The Aetosaurs (family Stagonolepididae or Aetosauridae) are an extinct clade of heavily armoured, medium to large sized, Late Triassic herbivorous archosaurs. A clade is group of organisms which share a common ancestor and which includes all decendents of that ancestor. ... The Triassic is a geologic period that extends from about 245 to 202 Ma (million years ago). ... A deer and two fawns feeding on some foliage In zoology, an herbivore is an animal that is adapted to eat primarily plant matter (rather than meat). ... Groups Archosaurus Chasmatosaurus Euparkeria Crurotarsi     Aetosauria     Crocodilia (crocodiles)     Phytosauria     Rauisuchia Ornithodira     Pterosauria     Marasuchus     Dinosauria        Aves (birds) Archosaurs (Greek for ruling reptiles) are a group of diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian (Lower Triassic). ...

The head is small relative to the large body, and quite unique in shape, being flat and blunt at the front, like the snout of a pig. The chisel-shaped teeth are small and leaf-like, indicating a herbivorous diet (Carroll 1988 p.273). Study of the braincase indicates that aetosaurs are actually closely related to crocodylomorphs (Gower & Walker 2002).

As with the rauisuchia, they had a "pillar-erect" erect limb posture (Heckert and Lucas 2002). The feet however resemble those of the phytosaurs in the retention of primitive characteristics (Carroll 1988 p.273). In other respects they have a typically crurotarsan (rauisuchian or crocodilian) body and large powerful tail. Although the fore-limbs are much smaller than the hind limbs, all aetosaurs were quadrapeds. Groups Family Prestosuchidae Family Rauisuchidae Family Poposauridae Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters) Triassic archosaurs. ... Phytosaurs - family Phytosauridae or Parasuchidae - were a group of large (2 to 12 meters long - average size 3 to 4 meters) semi-aquatic predatory thecodonts that flourished during the Late Triassic period. ... sub-taxa Family Phytosauridae Family Prestosuchidae Family Ornithosuchidae Family Stagonolepididae Family Rauisuchidae Family Poposauridae Superorder Crocodylomorpha    Sphenosuchia    Order Crocodylia Reference Benton, M. J. 2004, Vertebrate Paleontology Crurotarsi (cross-ankles) is a node-based taxon created by Paul Sereno in 1991 to supplant the old term Pseudosuchia. ... Groups Family Prestosuchidae Family Rauisuchidae Family Poposauridae Rauisuchia are a poorly known assemblage of predatory and mostly large (often 4 to 6 meters) Triassic archosaurs. ... Subfamilies Family Crocodylidae    Crocodylinae    Alligatorinae    Gavialinae Crocodylia (or crocodylians) is an order of large reptiles that scientists believe branched off from class Reptilia about 220 million years ago. ...

These animals were very heavily armoured (most certainly as a defense against predators), with large quadrangular, interlocking bony plates plates protecting the back and sides, belly, and tail (Carroll 1988 p.273). In life these plates were probably covered in horn (Colbert 1969 p.159).

Primitive genera, like the widespread Norian genus Aetosaurus, and the Carnian Coahomasuchus (Heckert and Lucas 1999) tended to be small, about a meter in length. However more advanced forms were larger - about 3 meters in length - with some, such as Typothorax and Paratypothorax, possessing broad turtle-like bodies, and others, like Desmatosuchus, a narrow-bodied genus upto 5 meters long, equipped with large spines over the shoulders, which added to the animal's defensive armament. The Norian Stage was a portion of the Triassic geological period. ... The Carnian is a stage on the geologic time scale occuring from 228 +/- 2 to 216. ...

Aetosaur fossil remains are known from Scotland, Germany, Greenland, the south-west and the eastern United States, Argentina, and Madagascar.

Since their armoured plates are often preserved, and as they often havce a wide geographic distribution but short stratigraphic range, Aetosaurs can serve, like phytosaurs, as important Late Triassic tetrapod index fossils (Heckert and Lucas 2002, Lucas 1998). The Late Triassic (also known as Upper Triassic, or Keuper) is the third and final of three epochs of the Triassic period. ... Index fossils (or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). ...

References

• Benton, M. J. (2000), Vertebrate Paleontology, 2nd ed. Blackwell Science Ltd
• Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
• Colbert, E H. (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
• Gower, D. J. and Walker, A. D. (2002), New data on the braincase of the aetosaurian archosaur (Reptilia: Diapsida) Stagonolepis robertsoni Agassiz, Zool. J. Linn. Soc. 136: 1-7. Archosaurian Anatomy and Palaeontology: Essays in Memory of Alick D. Walker, DB Norman & DJ Gower (eds.)
• Heckert, A. B and Lucas, S. G, (1999) A new aetosaur (Reptilia: Archosauria) from the Upper Triassic of Texas and the phylogeny of aetosaurs. Journal of Vertebrate Paleontology. Vol. 19, no.1, pp. 50-68
• -- -- (2002), South American occurrences of the Adamanian (Late Triassic: Latest Carnian) index taxon Stagonolepis (Archosauria: Aetosauria) and their biochronological significance, Journal of Paleontology, Vol. 76, no.5, pp. 852-863 online
• Lucas, S.G. (1998), Global Triassic tetrapod biostratigraphy and biochronology. Paleogeog. Palaeoclimatol., Palaeoecol. 143: 347-384

Robert L. Carroll (b. ... Edwin H. Colbert (1905 – 2001) was a distinguished vertebrate paleontologist and prolific researcher and author. ... Spencer G. Lucas is a paleontologist and stratigrapher, and curator of paleontology at the New Mexico Museum of Natural History and Science. ...

External links

• Archosauromorpha: Suchia (Aetosauridae) - Palaeos
• Aetosauria Translation and Pronunciation Guide - Ben Creisler
• Introduction to the Aetosauria - UCMP