Tetrapod Zoology

Last year Dave Hone (of Archosaurs Musings) and Mike Benton published their analysis of the English rhynchosaur Fodonyx spenceri. Ever one to make promises that I’m unable to fulfil (in the short term, at least), I vowed to cover the group at some stage. Neil Kelley recently reminded us that 2008 was the year in which the blogosphere began to reflect the global Scientific Triassicism movement. Here was the biblical sign I needed to dust off my files and, goddammit, publish on those rhynchosaurs.

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Also spurring me into action was the fact that I had to put my much-loved ‘The Age of the Rhynchosaur’ poster – previously occupying space on my office wall – into storage the other day. Yes, this poster really does exist: it was published by New Scientist magazine some time round about 1983, and here it is in all its glory. The main painting was done by Jenny Halstead, and I regret to say that I’m really not fond of her work (she illustrated several books on prehistoric life for her late husband, Beverley Halstead).

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Rhynchosaurs are an entirely Triassic group of herbivorous archosauromorph reptiles (more detail on all of this later) often likened to ‘reptilian pigs’, and often described as ‘tubby’, or as having ‘barrel-like’ bodies. Dale Russell suggested the term ‘owliguana’ as a vernacular term. Rhynchosaur fossils are very abundant in some assemblages and the anatomy and ontogeny of a few species is comparatively well known. Should you want to know more about rhynchosaurs, where should you go? As always with fossil animals that aren’t dinosaurs, the answer is not obvious. I haven’t seen anything that’s at all useful on the internet, and in the non-technical literature the only half-decent coverage of the group is that provided by Mike Benton in The Reign of the Reptiles (Benton 1990a) and in one of his chapters in The Book of Life (Benton 1993). I presume there’s some good stuff on the group in Nick Fraser’s Dawn of the Dinosaurs, but I have yet to see this volume (hint hint). There is an excellent and voluminous technical literature on rhynchosaurs: Mike Benton in particular has produced some very thorough, useful contributions* [adjacent image, showing Late Triassic rhynchosaur Hyperodapedon in life, and with skull in lateral and ventral views, from Benton (1983a)].

* Someone should definitely name a rhynchosaur taxon Bentonyx one day. Alas poor Mike, at the moment his only patronym is the procolophonoid Kapes bentoni Spencer & Storrs, 2002.

An ‘average’ rhynchosaur would be about 1 m long. Some (like Rhynchosaurus) were smaller (c. 50 cm long), and big ones (like Hyperodapedon) may have attained 2 m in length. Chatterjee (1969) said of rhynchosaurs that they might usually be ‘about as large as an Alsatian dog, but could attain the size of a cow’ (p. 203). I suppose he was thinking of a cow’s length, not its height. Rhynchosaurs were widespread: their fossils have been found in Britain, South Africa, Zimbabwe, Tanzania, Madagascar, India, Brazil, Argentina, Canada and the USA, though note that their Northern Hemisphere fossil record isn’t great. 15 species are currently regarded as valid, and another five wait in the wings as valid taxa still in need of a name (Hone & Benton 2008). In some assemblages, several taxa lived alongside one another: four species were contemporaneous in the Upper Triassic Santa Maria Formation of Brazil, for example (Langer & Schultz 2000). Rhynchosaurs did not make it past the extinction event that marked the end of the Carnian in the Late Triassic (or was this extinction in the early Norian? Hello Triassic workers – please advise).

The most distinctive rhynchosaur characters are found in the skull, and indeed their teeth and jaws are pretty incredible. Seen from above, the rhynchosaur skull is vaguely triangular, with a narrow snout and broad posterior region. In the most derived taxa, like Upper Triassic Hyperodapedon, the skull is nearly twice as wide as it is long. In these wide-skulled forms, gigantic supratemporal fenestrae virtually meet along the midline. Rhynchosaurs must have had large jaw muscles and a powerful bite.

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In all rhynchosaurs, the nostrils are united to form a large aperture on the midline. Another rhynchosaurian peculiarity is that the premaxillae are strongly down-turned relative to the maxilla. In the most basal known rhynchosaur, Mesosuchus browni from the Early Triassic of South Africa, there were two large, conical teeth in each premaxilla [reconstructed Mesosuchus skull shown here, from Dilkes (1998)]. In another Early Triassic, basal taxon – Howesia browni – the premaxillae are unknown, and may also have borne teeth. In all members of the mostly Middle and Late Triassic rhynchosaur clade Rhynchosauridae the premaxillae are toothless and form paired, tusk-like structures (remember the distinction between ‘rhynchosaur’ and ‘rhynchosaurid’). When the jaws are closed, these tusk-like premaxillae fit in between the dorsally curved anterior tips of the dentaries. The ‘tusks’ are striated and were presumably horn-covered in life; distinct ‘tide marks’ show that the tissues that covered them were not continuous with those on the rest of the face. Large hyoid bones indicate the presence of a large, muscular tongue.

The short-faced, wide-skulled appearance of rhynchosaurid rhynchosaurs was less developed in the basal rhynchosaurs Mesosuchus and Howesia. These basal taxa did possess all of the key rhynchosaur autapomorphies, but they had longer, narrower skulls, longer, slimmer limbs and much longer tails than rhynchosaurids (50 vertebrae as opposed to 25-30). Mesosuchus and Howesia would have superficially resembled big lizards or tuataras in proportions and look [life restoration of Mesosuchus shown here, from wikipedia].

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The large orbits of rhynchosaurs demonstrate large eyes and hence good eyesight. A large cavity within the nasal region suggests that the animals had good olfactory abilities (Benton 1990b), but the lack of what Benton (1983b, 1990b) termed a tympanic crest in the quadrato-squamosal area suggests that a tympanum was absent (ha – I told you there was an osteological correlate for the presence of the tympanum in fossil reptiles). It’s been speculated that bone-conducted hearing might have been employed, and that skin membranes behind the quadrate may have permitted the detection of airborne sounds (Benton 1983b, 1990b).

What does a rhynchosaur do?

The tusk-like premaxillae have typically been imagined as plant-gathering organs and the powerful, shearing jaws and occluding dentitions look well suited for a diet of tough plant material. Seed-ferns, conifers, cycads, ginkgos and ferns have all been suggested as possible food items. Rhynchosaurs would of course have been limited to plants growing at or near ground level, given that they couldn’t climb, nor could they have had much reach.

An alternative suggestion to herbivory is that rhynchosaurs ate unionid bivalves (the group that includes swan mussels). Chatterjee (1969) favoured this possibility both because unionid shells were common in rhynchosaur-bearing sediments, and because snails are sometimes eaten by some living reptiles (he wasn’t the first author to consider this idea however). That’s hardly convincing, and as Benton (1984) and Dilkes (1995) argued, the precision ‘blade and groove’ rhynchosaur jaw morphology does not compare at all well with the jaw and tooth morphology of extant shell-cracking animals. Furthermore, the weird teeth and jaws of rhynchosaurs were clearly specialised for shearing, not cracking.

And it’s this weird jaw and tooth morphology that we’ll be looking at next.

Refs – –

Benton, M. J. 1983a. Dinosaur success in the Triassic: a noncompetitive ecological model. The Quarterly Review of Biology 58, 29-55.

– . 1983b. The Triassic reptile Hyperodapedon from Elgin: functional morphology and relationships. Philosophical Transactions of the Royal Society of London B 302, 605-718.

– . 1984. Tooth form, growth, and function in Triassic rhynchosaurs (Reptilia, Diapsida). Palaeontology 27, 737-776.

– . 1990a. The Reign of the Reptiles. Kingfisher Books, London.

– . 1990b. The species of Rhynchosaurus, a rhynchosaur (Reptilia, Diapsida) from the Middle Triassic of England. Philosophical Transactions of the Royal Society of London B 328, 213-306.

– . 1993. Four feet on the ground. In Gould, S. J. (general editor) The Book of Life. Ebury Hutchinson (London), pp. 78-125.

Chatterjee, S. 1969. Rhynchosaurs in time and space. Proceedings of the Geological Society of London 1658, 203-208.

Dilkes, D. W. 1995. The rhynchosaur Howesia browni from the Late Triassic of South Africa. Palaeontology 38, 665-685.

– . 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Philosophical Transactions of the Royal Society of London B 353, 501-541.

Hone, D. W. E. & Benton, M. J. 2007. A new genus of rhynchosaur from the Middle Triassic of south-west England. Palaeontology 51, 95-115.

Langer, M. C. & Schultz, C. L. 2000. A new species of the Late Triassic rhynchosaur Hyperodapedon from the Santa Maria Formation of south Brazil. Palaeontology 43, 633-652.

Comments

  1. #1 Neil
    January 29, 2009

    Interesting stuff. Ive not really common across rhynchosaurs before. I guess they are another one on the list of interesting but ignored animals.

  2. #2 Christopher Taylor
    January 29, 2009

    And with all that wonderful coverage on yet another fossil group that usually gets given undeserved short shrift, surely it would be petty of me to criticise a totally minor detail of almost no relevance to the main point? Nevertheless…

    The plural of “tuatara” is “tuatara”.

  3. #3 Camel
    January 29, 2009

    Dear mister Naish!

    I know that you heavily concern study with the flying reptile – an pterosaur from jurassic period. I too think about this. My achievement – I have understood, as image life Nyctosaurus. I have written about this article. Can be, she is useful too you. Please, comment her.

    Excuse me, I have translated her program. So can be an errors. The original send any event in russian language.

    =======================

    Nyctosaurus is the only pterosaur to have lost its clawed “fingers”, with the exception of the wing finger, which is likely to have impaired its movement on the ground, leading scientists to conjecture that it spent almost all of its time on the wing and rarely landed.

    =======================

    Probably, landed he really much rarely, only for emergency – for instance, for postpone eggs. Probably, selected for this sandy beaches on secluded island.

    But conduct all period to lifes midair – seems the delirium. I have more idle time explanation lifestyle Nyctosaurus .

    …There is suggestions that he the water lifestyle.

    At such event he did not land, but sat on water. And specialized to towards water lifestyle so at he disappeared the wing a finger, not necessary to water environment. But has grown the mast to head that in at such condition was much well.

    He used this as sail, but not passive, as traditional human naves, but actively, as windserfing – a sowing on for water, he waved the head so his large membrane produced the strong airstream to sail in necessary trend, against winds even.

    But more often wind was passing. He did not make the regular run, but strolled. At tail winds he used its sail by classical manner. Under strong disadvantage winds (for instance, during storm) he be able lay the head on water sideways, sharply reducing area. (Again technology windserfing).

    At lifes on water needs was any manner of the moving. Paddle the paw they be able not. Since prevented the flying membrane. But head was vacant. They have found the way her use. Not only Nyctosaurus – a representatives such family, as Tapejaridae, Pteranodontidae – possible, used their own ridges for similar target.

    But they not so powerfully specialized. They the more varied lifestyle around coast. For instance, they happened to often to climb on stones. So they have saved the finger on wing, and ridges they had smaller.

    But Nyctosaurus preferred the wild sea. There he conducted nearly all time for melt – with such stick to fly was, probably, it is difficult. On water possible was be weakened. Why fly, when possible swim up to? So flew he few – only running away from dangers or in quest of new places (or marriage partner).

    It Is Received that pterosaurs invent the more progressive manner of the moving on water, than birds. The Birds possible to consider the analogue paddling boat, but pterosaurs – a sailfish. The Advantage and defect evident.

    My theory it is necessary to test.

    I this do can not. If you may, help me.

    Thank you.

    Gumerov K.V., enthusiast.

  4. #4 David Marjanović
    January 29, 2009

    I regret to say that I’m really not fond of her work

    Consider yourself in good company…

    What’s really bad are the red and green romerograms to the right, though. Urgh.

    ———————-

    Mr Gumerov, what you suggest is aerodynamically and biomechanically impossible…

  5. #5 Andreas Johansson
    January 29, 2009

    Who’s for a moratorium on popular dinosaur books till a decent number of popular works get written on other extinct groups?

    I’m particularly annoyed at the lack of popular books on non-mammalian synapsids.

  6. #6 Bill Parker
    January 29, 2009

    First off….any poster with an aetosaur on it is good in my book (hey we take what we can get!).

    Thanks for the post on rhynchosaurs, admittedly I somewhat neglect them because we do not find them in the Chinle Formation, although there is some material from the lower Dockum. So it is good to read up on them in this manner.

    As for the age of the Late Triassic non-marine faunal turnover in the southwest U.S. that was previously attributed to the Carnian/Norian boundary, several recent unpublished lines of evidence show that this turnover actually occurs solidly in the Late Norian. At this point in time we are not even sure if the Chinle Formation contains any Carnian age strata. It is possible that the Dockum does (and these may contain the rhynchosaur material), and the Newark (eastern US) certainly does.

    BTW…. notice how I can use Chinle and Dockum and not have to say Chinle (Arizona, eastern NM, Utah) and Chinle (western NM and Texas)? It is much easier to say one time that they are both Upper Triassic and then not have to continuously use geographic modifiers. Nomenclature does have it usages.

    Finally, Rhynchosaurs are known from the Norian of South America (beat you to it Randy).

  7. #7 Graham King
    January 29, 2009

    Hi Darren!
    I look forward to more on the anatomy (and biomechanics?) of rhynchosaurs. I’m glad to see you feature them – unglamorous by some standards, but surely remarkable and noteworthy creatures. (Hmm, head twice as wide as it is long… would ‘wide-heads’ suit as a popular name?)
    It seems to me those near-midline tusks must have been useful for something particular. Are there any analogues today? Or maybe they were specialised for eating a food-item that’s also extinct now…

  8. #8 Mike Keesey
    January 29, 2009

    “In all rhynchosaurs, the nostrils are united to form a large aperture on the midline.”

    Man, that is FREA… oh, wait.

    “It’s been speculated that bone-conducted hearing might have been employed, and that skin membranes behind the quadrate may have permitted the detection of airborne sounds.”

    Oo, oo! Can we get a post about the evolution(s) of external ears in tetrapods?

    “I’m particularly annoyed at the lack of popular books on non-mammalian synapsids.”

    Hear, hear! There is one, but it’s rather old by now (McLoughlin’s Synapsida). (Incidentally, while “non-mammalian synapsids” is perfectly correct, there is an equally correct and more succinct term for these animals: “stem-mammals”.)

  9. #9 Cory Trego-Erdner aka Moai
    January 29, 2009

    Sweet. Rynchosaurs are one of my favorite groups of fossil animals. Thanks for this, Darren!

  10. #10 Dr Vector
    January 29, 2009

    Good post. Are you going to cover the history of ideas about rhynchosaur phlogeny? Tuatara are in Sphenodontia or Rhynchocephalia (what is the relationship between those two terms, anyway?), and I have more than once found people who were confused and thought that tuatara were living rhynchosaurs. One particularly cringe-inducing time was when this idea was floated by an undergrad in a packed lecture hall following a department seminar. It wasn’t me, but that was just dumb luck, because at the time I was under the same misapprehension.

  11. #11 Andreas Johansson
    January 29, 2009

    Hear, hear! There is one, but it’s rather old by now (McLoughlin’s Synapsida).

    I just ordered a used copy from Amazon. At $1.5, it’d probably have been some sort of sin not to!

  12. #12 Mickey Mortimer
    January 29, 2009

    If basal forms had two large anterior premaxillary teeth where rhynchosaurids had apparently bony/horny premaxillary tusks, could they be homologous? I’m thinking of Peters’ deriving Pteranodon beaks from large anteriorly projecting teeth. Or is this developmentally impossible?

  13. #13 John Scanlon FCD
    January 29, 2009

    And it’s this weird jaw and tooth morphology that we’ll be looking at next.

    I see what you did there. Very Attenborough. :)

    I remember that New Scientist poster, but didn’t keep a copy (I was probably reading either in Fisher Library [Sydney Uni] or my brother’s copy, I’ve rarely bought the mag even when they printed my letters). At the time, I did not know the distinction between Rhynchocephalia and Rhynchosauria, and may have learned it from the poster. So if I was ever asked (and I don’t think I was), I’d use terms like ‘tuatara’, ‘Sphenodontian’, or ‘non-squamate lepidosaur’ and avoid Rhynchocephalia altogether. Not that I was much concerned with extinct groups at the time (fell into fossils four years further on).

    Sadly, none of either group is known in my country (not a hotbed of Scientific Triassicism); but it would be a shame not to mention the Miocene sphenodontine in New Zealand just published by Marc Jones et al. in Proc. Roy Soc. B (online) – from the same formation where we thought briefly, a few years back, there was a snake (turned out to be a fish jaw).

  14. #14 Raymond Minton
    January 29, 2009

    About the only thing I’d heard about rhynchosaurs were that they were favored prey of early predatory dinosaurs like Herrerasaurus (because there weren’t that many herbivorous dinosaurs at that point in the Triassic) and the S.J. Gould-edited volume. “The Book Of Life”, which, like your posting, pointed out the improbability of shellfish-eating rhynchosaurs on anatomical grounds. Good information!

  15. #15 Allen Hazen
    January 30, 2009

    As a Synapsid myself, I can’t help noting certain at leat superficial resemblances to Dicynodonts. Is there an interesting story to be told about the relative spatial and temporal distribution of Rhynchosaurs and Triassic Dicynodonts? (Are they found in the same deposits, suggesting direct beak-to-beak competition? Complementary distribution, suggesting some sort of partitioning of available herbivorous niches? Did one tend to replace the other over time? … My impression is that the last Dicynodonts tended to be big: so, same questions above for different size-classes.)

    I look forward to the next installment!

  16. #16 Dave Hone
    January 30, 2009

    Hi Darren, thanks for the link and promo stuff. would have covered the rhynchosaurs myself were it no for the fact that a) you promised to do it, and b) *you* are responsible for the title of the Musings which left rhynchosaurs out of my purview in any case! ;-)
    I did of course write up Fodonyx on the blog with photos of the specimen for any interested people: http://archosaurmusings.wordpress.com/2008/06/16/fodonyx-a-new-genus-of-rhynchosaur/

  17. #17 Darren Naish
    January 30, 2009

    Many thanks to all for comments. So the plural of tuatara is ‘tuatara’? I didn’t know this. Is this a general thing for maori words, as some sources state that the plural of moa is ‘moa’. Few know this, and the term ‘moas’ abounds. Thanks Bill for Carnian-Norian info.

    As for other comments, the answers will come. Phylogeny and the whole Rhynchocephalia issue will be covered in the third part. Given its confusing back-story, I personally wish that the term Rhynchocephalia was stricken from the record. Unfortunately Gauthier et al. co-opted it for the Gephyrosaurus + Sphenodontia group in 1988, so it remains widely used in the literature.

  18. #18 Christopher Taylor
    January 30, 2009

    Yeah, Maori words don’t have a distinct plural form (if anyone’s interested, what changes to indicate plural in Maori is the article – Maori is more like French than English in its use of articles – so one tuatara would be “te tuatara”, more than one would be “nga tuatara”). The reason why you’ll sometimes see Maori words pluralised (if that’s a word) is all part of our rich colonial history – until fairly recently, most English speakers simply didn’t give a damn. In recent years, people have moved more towards maintaining the original Maori usage. Funnily enough, the change has happened faster for some words than others (probably at least partially related to how commonly the word is used) – English speakers may have gotten used to using “moa”, “takahe” and “iwi” as plural, but I’m still hearing most people say “pukekos”.

  19. #19 David Marjanović
    January 30, 2009

    It seems to me those near-midline tusks must have been useful for something particular.

    Well, they’re the hook of a beak, not tusks.

    and I have more than once found people who were confused and thought that tuatara were living rhynchosaurs.

    That’s because Romer really believed rhynchosaurs and rhynchocephalians were sister-groups. AFAIK he even confused the tusks of the latter with the beak of the former, but the main argument was a misunderstanding of the multiple toothrows of rhynchosaurs.

    If basal forms had two large anterior premaxillary teeth where rhynchosaurids had apparently bony/horny premaxillary tusks, could they be homologous? I’m thinking of Peters’ deriving Pteranodon beaks from large anteriorly projecting teeth. Or is this developmentally impossible?

    Of course it is. Does David Peters really derive a beak (bone) from teeth (dentine, enamel…)?!? If so, he’s even more ignorant about basic vertebrate anatomy than I already used to think.

  20. #20 Ed Pardo
    February 2, 2009

    Thanks, Most popular paleontology books dedicate a paragraph or two to these animals so it’s difficult to get any details.

  21. #21 Alan Kellogg
    February 3, 2009

    At half a meter in length for some species it occurs to me that while most eras had things that are rinky dink, the Triassic had dinky rynchs.

  22. #22 Brian Smith
    January 31, 2010

    Hi, I found a rhychosaurus skull, south-west of Ladram Bay back in 1984 which extended the occurance of this species further south – I believe in the area I was searching there could be more remains to be found. My specimen was donated to Exeter Museum (EXEMS: 65/1984) and was classified as R. spenceri. I have heard that Rhychosaurs have since been reclassified – so not sure if mine is still the same species per reports in 1985-7. Any data on this? I could also show anyone interested where I found this skull. Brian

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