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Aetosaurs
Fossil range: Late Triassic
Desmatosuchus BW
Desmatosuchus, one of the largest aetosaurs at 16 feet long.
Scientific classification

Class:

Sauropsida

Subclass:

Diapsida

Infraclass:

Archosauromorpha

(Unranked) :

Crurotarsi

Order:

Aetosauria
Lydekker, 1889

Aetosauria (meaning "Eagle Lizards" in reference to the fact that their skulls somewhat resemble that of a bird) is an extinct clade of heavily armored, medium to large-sized Late Triassic quadrupedal archosaurs that were mostly herbivorous. They are widely characterized by their dorsal and ventral surfaces being encased in parallel rows of subrectangular osteoderms, some of which sported spikes in some species. Aetosaurs ranged in size from 1–5 meters (3–16 ft), with the average size being between 2.5–3 meters (8.2–9.8 ft). Some of the smallest aetosaurs were Aetosaurus and Coahomasuchus, reaching no more than a meter in length. Two distinct subdivisions of aeotosaurs are currently recognized, the Desmatosuchinae and the Aetosaurinae, based primarily on differences in the morphology of the bony scutes of the two groups.[1]:692

Aetosaurs first appeared during the Late Carnian epoch of the Late Triassic, roughly the same time that phytosaurs and ornithosuchids appeared. They quickly diversified, into various genera, with the most notable being Aetosaurus, Stagonolepis, and Desmatosuchus, and existed for nearly the entirety of the Late Triassic. Aetosaurs quickly became widespread across the the supercontinent Pangaea, with their fossils being found in Europe (Scotland, Germany), India, Northern Africa, North (United States) and South America (Argentina), Greenland and Madagascar.

DescriptionEdit

Desmatosuchus, PFNP

Aetosaur Desmatosuchus from Petrified Forest National Park, Arizona (artist's reconstruction). This is likely the animal that made the nests discovered there in 1996.

SkullEdit

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 bony nostril opening is huge in all aetosaur taxa and, at the back of the skull, the lower temporal opening is partially closed while the superior temporal opening faces laterally, rather than dorsally. The jaw joint is set below the level of the toothrow: a feature usually associated with herbivory. The premaxillary tips are moderately expanded, giving these animals a strange shovel-like snout tip. It has been suggested that this structure was used in digging or rooting up roots and soft vegetation.[2] While both the robust limbs, and some taphonomic data, indicates digging habits for some aetosaur taxa, it has yet to be determined whether the animals were eating objects that they found on or in the soil. Part or all of the premaxillary region is usually toothless but in Neoaetosauroides from the Los Colorados Formation of Argentina, teeth were present right up the expanded premaxillary tip.[3]

Aetosaurs differed quite considerably in limb and limb girdle anatomy and posture, indicating that diverse lifestyles were practiced within the group.

TeethEdit

Longosuchus BW

Longosuchus meani, an aetosaur from North America.

Aetosaur leg fossils

The fossilized leg bones of an aetosaur.

The tooth morphology of aetosaurs varies. Basal aetosaurs such as Aetosaurus and Aetosauroides have teeth that are gently recurved and somewhat compressed. Other aetosaurs have chisel-shaped teeth are small and leaf-like, indicating a probable herbivorous diet,[4]: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.[5] Study of the braincase indicates that aetosaurs are actually closely related to crocodylomorphs.[6] There is a report of a carnivorous aetosaur from Texas, however the only published material of this creature is an abstract.[7][8] This unique taxon reportedly has sharp, strongly recurved teeth. This feature differs strongly from teeth found in other aetosaurs that are subconical, and usually reported to be lacking in both denticles and wear facets. However, Small (2002)[5] reported that both denticles and wear facets are present in Desmatosuchus and Stagonolepis, although it should also be noted that these features are lacking in other aetosaurs. In Desmatosuchus, the teeth have bulbous crowns and a constriction at the crown-root junction. Most aetosaurs seem to lack evidence of tooth wear, suggesting that soft leaves formed most of the aetosaur's diet.[9] Teeth are also absent from the rostral part of the lower jaw, and the entire lower jaw has a distinctive, unique shape where the rostral end curves upwards like the prow of a boat.[10] The toothless jaw tips have led some paleontologists to suggest that aetosaurs might have had beaked jaw tips, or possibly a beaked lower jaw tip.

Posture and gaitEdit

As with the rauisuchians, aetosaurs had a "pillar-erect" erect limb posture.[11] The feet, however, resemble those of the phytosaurs in the retention of primitive characteristics.[4] 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.

ArmorEdit

Aetosaur armor cast

Three different types of osteoderms on display at a museum.

Desmatosuchus scutes1

Desmatosuchus scutes in their original matrix.

These animals were very heavily armored (most certainly as a defense against predators), with large quadrangular, interlocking bony plates, or osteoderms, protecting the back and sides, belly, and tail.[4] These plates can easily be distinguished from similar structures on other archosaurs in that they are flat, rectangular shape, presence of an anterior articulating surface, and lack of anteriorly- or posteriorly- projecting lappets.[12] In life, these plates were probably covered in horn,[13]:159 and some aetosaurs had spikes as well. Some aetosaurs, including Typothorax and Paratypothorax, had particularly wide paramedian osteoderms (osteoderms that run along either side of the dorsal midline) and particularly wide bodies. Different aetosaur genera have differently shaped scutes, and this unique feature among this group of archosaurs is commonly used as a diagnostic feature to differentiate one genera of aetosaur from another.[14] These osteoderms are supposedly diagnostic for all (or most) of the different aetosaur taxa, and differ in their shape, surface ornamentation, and possession of bumps, spikes and ridges. A consequence of this ability to differentiate taxa based on osteoderms, and of osteoderm abundance, is that aetosaur taxa have been employed by some authors as index fossils.

Basal genera, like the widespread Norian genus Aetosaurus and the Carnian Coahomasuchus,[12] tended to be small, about a meter in length. However more advanced forms were larger - about 3 meters in length - with taxa 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.

DietEdit

Within the skull, leaf-shaped chisel-like teeth are similar to those of other herbivorous reptiles, indicating that aetosaurs ate vegetation and digested it within their gut with the aid of symbiotic bacteria. More derived aetosaurs, including Typothorax, had bulky bodies that indicate a possible ability to ferment this vegetation. However, it has recently been suggested that aetosaurs may have also fed on.[15]

ReproductionEdit

Ischigualasto scene

A restoration with Stagonolepis.

At least some aetosaurs built nests and protected their eggs. In 1996, geologist Stephen Hasiotis discovered 220 million-year-old, fossilized, bowl-like nests in Arizona's Petrified Forest, in part of the Chinle Formation. The oldest such nests that have been found belonged to phytosaurs and aetosaurs. The nests are compacted and appear very similar to the nests of the modern day crocodiles who guard their nests. The nests were holes dug in the sand in the bank of an ancient river.

EvolutionEdit

The direct ancestor of the aetosaurs is unknown, however, they seem to be clearly suchian.[16] Aetosaur remains are not found in early Carnian deposits in Madagascar,[17] however they are prominent in the early late-Carnian faunas of North and South America. Aetosaurs most likely evolved from prestosuchids, as evidenced by the skeletal features the two groups have in common. Recent studies of the braincases of aetosaurs, however, has determined that aetosaurs are more similar to crocodylomorphs than to other major suchian groups.[18] Furthermore, the tail and body, while resembling crurotarsan in appearance, differ greatly from the head, which is relatively small for a creature of its size.

Distribution and remainsEdit

Aetosaur fossils were first documented by Louis Agassiz (1807-1873) in 1844, although the Stagonolepis specimen that he described was misinterpreted by him as an armored fish. A large number of new aetosaur genera have been named since the 1980s, including Paratypothorax,[19]Longosuchus, Redondasuchus, Lucasuchus, Acaenasuchus, Coahomasuchus, and Tecovasuchus. Several additional taxa await naming and description.

Aetosaur fossil remains are known from Scotland, Germany, Greenland, the southwest and the eastern United States, Argentina, and Madagascar. Since their armored plates are often preserved, and as they often have a wide geographic distribution, but a relatively short stratigraphic range, aetosaurs can serve, like phytosaurs, as important Late Triassic tetrapod index fossils.[20] However, most known aetosaur taxa are based on disarticulated material, with many good, articulated specimens remaining poorly known. The most complete published specimen is a 2.5 m long Typothorax coccinarum from the Bull Canyon Formation of New Mexico. However, the paper that describes the complete fossil is just four pages long, and of those, two are taken up by pictures and a third by the bibliography.[21]

Genera List Edit

Genus Status Age Location Description Images
Valid.

Late Triassic.

An herbivore that was protected by spikes and armor plating, Acaenasuchus was a stagonolepid aetosaur.
Nomen dubium.

Late Triassic.

An aetosaur whose name means "sturdy lizard." It may be a junior synonym of Stagonolepis.

Junior synonym.

N/A.

N/A. Junior synonym of Stagonolepis.
Valid.

Late Triassic.

A small (1.5 m, 5 ft long) but robustly armored herbivore with an upturned snout. Several species have been described. It had a wide range, skeletal remains have been discovered in both Europe and North America. Fossiliized scutes have been discovered in Madagascar as well.
Junior synonym.

N/A.

N/A. Junior synonym of Stagonolepis
Junior synonym. N/A N/A Junior synonym of Stagonolepis.
Valid.
Valid.
Lapsus calami. lapsus calami of Desmatosuchus
Valid. Late Triassic. North America.
Junior synonym. Rioarribasuchus is a possible junior synonym of Heliocanthus
Valid. Late Triassic. North America.
Valid.
Valid.
Valid.
Valid.

Might not be an aetosaur.

Valid. — possible junior synonym of Heliocanthus
Valid. Late Triassic. North America.
Valid. Late Triassic. Europe, Geopark of Paleorrota, Brazil.
Valid. Late Triassic. North America.
Valid.
Valid. Late Triassic. North America.


References Edit

  1. ^ Parker, W.G. et al. 2008. "A new desmatosuchine aetosaur (Archosauria; Suchia) from the Upper Triassic Tecovas Formation (Dockum Group) of Texas." Journal of Vertebrate Paleontology, 28 (2): 692-701.
  2. ^ Walker, A. D. 1961. Triassic reptiles from the Elgin area: Stagonolepis, Dasygnathus, and their allies. Philosophical Transactions of the Royal Society of London, Series B 248, 103-204.
  3. ^ Desojo, J. B. & Baez, A. M. 2005. El esqueleto postcraneano de Neoaetosauroides (Archosauria: Aetosauria) del Triasico Superior del centro-oeste de Argentina. Ameghiniana 42, 115-126.
  4. ^ a b c Carroll, R. L. (1988). Vertebrate Paleontology and Evolution, WH Freeman & Co.
  5. ^ a b Small, B.J. (2002). Cranial anatomy of Desmatosuchus haplocerus (Reptilia: Archosauria: Stagonolepididae). Zoological Journal of the Linnean Society 136(1): 97-111.
  6. ^ 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.)
  7. ^ Long, R. A. & Murry, P. A. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science, Bulletin 4, 1-254.
  8. ^ Murry, P. A. & Long, R. A. 1996. A diminutive carnivorous aetosaur from the Upper Triassic of Howard County, Texas. Journal of Vertebrate Paleontology 16 (Supp. 3), 55.
  9. ^ Parrish, J. M. 1994. Cranial osteology of Longosuchus meadei and the phylogeny and distribution of the Aetosauria. Journal of Vertebrate Paleontology 14, 196-209.
  10. ^ Sawin, H. J. 1947. The pseudosuchian reptile Typothorax meadei. Journal of Paleontology 21, 201-238.
  11. ^ 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. 19 (1): 50-68.
  12. ^ a b Heckert, AB, SG Lucas & JD Harris (1999), An aetosaur (Reptilia: Archosauria) from the Upper Triassic Chinle Group, Canyonlands National Park, Utah, in VL Santucci & L McClelland [eds.], Paleontology of the National Parks, Geologic Resources Division Technical Report NPS/NRGRD/GRDTR-99/03. Washington, D.C.: National Park Service.pp. 23-26
  13. ^ Colbert, E H. (1969). Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
  14. ^ Heckert, AB & SG Lucas (1999), A new aetosaur (Reptilia: Archosauria) from the Upper Triassic of Texas and the phylogeny of aetosaurs, J. Vert. Paleontol., 19: 50-68.
  15. ^ Small, BJ (2002), Cranial anatomy of Desmatosuchus haplocerus (Reptilia: Archosauria: Stagonolepididae), Zool. J. Linn. Soc., 136: 97-111
  16. ^ Palaeos website
  17. ^ Burmeister, KC, JJ Flynn, JM Parrish, RL Whatley & AR Wyss (2000), Biostratigraphic and biogeographic implications of new middle to Late Triassic fossil vertebrates; Morondava Basin, Madagascar, Abstracts of the 2000 Meeting of the Western Association of Vertebrate Paleontologists, Museum of Northern Arizona, Flagstaff.
  18. ^ Gower, DJ & AD Walker (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.]
  19. ^ Long, RA & KL Ballew (1985), Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park, Mus. N. Ariz. Bull., 54: 45-68.
  20. ^ (Heckert & Lucas, 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
  21. ^ Hunt, A. P., Lucas, S. G. & Reser, P. K. 1993. A complete skeleton of the stagonolepidid Typothorax coccinarum from the Upper Triassic Bull Canyon Formation of east-central New Mexico, USA. New Mexico Museum of Natural History and Science, Bulletin 3, 209-212.


External links Edit


Archosauromorphs
Primitive ArchosauromorphsEuparkeriidae • Erythrosuchidae • Proterochampsidae • Proterosuchidae • Choristodera • Prolacertiformes • Rhynchosauria • Trilophosauria

Crurotarsi ArchosaursOrnithosuchidae • Aetosauria • Phytosauria • Rauisuchia • Crocodylomorpha • Crocodilia

Avemetatarsalia and Ornithodira ArchosaursScleromochlus • Pterosauria • Dinosauromorpha • Dinosauria • Ornithischia • Saurischia • Aves

Avian ArchosaursAvialae • Archaeopteryx • Confuciusornis • Ichthyornis • Enantiornithes • Hesperornithes • Neornithes • Paleognathae • Neognathae

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