Please note that this is an accepted (in press) publication, has not yet undergone the proofing stage, and thus may still undergo some minor changes before final publication.
Knoll, F., Padian, K., and A. de Ricqles. 2009. Ontogenetic change and adult body size of the early ornithischian dinosaur Lesothosaurus diagnosticus: Implications for basal ornithischian taxonomy. Gondwana Research. Available online 15 April 2009, doi:10.1016/j.gr.2009.03.010.
Abstract: Questions about the taxonomic status, diversity, and pace of evolution of basal ornithischian dinosaurs persist in part because some historically important taxa have been based on incomplete material of uncertain ontogenetic status. We examined the morphology of critical “fabrosaurid” specimens and analyzed the bone tissues of small and large individuals. We conclude that the case for the existence of a non-heterodontosaurid ornithischian distinct from Lesothosaurus diagnosticus in the upper Elliot Formation of southern Africa is not conclusive and we suggest that this species and Stormbergia dangershoeki may actually represent ontogenetic stages of one taxon that reached maturity in approximately four years.
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So the bottom line is that ornithischians were ridiculously rare in the Late Triassic?
ReplyDeleteAlmost non-existant. One very scrapy, poorly diagnostic specimen of Pisanosaurus; one fragment of a possible heterodontosaurid, and then Eocursor. That's all. Three specimens, two very badly preserved and barely diagnostic, and the other right at the end of the Triassic if not in the early Jurassic.
ReplyDeleteI'm still not buying the presence of Triassic ornithischians 100%; however, Lesothosaurus and Stormbergia are Jurassic.
So you think that ornithischians are misplaced in current phylogenies? Because even if you discount the known Triassic ornithischian fossils, ornithischians MUST have been present if we have their phylogenetic position correct, as their sister-groups are all Late Triassic in age.
ReplyDeleteIn any case, why don't you buy Pisanosaurus? Its not just the teeth that place it as an ornithischian - the lower jaw is spot on for an ornithischian in every way, and particularly similar to heterodontosaurid lower jaws.
Let me switch the emphasis on one of your sentences, "because even if you discount the known Triassic ornithischian fossils, ornithischians must have been present IF we have their phylogenetic position correct, as their sister-groups are all Late Triassic in age".
ReplyDeleteI agree that all of the evidence for this positioning is extremely strong, except for one thing, the lack of good Triassic age ornithischian fossils. Because of this lack of specimens we are stuck with some sizeable ghost lineages and being forced to explain this paucity with statements such as they don't fossilize well, or maybe they were endemic to Gondwana, or...
I'm just saying that I am still dubious despite the phylogentic analyses. Sure Pisanosaurus has a coranoid process and an ornithischian-like dentition but hasn't this just gotten us into trouble in the past? I'd just like to see more and better specimens. Right now we have evidence of Triassic ornithischians because of phylogenetic hypotheses and two scrappy specimens; however, would it really be that shocking if Pisanosaurus turned out to be a non-ornithischian dinosauriform with a couple of convergences? As I said, I'm tentatively believing it is an ornithischian for now, but don't bet the farm just yet until better specimens are found.
Because of some unpublished proposed phylogenies we are going to have to start looking for lots of things in much older rocks. Some have started to show up, others have not. I suppose that we just need to keep at it, but the phylogeny is only a predictor of fossils, not the actual fossils on which presence or absence is based. We can only hope that in the future these predictions are confirmed.
Wouldn't this problem go away (sort of) if silesaurids turn out to be basal ornithischians?
ReplyDeleteBill - I think you miss the point. Of course science is conditional on present knowledge. Some new discovery could totally overturn what we know about the early evolutionary history of ornithischians. Nonetheless, right now, our best information suggests that ornithischians should have a Triassic record (you'd really have to change their phylogenetic position alot to get rid of that Triassic ghost lineage), and that Pisanosaurus is part of that lineage.
ReplyDeleteTo be honest, I think you're not giving Pisanosaurus enough credit. And no, I don't think its quite the same as the Revueltosaurus situation. The lower jaw isn't just *similar* to other ornithischians - its nearly identical in every way. And the teeth are a similar situation. Plus, the post-crania is exactly what one would expect in a very basal ornithischian dinosaur. In contrast, Revueltosaurus and other alleged Triassic ornithischian teeth were always *similar* to true ornithischians, but in a very general way. For example, none of the teeth were ever considered a spot-on imitation of Lesothosaurus or Heterodontosaurus. The phylogenetic mismatch between the crania and postcrania of Pisanosaurus was only a problem when heterodontosaurids were considered ornithopods. It now makes total sense with heterodontosaurids as the basal most clade of Ornithischia. Of course this could all change with the discovery of a complete Pisanosaurus skeleton that shows the taxon is a basal turtle, but thats just fantasy right now. We're scientists - we have to go with the best conclusions on the available evidence.
Zach - that would require that there is good character evidence for placing Silesaurus and friends as ornithischians. Which, as of right now, there isn't.
Randy,
ReplyDeleteGood points, but obviously your ideas regarding Pisanosaurus have changed a bit since our 2007 paper in which we stated:
"we tentatively consider Pisanosaurus an ornithischian dinosaur"
"the hypothesis that Pisanosaurus is a basal dinosauriform with jaw morphology convergent upon
ornithischians cannot be eliminated"
I guess that I have not kept up.