Friday, October 26, 2007

Ontogeny and taxonomy of pachycephalosaurs

There were two interesting talks on pachycephalosaurs at the SVP annual meeting in Austin, Texas. I don’t know if they were coincidental but both talks dealt with the possible synonymy of Dracorex, Stygimoloch and Pachycephalosaurus.

The first talk was by John Horner. Horner (2007) used comparative cranial morphology, computer tomography and osteohistology to hypothesize that Dracorex, Stygimoloch and Pachycephalosaurus all represent different stages in an ontogenetic series of a single taxon Pachycephalosaurus.

Aside from having a flat head, Dracorex differs from Stygimoloch and Pachycephalosaurus in having large supratemporal fenestrae. Dracorex also has extensive ornamentation along the squamosals. Stygimoloch has closed off its supratemporal fenestrae and have a well-developed frontoparietal dome incorporating the rostral part of the frontal and postorbital but not the lateral and caudal elements of the skull. Stygimoloch also has extensive cranial ornamentations along the squamosal. Pachycephalosaurus has an extreme doming of the frontoparietal with the incorporation of the prefrontal, squamosal, and postorbital into the dome. The squamosal ornamentations are not extensive as in Dracorex or Stygimoloch. However, all three taxa show very similar or near identical ornamentation patterns on the snout.

Bone histology and CT scans reveal the internal architecture of the domes in these pachycephalosaurs. The bones comprising the dome are highly spongy in both Dracorex and Stygimoloch but are completely solid in Pachycephalosaurus. Horner suggests that the bone was still growing in Dracorex and Stygimoloch while it had already stopped growing completely in Pachycephalosaurus.

Horner’s talk was followed by Robert Sullivan’s. Sullivan (2007) points out that while flat-headedness have been inferred to be the ancestral condition in pachycephalosaurs, small fully domed pachycephalosaurs occur much earlier in the fossil record with the flat-headed morphology occurring more frequently in younger strata. Because of this stratigraphic incongruence, Sullivan proposes the possibility that flat-headed morphology is an early ontogenetic stage that is delayed in the later larger mature individuals. If we are to accept this hypothesis and if doming and closing of the supratemporal fenestrae occurred later in ontogeny, then the taxonomic validity of many of the flat-headed pachycephalosaurs would be in doubt. This is particularly true for Dracorex and Stygimoloch as they are from the same formation as Pachycephalosaurus.

Sullivan also suggested that the squamosal ornamentations may have been rubbed off in the extremely old Pachycephalosaurus.

However, Robert Bakker, who was out in the hall at SVP with cast replicas of the skull or skull elements of Dracorex (whole skull), Stygimoloch (partial skull) and Pachycephalosaurus (can’t remember what parts of the skull he had), argued that there are substantial differences in the three skulls to distinguish them as separate genera. Dracorex in particular apparently has unique and diagnostic features in the snout. Bakker presented an analogy from the modern Serengeti where extremely similar but distinct species of antelope (? …or something) coexist. Good point. I wouldn’t be surprised if there were several species of pachycephalosaur living at the same place considering how relatively abundant pachycephalosaurs are at the Hell Creek Formation.

So who knows. I’ll just wait till these works get peer-reviewed and published before I make my mind up…

Horner, J. 2007. Synonomy consequences of dinosaur cranial ontology. J. Vert. Paleontol. 27: 92A.

Sullivan, R. 2007. Doming, heterochrony, and paedomorphosis in the Pachycephalosauridae (Ornithischia: Dinosauria): taxonomic and phylogenetic implications. J. Vert. Paleontol. 27: 154A.

Wednesday, October 24, 2007

Deinonychus antirrhopus

I painted an older sketch of Deinonychus antirrhopus using Photoshop. I must admit, I'm no where near a computer artist but I had quite a bit of fun with Photoshop. I didn't know you can do so much with it...

Anyway, the all-too-famous Deinonychus antirrhopus. One of my favourite dinosaurs thus far. I didn't draw flight feathers on this guy mostly because of the simpler integuments of Sinornithosaurus but its quite obviously outdated now that we know one of its closest relative Velociraptor was found to have quill knobs on its ulna. These are little bumps on the surface of the bone and are typically associated with flight feathers in modern birds.

Thursday, October 11, 2007

Wood-eating behaviour in hadrosaurs

I came across a really interesting article yesterday about some hadrosaur coprolites from the Upper Cretaceous Two Medicine Formation that contained woody materials (Chin 2007). This is direct evidence that at least some hadrosaurs ate wood. Coprolites at this locality regularly contain wood indicating that conifer wood was regularly ingested. Wood contains lignin which cannot be digested by vertebrate herbivores so there is no nutritional value on its own. Thus, in order for any animal to intentionally ingest wood it must have a very good reason of doing so as processing wood is such an effort, both in mechanical digestion (chewing) and chemical digestion. The absence of little twigs from the coprolites pretty much rules out accidental ingestion while foraging leaves.

On the other hand, the author found signs of fungal decomposition in the wood material. So apparently, the hadrosaur was eating fungus-infected, or in other words, rotting wood. Fungus de-lignifies wood and makes cellulose available for other organisms to consume. 'From an energetic standpoint, it is evident that partially degraded wood would provide a nourishing resource augmented by fungal tissues, associated microbial symbionts, and invertebrates. Decaying wood would require less energy to process and could be easily comminuted' (Chin 2007).

Modern megaherbivores consume fugus-decayed wood when high-quality browse is sparse. However, these Two Medicine coprolites were preserved during the rainy season. The author suggests that the high proportion of fungal degraded wood in the hadrosaur diet may have been because of a lack of vast grasslands. Modern large herds are largely supported by the fast regeneration of grass after consumption by a herd of herbivores. In a grass-poor Cretaceous world, large herbivorous dinosaurs obviously had to eat other sources of food, e.g. ferns, angiosperms, conifers AND decaying wood.

Chin, K. 2007. The paleobiological implications of herbivorous dinosaur coprolites from the upper cretaceous Two Medicine formation of Montana: Why eat wood? Palaios 22(5):554-566.