Earliest Dinosaurs
The first known dinosaurs were bipedal predators that were 1-2 metres (3.3-6.5 ft) long.
Spondylosoma may or may not be a dinosaur; the fossils (all postcranial) are tentatively dated at 230-232 Ma.
The earliest confirmed dinosaur fossils include saurischian ('lizard-hipped') dinosaurs "Nyasasaurus" 243 Ma, Saturnalia 225-232 Ma, Herrerasaurus 220-230 Ma, Staurikosaurus possibly 225-230 Ma, Eoraptor 220-230 Ma and Alwalkeria 220-230 Ma. Saturnalia may be a basal saurischian or a prosauropod. The others are basal saurischians.
Among the earliest ornithischian ('bird-hipped') dinosaurs is Pisanosaurus 220-230 Ma. Although Lesothosaurus comes from 195-206 Ma, skeletal features suggest that it branched from the main Ornithischia line at least as early as Pisanosaurus.
It is clear from this figure that early saurischians resembled early ornithischians, but not modern crocodiles. Saurischians are distinguished from the ornithischians by retaining the ancestral configuration of bones in the pelvis. Another difference is in the skull, the upper skull of the Ornithischia is more solid and the joint connecting the lower jaw is more flexible; both are adaptations to herbivory and both can already be seen in Lesothosaurus.
Read more about this topic: Evolution Of Dinosaurs
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