Stegomosuchus

Stegomosuchus
Temporal range: Early Jurassic
Stegomosuchus longipes skeleton replica
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Superorder:
Crocodylomorpha
Suborder:
Protosuchia
Genus:
Stegomosuchus

von Huene, 1922
Species

Stegomosuchus is an extinct genus of small protosuchian crocodyliform. 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

Stegomosuchus holotype

The holotype and only known specimen of Stegomosuchus (AM 900[1]) 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 10 feet (3.0 m) below the surface, in red sandstone used for building material.[2] This site is now called the Hoover Quarry; it has also yielded invertebrate trace fossils and dinosaur tracks (Eubrontes).[3][4] The rocks are now known to belong to the Portland Formation.[5][6] While thought to be Triassic when Stegomosuchus was originally described,[2] the Portland Formation is now known to date to the Early Jurassic, including the Hettangian and Sinemurian stages[7] (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.[2] Emerson and Loomis interpreted the impressions as showing another row of armor along the sides, but this was later shown to be a mistake.[8] AM 900 was a small animal, with a skull estimated at 35 millimetres (1.4 in) long and 27 millimetres (1.1 in) across, and a body length from snout to pelvis of 149 millimetres (5.9 in). Emerson and Loomis described the specimen as a new species of Stegomus (S. longipes), an aetosaur known from slightly older rocks.[2]

Classification

Model by Richard Swann Lull

The assessment of the material as belonging to a species of the aetosaur Stegomus held for about twenty years,[9][10] until Friedrich von Huene reclassified it, giving the species the new genus Stegomosuchus and new family Stegomosuchidae, in the Pseudosuchia.[11]

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.[8] The reassignment of Stegomosuchus was followed,[5][6][12][13] although Alfred Romer recommended passing over Stegomosuchidae for the more familiar Protosuchidae.[13] 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.[14]

Paleobiology

Reconstruction

By comparison to other protosuchids, Stegomosuchus was probably a terrestrial carnivore.[15]

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.[16] 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.[17]

Paleoenvironment

Batrachopus tracks, made by an animal like Stegomosuchus

The Hoover Quarry may represent a playa, or dry lake, a shallow lake, or a river setting.[3][4] 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.[6] 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 sauropodomorph dinosaur 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).[5][6]

References

  1. ^ Galton, Peter M.; Farlow, James O. (2003). "Dinosaur State Park, Connecticut, USA: history, footprints, trackways, exhibits". Zubía. 21: 129–173.
  2. ^ a b c d Emerson, B. K.; Loomis, F. B. (1904). "On Stegomus longipes, a new reptile from the Triassic sandstones of the Connecticut Valley". American Journal of Science. 17 (4): 377–380. Bibcode:1904AmJS...17..377E. doi:10.2475/ajs.s4-17.101.377. hdl:2027/hvd.32044107172959.
  3. ^ a b Collette, J. H.; Getty, P.R.; Hagadorn, J. W. (2006). "An Early Jurassic non-marine fossil assemblage from the Portland Formation, Hartford Basin, Massachusetts". Geological Society of America Abstracts with Programs. 38 (7): 68.
  4. ^ a b Collette, J. H.; Getty, P. R.; Hagadorn, J. W. (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. doi:10.4138/atlgeol.2011.003.
  5. ^ a b c Olsen, Paul E. (1988). "Paleontology and paleoecology of the Newark Supergroup (early Mesozoic, eastern North America)" (PDF). In Manspeizer, W. (ed.). Triassic-Jurassic rifting: continental breakup and the origin of the Atlantic Ocean and passive margins. Developments in Geotectonics. Vol. 22 (part A). Amsterdam, the Netherlands: Elsevier. pp. 185–230. ISBN 978-0-444-42903-2.
  6. ^ a b c d McDonald, Nicholas G. (1992). "Paleontology of the early Mesozoic (Newark Supergroup) rocks of the Connecticut Valley". Northeastern Geology. 14: 185–200.
  7. ^ Olsen, Paul E.; Kent, Dennis V.; LeTourneau, Peter M. (2002). "Stratigraphy and age of the Early Jurassic Portland Formation of Connecticut and Massachusetts: a contribution to the time scale of the Early Jurassic". Geological Society of America Abstracts with Programs. 34 (1): 61.
  8. ^ a b Walker, Alick D. (1968). "Protosuchus, Proterochampsa, and the origin of phytosaurs and crocodiles". Geological Magazine. 105 (1): 1–14. Bibcode:1968GeoM..105....1W. doi:10.1017/S0016756800046434.
  9. ^ Lull, Richard S. (1912). "The life of the Connecticut Trias". American Journal of Science. 33 (197): 397–422. Bibcode:1912AmJS...33..397L. doi:10.2475/ajs.s4-33.197.397.
  10. ^ Lull, Richard S. (1915). Triassic life of the Connecticut Valley. Bulletin. Vol. 24. Hartford, CT: Connecticut Geological and Natural History Survey.
  11. ^ von Huene, Friedrich (1922). "The Triassic reptilian order Thecodontia". American Journal of Science. 4 (19): 22–26. Bibcode:1922AmJS....4...22H. doi:10.2475/ajs.s5-4.19.22.
  12. ^ Galton, Peter M. (1971). "The prosauropod dinosaur Ammosaurus, the crocodile Protosuchus, and their bearing on the age of the Navajo sandstone of northeastern Arizona". Journal of Paleontology. 45 (5): 781–795.
  13. ^ a b Romer, Alfred Sherwood (1972). "The Chanares (Argentina) Triassic reptile fauna. XVI. Thecodont classification". Breviora. 395: 1–24.
  14. ^ Whetstone, K. N.; Whybrow, P. J. (1983). "A "cursorial" crocodilian from the Triassic of Lesotho (Basutoland), southern Africa". Occasional Papers of the Museum of Natural History, University of Kansas. 106: 1–37.
  15. ^ Kazlev, M. Alan (1998–2002). "Early Jurassic – the Lias epoch". Palaeos. Archived from the original on 15 January 2018. Retrieved 3 December 2009.
  16. ^ Lull, Richard S. (1904). "Notes on the probable footprints of Stegomus longipes". American Journal of Science. 17 (4): 381–382. Bibcode:1904AmJS...17..381L. doi:10.2475/ajs.s4-17.101.381.
  17. ^ Olsen, Paul E.; Padian, Kevin (1986). "Earliest records of Batrachopus from the Southwest U.S., and a revision of some Early Mesozoic crocodilomorph ichnogenera" (PDF). In Padian, Kevin (ed.). The Beginning of the Age of Dinosaurs: Faunal Change Across the Triassic-Jurassic Boundary. New York: Cambridge University Press. pp. 259–273. ISBN 978-0-521-36779-0.