The Tet Zoo guide to rhynchosaurs, part II

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In the previous rhynchosaur article we introduced these remarkable Triassic reptiles [adjacent Hyperodapedon restoration from wikipedia]. Their amazing, specialised jaws and teeth were mentioned and discussed in passing, but I skimped on the details. Make yourself some strong coffee and be sure you're not sitting in a comfortable chair, as here's where the going gets heavy. It's a story of shearing jaws, expanding tooth fields, and maxillary grooves...

Both the maxilla and dentary in rhynchosaurs possessed more than one row of teeth. You should imagine the occlusal surfaces of these bones as vaguely rectangular (the longest axis being the one parallel with the long axis of the body) and covered on their surfaces with parallel lines of conical tooth crowns (the tooth rows are also parallel to the long axis) [see image below. It shows the palatal surface of Hyperodapedon (from Chatterjee 1974). You can clearly see the multiple maxillary tooth rows. Note the groove extending along the length of the maxilla too]. In basal rhynchosaurs, the teeth were relatively low in number and spaced out, but they were abundant and closely packed in the most derived taxa. The teeth themselves were deep-rooted cylinders with large pulp cavities. They were firmly fixed to the jaw bones; therefore, rhynchosaurs exhibited what's known as ankylothecodonty or ankylosed thecodonty (this is where distinct sockets are absent, and the deeply rooted teeth are held in place by both soft tissues and the surrounding jaw bone). The tooth crowns were either conical or pyramidal. The Late Triassic rhynchosaurid Hyperodapedon typically possessed both crown shapes, whereas other taxa (such as Middle Triassic Ammorhynchus) had conical crowns only.

The teeth were not replaced in the conventional reptilian fashion. Instead, new teeth were added to the back of the toothrow during ontogeny: as the animal grew, the size of the occlusal regions increased (in other words, the tooth-bearing platforms on the maxillae and dentaries became steadily larger). As this happened, the new teeth at the back moved forward and hence into the active tooth field. As the teeth moved through the field, they became more solid as dentine was depositing within their pulp chambers, and they also became worn down by wear, until eventually their surfaces were flush with those of the bone surface. Old teeth also became resorbed at their bases. The dual effects of wear and resorption mean that the oldest, most anterior teeth are frequently tiny or even absent (Benton 1984, 1990).

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What makes things further complicated is that several tooth rows were operating in parallel at any one time. The tooth field did not just expand posteriorly, but lingually as well (lingually = on the side of the tongue, the opposite of labially), so new tooth rows were added to the tooth field during growth. When teeth from different rows came into close contact, the younger teeth eroded the older ones [in adjacent diagram, pm = premaxilla, m = maxilla, j = jugal, v= vomer, pl = palatine].

So, rhynchosaur teeth worked on a kind of 'conveyer belt' system, superficially similar to the (doubtless more familiar) solution to tooth wear evolved by elephants. In real contrast to the elephant system, however, the teeth themselves did not 'drift' through the tooth field: rather, the tooth field itself expanded backwards and sideways, all the while remodelling its bone and resorbing its older, worn teeth [Hyperodapedon skull below, from Benton (1983)].

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Rhynchosaurid rhynchosaurs all possess a groove extending along the length of the maxillary tooth field [see palatal diagram above]. When the jaws were closed, the blade-like tooth row of the dentary fitted into this groove, thereby indicating that rhynchosaurids used precise shearing when feeding. Some rhynchosaurids had two maxillary grooves, matched by two dentary blades. In some taxa (such as Fodonyx), the medial maxillary groove only developed as the animal matured, while in others (such as Stenaulorhynchus), two maxillary grooves were present even in juveniles (Langer et al. 2000). However, most species of Hyperodapedon (the most derived rhynchosaurid) lacked a medial maxillary groove at all stages of growth, and - in the dentary - had a reduced number of tooth rows medial to the main tooth row. There are two interpretations of this.

One is that Hyperodapedon was paedomorphic compared to Middle Triassic rhynchosaurids, and that its single maxillary groove and simple medial part of the dentary tooth field were secondarily simplified relative to the double-grooved, more complex tooth fields of more basal rhynchosaurids (Langer et al. 2000). The second interpretation is that the double-grooved maxillary tooth field is derived relative to the single-grooved condition, and that the single-grooved species of Hyperodapedon are simply plesiomorphic (Nesbitt & Whatley 2004). Ammorhynchus from the Middle Triassic of Arizona is like basal rhynchosaurs and Hyperodapedon in being single-grooved, yet was found by Hone & Benton (2008) to be positioned between the double-grooved taxa Fodonyx and 'Scaphonyx' sulcognathus. Furthermore, while most Hyperodapedon species were single-grooved, H. huenei is double-grooved, and it was recovered by Langer & Schultz (2008) as the most basal Hyperodapedon species.

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The distribution of the double-grooved condition would therefore appear to be complicated: it's possible that there were several convergences and reversals, but I wonder if the development of a medial (= second) maxillary groove was more developmentally labile than the literature currently indicates. Indeed, there seems to be some ambiguity as to whether all specimens referred to some taxa - like Fodonyx - exhibit the same condition. Nesbitt & Whatley (2004) noted that more work is needed to better determine the distribution and phylogeny of the double-grooved condition, and I strongly agree with them [adjacent image shows me posing with Fodonyx, photo courtesy Dave Hone].

I have a headache now, that's enough skull stuff, on to the postcrania...

Refs - -

Benton, M. J. 1983. 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.

- . 1990. 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.

Chatterjee, S. 1974. A rhynchosaur from the Upper Triassic Maleri Formation of India. Philosophical Transactions of the Royal Society of London B 276, 209-261.

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., Ferigolo, J. & Schultz, C. L. 2000. Heterochrony and tooth evolution in hyperodapedontine rhynchosaurs (Reptilia, Diapsida). Lethaia 33, 119-128.

- . & 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.

Nesbitt, S. J. & Whatley, R. L. 2004. The first discovery of a rhynchosaur from the upper Moenkopi Formation (Middle Triassic) of northern Arizona. PaleoBios 24, 1-10.

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Isn't "long axis" a bit ambiguous in a rhynchosaur skull?

If the teeth didn't move, does that mean the poor rhynchosaur was eventually left with a mouth full of worn-out teeth and starvation retirement?

By Andreas Johansson (not verified) on 02 Feb 2009 #permalink

If the teeth didn't move, does that mean the poor rhynchosaur was eventually left with a mouth full of worn-out teeth and starvation retirement?

Because new teeth were being added to the backs and insides of the tooth fields during life, only the oldest, most anterior parts of the tooth fields became 'worn out'. Indeed, there are old adult specimens with smooth anterior parts to the tooth fields. However, in extant reptiles that undergo 'conventional' reptilian tooth replacement, replacement slows in old individuals and may even stop (e.g., Erickson 1996). It's conceivable that really old rhynchosaurs might have gotten to the stage where the addition of new teeth stopped, leaving those teeth already on the tooth fields to wear away to nothing. Are there any such specimens? I don't know, but would like to.

Ref - -

Erickson, G. M. 1996. Toothlessness in American alligators, Alligator mississippiensis. Copeia 1996, 739-743.

So is this about the point where you have more information on a particular clade than any other source out there? I really appreciate your effort in doing this.

Thanks Cameron, very kind (part III to be posted soon, probably tomorrow). Part of me thinks I should tidy up texts like this and get them published (the rhynchosaur series, for example, could go as another one of those 'Fossils explained' articles for Geology Today). Another part of me says not to bother, because (1) it's time I could spend on something else, and (2) more people will see and read the Tet Zoo version anyway! Err.. if anyone wants to take this job on, however, let me know :) The borhyaenoid series could also do with publishing.

It's snowing here, and today southern England has had its heaviest snowfall in about 20 years. But 'heaviest snowfall' amounts to less than 10 cm. Even this, however, has been enough to bring the entire country to a grinding halt. Public transport has stopped entirely and London seems to be empty. Pathetic.

Err.. if anyone wants to take this job on, however, let me know :)

That sounds very, very tempting... if nobody claims it before May 9, I'll let you know.

But 'heaviest snowfall' amounts to less than 10 cm.

Reminds me of a King of the Hill episode where a few flakes amounted to "a nightmare". It's nothing to blink at in Maine, although the temperatures seem lower than usual. Since I lost my gloves, it's amazing that I still have fingers left after braving -30 C weather. How do those animals put up with this...

Fantastic stuff, thanks Darren. Your knowledge of these ignored groups is fabulous and I congratulate you on trying to communicate with the rest of us who can't get to grip with the material like you can! I am forever frustrated by my inability to get hold of enough of the literature (time and actual access problems) to really understand it.

I loved your series of fossils explained though I agree Geology Today is not an ideal place for them. The problem with Science blogs is that there is as far as I can see no good way to browse within it unless you know what you are searching for. I notice you put your blogs into categories but how do I search on those? A book or standalone site would be ideal but I know you haven't the time!

It's excellent stuff regardless and I would love to see some other groups covered like multituberculates, drepanosaurs, pareiasaurs etc

By Rstretton (not verified) on 02 Feb 2009 #permalink

Hi Rob - thanks for those comments. Getting hold of the literature is always a problem, even for those of us with institutional affiliations and lots of wonderful friends. Some works aren't available as pdfs, can't be located via inter-library loan, and were published by people who ignore requests for reprints or are no longer around, so tracking stuff down can sometimes make you tear your hair out.

As for searching for stuff on the site, I categorise what I can but the groupings are pretty broad (e.g., all reptiles except Mesozoic dinosaurs, birds and pterosaurs are lumped together in 'herpetology'). If you have a specific group in mind my advice is that you search for their name using the google custom search bar at top left. As a longish-term reader, it's unlikely that you've missed anything though.

Multis and pareiasaurs are destined to appear here at some stage. Protorosaurs (which include drepanosaurs according to some studies) are now long overdue but, as for drepanosaurs on their own, did you see the HMNH article from 2005?

Great data, looking forward to part III.

Darren, delighted to get my teeth into (groan) this nitty-gritty biomechanical info you've harvested, nipped into bite-size masses and sheared/ground down into comprehensible verbiage for regurgitation to feed us younglings...

Ok that was a metaphor taken way too far. I know, I know. :-D

PS Doesn't Hyperodapedon have a cute face though? Ahhh... That Wiki image alone should win rhynchosaurs more fans... as in "Can I adopt one?"
(Soft toy rhynchosaurs, any on the market yet? Plastic ones even?)

PPS If you feel so inclined (though it's lazy of me I know!) could you give the derivation of names you mention in your blog. eg I reckon Hyper- is er hyper (beyond, extreme?); -odape- is I haven't a clue; and -don is tooth.
So Hyperodapedon=Extreme something tooth? Tooth beyond something?
I know I could look these things up but I'm figuring you probably know a lot of these etymologies straight off already, and surely it's more energy-and-time-efficient for you to just tell us than for umpteen readers to have to go look it up...? ;-]

On the other hand it's fun to try working them out oneself...

By Graham King (not verified) on 12 Feb 2009 #permalink