Aetosaurs - History

History

Aetosaur material was first described by Swiss paleontologist Louis Agassiz in 1844. He named the genus Stagonolepis from the Lossiemouth Sandstone in Elgin, Scotland, but considered it to be a Devonian fish rather than a Triassic reptile. This may be because he considered the strata to be part of the Old Red Sandstone, and thus Paleozoic in age. Agassiz mistook the osteoderms for large rhomboidal scales, which he thought were arranged in a similar pattern to those of gars. He also thought that these supposed scales were very similar to those of the lobe-finned fish Megalichthys due to their large size.

English biologist Thomas Henry Huxley reconsidered the fish scales described by Agassiz and considered them to belong to a crocodilian. He first proposed this to the Geological Society of London in 1858, and went into more detail in an 1875 paper in the society's quarterly journal. By this time, new material had been uncovered from Elgin that indicated that Stagonolepis was not a fish, but a reptile. However, Stagonolepis was still known primarily by scutes and imprints of scutes, many of which were not well preserved.

More complete aetosaur remains were found from the Lower Stubensandtein of Germany in the 1870s. Among them were complete articulated skeletons of 22 aetosaurs. These specimens were found in a large sandstone outcrop near Stuttgart and were preserved together in an area less than 2 square metres in size. The animals were probably buried under lake sediment soon after they died, with the flow of water repositioning their bodies on the lake bed and putting them in close proximity to one another. In 1877, German paleontologist Oskar Fraas assigned these specimens to the newly erected genus Aetosaurus. Fraas named the genus after the skull's resemblance to the head of an eagle, with a narrow, elongate skull and a pointed snout.

The first aetosaurs from North America were also being described at this time. American paleontologist Edward Drinker Cope named Typothorax in 1875 and Episcoposaurus in 1877, both of which were from New Mexico. However, Cope considered these genera to be phytosaurs, crocodile-like aquatic archosaurs that were also common in the Late Triassic. While new material referable to Typothorax has been found in recent years throughout the American southwest, Episcoposaurus is no longer considered valid. Cope, along with later paleontologists such as Friedrich von Huene, recognized that remains of the type species E. horridus actually belonged to Typothorax. Another species, E. haplocerus, was reassigned to Desmatosuchus in 1953.

A third North American genus called Stegomus was named by Othniel Charles Marsh in 1896. Marsh had a long-time rivalry with Cope that was made famous in the Bone Wars of the late 19th century, in which the two tried to out-compete one another in the field and in scientific literature. Unlike Cope's aetosaurs, Stegomus was found from the eastern United States in Connecticut. Marsh also recognized Stegomus as an aetosaur rather than a phytosaur in his initial description of the genus. Like Cope, many paleontologists tended to consider aetosaur scutes to belong to phytosaurs during this time period.

Aetosauria was first named in 1889 by English naturalist Richard Lydekker and zoologist Henry Alleyne Nicholson. They considered Aetosauria to be one of three suborders of the order Crocodilia, the other two being Parasuchia (a Triassic group) and Eusuchia (a group including all post-Triassic crocodiles). At the time, Cope considered Aetosauria to belong to Rhynchocephalia, an order of reptiles that includes the living tuatara. He also thought that the tightly fitting osteoderms, which were thought to be fused to the ribs, indicated that aetosaurs were transitional between rhynchocephalians and turtles. Nicholson and Lydekker placed a single family within the suborder, Aëtosauridæ. They considered aetosaurs to be similar to living crocodilians apart from their elongated metatarsals, which were compared to those of dinosaurs. In fact, Marsh considered aetosaurs to be closely related to dinosaurs because both had similar metatarsals.

Aetosaur material continued to be described into the early 20th century, with notable paleontologists such as Barnum Brown and Charles Camp collecting specimens. However, aetosaur remains were still being confused with those of phytosaurs. During this time, aetosaurs were usually considered to be members of Pseudosuchia, a now obsolete group consisting of various Triassic archosaurs including phytosaurs. However, similarities were often observed between aetosaurs and Crocodilia, the group to which aetosaurs were initially referred in the 19th century. While the skull had many features in common with pseudosuchians, parts of the postcranial skeleton, in particular the scutes, seemed to be similar to those of crocodilians.