Eoraptor

Eoraptor is one of the earliest known theropod dinosaurs. It was a very small omnivore or carnivore which lived in the Lower Triassic, about 231-228 million years ago (mya).  It is the only one of the possible early dinosaurs which can be exactly dated by a layer of volcanic ash close beneath the fossil. 

Eoraptor was a small, lightly-built archosaur that walked on two long legs. It was about  it had light, hollow bones, a long head with dozens of small, sharp teeth, and five fingers on its grasping hands (two of the fingers on each hand were very small).

The fossil was found in the Ischigualasto Formation in the Argentine. This contains some of the oldest known dinosaur and archosaur remains. They are of top quality, number and importance. It is the only place in the world where nearly all of the Triassic is represented in an undisturbed sequence of strata. What is now badlands was then a volcanically active floodplain dominated by rivers, with a strongly seasonal rainfall. 

Rhynchosaurs and cynodonts are by far the most common among the tetrapod fossils in the park. Dinosaurs make up only 6% of the findings,Rogers R.R. et al 1993 while archosaurs make up 19% of the findings.The Ischigualasto tetrapod assemblage (late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins. Science, 260, 794-797. but these include early samples of the two major lineages of dinosaurs (ornithischians and saurischians). The carnivorous Herrerasaurus is the most numerous of these dinosaur fossils.

 Teeth 
The teeth of Eoraptor  are of two types. There are  serrated, recurved teeth in the maxillae (upper jaws), like the teeth  of theropods, and leaf-shaped teeth in the dentary (lower jaw), like the teeth of basal sauropodomorphs.Sereno P.C. et al 1993.  Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria.  Nature 361, 64-66.

 Cladogram 
Below is a cladogram based on the phylogenetic analysis conducted by Sues et al. in 2011, showing the relationships of Eoraptor:Sues, Hans-Dieter et al 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society B 278 (1723): 3459–3464

A large analysis of early dinosaurs by Matthew Baron, David Norman and Paul Barrett (2017) found Eoraptor to be the earliest diverging member of Theropoda, within the larger clade Ornithoscelida.Baron, M.G., Norman, D.B., and Barrett, P.M. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543: 501–506. 

 References 

Category:Theropods
Category:Triassic dinosaurs