Olsen and Baird (1986) argued on phylogenetic grounds that Atreipus was made by either an ornithischian
or a member of the sister-group of Dinosauria, which we would now term a non-dinosaurian dinosauromorph.
In recent years, scientists have found evidence of bristle- and quill-like strands--perhaps early versions of feathers--that speckled the hides of ornithischian
When they analyzed the tracks, the researchers determined that ornithischians
changed their way of walking as surface conditions changed.
Second, this Maastrichtian Marilia Formation fauna, when considered in the light of the Late Cretaceous discoveries, was different by the lack of ornithischian
records in Brazil, but which are found in Argentina.
It means that feathers may have evolved, or developed over a long period of time, from structures on an ancestor of both saurichian and ornithischian
34C; impressions of teeth: YPM VP 008668, VP 008691) closely resemble those of the teeth of the basal ornithischian
dinosaur Lesothosaurus diagnosticus from the Upper Elliot Formation of southern Africa (Sereno 1991).
In North America, for instance, sauropodomorphs and ornithischians
didn't even show up until after the Triassic was over, Irmis reports in an upcoming issue of Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
Olsen and Baird (1986) argued that either an early ornithischian
dinosaur or a non-dinosaurian archosaur converging on ornithischians
in the structure of its feet left the distinctive tracks of Atreipus.
Dinosaurs are divided into two main orders: saurischians, which have forward-pointing pubic bones, and ornithischians
, which have backward-pointing pubic bones.