It isn’t every day that your friends make the cover of Science magazine. Belated congrats to my friend Randy Irmis and his colleagues Sterling Nesbitt, Kevin Padian and others for their neat work on the dinosauromorph assemblage of Hayden Quarry, New Mexico (Irmis et al. 2007). Exciting stuff. Why? Well…
At Hayden Quarry, Norian-aged sediments of the Chinle Formation preserve temnospondyls, drepanosaurids, aetosaurs and diverse other crurotarsans, and dinosauromorphs. The big deal is this: despite the Late Triassic date of the assemblage, Irmis et al. (2007) have been able to demonstrate the presence of both dinosaurs and non-dinosaurian dinosauromorphs. Everyone knows what dinosaurs are (or at least I hope they do), but what are dinosauromorphs? Dinosauromorpha was coined by Benton (1985) and later defined (Sereno 1991) to include all those ornithodiran archosaurs closer to dinosaurs than to pterosaurs: essentially, it is the clade that includes dinosaurs and all their close ‘proto-dinosaur’ relatives. The latter animals are best known for little Marasuchus from the Middle Triassic Chañares Formation of Argentina: they were dinosaur-like, but lacked the specialisations used to differentiate dinosaurs proper.
[The Science cover shown at top depicts Donna Braginetz’s reconstruction of various of the Hayden Quarry dinosauromorphs: Dromomeron is at bottom left, the Hayden Quarry silesaurid is adjacent to it, at top left the basal dinosaur Chindesaurus has a small crocodylomorph in its mouth, and at top right a coelophysoid theropod approaches. Nice work, Donna!]
By the Norian, the dinosaur radiation was well underway. But it was thought that the non-dinosaurian dinosauromorphs had all but gone; their places filled by the all-new dinosaurs. Hayden Quarry demonstrates that this was not so, and that non-dinosaurian dinosauromorphs were still hanging on and living alongside dinosaurs. This is a big deal because previous ideas about the early evolution of dinosaurs have argued either that dinosaurs were competitively superior to their close relatives, and hence able to push such groups into extinction and take over their ecological roles, or that dinosaurs were ‘victors by default’ that only rose to ascendance once extinction events had knocked out most of the competition (Benton 1983). If dinosaurs and non-dinosaurian dinosauromorphs actually lived alongside one another for a long time, these models become redundant: early dinosaurian success did not depend on the replacement or extinction of these other forms. Does this simply mean that there was enough room for everyone in the Late Triassic, and that the diversification of dinosaurs was not dependent on the changing fortunes of contemporary groups? No doubt we’ll see a lot more on this topic in the future.
What were these late-surviving non-dinosaurian dinosauromorphs? The first of them is the new taxon Dromomeron romeri: said quickly, that name has a certain poetic euphony about it. Shared characters of the hindlimb bones indicate that Dromomeron is a close relative of Lagerpeton chanarensis from the Middle Triassic Chañares Formation of Argentina, and the two group together in a dinosauromorph clade in Irmis et al.’s study. The name Lagerpetonidae Arcucci, 1986 already exists for this clade: Arcucci (1986) spelt it Lagerpetonidae, but it should really be Lagerpetontidae I think. Finding lagerpetonids hanging on as late as the Norian is a big surprise [in the adjacent image, I’ve enlarged the Dromomeron section of the scene shown on the cover of Science].
Lagerpetonids were unusual little beasts: they had very long hindlimbs and really weird feet [left foot of Lagerpeton shown in adjacent image. Scale bar: 20 mm]. In Lagerpeton, the foot was tetradactyl, but digits I and II were strongly reduced, meaning that the animal only walked on digits III and IV. Sereno & Arcucci (1993) suggested that this foot morphology, combined with the relatively small size of the pelvis and anteriorly inclined neural spines on some of the dorsal vertebrae, indicated that Lagerpeton was a saltator: a jumping archosaur. Unfortunately we don’t know what the forelimb anatomy of these animals was like, so we’re not sure whether they were quadrupedal or bipedal.
The Hayden Quarry assemblage includes another non-dinosaurian dinosauromorph: it’s a silesaurid, a group that were all but unknown prior to 2003 but have fast become poster-children for 21st century dinosauromorph research. First named for Silesaurus opolensis from the Carnian of Poland (Dzik 2003), silesaurids are also known from the Carnian Caturrita Formation of Brazil where Sacisaurus agudoensis was discovered (Ferigolo & Langer 2007). Eucoelophysis baldwini from the Chinle Formation – originally described as a coelophysoid theropod – is another silesaurid, as is Pseudolagosuchus major from the Middle Triassic of Argentina (Nesbitt et al. 2007). The new Hayden Quarry silesaurid (represented by a partial jaw, part of a pelvis, and some hindlimb material) might be referable to Eucoelophysis. These taxa all share a distinctive femoral morphology and, in those taxa for which jaws are known, exhibit subtriangular, coarsely serrated teeth that have swollen crowns marked with striations, a constricted crown-base, and are partially fused to the jaw bones. These teeth – superficially similar to those of ornithischians – suggest that silesaurids were predominantly herbivorous. The elongate forelimbs of Silesaurus suggest that it was quadrupedal: it was a gracile, reasonably long-legged archosaur with a total length of about 2 m.
What has received a lot of attention is the fact that silesaurids sport a toothless prong at the tip of the lower jaw; a structure that bears a superficial resemblance to the ornithischian predentary bone. The predentary is a single, U-shaped structure that has grooves on its posterior surface for reception of the two dentary bones*. The silesaurid dentary prong is strongly up-curved in Silesaurus but straighter in Sacisaurus, and in the latter the prong is reported to be formed from two separate, paired ossifications, both of which are separate from the adjacent dentaries (the rostral end of the dentary is not present in Eucoelophysis and the Hayden Quarry silesaurid). Ferigilo & Langer (2007) went as far as arguing that the paired prong of Sacisaurus was homologous with the ornithischian predentary, and that the ornithischian predentary must have evolved from paired rostral structures like those of Sacisaurus. For this to happen however, silesaurids would have to be basal members of Ornithischia (or at least the ornithischian sister-group), and hence part of Dinosauria. This isn’t supported by the character data: silesaurids instead appear to be non-dinosaurian dinosauromorphs (Irmis et al. 2007), so at the moment any similarity that the silesaurid dentary prong might have with the ornithischian predentary must be assumed to be convergent. Furthermore, it’s difficult to be confident that the prong of Sacisaurus really was separated from the dentaries and thus similar to an ornithischian predentary: the junction that Ferigilo & Langer (2007) reported looks like a crack [life restoration of Silesaurus shown in adjacent image].
* Predentary bones evolved independently in several fishes and also in some birds, including hesperornithines and teratornithids.
In recent years, the base of the dinosauromorph tree has become populated by some very odd, fascinating archosaurs. They pose new questions about dinosaur ancestry: was the ancestral condition for dinosaurs bipedality (as usually posited for Marasuchus) or quadrupedality (as posited for the silesaurids)? Were dinosaurs ancestrally carnivorous (like – it is assumed – Marasuchus) or herbivorous (like the silesaurids)? But rather than being a flash-in-the-pan, Middle Triassic phenomenon, we now know that non-dinosaurian dinosauromorphs hung on well into the Late Triassic, to survive and thrive alongside their better-known cousins.
And.. huh, so much for that goodbye ðŸ™‚ I blame Randy.
Refs – –
Arcucci, A. B. 1986. New materials and reinterpretation of Lagerpeton chanarensis Romer (Thecodontia, Lagerpetonidae nov.) from the Middle Triassic of La Rioja, Argentina. Ameghiniana 23, 233-242.
Benton, M. J. 1983. Dinosaur success in the Triassic: a noncompetitive ecological model. The Quarterly Review of Biology 58, 29-55.
– . 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84, 97-164.
Dzik, J. 2003. A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland. Journal of Vertebrate Paleontology 23, 556-574.
Ferigolo, J. & Langer, M. C. 2007. A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology 19, 23-33.
Irmis, R. B., Nesbitt, S. J., Padian, K., Smith, N. D., Turner, A. H., Woody, D. & Downs, A. 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317, 358-361.
Nesbitt, S. J., Irmis, R. B. & Parker, W. G. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5, 209-243.
Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11 (Supplement to Number 4), Memoir 2, 1-49.
– . & Arcucci, A. B. 1993. Dinosaurian precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis. Journal of Vertebrate Paleontology 13, 385-399.