Welcome to the third, and last, of the rhynchosaur articles. The other two are mandatory reading: part I is a general intro, part II is on jaws and teeth. This time round, we look at the form and function of the postcranial anatomy (well, predominantly at the limbs actually), and also at rhynchosaur phylogeny and at their place in the grand scheme of things [life restoration of Hyperodapedon shown above, from Benton (1983)].
We begin with the forelimb. The rhynchosaur humerus is stout, with large crests for muscle attachment and a wide, flaring distal end. The rest of the forelimb appears fairly typical for a diapsid reptile. Many sources state that the rhynchosaur hindlimb appears well suited for scratch-digging (this is where curved claws are used, in a raking motion, to shift sediment), but few elaborate on it. An excellent evaluation of rhynchosaur hindlimb function was provided by Benton (1983), and most of the following is taken from that discussion. While illustrations usually depict rhynchosaurs as ground-hugging animals with short, fully sprawling limbs, the details of their limbs and limb girdles show that their limbs were semi-erect and that their bodies were typically held well up off the ground.
In the shoulder girdle, the glenoid (= socket) is located far posteriorly and faces backwards and sideways (rather than being located closer to the middle of the girdle and facing fully sideways). These features and others indicate that a semi-erect forelimb gait could be adopted. In the hindlimb, the acetabulum (= hip socket) is broad and shallow, and faces downwards and backwards as well as sideways. When the in-turned head of the femur is articulated with the acetabulum, a semi-erect (rather than fully sprawling) posture seems to work best. The location of the distal condyles on the distal end of the femur (rather than on its ventral surface) indicate that the femur normally functioned in a near-vertical (rather than near-horizontal) pose, and the straight, column-like tibia also indicates a semi-erect stance. Rhynchosaur hands and feet were not close together and directly underneath the body, as they are in erect-limbed tetrapods, but they were closer together than those of truly sprawling tetrapods [diagram below, from Benton (1983), shows Hyperodapedon hindlimb in lateral and anterior view. The two lateral views show some of the inferred musculature].
The hindlimb bones are thin anteroposteriorly, but broad mediolaterally. Large attachment areas were available for well developed flexors and extensors of the foot. Combined with the near-parasagittal (parasagittal = parallel to the long axis of the body) movement of the femur, this shows that the long, strongly clawed feet could have functioned well in digging.
The foot claws of rhynchosaurids were not long and pointed, but very deep and narrow, and borne on digits that allowed a wide range of flexion and extension. As you can see from this diagram [below] from Benton (1983), these claws were similar in shape to those of some extant scratch-diggers, like golden moles and marsupial moles [in diagram below, from Benton (1983), rhynchosaur foot claws and a single hand claw (all at left) are compared to those of (from top to bottom) marsupial mole, pangolin, long-nosed armadillo, pygmy anteater, aardvark and golden mole]. It is inferred from these details that rhynchosaurids used their feet to break up and shift sediment, perhaps while they were foraging for roots, tubers and other plant structures. Longer, slimmer unguals were present in Mesosuchus, so this basal rhynchosaur may not have indulged in the same behaviour.
The three ages of rhynchosaur
Rhynchosaurs can be imagined to fall into three rough groups, two of which are grades, not clades. Basal rhynchosaurs – namely Mesosuchus, Howesia and Noteosuchus from the Early Triassic – share with all later forms a united naris, down-turned premaxillae and the presence of several maxillary tooth rows, but they were less ‘extreme’ in morphology than the more derived rhynchosaurs, all of which are united in Rhynchosauridae.
Basal rhynchosaurids form the second group. These animals had deeper, shorter skulls than basal rhynchosaurs, a more robust body, stockier limbs that were more similar in length than those of basal rhynchosaurs, and a much shorter tail. The several Rhynchosaurus species belong here, as does the African Stenaulorhynchus and the English Fodonyx. Traditionally, these taxa were classified together as the Rhynchosaurinae (Chatterjee 1969, 1974). However, this group is evidently paraphyletic and has now been abandoned [skeleton of basal rhynchosaur Mesosuchus shown below, from Dilkes (1998)].
Finally, hyperodapedontines form the third group. These are the ‘most extreme’ rhynchosaurs: the biggest ones, and the ones with the broadest, shortest, deepest skulls and the most complicated tooth fields. Their fore- and hindlimbs were nearly equal in length, and their bodies were bulkier than those of other rhynchosaurs. Langer & Schultz (2000) defined Hyperodapedontinae as a branch-based taxon that includes all rhynchosaurs closer to Hyperodapedon than to Fodonyx. The branch-based definition of Hyperodapedontinae means that Ammorhynchus and ‘Scaphonyx‘ sulcognathus are both part of the clade if we follow the topology recovered by Hone & Benton (2008). Several hyperodapedontine species previously regarded as representing distinct genera, including Scaphonyx from Brazil and Paradapedon from India, were found by Langer & Schultz (2000) to be nested within Hyperodapedon. Isalorhynchus from Madagascar has also been shown to be referable to Hyperodapedon (Langer et al. 2000).
What are rhynchosaurs?
Like so many other Triassic reptile groups, rhynchosaurs have been moved all about the tetrapod family tree since their recognition in 1842. On naming Rhynchosaurus articeps in that year, Richard Owen initially described some rhynchosaur material as that of a ‘labyrinthodont’ (= temnospondyl). He later realised this mistake and came to regard rhynchosaurs as close relatives of dicynodonts, as the tusk-like caniniforms of the latter were wrongly thought to be homologous with the paired premaxillary beaks of rhynchosaurs*. An affinity with Sphenodon (tuatara) was also suggested, as the beak-like premaxillae of the tuatara was, again, thought homologous with the beaks of rhynchosaurs. Thomas Huxley also supported an affinity between rhynchosaurs and Sphenodon when he described Hyperodapedon in 1869, and he proposed the name Sphenodontina for this group [Sphenodon shown here, from wikipedia].
* Incidentally, I’ve just noticed that Owen referred to big teiids as ‘New World monitors’. How cool.
While numerous different classification schemes were proposed during the course of the 20th century, it was mostly thought by the 1960s and 70s that tuatara and rhynchosaurs were close relatives, that they formed a group termed Rhynchocephalia, and that Rhynchocephalia was part of Lepidosauria (the group that also includes lizards and snakes). Yes yes, I know that things were actually much more complicated, but that’ll do as a review: for more complete retellings see Benton (1985, 1990) and Dilkes (1998).
Numerous studies published since the 1980s have shown that rhynchosaurs and the members of the tuatara clade were not really closely related (R. Burckhardt had argued as early as 1900 that the supposedly similar premaxillary beaks of tuatara and rhynchosaurs were actually completely different). Tuatara and kin – now termed the Sphenodontia* – are lepidosaurs (and hence close to squamates), while rhynchosaurs are archosauromorphs: part of the same group as archosaurs and their relatives. Archosauromorpha has been defined as a branch-based taxon that includes all taxa closer to Protorosaurus than to Lepidosauromorpha** (Dilkes 1998, p. 528) and autapomorphies of the clade include slender cervical ribs, a notch on the leading margin of the interclavicle, and an ilium with a small anterior and large posterior process (Dilkes 1998).
* Sphenodontians are closely related to Gephyrosaurus from the Early Jurassic and both form a clade that needs a name. Gauthier et al. (1988) decided to co-opt Rhynchocephalia for this purpose. Given the confusing history of this name (in addition to sphenodontians and rhynchosaurs, ‘Rhynchocephalia’ of old authors might also include choristoderes, thalattosaurs and/or younginiforms), I think this is a really dumb idea and would have preferred a new, more ‘sphenodontian-themed’ name, like Sphenodontiformes or Sphenodontomorpha. Anyway, too late to change this now.
** A definition using an archosaur specifier would have been more sensible in my opinion.
Within Archosauromorpha, the distribution of characters indicate that rhynchosaurs are closest to the Prolacerta + archosaur clade, with trilophosaurs and protorosaurs being successively more distant. Not all studies agree on this topology: Prolacerta was recovered as the sister-taxon to a rhynchosaur + archosaur clade by Müller (2003). In the diagram above (it depicts a very simplified ‘consensus’ cladogram), note that Lepidosauria and Archosauromorpha are both branch-based clades that, together, form the node-based clade Sauria. Rhynchosauria and Archosauria are both node-based (I’ve used Archosauria here in the crown-group sense, in which case the more inclusive clade – corresponding to Archosauria of tradition – is Archosauriformes. Archosauriformes is also node-based, so even this name does not correspond to the archosaur total group. How dumb). So far as I can tell, there is no published phylogenetic definition of Protorosauria.
The basal members of all archosauromorph clades seem to have been long-tailed quadrupeds, superficially similar to living monitor lizards, and generally somewhere round about 50 cm in length. After appearing in the Late Permian, these basal forms had produced a weird and wonderful assemblage by the end of the Early Triassic. In fact, the development of so many ‘extreme’ morphologies among Triassic archosauromorphs suggests that some pretty heavy selection pressure was acting on these Permo-Triassic generalists. Look at rhynchosaurid rhynchosaurs, trilophosaurs, Teraterpeton and Tanystropheus*. You see, this is why we need time travel to be invented.
* Drepanosaurids and Longisquama might be in there too, but this was contested by Senter (2004). I’m currently pretty sceptical of his argument.
So, that’s rhynchosaurs done at last. Only trilophosaurs, protorosaurs, thalattosaurs, choristoderes and all those Triassic archosaurs to go… Oh yeah, and all the rest of those temnospondyls.
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.
- . 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84, 97-164.
- . 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. 1969. Rhynchosaurs in time and space. Proceedings of the Geological Society of London 1658, 203-208.
- . 1974. A rhynchosaur from the Upper Triassic Maleri Formation of India. Philosophical Transactions of the Royal Society of London B 276, 209-261.
Dilkes, D. W. 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.
Gauthier, J. A., Estes, R. & de Queiroz, K. 1988. A phylogenetic analysis of the Lepidosauromorpha. In Estes, R. & Pregill, G. (eds) Phylogenetic Relationships of the Lizard Families: Essays Commemorating Charles L. Camp. Stanford University Press (Stanford), pp. 15-98.
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., Boniface, M., Cuny, G. & Barbieri, L. 2000. The phylogenetic position of Isalorhynchus genovefae, a Late Triassic rhynchosaur from Madagascar. Annales de PalÃ©ontologie 86, 101-127.
- . & 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.
Müller, J. 2003. Early loss and multiple return of the lower temporal arcade in diapsid reptiles. Naturwissenschaften 90, 473-476.
Senter, P. 2004. Phylogeny of Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology 2, 257-268.