Stegomosuchus longipes, the terrestrial croc relative of Massachusetts

If you're the kind of person who reads this blog regularly, you're probably also the kind of person who's got at least one rock laying around. Maybe you've got dozens. Maybe you've got too many. Who am I to judge? The point is you've got rocks. Odds are, though, there isn't a potential type specimen in your yard.

The Portland Formation (or Portland Group in the Hartford Basin, per Weems et al. 2016; it can get complicated) has lots of tetrapod tracks, but not so many tetrapod bodies. I *think* there's eight: four Anchisaurus (each of which having been given its own species of Anchisaurus, Ammosaurus, or Yaleosaurus at one time or another, but now boiled down to Anchisaurus polyzelus), a fifth very fragmentary sauropodomorph, the type of the coelophysoid Podokesaurus holyokensis, another very fragmentary coelophysoid that may be Podokesaurus, and the subject of the current post, a small crocodylomorph known as Stegomosuchus longipes.

The classic source of body fossils in the Portland is from quarries or construction, and Stegomosuchus is no exception. The type and only known specimen was found shortly before the turn of the 20th century in the Hines (=Hoover) Quarry near East Longmeadow, Massachusetts, not all that far from Springfield Armory and the original type of our old friend Anchisaurus polyzelus. This quarry, no longer in operation, is also a notable source of trace fossils (Collette et al. 2011). The blocks hosting the specimen were taken by a G. B. Robinson, who put them in his door yard. There the blocks sat for "about seven years exposed to the weather" until Mr. and Mrs. E. D. White saw them and got them into the hands of Benjamin Kendall Emerson and Frederic Brewster Loomis, geologists at Amherst College. As far as serendipity goes, this is pretty good. It's right up there with Samuel Scudder arriving at the Florissant quarries and appropriating the type-specimens-to-be of Petrolystra gigantea from the hands of a child, or Titus Ulke naming a new species of algae from a block of Indiana Limestone in the steps of a Washington, D.C. church.

From Emerson and Loomis (1904).

Emerson and Landis described the former landscaping as a new species of Stegomus, another armored reptile from New England. The fossils were in three blocks, a part, counterpart, and a chip from the pelvic region. (By the time of Collette et al. 2011, only the part and counterpart were mentioned, so the chip has either been lost or permanently affixed). The individual was a little critter; "snout-to-vent" is about 149 mm (a little less than 6 in), and the skull is 35 mm long and 27 mm across (about 1.4 by 1 in). As the figures show, the specimen is articulated and includes much of the skeleton, dominated by two rows of scutes running along the back. So far, so good. There is one slight problem, though: the bones are largely leached, so the specimen is basically a natural cast. A natural cast of a skeleton is much better than nothing at all, though, so Emerson and Landis soldiered on. They noted that the skull is rather different from the skull of the type species of Stegomus, but in 1904 there weren't enough fossils to compare with for them to realize the significance. Emerson and Landis were impressed by the long slender limbs, which inspired the species name "longipes", and interpreted their new species as an agile terrestrial animal. Conveniently enough, the Portland also has tracks (Batrachopus) of a five-fingered, four-toed animal, which just suit an animal of the form of S. longipes (Lull 1904).

 The name "Stegomus longipes" held until 1922, when Friedrich von Huene gave it another look while classifying Triassic "thecodonts". He realized he was dealing with two different things: original flavor Stegomus arcuatus he decided was a stagonolepid (=aetosaur, although note that in 1922 Aetosauridae and Stagonolepidae [sic] were two different things), while "S. longipes" warranted a new genus (Stegomosuchus) and family (Stegomosuchidae). Further resolution was provided by Walker (1968), who noted the resemblance to the roughly contemporaneous terrestrial croc relative Protosuchus. (He also proposed that the type was a juvenile, which isn't too surprising.) He assigned the genus to Protosuchidae; Huene's Stegomosuchidae is an older name than Protosuchidae, but people have been hesitant to switch names because Protosuchidae is well-established (Romer 1972) and because of the whole poorly-preserved-natural-cast thing (Whetstone and Whybrow 1983). At any rate, it's certainly a Protosuchus-like animal, but given the preservation and incompleteness, there aren't a lot of details to work with.

Protosuchus by Nobu Tamura (Wikimedia Commons).

References

Collette, J. H., P. R. Getty, and J. W. Hagadorn. 2011. Insights into an Early Jurassic dinosaur habitat: ichnofacies and enigmatic structures from the Portland Formation, Hoover Quarry, Massachusetts, U.S.A. Atlantic Geology 47:81–98.

Emerson, B. K., and F. B. Loomis. 1904. On Stegomus longipes, a new reptile from the Triassic sandstones of the Connecticut Valley. American Journal of Science (4th series) 17(4):377–380.

Huene, F. v. 1922. The Triassic reptilian order Thecodontia. American Journal of Science (5th series) 4(19):22–26.

Lull, R. S. 1904. Notes on the probable footprints of Stegomus longipes. American Journal of Science (4th series) 17(4):381–382.

Romer, A. S. 1972. The Chanares (Argentina) Triassic reptile fauna. XVI. Thecodont classification. Breviora. 395:1–24.

Walker, A. D. 1968. Protosuchus, Proterochampsa, and the origin of phytosaurs and crocodiles. Geological Magazine 105(1):1–14.

Weems, R. E., L. H. Turner, and S. G. Lucas. 2016. Synthesis and revision of the lithostratigraphic groups and formations in the Upper Permian?–Lower Jurassic Newark Supergroup of eastern North America. Stratigraphy 13(2):111–153.

Whetstone, K. N., and P. J. Whybrow. 1983. A "cursorial" crocodilian from the Triassic of Lesotho (Basutoland), southern Africa. Occasional Papers of the Museum of Natural History, the University of Kansas 106:1–37.