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.

This is what we picture crocodiles as today. In the Mesozoic however, dog-like, fully marine, and even herbivorous crocs were running and swimming around! Image source: news.com.au

This is what we picture crocodiles as today. In the Mesozoic however, dog-like, fully marine, and even herbivorous crocs were running and swimming around! Image source: news.com.au

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).

A sample of the morphological diversity seen in Mesozoic crocodiles, with the  lower jaws highlighted in anatomical position. From top to bottom the animals  are: Goniopholis (Jurassic to Cretaceous), Simosuchus (Cretaceous), Dakosaurus  (Jurassic to Cretaceous), Cricosaurus (Jurassic to Cretaceous) and  Mariliasuchus (Cretaceous). The silhouettes are not to scale. Image created by  Tom Stubbs using silhouettes from http://phylopic.org. Image source EurekAlert! media release.

A sample of the morphological diversity seen in Mesozoic crocodiles, with the
lower jaws highlighted in anatomical position. From top to bottom the animals
are: Goniopholis (Jurassic to Cretaceous), Simosuchus (Cretaceous), Dakosaurus
(Jurassic to Cretaceous), Cricosaurus (Jurassic to Cretaceous) and
Mariliasuchus (Cretaceous). The silhouettes are not to scale. Image created by
Tom Stubbs using silhouettes from http://phylopic.org. Image source EurekAlert! media release.

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.

A sample of jaws from the Mesozoic crocodile fossil record. From top to bottom  jaws are from: Kaprosuchus (Cretaceous) (image by Carol Abraczinskas),  Simosuchus (Cretaceous), Mariliasuchus (Cretaceous) (courtesy of The American  Museum of Natural History), Dakosaurus (Jurassic to Cretaceous) and Cricosaurus  (Jurassic to Cretaceous) (courtesy of Jeremías Taborda). Image created by Tom Stubbs. Image source EurekAlert! media release.

A sample of jaws from the Mesozoic crocodile fossil record. From top to bottom
jaws are from: Kaprosuchus (Cretaceous) (image by Carol Abraczinskas),
Simosuchus (Cretaceous), Mariliasuchus (Cretaceous) (courtesy of The American
Museum of Natural History), Dakosaurus (Jurassic to Cretaceous) and Cricosaurus
(Jurassic to Cretaceous) (courtesy of Jeremías Taborda). Image created by Tom Stubbs. Image source EurekAlert! media release.

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.

These two graphs show how morphological (top) and biomechanical (bottom) disparity of the mandible changed over the course of the Mesozoic. Image modified from Stubbs et al. 2013.

These two graphs show how morphological (top) and biomechanical (bottom) disparity of the mandible changed over the course of the Mesozoic. Image modified from Stubbs et al. 2013.

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

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About Travis Park

I am a PhD student in Palaeontology at Monash University and Museum Victoria in Melbourne, Australia, where I'm studying fossil whales. Other areas of interest include fossil penguins, seals, dromornithids, dinosaurs... basically fossil vertebrates in general! I'll be blogging about anything palaeo related that interests me, but I'll try to ensure Australian palaeontology gets its fair share! If there's anything specific you would like me to blog about, drop me an email and I'll have a stab at it!

8 responses to “Crazy Crocodylomorphs: Mesozoic Mandibular Morphological Madness!”

  1. protohedgehog says :

    I haz a question! If “the mandible plays such a fundamental role in the animal’s life, any change in morphology will likely represent an evolutionary adaptation” is true, then what degree does an [reduced] outline of the mandible faithfully represent the functional variety and performance of the mandible..? And if a landmark slides a bit wrt the grand mean, does that represent a functional change, or just background geometric shifts? Null hypothesis?

    • Travis Park says :

      To truthfully answer your original comment Protohedgehog, I don’t honestly know! If someone hasn’t already tested this, then it could be potentially worthwile looking at as a measure of the robustness of using the landmark method. Most people just go ahead and use these techniques without actually questioning the assumptions they are based on.

      I have a colleague who is testing a few of the assumptions of FEA, it’s not particularly flashy work, but somebody’s got to do it!

  2. Larry Martin says :

    I collect in uppermost Triassic deposits in northeastern New Mexico. They are found in uppermost Bull Canyon Formation layers. I regularly find crocodylomorph teeth & some bone elements. Yes, it is an amazing group. I’ve lived through Revueltosaurus callenderi being described as an Ornithischian dinosaur until being redescribed as a crocodylomorph in the last decade or so. And now it seems there simply were no Ornithischians in the Triassic, according to prevalent thought. Along with Revueltosaur teeth, I also find similarly shaped teeth, although more slender in appearance, that seem to be morphologically different. I personally believe I am finding Ornithischian teeth. Any thoughts?

    • protohedgehog says :

      Plenty of ornithischians around in the Late Triassic – you had Pisanosaurus for one! 🙂 And Eocursor parvus, one Richard Butler named a few years ago. Here’s a neat li’l OA paper on early ornithschians: http://socrates.berkeley.edu/~irmisr/trornith.pdf

      • Larry Martin says :

        Thanks for the response! I’m not sure how I got the idea that the prevailing thought had become that there were no Ornithischian in the upper Triassic. Seems like around the time it was determined that Revueltosaurus was a Crocodylomorph & not an Ornithischian. I thought it was just my own pet theory that YES THERE WERE Ornithischians in the upper Triassic. I must say, the dentition of Revueltosaurus, as seen from the U Trias of Arizona, is sure a strange dental design. I’m thrilled whenever I find one of those teeth, usually in matrix I process & search. I just love those teeth! Thanks for adding that link to the article, very much appreciated!!!

    • Travis Park says :

      Thanks Protohedgehog for fielding that one, I would have guessed there were Ornithischians around in the Late Triassic, but wouldn’t have known any species names!

      The group really is quite fascinating, I only really know the most basic information about the group but I will definitely be reading up on them more whenever I get the chance and would ultimately love to do some research on them in the future!

      I’ve noticed that happen with quite a few Late Triassic taxa, they are originally described as crocodylomorph or as a dinosaur, then years later have been reassigned to the other group. Not sure why this is the case, maybe an improvement in analytical methods or perhaps better fossils to compare, or maybe a combination of both!

      Thanks for taking the time to comment.

      Travis

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