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For the love of crocodylomorphs

The views expressed are those of the author and are not necessarily those of Scientific American.


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Brilliant life restoration of the recently described Cretaceous baurusuchid Aplestosuchus (with another crocodylomorph - a sphagesaurid - in its mouth), by Rodolfo Nogueira. Aplestosuchus was described by Godoy et al. (2014); image CC BY.

Crocodiles, alligators and gharials are the modern members of a far grander, far more diverse clade of archosaurian reptiles termed Crocodylomorpha. It’s gradually becoming better known that, in additional to including amphibious, long-skulled taxa like the living ones, the group encompasses an incredible array of terrestrial and semi-terrestrial omnivores, herbivores, carnivores and insectivores. Some had short, deep skulls and complex, heterodont teeth (various notosuchians), or leaf-shaped teeth (some sphenosuchians); others had deep, narrow skulls and laterally compressed blade-like teeth (various sebecosuchians); others had ridiculous, enormously long jaws and snouts and are termed ‘duck-faced’ crocodylomorphs (stomatosuchids and nettosuchine caimans). There were marine, flippered crocodylomorphs that had tail flukes (some thalattosuchians), and gigantic aquatic or amphibious kinds that must have looked like souped-up, super-sized versions of the gharials, alligators and crocodiles we’re familiar with (some tethysuchians and extinct gavialoids and alligatoroids). [Images below by Smokeybjb and Dmitry Bogdanov.]

Some other highlights of crocodylomorph diversity. At left, dentition of the notosuchian Pakasuchus (image by Smokeybjb, image CC BY-SA 3.0). At right, life restoration of the Cretaceous neosuchian Stomatosuchus (image by Dmitry Bogdanov, CC BY 3.0).

An issue that needs to be covered just about every time crocodylomorph diversity gets discussed is the terminology we use for the various relevant clades; it’s confusing, and it’s made all the more so by the fact that some workers are currently arguing that we should give up on the current consensus and change it again! No, BAD idea!

Highly simplified 'consensus' phylogeny for Crocodylomorpha and kin to help show the different meanings of the terms Crocodylia, Crocodyliformes and Crocodylomorpha. Image by Darren Naish.

Ok… the clade that includes crocodiles, alligators and gharials and all of their extinct relatives is termed Crocodylomorpha. Crocodylomorpha is itself part of a more inclusive archosaur clade that includes the largely Triassic aetosaurs, rauisuchians and so on, variously termed Pseudosuchia or Crurotarsi depending on who you ask. Crocodylomorpha includes the archaic, long-legged sphenosuchians of the Triassic and Jurassic as well as the more ‘classically crocodilian’ lineages grouped together as the Crocodyliformes (sphenosuchians are crocodyliform-like, but differ conspicuously from them in possessing a typical archosaurian pelvis where the pubis contributes to the border of the acetabulum). This latter group – Crocodyliformes – includes virtually all the famous extinct lineages, all the protosuchians, sebecosuchians, notosuchians, tethysuchians, archaic neosuchians and so on as well as the crown-clade: the group that includes living gavialoids, crocodyloids and alligatoroids (all of which debuted in the Late Cretaceous).

Late Jurassic geosaurine metriorhynchid thalattosuchians from Europe, illustrated to scale from Young et al. (2012). Life reconstructions by Dmitry Bogdanov.

There are indications that thalattosuchians – the famous and awesome sea-going crocs of the Mesozoic – might be non-crocodyliform crocodylomorphs: we’re waiting for a key paper to appear on that subject (or is it already out?). Anyway, the crown-clade* within Crocodyliformes is termed Crocodylia (forget ‘Crocodilia’: it doesn’t have a formal meaning in the majority of current studies). Other schemes of nomenclature are out there, but what I’ve just described is the one that’s being used most widely.

* The ‘crown’ is the bit of a phylogenetic tree delimited by extant species only. ‘Crown’ does not just mean ‘that clade of most recently evolved species’ – it has to include extant species by definition. The rest of the lineage is termed the stem.

I’m still planning to produce a grand, group-by-group review of crocodylomorph diversity here some time. But, then, I plan a lot of things, and you might have to wait for the book I’m currently working on. Anyway, suppose you think crocodylomorphs are awesome, and suppose you think that this fact needs to become better appreciated by humanity at large. I would strongly recommend the wearing of the following t-shirt design…

Buy it now from the Tet Zoo redbubble shop, and show the love and look awesome by wearing it to TetZooCon (happening July 12th at the London Wetland Centre: full programme now online).

I think I really like the green one!

Tet Zoo has now covered crocodylomorphs living and extinct on quite a few occasions. Here, I’ve listed them all together, grouped according to their position on the cladogram…

Crocodylomorphs in general

Thalattosuchians

Notosuchians

Archaic neosuchians

Crocodylians

Refs – -

Godoy, P. L., Montefeltro, F. C., Norell, M. A. & Langer, M. C. 2014. An additional baurusuchid from the Cretaceous of Brazil with evidence of interspecific predation among Crocodyliformes. PLoS ONE 9 (5): e97138.

Young, M. T., Brusatte, S. L., de Andrade, M. B., Desojo, J. B., Beatty, B. L., Steel, L., Fernández, M. S., Sakamoto, M., Ruiz-Omeñaca, J. I. & Schoch, R. R. 2012b. The cranial osteology and feeding ecology of the metriorhynchid crocodylomorph genera Dakosaurus and Plesiosuchus from the Late Jurassic of Europe. PLoS ONE 7(9): e44985.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.



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  1. 1. DavidMarjanovic 7:11 am 06/9/2014

    some workers are currently arguing that we should give up on the current consensus and change it again!

    Details, please!

    we’re waiting for a key paper to appear on that subject (or is it already out?)

    AFAIK, only the SVP abstract (from last year) is out.

    Link to this
  2. 2. naishd 7:18 am 06/9/2014

    Hey, David. Details on suggested change to nomenclature: I have in mind Jeremy Martin and Michael Benton’s argument that Crocodylia should be applied to Crocodyliformes, not to the crown…

    Martin, J. E. & Benton, M. J. 2008. Crown clades in vertebrate nomenclature: correcting the definition of Crocodylia. Systematic Biology 57, 173-181. (free pdf here)

    Martin and others have published several papers where Crocodylia is used in this way, now meaning that we have two different uses of Crocodylia in the contemporary literature.

    As goes the ‘thalattosuchians might not be crocodyliforms’ paper – I think I heard a few months ago that a copy was in review, in which case the publish paper should be out soon (or is already out).

    Link to this
  3. 3. irenedelse 7:18 am 06/9/2014

    “An issue that needs to be covered just about every time crocodylomorph diversity gets discussed is the terminology we use for the various relevant clades; it’s confusing, and it’s made all the more so by the fact that some workers are currently arguing that we should give up on the current consensus and change it again! No, BAD idea!”

    Are you referring to the redefinition of Crurotarsi to include both Archosaurs and Phytosaurs? I thought that ship had sailed… Which is a shame, since using Pseudosuchia for the group including true crocodiles is a bit confusing. (Wasn’t that topic recently discussed at Tet Zoo?)

    Anyway, crocodylomorphs are truly awesome. The Mesozoic (and Cenozoic, too, for a large part) looks even more scary than with just the dinosaurs!

    Link to this
  4. 4. irenedelse 7:25 am 06/9/2014

    Reference for the redefinition of Crurotarsi, according to Wikipedia: Nesbitt, S.J. (2011). “The early evolution of archosaurs: relationships and the origin of major clades”. Bulletin of the American Museum of Natural History 352: 1–292. Link to PDF here.

    Link to this
  5. 5. naishd 7:34 am 06/9/2014

    “Are you referring to the redefinition of Crurotarsi to include both Archosaurs and Phytosaurs?” (comment # 3)

    Nope – as now explained in comment # 2, I was referring to uses of Crocodylomorpha, Crocodyliformes and Crocodylia, since some authors think we should use Crocodylia in place of Crocodyliformes.

    The Pseudosuchia-Crurotarsi issue is one I was hoping to avoid here! Crurotarsi (which is infinitely superior to Pseudosuchia for croc-line archosaurs) was phylogenetically defined with phytosaurs and croc-line archosaurs as internal specifiers, the complication then being that phytosaurs now seem to be outside crown-Archosauria (Nesbitt 2011: cited above)… in which case Crurotarsi becomes the name for a far more inclusive clade that includes ALL crown-archosaurs as well as phytosaurs, not the croc lineage alone. The sensible course of action here is to ignore the offending definitions (Sereno & Arcucci 1990, Nesbitt 2011) and come up with a new one.

    Meanwhile, some people are still using Pseudosuchia.

    Refs – -

    Sereno, P. C. & Arcucci, A. B. 1990. The monophyly of crurotarsal archosaurs and the origin of bird and crocodile ankle joints. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 180, 21–52.

    Link to this
  6. 6. irenedelse 7:48 am 06/9/2014

    @Darren:

    Sorry for that! I seem to have posted at the same time as David.

    “The sensible course of action here is to ignore the offending definitions (Sereno & Arcucci 1990, Nesbitt 2011) and come up with a new one.”

    I would think so too. If the issue is where Phytosaurs fit, allowing it to spread to the definition of the much larger croc-line group seems a bit inelegant.

    Link to this
  7. 7. darkgabi 8:32 am 06/9/2014

    well, since nesbitt’s work is the most recent one and he ressurected pseudosuchia, i guess this is the name to use so far. unless people suggest another one. which i think it’s unnecessary. pseudosuchia may sound weird for lay ears cuz it includes “real” crocs, but names have ceased to carry a definition in themselves. i mean, i see no problem using pseudosuchia. and i think this is the name archosaur workers have been using since then. the same thing goes for crocodylia… croc workers are using it for the crown group, despite of what nomenclature people who are not really involved with the group itself think.

    Link to this
  8. 8. ekocak 9:32 am 06/9/2014

    Lay person question: Since we know so many other extinct archosaurs had fur/hair-like integument, is it possible any of these animals had such a thing? Or are they too basal for that? It would make sense that surviving aquatic ones would be bare-scaled. Just curious.

    Link to this
  9. 9. Mark Young 9:53 am 06/9/2014

    Eric Wilberg’s description of the Oregon croc was accepted in JVP. Not sure when it’ll be out. I think the phylogenetic analyses has thalattosuchians as non-crocodyliform crocodylomorphs.

    Link to this
  10. 10. irenedelse 10:37 am 06/9/2014

    @ ecocak:

    I don’t know about hair or feathers, but many of the fossil crocodylomorphs (and crurotarsans too) had preserved osteoderms. So they had armoured skin on at least part of their body. Aetosaurs come to mind of course!

    And then there’s fast-swimming pelagic species, where I would expect to find smoother skin, similar to that of sharks or ihtyosaurs.

    Link to this
  11. 11. Andreas Johansson 10:49 am 06/9/2014

    You repeatedly mention neosuchians, but there’s no Neosuchia on the cladogram. Acc’d to WP it’s everything crownward of Notosuchia – are you using it in the same sense? (The WP page rather confusingly suggests the taxon may be polyphyletic, despite giving it a definition that bloody well cannot be .)

    Also acc’d to WP, plain old Suchia includes all of Pseudosuchia except the ornithosuchids. Might’a been a nice name for the croc total group but oh well.

    Link to this
  12. 12. Andreas Johansson 10:51 am 06/9/2014

    Surfing along on WP, I notice there’s also a Paracrocodylomorpha, that by definition includes Crocodylomorpha. Gah!

    Link to this
  13. 13. naishd 10:59 am 06/9/2014

    Andreas (comment # 11): yeah, sorry – I hope it’s understandable that I can’t explain exactly what those groups names mean, at least not without adding many hundreds of words to the article.

    Neosuchia = the crocodyliform clade that includes Crocodylia and the lineages closer to it than to groups like Notosuchia. Its content varies quite a bit according to the favoured topology, but it typically includes tethysuchians, stomatosuchids, atoposaurids, goniopholidids, hylaeochampsids and bernissatiids… as well as crocodylians. As discussed in the Tet Zoo thalattosuchian article, thalattosuchians are part of this clade in some studies, but not in others.

    Link to this
  14. 14. irenedelse 11:10 am 06/9/2014

    Follow-up to my comment #10: yes indeed, the Metriorynchids, advanced sea-going crocs included in the Thalattosuchian clade, had no osteoderms! It’s in the Tet Zoo post on Thalatosuchians mentioned in the list below today’s post. Duh! ;-)

    Link to this
  15. 15. ekocak 11:17 am 06/9/2014

    @Irenedelse, Darren

    Does this imply then that if saltwater crocs continue to evolve in the direction of a marine lifestyle they’d follow suit? Do they have reduced osteoderms compared to other crocodiles?

    Link to this
  16. 16. irenedelse 11:45 am 06/9/2014

    @ Ethan:

    Yes, salties have a reduced osteoderm complement (check the installment in TZ crocodiles of the world series cited at the end of this article). So does the American crocodile, Crocodylus acutus, who is also adept at long distance swimming.

    Link to this
  17. 17. ekocak 11:50 am 06/9/2014

    @irenedelse

    Just did that, very cool. Really cool that you can make a prediction like that based on evolutionary history.

    Link to this
  18. 18. irenedelse 11:50 am 06/9/2014

    Oops, formatting error, sorry. Link to the saltwater croc article is in the links above anyway.

    Link to this
  19. 19. irenedelse 12:33 pm 06/9/2014

    @ekocak #17:

    Charlie Darwin must have been on to something, with that theory of evolution… ^^°

    Link to this
  20. 20. Mark Young 12:55 pm 06/9/2014

    Dyrosaurids and the thalattosuchian Pelagosaurus also had reduced osteoderm cover.
    Skin surface – well sharks don’t have a smooth surface, it’s rough (placoid scales). And the jury is still out on whether ichthyosaurs had a smooth integument. Mosasaurids also had a rough surface (created by scales, and in some cases scales and osteoderms).

    Link to this
  21. 21. irenedelse 1:43 pm 06/9/2014

    @Mark Young #20:

    I understand that, and my offhand comment on Metriorhynchids having a “smoother” skin than what we are used to see on extant crocs was probably misleading. I was referring to the fact that the surface texture on sharks exhibits ‘dips and bumps’ on a smaller scale compared to that of Crocodylians, which reduces drag and makes for more efficient swimming.

    From what I’ve seen here and elsewhere on the net, Metriorhynchids reconstitutions show them in a similar fashion, streamlined and with a skin devoid of prominent rugosities.

    Link to this
  22. 22. Tayo Bethel 6:58 pm 06/9/2014

    @NaishD:

    Is there any up-to-date info on ornithosuchids? In a lot of cases they only get a passing mention.

    Link to this
  23. 23. DavidMarjanovic 7:13 am 06/10/2014

    Pseudosuchia, Paracrocodylomorpha, Paraves and Eosauropterygia will all run counter to Recommendation 11G (at the bottom of this page).

    Mosasaurids also had a rough surface (created by scales, and in some cases scales and osteoderms).

    Osteoderms in a mosasaur?

    Link to this
  24. 24. irenedelse 7:47 am 06/10/2014

    @ DM:
    Well, according to Lindgren et al. (2009):

    “A newly discovered Plotosaurus skeleton with integument preserved in three dimensions represents not only the first documented squamation in a mosasaurine mosasaur but also the first record of skin in an advanced member of the Mosasauroidea. The dermal cover comprises keeled and possibly osteoderm-reinforced scales that presumably contributed to an anterior–posterior channelling of the water flow and a reduction of microturbulent burst activities along the surface of the skin.”

    So, “possibly osteoderm-reinforced scales” is how they interpret that specimen.

    Lindgren J., Alwmark C., Caldwell M.W. and Fiorillo A.R., Biol. Lett. 2009 vol. 5 no. 4 528-531.

    Link to this
  25. 25. naishd 9:03 am 06/10/2014

    The mosasaur paper mentioned in comment # 24 was the focus of interest in the Tet Zoo ver 2 article Mosasaurs might have used the same microscopic streamlining tricks as sharks and dolphins.

    Link to this
  26. 26. naishd 9:10 am 06/10/2014

    Speaking of anguimorph skin (assuming for now that mosasaurs are anguimorphs, ha ha), who else saw the amazing chunk of Komodo dragon skin that recently showed up on twitter? (courtesy Ainy Rainwater)

    Link to this
  27. 27. Tayo Bethel 11:02 am 06/10/2014

    Just how commonare osteoderms in squamates?

    Link to this
  28. 28. irenedelse 2:52 pm 06/10/2014

    I saw the Komodo dragon skin picture on Twitter too, very cool! I suppose it’s from moulting?

    Which reminds me, the presence of osteoderms in the Komodo dragon skin was a factoid often mentioned in natural history books for kids when I grew up, most often with the added comment that it made their skin unsuitable for use as leather. I remember thinking “Lucky animal!” then.

    Link to this
  29. 29. Heteromeles 3:32 pm 06/10/2014

    Just in the interest of stirring up an esoteric and pointless discussion, why did alligator, gharial, and caiman-derived words roots get discarded as possible phylogenetic terms for this clade? It’s not like the clade crown is all crocodiles. Wouldn’t it be less confusing to use a richer assortment of root words?

    Link to this
  30. 30. irenedelse 3:57 pm 06/10/2014

    @DM:

    “Pseudosuchia, Paracrocodylomorpha, Paraves and Eosauropterygia will all run counter to Recommendation 11G (at the bottom of this page).”

    Wonder why not many clade names use the Pan- prefix. It’s short, clear, and common enough in other areas of knowledge to be understood intuitively by non-taxonomists, which is surely a bonus for educational purposes.

    Is it the existence of genus Pan? I doubt anyone would seriously be confused by say, “Pansuchia”, and think of “chimp crocodiles”!

    Link to this
  31. 31. Christopher Taylor 8:52 pm 06/10/2014

    Wonder why not many clade names use the Pan- prefix.

    A lot of the names being discussed here are artefacts of pre-’cladistic’ taxonomic practices, when it was common for a higher taxon to be divided between a monophyletic ‘upper’ group and an often implicitly paraphyletic ‘lower’ group (so, for instance, Sauropterygia might be divided between Neosauropterygia, containing the more derived forms, and Eosauropterygia, containing the more plesiomorphic ‘ancestral’ forms). When the practice became standard that all taxon names should refer to a monophyletic clade, a lot of older paraphyla were re-defined to refer to the monophylum containing their original members, rather than coining a new name for the equivalent clade. So, for instance, ‘Pseudosuchia’ was originally the name for an assortment of basal archosaurs. When it was re-defined to refer to the clade containing all the old pseudosuchians, that clade just happened to include the eusuchians as well.

    The ‘Pan-’ prefix has seen other uses than the common total-group one. In the past, the phylum Arthropoda was commonly used to cover the Onychophora and Tardigrada. Modern researchers usually exclude those two taxa from the arthropods, but the name ‘Panarthropoda’ is in use for the clade containing all three groups. Panarthropoda in this sense can be read as ‘everything that has been regarded before as an arthropod’.

    Link to this
  32. 32. Maximoose 12:48 am 06/11/2014

    Notosuchia are the first heterodont, occuring outside mammals or therapsids, that I’ve heard of. Are there any other non mammalian heterodonts?

    Link to this
  33. 33. Christopher Taylor 1:43 am 06/11/2014

    The aptly named Heterodontosaurus springs immediately to mind.

    Link to this
  34. 34. Christopher Taylor 1:46 am 06/11/2014

    And there are, of course, these heterodonts, but that’s probably not what you meant.

    Link to this
  35. 35. Maximoose 3:21 am 06/11/2014

    Thanks for the info Christopher!

    Link to this
  36. 36. naishd 4:48 am 06/11/2014

    Thanks for comments. Some quick responses…

    The Pan- prefix has caught on, but not in a big way. You’ll see papers and discussions that refer to Pan-Aves, or Pan-Sphenisciformes (or Pansphenisciformes), but it isn’t widespread, being specific to particular sets of organisms (e.g. ‘Pansphenisciformes’ has been used a fair bit in recent penguin literature due to the influence of papers by Julia Clarke and colleagues). I googled ‘Pan-Squamata’ and essentially found zero presence, and found ‘Pan-Mammalia’ perhaps being used once. As a convention I think it’s a good idea, and handy shorthand, but I’m not so keen on the idea that there should be a set of formal renamings as a result. And, yes, the idea that there might be ‘Panpan’ has been mentioned (not that it’s a likely genuine problem).

    Chris is right (comment # 31) on the application of ‘grade names’ for clades – Pseudosuchia, for example, was specifically meant to exclude crocodylians, so you might argue that co-opting it for a clade that includes crocodylians is not wise. There are fundamentally different ideas here. Some phylogeneticists are big on the idea that historical names have to be used in formal nomenclature, whereas others are of the opinion that they should be abandoned if their usage has been inconsistent or confusing (examples: Pseudosuchia, Eosuchia).

    Link to this
  37. 37. DavidMarjanovic 5:26 am 06/11/2014

    Has Eosuchia been used at all since *ehem* Reisz et al. (2010) found Apsisaurus to be a varanopid theropsid rather than a diapsid sauropsid?

    Link to this
  38. 38. naishd 5:47 am 06/11/2014

    Maximoose (comment # 32): heterodonty is far more widespread than typically implied by textbooks, there being numerous vertebrates (teleost fish, lizards, rhynchocephalians, crocodylomorphs, pterosaurs, dinosaurs and others) that have heterodonty to a degree. Sometimes this is relatively subtle (e.g., heterodonty is clearly present in the dinosaur Tyrannosaurus, but all the teeth – varying from slim and U-shaped in cross section to giant and banana-shaped to short and blunt-tipped – are all essentially modified versions of the same basic tooth type), but sometimes it really has to be classed as similar in degree to that of mammals (e.g., inciform, caniniform and molariform teeth occupy the jaws of some crocodylomorphs and lizards).

    Link to this
  39. 39. naishd 5:52 am 06/11/2014

    Eosuchia (comment # 37): it was used a fair bit before 2010 – my point being that it was a tremendously problematic term to ever use in a phylogenetic context. Oh, Sauria is another example. Judging from google scholar, Eosuchia is used in some context in Carroll’s chapter in the Gaffney turtle book (which I haven’t seen yet)… published in 2013.

    Link to this
  40. 40. irenedelse 8:44 am 06/11/2014

    @Tayo Bethel:

    “Just how commonare osteoderms in squamates?”

    Off the top of my head, they occur in the Komodo dragon and in the Gila monster… Off the top of Google, quite a number of groups exhibit osteoderms, according to Phylogenetic Relationships of the Lizard Families, Estes et al., 1988, p. 177.

    Link to this
  41. 41. ekocak 3:40 pm 06/11/2014

    I think Caiman lizards (Dracaena) have heterodont teeth for crushing snails as well as osteoderms.

    Link to this
  42. 42. Shuhray 8:15 pm 06/13/2014

    Write about temnospondyls, we like temnospondyls.

    Link to this
  43. 43. DavidMarjanovic 4:36 pm 06/14/2014

    We do! :-)

    Link to this
  44. 44. naishd 4:52 pm 06/14/2014

    Yes yes, more temnospondyls. I do need to get back to them.

    Link to this
  45. 45. DavidMarjanovic 6:39 am 06/15/2014

    :-) :-) :-)

    (Stupid smiley incapable of expressing basic happiness. It almost looks like an evil grin.)

    Link to this

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