The Tet Zoo guide to rhynchosaurs, part II

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).

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)].

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.

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.