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Re: New ankylosaur paper- Bissektipelta gen. nov.
Me debate Ken Carpenter regarding ankylosaurs? Perhaps not the brightest
thing to do, but let's see where it leads. He wrote-
> The reason is simple. There is nothing about the Amtosaurus magnus
specimen (a partial, weathered
> braincase) that allows it to be separated from any other ankylosaur
braincase. It could just as likely belong > to one previously named.
Bissektipelta, another braincase, differs from any known braincase.
If that were the case, I'd agree Amtosaurus was a nomen dubium. But from
what Parish and Barrett write, it's not. As a disclaimer, note all these
character distributions and morphologies are taken directly from that paper
(unless noted), as my knowledge of both ankylosaurs and braincases are both
very limited, and most of my ankylosaur refs are at home anyway. So,
keeping those in mind...
Parish and Barrett state Amtosaurus can be distinguished from Bissektipelta
by two exits for the hypoglossal nerve, the higher angle between the ventral
basioccipital and basisphenoid surfaces, and quite possibly the more
anteriorly placed basipterygoid processes. So that's one other taxon
Bissektipelta can be separated from. What about others?
Well, Pawpawsaurus, Panoplosaurus, "Acanthopholis", Pinacosaurus, Tarchia
and Saichania have one hypoglossal opening, so are distinct from Amtosaurus.
Edmontonia, Silvisaurus and Sauropelta are further differentiable by their
sub-hemispherical occipital condyle (Vickaryous et al., 2001). Cedarpelta,
Shamosaurus and Ankylosaurus have a transversely convex ventral surface of
the basioccipital-basisphenoid ridges without longitudinal ridges, unlike
Amtosaurus. Talarurus lacks a dorsally flexed endocranial floor,
distinguishing it. Maleevus is distinguished by having the anterior foramen
for XII placed ventral to the posterior one. Euoplocephalus can be
distinguished by the fenestra ovale not being positioned dorsally to the
metotic fissure (Vickaryous and Russell, 2003). So which taxa examined by
the authors are left? Gargoyleosaurus, Gastonia, Minmi, Nodocephalosaurus,
Polacanthus(?), Shanxia and Struthiosaurus. Parish and Barrett state most
ankylosaurs besides Amtosaurus and Polacanthus(?) have the anterior XII
foramen placed ventral to the posterior foramen. Does this include any of
these other taxa? The same could be asked for many of the other characters
I listed above. So from my understanding, Amtosaurus can be distinguished
from most ankylosaurs by the combination of characters listed by Kurzanov
and Tumanova, minus the high occiput and braincase. The question that needs
to be answered now, is can Amtosaurus be told apart from the seven remaining
comparable taxa? It's a question however, that I lack the knowledge to