Stegomosuchus

''Stegomosuchus'' is an extinct genus of small protosuchian crocodylomorph. It is known from a single incomplete specimen discovered in the late 19th century in Lower Jurassic rocks of south-central Massachusetts, United States. It was originally thought to be a species of ''Stegomus'', an aetosaur (a type of armored herbivorous reptile), but was eventually shown to be related to ''Protosuchus'' and thus closer to the ancestry of crocodilians. ''Stegomosuchus'' is also regarded as a candidate for the maker of at least some of the tracks named '' Batrachopus'' in the Connecticut River Valley.

Discovery

The holotype and only known specimen of ''Stegomosuchus'' ( AM 900) was discovered at what was then known as the Hines Quarry, east of East Longmeadow, Massachusetts, just before the turn of the 20th century. It was found below the surface, in red sandstone used for building material. This site is now called the Hoover Quarry; it has also yielded invertebrate trace fossils and dinosaur tracks (''Eubrontes''). The rocks are now known to belong to the Portland Formation when ''Stegomosuchus'' was originally described, the Portland Formation is now known to date to the Early Jurassic, including the Hettangian and Sinemurian stages (approximately 200 to 190 million years ago).

Its discoverer, G. B. Robinson, took home the blocks containing the specimen and placed them in his door yard, where they were exposed to the elements for "about seven years." The fossil was then found and obtained by Mr. and Mrs. E. D. White, and the specimen was then described by B. K. Emerson and F. B. Loomis in 1904. At that point, the specimen was in three blocks. The bones had been largely preserved as impressions, and the two main blocks had upper and lower impressions of the skull, twenty-eight pairs of armor plates situated along the spine up to the pelvis, right arm (minus the hand) and shoulder blade, and left foot. Emerson and Loomis interpreted the impressions as showing another row of armor along the sides, but this was later shown to be a mistake. AM 900 was a small animal, with a skull estimated at long and across, and a body length from snout to pelvis of . Emerson and Loomis described the specimen as a new species of ''Stegomus'' (''S. longipes''), an aetosaur known from slightly older rocks.

Classification

The assessment of the material as belonging to a species of the aetosaur ''Stegomus'' held for about twenty years, until Friedrich von Huene reclassified it, giving the species the new genus ''Stegomosuchus'' and new family Stegomosuchidae, in the Pseudosuchia.

Its classification was further reassessed by Alick Walker over forty-five years later, who reinterpreted ''Stegomosuchus'' as a close relative of ''Protosuchus'', a crocodile-like reptile of similar age. In making this change, he noted that Stegomosuchidae should have priority over Protosuchidae. He also regarded AM 900 as a juvenile. The reassignment of ''Stegomosuchus'' was followed, although Alfred Romer recommended passing over Stegomosuchidae for the more familiar Protosuchidae. Whetstone and Whybrow (1983) agreed that a protosuchian identity was probable, but found AM 900 too poorly preserved and lacking too many important parts of the body to classify further.

Paleobiology

By comparison to other protosuchids, ''Stegomosuchus'' was probably a terrestrial carnivore.

In the same issue of ''The American Journal of Science'' that contained the description of ''Stegomus longipes'', Richard Swann Lull published a short article in which he proposed that ''S. longipes'' had produced previously known tracks named ''Batrachopus gracilis'', also from the Connecticut River Valley. This assessment has been followed, although the ichnospecies assigned to ''Batrachopus'' have since been consolidated; ''B. deweyi'' is now the name for the tracks in question.

Paleoenvironment

The Hoover Quarry may represent a playa, or dry lake, a shallow lake, or a river setting. The Portland Formation as a whole is composed of reddish shallow–water and gray or black deeper water rocks in the lower part of the formation, and coarser red rocks from river or alluvial settings in the upper part of the formation. It has yielded fossils of algal structures, pollen, trees and smaller plants, bivalves, clam shrimp, ostracodes, beetles, invertebrate traces, several genera of fish (''Acentrophorus'', '' Redfieldius'', ''Semionotus'', and the coelacanth ''Diplurus''), the theropod dinosaur ''Podokesaurus'', the prosauropod dinosaurs ''Ammosaurus'' and ''Anchisaurus'', vertebrate coprolites (fossilized droppings), and several vertebrate track genera ('' Batrachopus'', the theropod tracks '' Anchisauripus'', ''Eubrontes'', and ''Grallator'', the prosauropod tracks ''Otozoum'', and the ornithischian dinosaur tracks ''Anomoepus'').

References

External links

''Stegomosuchus''
in the Paleobiology Database

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Early Jurassic crocodylomorphs
Terrestrial crocodylomorphs
Early Jurassic reptiles of North America
Taxa named by Friedrich von Huene
Fossil taxa described in 1922
Prehistoric pseudosuchian genera