Aetosaur
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 Stagonolepis
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Acaenasuchus |
The Aetosaurs (family Stagonolepididae order Aetosauria) are an extinct clade of heavily armoured, medium to large sized, Late Triassic herbivorous archosaurs.
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), although peg-like teeth and a keratinous snout have been described in at least one species as possible adaptations for feeding on colonial insects (Small 2002). 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 quadrupeds.
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 up to 5 meters long, equipped with large spines over the shoulders, which added to the animal's defensive armament.
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 have 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).
[edit] 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
- Small, B.J. (2002), Cranial anatomy of Desmatosuchus haplocerus (Reptilia: Archosauria: Stagonolepididae. Zoological Journal of the Linnean Society 136(1):97-111.
[edit] External links
- Archosauromorpha: Suchia (Aetosauridae) - Palaeos
- Aetosauria Translation and Pronunciation Guide - Ben Creisler
- Introduction to the Aetosauria - UCMP
