An Albert Einstein anecdote is appropriate to today’s discussion.
One of his students staood up 15 minutes into an exam saying, “The questions in this year’s exam are the same as last year’s exam.” Einstein replied, “Don’t worry; the answers are different this year.”
It’s got to be difficult telling students
how basal dinosaurs are related. The answers are different this year. Do they traditionally split into Saurischia and Ornithischia? Or do ornithischians nest with theropods, as Baron, Norman and Barrett 2017 proposed a few months ago. Or do they split into Theropoda and Phytodinosauria, as recovered here in the large reptile tree (LRT)?
Dr. Thomas R. Holtz (U of Maryland, PhD from Yale U) is often seen on TV and YouTube as a popularizer/explainer of all things dinosaur. Recently he uploaded a web page that showed several options for dinosaur and outgroup relations. This was part of his lecture series.
“Dinosauria is comprised of three major clades: Ornithischia, Sauropodomorpha, and Theropoda. Traditionally, sauropodomorphs and theropods were recognized to form a clade Saurischia. However, recent discoveries have reduced the support for this hypothesis, and alternative relationships are possible.”
Things would be easier and more logical
if Holtz knew the precise outgroup for the Dinosauria. Unfortunately he does not. He bought into the Avemetatarsalia hypothesis, when that was invalidated 17 years ago (Peters 2000).
In the LRT
Crocodylomorpha and Poposauridae are successively more distant outgroups to the Dinosauria. In Holtz’s view crocs are distantly related with a common ancestor close to Euparkeria. Pterosaurs, Lagerpeton, Lagosuchus and Silesaurus are closer relatives. In the LRT pterosaurs are lepidosaurs, Lagerpeton is a sister to Tropidosuchus and Lagosuchus is a theropod and Silesaurus is a poposaur.
Holtz also believes in the clade Ornithodira
even though that was also invalidated 17 years ago (Peters 2000). Holtz reports, “Unfortunately, at present we have no pterosaur-lineage animals which are not already highly derived for flight, so we can’t yet trace the transformations from walking to flying in this group.” This is wrong. Peters 2000 listed a half dozen taxa with a gradual accumulation of pterosaur traits, even when tested against archosaurs. The concept of the clade Ornithodira survives to this day due to taxon exclusion. And Peters exclusion, even published work in academic journals. Apparently no one wants to test what happens when various tritosaurs are entered into the taxon list.
Holtz believes that very small dinosaurormphs
left footprints in the Early Triassic. This was invalidated earlier here.
Holtz believes that dinsoauromorphs
- had a parasagittal stance with erect hind limbs, but several clades develop this
- had simple hinge ankle joints, but both mammals and tritosaur lepidosaurs had this
- had a digitigrade posture, but both mammals and tritosaur lepidosaurs had this
See what happens
when Holtz tries to pull a “Larry Martin“? Larry would have given the same answers with a wry smile. Holtz needs to base his conclusions on a large gamut phylogenetic analysis that considers all possible candidates, not a short list of convergent traits.
Holtz reports, “No recent computer-generated phylogenetic analysis has supported [Phytodinosauria]. This is wrong. The LRT recovered the clade Phytodinosauria six years ago. Holtz also reports, “but possible support for this arrangement may exist in the enigmatic Chilesaurus.” Yes. And you heard that first here two years ago.
several Late Triassic dinosaurs of uncertain position.
In the LRT
none of the taxa listed by Holtz nests in an uncertain position… and he would discover that, too, if he also ran a large gamut phylogenetic analysis. He has access to all the literature and specimens, more so than I do. Instead of leaving dinosaur origins as a big question for his students, Holtz could find out for himself and provide an unequivocal answer. This is science. Anyone can do it, whether PhD or independent researcher.
Baron MG, Barrett PM 2017. A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220 pdf online
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.
Peters D 2000b. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293–336.