Friday 5 April 2013

The first Dinosaur?

The earliest known Dinosaurs appear in the fossil record appear in the Late Carnian (beginning of the Late Triassic, 230 million years ago) of Argentina. By the end of the Carnian Theropods, Sauropods and Ornithischians are all known. The Silesaurids, considered the closest relatives of the Dinosaurs, appear in the fossil record in the Late Anisian, about 245 million years ago, in South America and southern Africa, suggesting that Dinosaurs originated in southern Pangea in the Middle-to-Late Triassic (in the Triassic all the continents were fused into one single supercontinent, Pangea; with South America, Africa, Australia, India and Antarctica making up the southern part of this supercontinent).

In a paper published in the journal Biology Letters on 5 December 2012, a team of scientists led by Sterling Nesbitt of the Burke Museum and Department of Biology at the University of Washington, describe a new possible Dinosaur from the Late Anisian Manda Beds of the Ruhuhu Basin in southern Tanzania.

The putative new Dinosaur is named Nyasasaurus parringtoni, after the nearby Lake Nyasa and the discoverer of the type specimen, Rex Parrington. It is a very incomplete specimen, described from a partial humerus (upper arm bone), three sacral vertebrae (vertebrae from the hip region) and three partial presacral vertebrae (vertebrae from before the hip region). A second specimen is also referred to the species; this comprising three cervical (neck) and two posterior presacral vertebrae.

Sacral and posterior presacral vertebrae of Nyasasaurus parringtoni. (Top) Sacral vertebrae in right lateral view. Scale bar is 1 cm, arrow points toward the head. (Middle) Interpretive drawing of (top). Abbreviations: sacral vertebra number; sr1–3, sacral rib number; st, striations. (Bottom left) Posterior presacral vertebrae in right lateral view. Scale bar is 1 cm, arrow points towards the head, abbreviation: st, striation. (Bottom right) Partial posterior presacral vertebra in dorsal view. Scale bar is 1 cm, arrow points towards the head, abbreviation: hyp, hypantrum. Nesbitt et al. (2012).

Nyasasaurus parringtoni is considered to be a Dinosaur on the basis of its humerus, which shows a number of features seen in Dinosaurs but not other Archosaurs. It has an elongated deltopectoral crest (muscle attachment in the shoulder) which is laterally deflected and has a notch at the apex, and a pointed expansion on its inside surface, and a depression on the opposite edge. The specimen also has only three cervical vertebrae, which is a feature found in, but not exclusive to, Dinosaurs.

The right humerus of Nyasasaurus parringtoni. (Left) Anterior view. Abbreviations: Dep, depression; dp, deltopectoral crest; hs, histology section. (Right) Posterior view. Abbreviations: dp, deltopectoral crest; no, notch; r, ridge. Scale bar is 1 cm. Nesbitt et al. (2012).

The partial nature of the specimen means that Nyasasaurus parringtoni cannot be classified as a Dinosaur with 100% certainty unless more material comes to light, however it did live in roughly the time and place when palaeontologists believe the first Dinosaurs to have lived, and if not actually a Dinosaur it was probably very closely related to them.

Anterior cervical vertebrae of Nyasasaurus parringtoni. (Top) Left lateral view. (Bottom) Interpretive drawings of (top). Abbreviations: Dep, depression; df, deep fossa; dia, diapophysis; epi, epipophysis; ns, neural spine; par, parapophysis; pre, prezygapophysis; pz, postzygapophysis. Scale bars are 1 cm. Arrows point towards head. Nesbitt et al. (2012).

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