Tetrapod Zoology

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Among the many, many groups I have yet to cover on Tet Zoo are stem-group synapsids: Synapsida is the tetrapod clade that includes mammals and all of their relatives, and there is a long tradition of referring to non-mammalian synapsids as ‘mammal-like reptiles’ (other names include protomammals and paramammals). Because synapsids are not part of Reptilia*, referring to them as ‘mammal-like reptiles’ is both technically incorrect and misleading, hence the push to use their proper name.

* Reptilia and Synapsida are sister-taxa within the tetrapod clade Amniota.

The photo here was provided by Matt Wedel and shows the mounted skeleton of the Early Permian non-mammalian synapsid Cotylorhynchus hancocki, as mounted at the Oklahoma Museum of Natural History. Because I haven’t had time to finish any proper articles lately, this provides me with a good excuse to talk briefly about Cotylorhynchus and its relatives. Or, not so briefly, as it turns out…

Cotylorhynchus is the best known representative of the basal synapsid clade Caseidae, a group that is odd for lots of reasons. While caseids are among the most basal of all synapsids, they don’t make their first appearance until relatively late in basal synapsid history: they debut in the Early Permian, whereas four of the six other basal synapsid clades (varanopids, ophiacodontids, edaphosaurids and sphenacodontids) were all present in the Carboniferous*. Caseids must therefore have a long ghost lineage that extends back to the Carboniferous (Reisz et al. 1998), and the very earliest, most primitive members of the group remain unknown.

* All of these basal synapsids were formerly grouped together as Pelycosauria: because this group is paraphyletic, ‘pelycosaur’ is only used informally today. Eupelycosauria is a formal clade name however – it’s the sister-group within Synapsida of Caseasauria [read on].

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Caseids almost certainly descended from small carnivorous forms, but experts have disagreed as to whether their closest relatives were the eothyridids or varanopids. Reisz (1980) noted that caseids shared with eothyridids the same sort of tilted anterior margin of the premaxilla (where the dorsal surface of the snout tip extends further rostrally than the mouth), an elongated external nostril, and a shortened face where the maxilla contributed to the margin of the orbit (in other basal synapsids, the lacrimal and jugal keep the orbit and maxilla well apart). Consequently he grouped the two together in a clade, for which the name Caseasauria Williston, 1911 already exists. I think that eothyridids look like good ‘proto-caseids'; unlike caseids, they possessed enlarged caniniform teeth and were presumably predatory [adjacent image is life restoration of Cotylorhynchus].

Indeed, caseids are unusual among basal synapsids in that all known members of the group were herbivorous, as is demonstrated by their proportionally small skulls, spatulate teeth, absence of any sort of caniniform region in the dentition, and by their massive broad bodies* (Sues & Reisz 1998). So far as we can tell, these features were true of Oromycter dolesorum, the oldest and apparently most basal caseid (Reisz 2005). Probably c. 1 m long, it was much smaller than some of the later members of the group: Cotylorhynchus romeri (the presumed ancestor of the slightly larger, geologically younger C. hancocki) from the late Early Permian reached 3.5 m in length and had an estimated weight of 330 kg (Stovall et al. 1966). It was gigantic compared to many of its relatives and contemporaries, and in fact it and C. hancocki were the largest ‘pelycosaurs’ and largest terrestrial vertebrates of their time.

* Some caseids, such as Caseopsis agilis, were slimmer-bodied than the best known forms, but were still otherwise similar.

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As you can see from the accompanying picture, Cotylorhynchus had a massive scapulocoracoid, enormous flaring ends on its humeri, stout forearm bones, and broad, robust hands with large claws. Large retractor processes on the ventral surfaces of the unguals show that caseids could flex their claws with a very powerful motion, and the articulatory surfaces of phalanges weren’t perpendicular to the bone’s long axis, but oblique to it, thereby providing a much larger surface area for flexor muscles. These features all suggest that ‘in life the animal did considerable digging for its food supply’ (Stovall et al. 1966, p. 23). I wonder if they constructed burrows. Articulated specimens confirm that the digits had a considerable range of motion: in a specimen kept at the Cleveland Museum of Natural History, the toes are folded right under the rest of the foot, with the claws pointing backwards towards the tail. One odd thing I’d like to know more about concerns the apparent variation that exists in the size of the hand claws of Cotylorhynchus: some specimens seem to have rather longer unguals than others (look at figs. 13 and 14 in Stovall et al. (1966)). Is this genuine variation? If so, it is sexual or ontogenetic or what?

Why is the early history of caseids so poorly known? Olson (1968) proposed that these animals spent the early part of their evolutionary history in areas that were well away from the lowland/deltaic environments that were best incorporated into the fossil record. This hypothesis is supported by the discovery of the new basal caseid Oromycter in an upland depositional setting, and specimens from upland environments at the rich Bromacker site in Germany provide further support for this (Reisz 2005). The inference is that it took caseids a considerable time to change from living in remote upland habitats to the better-sampled, lowland flood-plain environments where most of our Carboniferous and Permian fossils come from.

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Despite the fact that they were so archaic within the synapsid family tree, caseids became the most abundant Early and Middle Permian herbivores in North America at least, out-living their herbivorous cousins the sail-backed edaphosaurids. Ennatosaurus from Russia shows that caseids survived to near the end of the Middle Permian to live alongside dinocephalians and other therapsid synapsids [image of Ennatosaurus above borrowed from here].

As always there’s much more to say, but that’ll have to do. This was only meant to be a picture of the day submission, but I suppose I got carried away. I must do more stem-group synapsids in the future.

Refs – –

Olson, E. C. 1968. The family Caseidae. Fieldiana: Geology 17, 223-349.

Reisz, R. R. 1980. The Pelycosauria: a review of phylogenetic relationships. In Panchen, A. L. (ed) The Terrestrial Environment and the Origin of Land Vertebrates. Academic Press (London/NY), pp. 553-592.

– . 2005. Oromycter, a new caseid from the Lower Permian of Oklahoma. Journal of Vertebrate Paleontology 25, 905-910.

– ., Dilkes, D. W. & Berman, D. S. 1998. Anatomy and relationships of Elliotsmithia longiceps Broom, a small synapsid (Eupelycosauria: Varanopseidae) from the Late Permian of South Africa. Journal of Vertebrate Paleontology 18, 602-611.

Stovall, J. W., Price, L. I. & Romer, A. S. 1966. The postcranial skeleton of the giant Permian pelycosaur Cotylorhynchus romeri. Bulletin of the Museum of Comparative Zoology 135, 1-30.

Sues, H.-D. & Reisz, R. R. 1998. Origins and early evolution of herbivory in tetrapods. Trends in Ecology & Evolution 13, 141-145.

Comments

  1. #1 Phil Hore
    July 16, 2007

    what a wonderful group of animals and so often completly ignored.
    personally I’ve been arguing they are more ‘reptile-like mammals’ than ‘mammal-like reptiles’ for years!

  2. #2 johannes
    July 16, 2007

    Nice post on caseids. Fascinating animals; they are so ugly that they become beautiful again, like french airplanes of the interwar years (look here: jnpassieux.chez-alice.fr/html/FarmanF120.php for an example that had a diminutive radial engine attached at the same position were caseids had their equally tiny heads:-)). What is frustrating about stem synapsids, however, is the fact that the anatomically most primitive ones, like eothyridids, appear so late in the fossil record.

    BTW, if we are already in the Permian: Give us gorgonopsians, please!

  3. #3 Laelaps
    July 16, 2007

    Absolutely wonderful post, Darren; thank you for writing this one up. Like you mentioned, despite the terms inaccuracy “mammal-like reptile” still seems to be in wide use, and a clarification in more popular works needs to be made. Like Phil said, thanks for giving another often-ignored group some much-needed attention.

  4. #4 chris wemmer
    July 16, 2007

    Wow, a fascinating group. Their robustness and sprawling suspension is somehow tortoise-like, no? It’s hard not to believe they were burrowers. They had to be slow-pokes, and digging burrows with those spades in front would offer protection against predators, and perhaps provide a means of regulating body temperature. Neat creatures.

  5. #5 Sordes
    July 16, 2007

    Johannes, if you´re interested in Gorgonopsians you could like this photo of a Dinogorgon quinquemolaris-skull I made some time ago: http://bestiarium.kryptozoologie.net/?p=79

  6. #6 Mike Keesey
    July 16, 2007

    other names include protomammals and paramammals

    But the best term (for both succinctness and precision) is “stem-mammal”.

    “Protomammal” makes it sound like they are the ancestors of mammals, when most were not. It also sounds like they appeared before mammals, when many were contemporaneous. As noted elsewhere on this blog, I think, some were still around even into the Cretaceous.

    “Paramammal” is a bit better, but to me it makes it sound like their similarity to mammals may only be due to coincidence. At the very least, it emphasizes the similarity without any hint that that similarity is due to common ancestry.

    “Stem-mammal”, on the other hand, says exactly what the group is: all members of the total group (Pan-Mammalia/Synapsida/Theropsida) which are not members of the crown group (Mammalia).

  7. #7 Will Baird
    July 16, 2007

    Woohoo! Finally, some hot synapsid action!

    Okay, a bit of an evolutionary origination query: in reading, it seems a bit of question whether or not the amniotes are paraphylitic? The reason I am asking is that while reading up a bit, I get the impression that one of three scenarios took place: there were three separate originations out of the amphibians (synapsids, diapsids, anapsids); there were two (synapsids and diapsids with the anapsids deriving out of the diapsids); or there was a single event from the amphibians and the th events of the *apsids were all subsequent.

    I would think that it was the last, but some of the impressions I’ve been getting make it seem that the amniotes are not monophylitic at all. umm. Can I get a clarification from those that are pros int he area?

    Again, thanx for more on our synapsid paleo-cousins, Darren!

  8. #8 Zach Miller
    July 16, 2007

    Great post! How did those things even operate with such tiny heads?! Isn’t some variation on “Pelycosauria” still around to include Dimetrodon, Edaphosaurus, and their cousins?

  9. #9 cameron
    July 16, 2007

    While some large sauropods and tortoises probably have similarly tiny heads, I’ve long wondered how Cotylorhynchus managed to get by without a long neck. Something as simple as getting a drink of water must have been a hilariously awkward undertaking, if it ever did drink that is. I yearn for the day to see a CGI caseid bumbling around, well, and many other things.

    Sigh, too bad people out there don’t pay attention to all the fascinating beasties out there besides dinosaurs.

  10. #10 Darren Naish
    July 16, 2007

    But the best term (for both succinctness and precision) is “stem-mammal”.

    “Protomammal” makes it sound like they are the ancestors of mammals, when most were not. It also sounds like they appeared before mammals, when many were contemporaneous. As noted elsewhere on this blog, I think, some were still around even into the Cretaceous.

    “Paramammal” is a bit better, but to me it makes it sound like their similarity to mammals may only be due to coincidence. At the very least, it emphasizes the similarity without any hint that that similarity is due to common ancestry.

    “Stem-mammal”, on the other hand, says exactly what the group is: all members of the total group (Pan-Mammalia/Synapsida/Theropsida) which are not members of the crown group (Mammalia).

    Yeah, ok. But – to clarify – I was specifically referring to vernacular names that have historical precedent in the literature. Out of interest, has anyone ever referred to stem-group synapsids as ‘stem-mammals’? There have been a few efforts to get the name Mammalia extended all the way down to the synapsid root (Ax 1987 for example, I think), but they clearly haven’t been successful.

  11. #11 Alan Kellogg
    July 16, 2007

    Phil More,

    Agree regarding reptile-like mammals. As a matter of fact, there might be a case for amphibian-like mammals.

    For my part I reserve “proto-mammals” for the ancestors of marsupials — which would include the triconodonts/monotremes. Quasi-mammals would include groups such as the multituberculates and docodonts.

  12. #12 Allen Hazen
    July 17, 2007

    A synapsid post! The mammalian chauvinist thanks you!

    (i) Neat that the life reconstruction of Cotylorhyncus is from the same angle as the photo of the mounted skeleton. Lucky accident, or did Matt Wedel have this in mind when he took the photo?

    (ii) The most striking thing about the skull of Casea is its IMMENSE nostrils. Any speculation about function? (Totally groundless speculation: Basal amniotes had no very efficient air-pumping mechanism — synapsids of the Therapsid line seem to have evolved the mammalian system with a muscular diaphragm, suggesting that more basal forms weren’t good at deep breathing — and the caseids, as recent elvovers of large body size, needed air any way they could get it: enlarging the diameter of the airways evolved in response. As I said, TOTALLY groundless speculation!)

    (iii) (To Will Baird) Available evidence is that amniotes arose from one branch of the paleozoic tetrapod tree (somewhere not TOO far from Seymouriamorphs). So whther they are mono- or poly- becomes something of a matter of definition: do we draw the line between amniote and pre-amniote just above or just below the last common ancestor. Similar questions arise with mammals: do multituberculates have a MAMMALIAN common ancestor with us or not? Even if not, though, they are now seen as having a more recent (and more mammal-like) common ancestor with us than some early 20th C. speculations suggested.

  13. #13 Anne-Marie
    July 17, 2007

    Great post, and good point about the incorrectness of “mammal-like reptiles”, I had actually never thought about that before but you’re very right, as usual. ;)

    Is the museum you mentioned the one in Norman, Oklahoma? I got to spend the night there over Christmas break, they have some fantastic exhibits. We got to tour some of the collections, they’ve got some magnificent Australasian bird specimens, and I even got my picture taken with their voucher Jackalope. ;)

  14. #14 johannes
    July 17, 2007

    > there were three separate originations out of the amphibians (synapsids, diapsids,
    > anapsids); there were two (synapsids and diapsids with the
    > anapsids deriving out of the diapsids); or there was a single event from the amphibians
    > and the th events of the *apsids were all subsequent.

    Most sources tend to support your third alternative, in other words: monophyly of amniotes. Try palaeos or Tree of life for a start.

    BTW: Be careful when using the term “amphibians” for stem tetrapods. It is by no means clear which stem tetrapod clade is the ancestor of lissamphibians (Temnospondyls? Lepospondyls?), but either alternative would leave many other stem tetrapod clades that are not closer related to lissamphibians than to mammals, birds or turtles.

  15. #15 R. A. W.
    July 17, 2007

    Fat-bodied, lived in upland environments, small, obligate herbivores, possibly good digging skills?

    Sounds like a marmot analogue.

  16. #16 Dr Vector
    July 17, 2007

    Neat that the life reconstruction of Cotylorhyncus is from the same angle as the photo of the mounted skeleton. Lucky accident, or did Matt Wedel have this in mind when he took the photo?

    Lucky accident! I had never seen that life restoration when I took the photo. I was just trying to find an angle that showed how deeply, deeply weird this animal is. Unfortunately the photo does not convey the scale very well–that skeleton is probably 9 or 10 feet from nose to tail.

    I just posted some more photos of the specimens on display at the OMNH. Click the “posted by” link below.

    It is very strange to stand in front of that fat, squat, pin-headed neckless freak, which looks like nothing so much as a microcephalic chuckwalla, and think that when it was alive, it was the largest terrestrial vertebrate of all time–the Amphicoelias of the Early Permian.

    Great post, Darren. There are so many interesting but obscure clades from the Late Paleozoic that you could probably spend a year doing nothing but. I know that your mandate is broader, and I appreciate it, but this post shows that there is a market for these weirdos. Keep ‘em coming.

  17. #17 Louis
    July 17, 2007

    I never noticed some Caseids had such small heads.

    Great post on an underrated group of Synapsids.

  18. #18 Alexandra Lynch
    July 18, 2007

    Thanks for writing what you did. My education in biology ended with the obligatory class in college, but the wonder of life’s variety, past and present, great and small, still enthralls me.

    And I have been voted “coolest mom ever” by a five year old for being able to look at a cast of a dinosaur skeleton and at a glance name the species (thanks to Peter Dodson’s book, I knew it was C. belli.)

  19. #19 Mike
    July 18, 2007

    I thought that skeleton was missing its head, until I saw the life restoration and took another look at the photo. That is indeed one deeply, deeply weird animal.

    But then all those early synapsids are deeply weird to me. If I were completely unfamiliar with them and dinosaurs and had to pick one group to have been the work of evolution and another the work of an artist’s imagination, I’d pick those early synapsids as the work of an artist. They seem much more alien than the dinosaurs which seem to make more ‘sense’ as functioning animals.

  20. #20 Anthony Docimo
    July 19, 2007

    most interesting. now I finally know something about those tiny-headed skeletons. thank you.

    >Caseids must therefore have a long ghost lineage that extends back to the Carboniferous
    If I may suggest something…maybe the Caseids arose from another synapsid clade during the Permian and (not sure what the technical term is) evolved in a more basal direction while/followed-by specializing as diggers.

    tis just a thought.

  21. #21 Anthony Docimo
    July 21, 2007

    Did *any* Caseids survive to the Triassic?

  22. #22 Darren Naish
    July 21, 2007

    Did *any* Caseids survive to the Triassic?

    Not so far as we know, no.

    Many thanks to everyone for their comments – sorry that I don’t have time to respond to everything.

  23. #23 David Marjanovi?
    July 29, 2007

    - Varanopseidae is incorrect; Varanopidae is correct.
    – Please, please, drop the name Reptilia. It only sows confusion and comes with centuries of baggage. Just say Sauropsida; it’s the same anyway.
    – Eupelycosauria was a bad idea — so bad that the PhyloCode now recommends against it.
    – Caseids are mind-boggling, but the edaphosaurids had a way cooler dentition: incisor analogues outside (premaxilla, maxilla, dentary), molar analogues inside (pterygoid, coronoid III, prearticular — yes, prearticular, like in lungfish — yes, prearticular, a bone that we have in the middle ear).
    – No, Ennatosaurus is not Late Permian. It is Middle Permian. There is no Late Permian in Russia (according to A Geologic Time Scale 2004), unfortunately.

    —————-

    – A long row of reversals leading to caseids? There’s no evidence for that. It would be a mightily unparsimonious hypothesis. They, plus Eothyrididae, really must have a ghost lineage that leads way back into the Carboniferous.
    – Are caseid nostrils really enormous relative to body size? Or only to skull size, which would be expected given the absurdly tiny skull?
    – No, there is no version of Pelycosauria around anymore. Dimetrodon is more closely related to you than to Edaphosaurus.

    Fat-bodied, lived in upland environments, small, obligate herbivores, possibly good digging skills?

    Sounds like a marmot analogue.

    Ten times the length of a marmot, not small!

    [from Darren: thanks David, I will correct the article in view of some of these comments. But I'm not dropping Reptilia until everyone else does]

  24. #24 johannes
    August 3, 2007

    > Caseids are mind-boggling, but the edaphosaurids had a way cooler
    > dentition

    When discussing the early permian terrestrial herbivorous “giants”, we should not forget Diadectes. This animal is really weird and fascinating: A terrestrial herbivorous tetrapod that is not even an amniote.

  25. #25 Mike Keesey
    May 25, 2009

    There have been a few efforts to get the name Mammalia extended all the way down to the synapsid root (Ax 1987 for example, I think), but they clearly haven’t been successful.

    To be clear, I’m not advocating this. “Stem-X” means “everything closer to X, a crown group, than to any other crown group, but not part of X”. Or, put another way, “pan-X (total group) minus X”.

  26. #26 David Marjanović
    May 25, 2009

    I wrote:

    There is no Late Permian in Russia (according to A Geologic Time Scale 2004), unfortunately.

    But Benton and friends doubt this…

    This animal is really weird and fascinating: A terrestrial herbivorous tetrapod that is not even an amniote.

    Well, no, but almost. It may well have laid amniotic eggs (…if the alleged traces of lateral-line canals in various diadectomorphs are misinterpreted, that is).

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