Episode 3: Bernissartids, the button-toothed Crocodyliformes

3/3 - this, ladies and gentlemen, is the end. At least, until the inevitable prequels where I'll ignore the canon of the expanded universe and do my best to tarnish everything you liked about the original trilogy.

Here we are then, the last instalment of the Wealden Crocodyliformes Trilogy. Following the posts on atoposaurids and goniopholidids, today we're going out with a bang by covering a newly described Wealden crocodyliform unleashed on the world this morning. The study was written up by my University of Portsmouth chums and colleagues Steve Sweetman, Ulysse Pedreira-Segade and Steven Vidovic (Sweetman et al. 2014), and Steve V. has covered some aspects of his involvement at his blog. The paper is open-access so, for the full skinny on the discovery, you should head here.

This most recently identified Wealden crocodyliform is among the most sophisticated and unusual of all Wealden crocs. Named Koumpiodontosuchus aprosdokitii, it is known from a well-preserved skull which was recovered in circumstances owing much to chance and good fortune (Sweetman et al. 2014). This animal is currently only known for certain from the Wessex Formation of the Isle of Wight, specifically from fossil-rich cliffs next to the seaside village of Yaverland, and the only known skull of it is broken in half. The posterior half was discovered in March 2011 by holidaying fossil hunters, who took it to the local dinosaur museum (Dinosaur Isle, of Sandown) to have it identified. Another family, on a fossil-hunting holiday three months later, then found the front half of the skull. They took this to the same museum where, by chance, the same museum staff who’d handled the first piece were on hand. It was realised that each piece belonged to the same specimen, and the first half was rapidly brought back to the museum to check the degree of articulation. Remarkably, the join between the broken pieces was near perfect – clearly neither chunk had been exposed to weathering effects very long before being discovered – and the entire skull could be seen. Each piece was then donated to the museum to allow its study. Given the chain of events and people involved in the discovery of Koumpiodontosuchus, it’s easy to imagine how only single halves of the skull might be known to science, or even neither. This is clearly yet another story which stresses the importance of amateur fossil hunters to Wealden fossil discoveries, and the benefits of responsible collecting.

Holotype skull and mandible of the button-toothed crocodyliform, Koumpiodontosuchus aprosdokitii. From Sweetman et al. 2014.

Button-toothed crocodiles in context

Koumpiodontosuchus is a member of Bernissartidae, a group named by Sweetman et al. (2014) which only contains two species: Koumpiodontosuchus and Bernissartia fagesii. The latter is a famous, small Jurassic and Cretaceous crocodyliform known from France, Denmark, Spain, Portugal and particularly Belgium, where a spectacular complete skeleton has been unearthed. Indeterminate species of Bernissartia also seem to occur in the Ashdown Formation of Hastings (Salisbury and Naish 2011), but this identification may eventually warrant reappraisal now that Koumpiodontosuchus has been discovered. Bernissartid remains are not new, some of the first material of these animals being documented in the 1850s and Bernissartia itself being named from Belgian fossils in the 1880s. Isolated teeth, likely referable to Koumpiodontosuchus, have been found in Wealden deposits since at least the 1970s (Buffetaut and Ford 1979), so were clearly present across the entire geographic and stratigraphic range of the Wealden Supergroup.

Bernissartia has long been a bit of an oddball among Crocodyliformes, possessing some unusual anatomy and being of uncertain placement in crocodyliform systematics. The discovery of Koumpiodontosuchus provided a bit of light on this front, suggesting that Bernissartia was part of a group containing at least one other similar species, and that they occupy an evolutionary place between atoposaurids and the goniopholidid + Eusuchia radiation. This position isn’t too surprising, as there are a number of features in bernissartids which link them to Eusuchia – see below. Bernissartidae is primarily defined by dental characteristics, with the most obvious one also being the namesake of Koumpiodontosuchus: “button-toothed crocodile” (if anyone wants a common name for these Crocodyliformes, this is the one to use). The posterior teeth of bernissartids are rather globose – wide, short and blunt – and distinctive compared to the dentitions of most other Crocodyliformes. It’s these teeth which, even in isolation, betrayed the presence of bernissartids in the Wealden well before the more substantial Koumpiodontosuchus fossil was discovered. Their other teeth are quite different to this, however. The mid-region dentition is rather conical in shape; ‘pseudocanines’ erupt about 25 % of the jaw length from the jaw tip, and conical teeth emerge procumbently from the jaw tips themselves. Koumpiodontosuchus has two large pseudocanines on its lower jaw, which erupt so close to each other that they share a single, enlarged tooth socket. Bernissartia, by contrast, only possesses one.

The new Wealden bernissartid Koumpiodontosuchus aprosdokitii foraging for molluscs. It's eating a mud snail, Viviparus cariniferus, while tiny (6 mm long) physid gastropods Prophysa crawl over pond scum in the lower left of the image. Dragonflies provide scale, while unnamed tetanurans (based on findings of Benson et al. 2009) prowl around the background. This reconstruction is featured in Sweetman et al. (2014).

Bernissartids packed this sophisticated dentition into relatively tiny jaws: these were not big crocodyliforms. Indeed, with body lengths of approximately 600 mm, bernissartids were probably the smallest crocodyliform species in the entire Wealden succession. Like goniopholidids, bernissartids bore osteoderm shields on their backs and bellies, but the dorsal series was rather more complex than those of other Wealden crocodyliforms. Rather than possessing two rows of interlocking osteoderms as we saw in goniopholidids and atoposaurids, bernissartids possess four rows of osteoderms along their backs. These comprise two sets of rectangular, double-keeled scutes along the midline, and laterally bordering square osteoderms with single keels (Salisbury and Frey 2001). None of these interlocked, and – based on what we’ve discussed for other Wealden Crocodyliformes – it’s worth considering what impact this had on bernissartid locomotion. Rather than supporting their trunks with scutes, it seems that bernissartids developed procoelus trunk vertebrae (that is, vertebrae with centra extending into the corpus of the vertebra behind) to support their bodies when walking (Salisbury and Frey 2001). This feature, along with their relatively complex osteoderms, is shared with eusuchians and are some of the reasons why these animals have classically been allied to these Crocodyliformes. Of further interest here is the biconvex nature of the first bernissartid tail vertebra – this has further implications for their locomotion, which we’ll get to below.

The bit on palaeoecology

Ecologically, it seems that bernissartids had a preference for hard shelled prey. Their blunt posterior dentition has been labelled as ‘tribodont’ – literally meaning ‘crushing teeth’ – and, like slamming a couple of anvils together, are ideally shaped to crunch hard shells. Some confirmation of this idea is seen in the wear facets often seen on tribodont bernissartid teeth. Classically, their prey was largely considered to comprise molluscs such as the freshwater snails and clams populating Wealden streams and lakes (Buffetaut and Ford 1979). Recently, a broader diet has been postulated for bernissartids however, the logic being that hard shells are hardly restricted to molluscs even in freshwater settings (Sweetman et al. 2014). Insects and crayfish probably formed as much of their diet as molluscs, all of which were likely procured or extracted from soft-substrates with the procumbent anterior teeth. We should not forget the savage-looking pseudocanines of these animals however: these would be of little use against hard prey items, but may have allowed for spearing relatively soft-animals. Perhaps bernissartids are best viewed as rather opportunistic feeders, primarily taking hard-shelled prey but not turning their noses to other types of food when the opportunity arose.

If gastropods like this Wealden mud snail, Viviparus cariniferus, had nightmares, they contained bernissartids.

Where was most of this prey caught? There is evidence that bernissartids were equally at home in water and on land. Their biconvex first tail vertebra suggests their tails were capable of considerable movement for providing burst propulsion through water and, unlike most other Wealden Crocodyliformes, their lack of interlocking osteoderms facilitated lateral trunk motion (Salisbury and Frey 2001). While compromising overall speed, this may have permitted greater amounts of manoeuvrability – ideal for pursing nimble, if relatively slow, aquatic arthropods. We’ve already mentioned that the reinforced trunk vertebrae of bernissartids would provide ample reinforcement for terrestrial locomotion, and their small size is relevant here as well. Like the small-bodied atoposaurids, and unlike the big goniopholidids, bernissartids had relatively small amounts of weight to lug around on land and could likely sustain long periods of terrestrial locomotion without tiring. It’s possible, therefore, that they found much of their prey on land as well as in water, perhaps enjoying the beetles, cockroaches and other tough-shelled terrestrial insects known to occur in Wealden deposits.

It’s worth pointing out that bernissartids may not be the only Wealden Crocodyliformes adapted for hard-shelled prey. The poorly known, 1.5 m long Wealden eusuchian Hylaeochampsa vectiana also has large posterior teeth ideal for smashing shelled prey (Clark and Norell 1992), although the dentitions of other hylaeochampsids are complex and it’s possible Hylaeochampsa had a very varied diet. As discussed for other Wealden Crocodyliformes, it’s likely that the size difference between the bernissartids and Hylaeochampsa would prevent too much overlap in prey preference: the latter may have been capable of eating large molluscs or even small armoured vertebrates, which were probably unavailable to bernissartids. There's lots more we could say here, but I'd best not - maybe Hylaeochampsa will warrant dedicated discussion at a later date.

The end

References