prints of this image can be obtained from my print store.
I assume that most people want to read new stuff when they're browsing online, and I try to take the same attitude to my own blogging. I'd much rather offer fresh perspectives or cover rarely-discussed issues than simply rehashing the same tried and tested material that we've all picked up from elsewhere. This explains why the image above, showing the famous Lower Cretaceous British spinosaurid Baryonyx walkeri, has sat on my hard-drive since November. Drafted for a project which needed a picture of a theropod, it's been collecting dust for ages while I've been wondering what to say about it here. Problem is, we all know too much about Baryonyx. It's well known that the 1986 description of Baryonyx provided the first real look anyone had ever had into spinosaur anatomy; that spinosaurids have kinda weird, superficially-crocodile like skulls which permitted feeding on a variety of prey which included fish and other dinosaurs; that the forelimb anatomy of this group, formed of robust arms and large claws, is particularly interesting... The list of things we all know about Baryonyx goes on. Is there anything left to say about this animal which hasn't been poured over dozens of times before?

Hopefully, this
There may be one component of Baryonyx palaeontology which, while hardly unknown, at least doesn't get mentioned too often. When considering the discovery of Baryonyx, we imagine the story starting in 1986 or perhaps, in 1983, when the holotype specimen was found. Eric Buffetaut's (2010) research into early spinosaurid discoveries suggested this tale warrants a prequel however, one which started a whopping 160 years before Baryonyx was found. As with other British dinosaurs such as Iguanodon and Cetiosaurus, Baryonyx also has a long history which dates back to the first dinosaur discoveries, and it even stars A-list 19th century palaeontologist celebrities.

Our story begins around 1820, when Gideon Mantell recovered, or was given, a series of conical, sharp teeth from the same Tilgate Forest quarries which would later yield the first bones of Iguanodon. These teeth came in three flavours, all variations on a conical shape with differing degrees of slenderness and curvature. They all clearly belonged to carnivorous animals. Of interest here are teeth that Mantell characterised as being paritcularly slender, laterally compressed, with carinae on the anterior and posterior margins and distinct grooves on the lingual and labial surface. To Mantell, the teeth resembled those of long-snouted crocodiles or perhaps, at the suggestion of his mentor in comparative anatomy, William Clift, monitor lizards. Mantell first put these throughts into print in 1822 and in several other publications throughout the 1820s, favouring their crocodilian identification and making repeated positive comparisons to long-snouted Crocodyliformes such as gharials and the extinct teleosaurs.

Cuvier's (1824) illustration of Mantell's Wealden 'crocodile' tooth, left, compared to the actual specimen. From Buffetaut (2010). Image borrowed from here.
Mantell was not the first to illustrate these strange Wealden teeth however. This accolade belongs to Baron Georges Cuvier who, having been sent teeth for analysis by Mantell, proceeded to illustrate them in 1824 (above). Cuvier was not above illustrating unusual fossil specimens even when his peers had more claim to the prestige of publishing them first, having also done this with Iguanodon material. Cuvier's 1824 illustration and discussion of Mantell’s alleged crocodyliform teeth agreed with Mantell’s identification, also positively comparing them with the teeth of animals we would ultimately call teleosaurs. Far from being miffed with Cuvier partially scooping his discovery, Mantell seemed chuffed that he and Cuvier were in agreement in documents published in the late 1820s. Perhaps this came as a relief after Mantell's Iguanodon teeth were identified those of a rhinoceros by Cuvier: having one of the leading comparative anatomists in the world shoot your ideas down - even if he's wrong - can't do much for one's self esteem.

A few years later saw entry of Sir Richard Owen into discussions of Mantell’s alleged crocodyliform Wealden teeth. In Owen’s seminal Odontography, a major overview of animal teeth published from 1840-1845, he named them Crocodilius (Suchosaurus) cultridens. Like Cuvier and Mantell, Owen also made several favourable comparisons between the Wealden teeth and other fossil Crocodyliformes, including teleosaurs*. In a contemporary work, Report on British Fossil Reptiles (1842), Owen noted some subtle differences between the cross-sectional shape and carinae position of the Wealden teeth and those of crocodiles, although he maintained a crocodyliform referral for the teeth at this time. Later, Owen seemed less certain about this identification, noting in an 1878 publication that his subgenus Suchosaurus had a ‘nearer affinity or transition to the Dinosaurian order than does any of the Mesozoic Crocodilia’. This was the first clear indication that all was not as seemed with Mantell's early Wealden discovery. However, an 1884 Owen manuscript which basically rehashed his 1878 discussion of Suchosaurus, contained none of the doubt he expressed in 1878. Clearly, Owen had changed his mind about the similarity of the Mantell's 'crocodile' teeth to those of dinosaurs.

*The story becomes somewhat complicated here by the referral of a vertebra to Suchosaurus, which was suggested as early as 1888 to represent an iguanodont and remains that way today. However, this was not immediately accepted by 19th century naturalists and it is not is not always clear if they are discussing the vertebra or teeth when considering the affinities of Suchosaurus.

Most 19th century palaeontologists followed Owen’s lead in considering Suchosaurus a crocodile-like animal, but others were not convinced. Felix Plieninger (1846) and John Hulke (1979) both suggested that Suchosaurus had greater affinity with Dinosauria than other reptiles. Plieninger was definitely basing his discussion on the Suchosaurus teeth, although it's not clear whether Hulke is considering these or the vertebra also referred to this taxon. Other authors, such as Heinrich Georg Bronn (1849) considered Suchosaurus a reptile of uncertain identity, at least for a while (he later agreed with Owen). The mystery surrounding Suchosaurus was more-or-less left there however, as these were among the last discussions Suchosaurus was to receive in palaeontological literature. Although a second Suchosaurus species was named in 1897-1898 for a Portuguese tooth, the taxon fell out of regular use in discussions of Wealden animals – even its Crocodyliformes – in the 20th century.

Back to the future
Fast forward to the 1980s and, to everyone’s delight, Baryonyx walkeri was discovered and described from a fairly complete skeleton found in Surrey. Among the many surprises associated with its discovery were its slender, laterally compressed and grooved teeth, a distinctive dentition that saw any similar isolated tooth from the Wealden being allocated to Baryonyx (e.g. Martill and Hutt 1996 - see below). It didn’t take long the penny to drop: by 2003 it was realised that the teeth of Baryonyx were very, very similar to those of Suchosaurus (Milner 2003). After nearly 200 years, the real identity of Suchosaurus was revealed: a spinosaurid theropod. This meant that, far from being relatively new discoveries for the Wealden, spinosaurs were actually among the very first animals to be documented from the British Lower Cretaceous. Hats must be tipped to the likes of Plieninger and Hulke and, to a lesser extent, Owen and Bronn, for their insightful taxonomic comments on the very fragmentary material they had to work with. Each of them saw, to greater and lesser extents, past the crocodile-like appearance of the Suchosaurus teeth to suggest they may be crocodile-like dinosaurian reptiles. That's pretty good going considering what they had to work with.
Isolated teeth attributed to baryonychine theropods from the Wessex Formation, Isle of Wight. From Martill and Hutt (1996).
Unsurprisingly, several authors have mentioned the likelihood that Suchosaurus and Baryonyx are one and the same (e.g. Buffetaut 2010; Mateus et al. 2011). Although their teeth do differ in subtleties of carinae and groove development, it has long been noted (even by Mantell way back in the 1820s) that such particulars of dentition are readily worn away in life or taphonomy. It is therefore possible, maybe probable, that Suchosaurus and Baryonyx are synonyms, the teeth of the former simply being worn versions of the latter. If so, the animal we know as Baryonyx was actually one of the first dinosaurs ever found. But if it's likely that Baryonyx and Suchosaurus are the same animal, why are we still talking about Baryonyx instead of resurrecting Suchosaurus as the Wealden spinosaurid genus? There are several reasons. Firstly, the dentition of Baryonyx is not unique among baryonychines, creating the (admittedly unlikely) possibility of Suchosaurus being synonymous with another baryonynchine taxon. A second possibility, that Suchosaurus is a second Wealden baryonychine, presents another problem. But most important is our third reason: the type material of Suchosaurus possesses no real defining features, giving us nothing to diagnose this genus with. This echoes the situation of many ‘classic’ dinosaur species of course, and palaeontologists sometimes work taxonomic magic to transfer their name to other, diagnostic specimens. There seems little reason to do this for Suchosaurus however, it being a fairly obscure and under-discussed animal for much of the 20th century. Unlike Iguanodon or Allosaurus, which have received name transfer treatment in the past, Suchosaurus is not a familiar animal with a poor type specimen, so it isn’t too criminal to refer the type specimen to Spinosauridae indet. and let the name Suchosaurus slip into the nomen dubium realm of obscurity.

So there we have it, then: Baryonyx and Wealden spinosaurids have, from a certain point of view, been known for as long as any other dinosaur you care to mention, and some folks had an inkling of spinosaurid's superficially crocodile-like morphology even when they were only known from teeth. For more on this story and the early discoveries of spinosaurids, be sure to check out Buffetaut (2010), Mateus et al. (2011). To find out happened to the rest of the 'crocodile' teeth handled by Mantell in the 1820s, read Salisbury and Naish (2011). Further, exciting news on the developing science of Wealden spinosaurs can be found in this near-recent blog post over at Mark Wildman's Saurian.

References
  • Bronn, H. G. (1849). Index Palaeontologicus, 2. Abetheilung. Schweizerbart, Stuttgart.
  • Cuvier, G. (1824). Recherche sur les ossemens fossiles, tome V, 2ème partie. Dufour et E. d'Ocagne, Paris.
  • Hulke, J. W. (1879). Vectisaurus valdensis, a new Wealden dinosaur. Quarterly Journal of the Geological Society, 35(1-4), 421-424.
  • Mantell, G. A., & Mantell, M. A. (1822). The fossils of the South Downs; or illustrations of the geology of Sussex. Lupton Relfe, London.
  • Martill, D. M., & Hutt, S. (1996). Possible baryonychid dinosaur teeth from the Wessex Formation (Lower Cretaceous, Barremian) of the isle of Wight, England. Proceedings of the Geologists' Association, 107(2), 81-84.
  • Mateus, O., Araújo, R., Natário, C., & Castanhinha, R. (2011). A new specimen of the theropod dinosaur Baryonyx from the early Cretaceous of Portugal and taxonomic validity of Suchosaurus. Zootaxa, 2827, 54-68.
  • Milner, A. C. (2003). Fish-eating theropods: a short review of the systematics, biology and palaeobiology of spinosaurs. Journadas Internacionales sobre paleontologiá de Dinosaurios y su Entoro, 2, 129-138.
  • Owen, R. (1840–1845). Odontography. Hippolyte Bailliere, London.
  • Owen, R. (1842). Report on British fossil reptiles. Part II. Reports of the meetings of the British Association for the Advancement of Science, 11, 61-204.
  • Plieninger, T. (1846). Über ein neues Sauriergenus und die Einreihung der Saurier mit flachen, schneidenden Zähnen in eine Familie. Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg, 2(1), 148-154.
  • Salisbury, S. W. & Naish, D. (2011). Crocodilians. In Batten, D. J. (ed.) English Wealden Fossils. The Palaeontological Association (London), pp. 305-369.

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