Crazy Crocodylomorphs: Mesozoic Mandibular Morphological Madness!

When most people hear the word Mesozoic, they immediately think of dinosaurs. That’s fair enough, the “terrible lizards” have had the most research and media attention devoted to them out of all the Mesozoic vertebrate groups. But that doesn’t mean that other critters that were roaming the land and sea during that era weren’t as cool or as interesting as the dinosaurs. One particularly striking example of this is the crocodiles, or more precisely the broader group to which they belong, the crocodylomorphs. These, in turn, belong to an even broader group known as the crurotarsans, so named due to a specialized articulation between their fibula and tarsus (ankle bones). Crurotarsans were hit particularly hard in the end-Triassic extinction event 201 Ma; the only surviving members of the croup were those wily crocodylomorphs.

The crocodilians we are familiar with today are all part of the same group (Neosuchia) which first appear in the Late Cretaceous and are today all more or less the same in their morphology and lifestyle. In the Mesozoic however, things could not have been more different. These crocodylomorphs were far more diverse occupying ecological niches that saw dog-like and even herbivorous forms running around on land whilst in the seas there were obligate marine forms that were almost like sharks or killer whales. These ancient crocodiles would have been truly spectacular to see alive, their fossils are certainly impressive enough! Whilst this disparity (this term is used to distinguish morphological diversity i.e. lots of different body plans from taxonomic diversity i.e. lots of different species in sheer numbers, but who may all have similar body plans) has been studied in terms of variation in cranial (skull) cladistic characters it has yet to be quantified using morphological and biomechanical variation of the mandible (jawbone). That has all changed with the publication of a new (open access) paper in the journal Proceedings of the Royal Society B (Biological Sciences).

The team, led by Bristol University PhD student Tom Stubbs have examined the diversification of Mesozoic crocodylomorph feeding ecologies by quantifying morphological and biomechanical disparity in the mandible. As Tom explains: “The ancestors of today’s crocodiles have a fascinating history that is relatively unknown compared to their dinosaur counterparts. They were very different creatures to the ones we are familiar with today, much more diverse and, as this research shows, their ability to adapt was quite remarkable. Their evolution and anatomical variation during the Mesozoic Era was exceptional. They evolved lifestyles and feeding ecologies unlike anything seen today.”

A whopping one hundred and seven mandibles were examined for the study, giving them examples of the complete spectrum of shapes and sizes the Mesozoic crocodylomorphs occupied. Why the mandible I hear you ask? There are several reasons for this: one is that the mandible is particularly well suited to the types of analyses the team were planning to conduct; secondly, as the mandible plays such a fundamental role in the animal’s life, any change in morphology will likely represent an evolutionary adaptation; and lastly, as the mandible is made up of fewer parts than the complex skull, it is more likely to preserve complete, giving researchers a larger sample size.

So what did they find? Well, there were several interesting results. Firstly, Late Triassic taxa had high disparity both in terms of their morphology and their biomechanics. This shows that crurotarsans living at this time occupied many ecological niches and employed many different feeding ecologies. Secondly, morphological disparity declined following the Late Triassic extinction event, and remained low throughout the Jurassic. Crocodylomorphs were predominantly marine during this period, the end Triassic extinction event and (potentially also) the rise of the dinosaurs preventing them from remaining successful on land. However the breakup of the supercontinent Pangaea caused the formation of new epicontinental seas where one group of crocodylomorphs in particular, the thalattosuchians, thrived. The hydrodynamic demands of living in water meant that most taxa had a similarly shaped, elongated and dorsoventrally flattened skull morphology.

The third key finding concerns the final Mesozoic period, the Cretaceous. During this period crocodylomorphs radiated into the terrestrial realm once again. With this radiation came a whole host of new morphological disparity, with taxa occupying ecological niches that had remained vacant since the Late Triassic. Interestingly, despite this proliferation of morphological disparity, biomechanical disparity did not increase. The authors proffer some potential theories as to why this may have been the case: the lifting of hydrodynamic constraints meant they were now free to evolve these new morphologies; alternatively they have evolved new biomechanical disparity in other anatomical regions, releasing the mandible from selective pressures, making mandibular evolution less significant.

This excellent study reveals that morphological and biomechanical disparity are not as entwined as you would intuitively think, but have a rather more complex relationship. Factors such as diet and habitat affect these two measures of disparity differently. Co-author in the study, Dr. Stephanie Pierce, from the Royal Veterinary College, sums it up: “Our results show that the ability to exploit a variety of different food resources and habitats, by evolving many different jaw shapes, was crucial to recovering from the end-Triassic extinction and most likely contributed to the success of Mesozoic crocodiles living in the shadow of the dinosaurs.”

A really cool paper indeed, not least because there are several parallels with what I’m hoping to do in my PhD project with fossil cetaceans (so if any of you had any ideas about doing that, I’ve got dibs!), in addition to some new ideas reading this paper has given me. I reckon the new Jurassic World movie would benefit with having a few crocodylomorphs in it. I’m sure you lot wouldn’t complain either.

Quotations were taken from the EurekAlert! Media release.

Reference

Tom Stubbs, Stephanie Pierce, Emily Rayfield and Phil Anderson (2013) Morphological and biomechanical disparity of crocodile-line archosaurs following the end-Triassic extinction. Proceedings of the Royal Society B: Biological Sciences 280: 20131940. http://dx.doi.org/10.1098/rspb.2013.1940