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Description

A selection of early saurischian dinosaurs including herrerasaurs and basal theropods, to scale.

Dinosaurs dominated the terrestrial environments during the Mesozoic, but this largely occurred following the Triassic-Jurassic extinction event. During the latter stages of the Triassic, early dinosaurs played second fiddle to the pseudosuchians and the therapsids (precursors to the mammals). Of course the circumstances were more complicated as there were a series of extinction events which affected different groups to varying levels. For example the herbivorous therapsids (kannemeyerids Last of the dogtooths - Stahleckeriid Dicynodonts and traversodontids Dog jaws 1 - Traversodontids ) disappeared from the Northern continents at the end of the earliest stage (Carnian) of the Late Triassic while the pseudosuchian aetosaurs Armadillodiles - Aetosaurs and phytosaurs Copy-crocs 1 - Phytosaurs survived right to the end. It was during the Carnian that Staurikosaurus, one of the earliest confirmed dinosaurs, first occurred.

The earliest dinosaurs share many similar features which makes taxonomic placement difficult. Often they were lightly built carnivores or possibly omnivores with relatively small heads, a moveable joint in the jaw, and up to five digits on each limb. All known fossils originate from southern USA or South America. Superficially they resemble theropods but they shared a mixture of features. Herrerasaurs (Herrerasaurus, Staurikosaurus, Gnathovorax) were some of the more familiar of the early dinosaurs. Although obviously carnivores, their closest relatives are a point of contention. At times they have been considered to be basal theropods, basal sauropodomorphs, basal saurischians or possibly not even dinosaurs at all. Currently the consensus is that herrerasaurs are either basal saurischians or the sister group to the sauropodomorphs, and are not the direct ancestors to any group. Interestingly one former member, Eoraptor is now considered a basal sauropodomorph.
Similarly a selection of other small carnivorous dinosaurs may be closely aligned to the herrerasaurs or actually basal theropods (Tawa, Daemonsaurus, Eodromaeus). A notably unusual member was Daemonosaurus which had a particularly short skull with long protruding teeth and large orbits. Not surprisingly this species lacks any clear unambiguous theropod features and thus has been previously interpreted as a basal saurischian, silesaur or even ornithiscian.

Description

A selection of early saurischian dinosaurs including herrerasaurs and basal theropods, to scale.

Dinosaurs dominated the terrestrial environments during the Mesozoic, but this largely occurred following the Triassic-Jurassic extinction event. During the latter stages of the Triassic, early dinosaurs played second fiddle to the pseudosuchians and the therapsids (precursors to the mammals). Of course the circumstances were more complicated as there were a series of extinction events which affected different groups to varying levels. For example the herbivorous therapsids (kannemeyerids Last of the dogtooths - Stahleckeriid Dicynodonts and traversodontids Dog jaws 1 - Traversodontids ) disappeared from the Northern continents at the end of the earliest stage (Carnian) of the Late Triassic while the pseudosuchian aetosaurs Armadillodiles - Aetosaurs and phytosaurs Copy-crocs 1 - Phytosaurs survived right to the end. It was during the Carnian that Staurikosaurus, one of the earliest confirmed dinosaurs, first occurred.

The earliest dinosaurs share many similar features which makes taxonomic placement difficult. Often they were lightly built carnivores or possibly omnivores with relatively small heads, a moveable joint in the jaw, and up to five digits on each limb. All known fossils originate from southern USA or South America. Superficially they resemble theropods but they shared a mixture of features. Herrerasaurs (Herrerasaurus, Staurikosaurus, Gnathovorax) were some of the more familiar of the early dinosaurs. Although obviously carnivores, their closest relatives are a point of contention. At times they have been considered to be basal theropods, basal sauropodomorphs, basal saurischians or possibly not even dinosaurs at all. Currently the consensus is that herrerasaurs are either basal saurischians or the sister group to the sauropodomorphs, and are not the direct ancestors to any group. Interestingly one former member, Eoraptor is now considered a basal sauropodomorph.
Similarly a selection of other small carnivorous dinosaurs may be closely aligned to the herrerasaurs or actually basal theropods (Tawa, Daemonsaurus, Eodromaeus). A notably unusual member was Daemonosaurus which had a particularly short skull with long protruding teeth and large orbits. Not surprisingly this species lacks any clear unambiguous theropod features and thus has been previously interpreted as a basal saurischian, silesaur or even ornithiscian.