Herrerasaurus – Dinosaur or not?

The first fossils of this prehistoric animal were found in 1959 by a goat herder (Victorino Herrera) – and this animal was named after him.  It was not until 1988 that skull material of this Triassic prehistoric animal was found and the name Herrerasaurus formerly assigned.

Known from strata close to the city of San Juan, the same formation that yielded evidence of the early Saurischian dinosaur Eoraptor (E. lunenesis), scientist have struggled to place Herrerasaurus within the Dinosauria clade.  It as been assigned basal Theropod status.  The animal, although one of the largest terrestrial vertebrates known from the mid Triassic (approximately 228 million years ago) may not have been a dinosaur at all.  One of the diagnostic features of Dinosauria are the number of sacral vertebrae that are attached to the hip bones.  Dinosaurs have at least three vertebrae attached to the sacrum, Herrerasaurus only had two.  Three vertebrae attached to the sacrum is a trait shared between Dinosauria and some other Archosaurs, but Herrerasaurus seems to be the exception to this rule.

This animal does possess some primitive Dinosaurian features but also a number of other strange aspects of anatomy – such as the fenestra (hole) in the lower jaw and the numerous fenestrae in the skull.  The size of this meat-eater has also been the subject of considerable conjecture with estimates ranging from 2 metres in length right up to lengths in excess of 6 metres.

An Illustration of Herrerasaurus

Herrerasaurus

Picture Credit: Everything Dinosaur

Most scientists now place Herrerasaurus in the Theropoda, ascribing it to a basal Theropod.  It lived during a time when the dinosaurs were yet to establish themselves as the dominant large terrestrial animals, only about 5% of vertebrate fossil material recovered from the mid Triassic rocks of north-western Argentina has been assigned to the Dinosauria, the bulk of the fossils found are from synapsids or other Archosaurs.

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