Aetosaur
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| Aetosaurs Fossil range: Late Triassic |
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 Stagonolepis
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Acaenasuchus |
The Aetosaurs (literally "Eagle Lizards") ( 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 distinctive 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 & 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 crocodylian) 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 & 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 & 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, Zoological Journal of the Linnean Society 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 76 (5): 852-863. online
- Lucas, S.G. (1998). Global Triassic tetrapod biostratigraphy and biochronology. Paleogeography, Palaeoclimatology, Palaeoecology 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
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