The dawn of the dinosaurs begins in the early Triassic period (248-205 million years ago), after the great mass extinction of the Permian period which occurred 248 million years ago. This mass extinction opened up many vacant ecological roles and the Triassic era saw the evolution of the very first true dinosaurs. Up to this point, dinosaur predecessors, the archosaurian dinosauromorphs, were extant. Fossilised footprint evidence suggests the presence of dinosauromorphs as early as 250 million years ago. Dinosauromorphs, although not considered true dinosaurs, resembled dinosaurs in appearance and took both bipedal (two legged) and quadrupedal (four legged) forms. Dinosauromorphs include the silesaurians and lagerpetonids among others.

Image of a silesaurus taken from skeletaldrawing.blogspot.com

Image of a lagerpeton taken from dinopedia.wikia.com

Commonly mistaken for an early dinosaur, the Dimetrodon, which lived during the Permian period, is actually a synapsid. A synapsid has one fenestra on each lateral side of the skull (a fenestra is an opening in the skull) unlike the dinosaurs which are diapsids which have two openings in the skull, one on each lateral side of the skull and the other on the top surface. In fact all mammals are synapsids which makes the Dimetrodon a closer living relative to us than to the dinosaurs.

The Dimetrodon is not an early dinosaur. Image taken from en.wikipedia.org

True dinosaurs begin to appear in the Triassic period. To define true dinosaurs it must be recognised that several key features are common to the dinosaurians or ‘synapomorphies’, which separate them from other archosaurs.

Perhaps the most obvious feature being the hip socket with a hole through it – allowing for the hind limbs to be held directly beneath the body – for dinosaurs to take an ‘erect’ posture rather than the sprawling posture of other archosaurs.

Archosaur sprawling posture

The earliest dinosaur bones to be discovered are those of the carnivorous Eoraptor, Eodromaeus, Herrerasaurus , Panphagia and the herbivorous Pisanosaurus, all of these specimens were found in a bone bed located in Argentina and are believed to be 228 million years old. This would indicate the diversity and success of the early carnivorous dinosaurs, most of those mentioned above are under a meter in length, however the Herrerasaurus was comparable to a modern tiger in size.

Dinosaurs were still gaining ground in the  Triassic era and competing with archosaurs. However, dinosaurs were soon to become more widespread and fossils of the Triassic therapod Coelophysis, laid down in late Triassic New Mexico, indicated that dinosaur range was spreading. Coelophysis numbers also indicate that this was a common predator at the time.

Coelophysis. Image taken from morrisonplanetarium.org

More success came with the evolution of the prosauropods such as Plateosaurus. Which developed to be the largest of the herbivores that had evolved up to that time.

Plateosaurus could weigh up to three tonnes. Image taken from en.wikipedia.org

For the dinosaurs it was a slow rise to success with many of the archosaurs developing similar body plans, the pseudosuchians in particular were a commonplace group of archosaurs during the Triassic, and it must be emphasised that dinosaurs by no means had a monopoly on the ecosystem. Once the dinosaurs had become firmly established during the Triassic period they soon faced devastation from the end Triassic extinction, which will be discussed in a later post.

In future blog posts I also plan to further investigate the evolution of the archosaurs and trace dinosaur history back even further.