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.
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).
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.
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].
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.