Early Evolution Of Dryosaurids And Rhabdodontids

[DD1991]

Although Callovosaurus is now securely identified as the oldest dryosaurid, the confirmation of the dryosaurid classification of Callovosaurus by iguanodont guru Andy McDonald creates a ghost lineage for other groups of non-ankylopollexian iguanodonts, including the rhabdodonts (for which there are no Jurassic representatives yet). Since the Jurassic form Yandusaurus is recovered as a non-iguanodont ornithopod by Butler et. al. (2008), and Othnielosaurus, Agilisaurus, and Hexinlusaurus are recovered outside Cerapoda, is it possible that the lack of any Jurassic rhabdodontids is an artifact of sampling (considering that knowledge of Middle Jurassic ornithopods is still in its infancy)? The important thing to note is that Callovosaurus was fossilized in an island environment surrounded by ocean, and most other Middle Jurassic neornithischians are known from China and Argentina.

[Regg Cato]

I think a phylogenetic analysis creating a ghost lineage is not sufficient evidence to suggest the presence of a group at a particular time, but I'm not really sure what you're trying to understand in posing this question. Acknowledging that our understanding of middle Jurassic ornithopods is still incomplete, of course it's perfectly possible that sampling is to blame for absence of Rhabdodontids from that period. That being said, recognition of Callovosaurus as a Dryosaurid only extends the temporal range of the group back about ten million years, which is not an exceptionally significant period of time. The middle Jurassic was until recently a poorly-documented age for most dinosaur groups, (outside of Chinese faunas, which seemed to get a head start on things, and even then our understanding of lineages at that time is incomplete and likely inaccurate) and is still a problematic age.

But as for anything, one could say "it probably exists, we just haven't found you yet."

Reminds me of a Michael Buble song...