Are Aetosaurs Dinosaurs?

Aetosaurs were a group of armour plated reptiles that evolved in the Early Triassic and survived to the end of the Triassic period.  Although many of these mainly herbivorous animals, shared the environment with dinosaurs they were not members of the Dinosauria.  Dinosaurs and Aetosaurs are members of the Archosaur group (means ruling reptiles).  These reptiles evolved in the Permian and diversified in the Triassic into a a variety of forms.  Early on in their history, the Archosaurs split into two distinct clades, both clades have extant representatives.  You can still see examples of Archosaurs today.

The first clade was the Ornithodirans – Pterosaurs, Dinosaurs and Birds

The second clade was the Crocodylotarsians – otherwise known as the crocodile-group.  Representatives of this group were the Aetosaurs, Phytosaurs, Rauischians and the Crocodiles of which the descendants are still around today.  By the beginning of the Jurassic all the Crocodylotarsians, except the crocodiles were extinct.

Unlike the Dinosauria and their ankle bones with an upward projection, allowing the weight to be supported in an upright posture, the Crocodylotarsians had a different ankle joint arrangement.  The structure of the ankle joint gave this type of Archosaur their name as Crocodylotarsians means “crocodile ankle”.  These ankle joints allowed these reptiles to twist their feet to the side whilst walking, a more sprawling or semi-upright stance.

Aetosaurs belonged to the crocodile-type of Archosaur.  Most Aetosaurs had large bony plates covering their bodies.  This formed a heavy, protective body armour.

A Close up of an Aetosaur – Paratypothorax

Picture Credit: Everything Dinosaur

Ironically, although Aetosaurs are not closely related to birds, since birds belong in the Ornithodiran clade of Archosaurs, the Aetosaur name means “Eagle Lizards” as when the first skulls of these animals were studied; it was remarked how closely the fossils resembled the skulls of eagles.

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