Ornithischians Of The Late Triassic And Early Jurassic

The clade Ornithischia is well established and defined, but its earliest members remain largely a mystery. Some are known only from teeth. The best fossil clues to their anatomy are fragmentary at best. Of about 15 specimens tentatively assigned to this group, paleontologists are most confident in the ornithischian nature of the following three taxa.

Lesothosaurus (Early Jurassic, Lesotho). This dinosaur from Africa is the best understood of the basal ornithischians. Paleontologist Peter Galton first described it in 1978, giving it a name that means "Lesotho lizard." It is perhaps the most primitive of all known ornithischians and is understood from four skulls and associated skeletal material. The skull material includes a nearly complete juvenile skull, described by French paleontologist Fabien Knoll in 2002. The skulls clearly show the cheek dentition, made up of leaf-shaped teeth; a small, bony beak on the upper jaw; a predentary; and the characteristic eyelid bone, in this case jutting backward and over the top of the eye socket. Lesothosaurus was small, about 3 feet (0.9 m) long, with long legs and a lightly built body. British paleontologist Michael Benton pointed out that the tooth wear seen in the jaw of Lesothosaurus suggests that it used an up-and-down chopping motion but had not yet adapted the back-and-forth and sideways jaw mobility that characterizes true chewing motion as seen in later ornithischians.

Pisanosaurus (Late Triassic, Argentina). Pisanosaurus was a small herbivore, possibly measuring only 3 feet (0.9 m) long. It is the oldest known plant-eating dinosaur and was nearly contemporaneous with the "prosauropods" of South Africa and South America. The fossil material for Pisanosaurus is fragmentary and difficult to diagnose.

Lethososaurus

It was once thought to be an ornithopod (a later line of ornithischi-ans) by paleontologists who studied it between 1967 and 1986, but cladistic analysis by American paleontologists David Weishampel and Paul Sereno in the early 1990s found it to be the most basal of all known ornithischians. Parts of the upper and lower jaw provide the best clues to its affinity, but doubts persist because other portions of the specimen are poorly preserved and incomplete. Traits that would unambiguously place it among the basal ornithischians, such as the sacral vertebrae, better pelvic material, and limbs, are missing from this curious and puzzling early dinosaur.

Technosaurus (Late Triassic, Texas). Technosaurus was described in 1984 by Indian paleontologist Sankar Chatterjee, who has spent much of his career working with fossil material from Texas. It is known only from a partial dentary—the lower jaw. It was named after Texas Tech University in Lubbock, Texas, where Chatterjee teaches. The teeth are leaf-shaped and positioned in the cheek region. The teeth are closely packed, forming a rudimentary dental battery, a feature also seen in Lesothosaurus.

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