Stegomosuchus
Encyclopedia
Stegomosuchus is an extinct genus
of small protosuchia
n 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 crocodilia
ns. Stegomosuchus is also regarded as a candidate for the maker of at least some of the tracks named Batrachopus in the Connecticut River Valley.
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 10 feet (3 m) below the surface, in red sandstone
used for building material. This site is now called the Hoover Quarry; it has also yielded invertebrate trace fossil
s and dinosaur
tracks (Eubrontes
). The rocks are now known to belong to the Portland Formation
. While thought to be Triassic
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 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.
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.
, 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
, ostracod
es, beetle
s, invertebrate traces, several genera of fish (Acentrophorus
, Redfieldius, Semionotus
, and the coelacanth
Diplurus
), the theropod
dinosaur Podokesaurus
, the prosauropod dinosaurs Ammosaurus
and Anchisaurus
, vertebrate coprolite
s (fossilized droppings), and several vertebrate track genera (Batrachopus, the theropod tracks Anchisauripus, Eubrontes
, and Grallator
, the prosauropod tracks Otozoum
, and the ornithischia
n dinosaur tracks Anomoepus
). By comparison to other protosuchids, Stegomosuchus was probably a terrestrial carnivore
.
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.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of small protosuchia
Protosuchia
Protosuchia is a group of extinct Mesozoic crocodyliforms. They were small in size and terrestrial. In phylogenetic terms, Protosuchia is considered an informal group because it is a grade of basal crocodyliforms, not a true clade....
n crocodylomorph
Crocodylomorpha
The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...
. It is known from a single incomplete specimen discovered in the late 19th century in Lower Jurassic rocks of south-central Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. It was originally thought to be a species of Stegomus, an aetosaur
Aetosaur
Aetosaurs are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae...
(a type of armored herbivorous reptile), but was eventually shown to be related to Protosuchus
Protosuchus
Protosuchus is an extinct genus of carnivorous crocodylomorph from the Early Jurassic. The name Protosuchus means "first crocodile", and is among the earliest animals that resemble crocodilians...
and thus closer to the ancestry of crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
ns. 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 holotypeHolotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as 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
East Longmeadow, Massachusetts
As of the census of 2010, there were 16,187 people, 5,248 households, and 3,988 families residing in the town. The population density was 1,087.1 people per square mile . There were 5,363 housing units at an average density of 413.5 per square mile...
, Massachusetts, just before the turn of the 20th century. It was found 10 feet (3 m) below the surface, in red sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
used for building material. This site is now called the Hoover Quarry; it has also yielded invertebrate trace fossil
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s and dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
tracks (Eubrontes
Eubrontes
Eubrontes is the name of fossilised dinosaur footprints dating from the Late Triassic and Early Jurassic. They have been identified from France, Poland, Slovakia, Italy, Spain, Australia and the USA...
). The rocks are now known to belong to the Portland Formation
Portland Formation
The Portland Formation is a geological formation in the northeastern United States. It dates back to the Early Jurassic.-Vertebrate fauna:Dinosaur coprolites located in Massachusetts, USA. Ornithischian tracks located in Massachusetts and Connecticut, USA. Theropod tracks located in Massachusetts...
. While thought to be Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
when Stegomosuchus was originally described, the Portland Formation is now known to date to the Early Jurassic
Early Jurassic
The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...
, including the Hettangian
Hettangian
The Hettangian is the earliest age or lowest stage of the Jurassic period of the geologic timescale. It spans the time between 199.6 ± 0.6 Ma and 196.5 ± 1 Ma . The Hettangian follows the Rhaetian and is followed by the Sinemurian.In Europe stratigraphy the Hettangian is a part of the time span in...
and Sinemurian
Sinemurian
In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...
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
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, right arm (minus the hand) and shoulder blade
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
, 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 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.
Classification
The assessment of the material as belonging to a species of the aetosaur Stegomus held for about twenty years, until Friedrich von HueneFriedrich von Huene
Friedrich von Huene was a German paleontologist who named more dinosaurs in the early 20th century than anyone else in Europe.-Biography:...
reclassified it, giving the species the new genus Stegomosuchus and new family Stegomosuchidae, in the Pseudosuchia
Pseudosuchia
Pseudosuchia is the name originally given to a group of prehistoric reptiles from the Triassic period. The name has been variously interpreted, and it is still sometimes, if infrequently, used in scientific literature today. A more commonly used name, Crurotarsi, is often substituted for...
.
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
Alfred Romer
Alfred Sherwood Romer was an American paleontologist and comparative anatomist and a specialist in vertebrate evolution.-Biography:...
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 and paleoecology
The Hoover Quarry may represent a playa, or dry lakeDry lake
Dry lakes are ephemeral lakebeds, or a remnant of an endorheic lake. Such flats consist of fine-grained sediments infused with alkali salts. Dry lakes are also referred to as alkali flats, sabkhas, playas or mud flats...
, 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
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
settings in the upper part of the formation. It has yielded fossils of algal
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
structures, pollen, trees and smaller plants, bivalves
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
, clam shrimp
Clam shrimp
Clam shrimp are a taxon of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant, and known from the fossil record, from at least the Devonian period and perhaps before...
, ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
es, beetle
Beetle
Coleoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...
s, invertebrate traces, several genera of fish (Acentrophorus
Acentrophorus
Acentrophorus is an extinct genus of prehistoric bony fish.-See also:* Prehistoric fish* List of prehistoric bony fish...
, Redfieldius, Semionotus
Semionotus
Semionotus is an extinct genus of ray-finned fish found throughout Northern Pangaea during the late Triassic, becoming extinct at the start of the Jurassic.-External links:...
, and the coelacanth
Coelacanth
Coelacanths are members of an order of fish that includes the oldest living lineage of Sarcopterygii known to date....
Diplurus
Diplurus
Diplurus is a genus of prehistoric coelacanth fish which lived during the Triassic period....
), the theropod
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
dinosaur Podokesaurus
Podokesaurus
Podokesaurus was a small carnivorous dinosaur of the Early Jurassic Period , and as such is one of the earliest known dinosaurs to inhabit the eastern United States . The small, bipedal carnivore was about 90 cm long and 0.3 m tall...
, the prosauropod dinosaurs Ammosaurus
Ammosaurus
Ammosaurus is a genus of sauropodomorph dinosaur from the Early and Middle Jurassic Period of North America. At 4 meters in length, it was small compared to some other members of its suborder, which included the largest animals ever to walk the Earth...
and Anchisaurus
Anchisaurus
Anchisaurus is a genus of basal sauropodomorph, and was an early herbivorous dinosaur. Until recently it was classed as a member of Prosauropoda. The name comes from the Greek αγχι/agkhi...
, vertebrate coprolite
Coprolite
A coprolite is fossilized animal dung. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour rather than morphology. The name is derived from the Greek words κοπρος / kopros meaning 'dung' and λιθος / lithos meaning 'stone'. They...
s (fossilized droppings), and several vertebrate track genera (Batrachopus, the theropod tracks Anchisauripus, Eubrontes
Eubrontes
Eubrontes is the name of fossilised dinosaur footprints dating from the Late Triassic and Early Jurassic. They have been identified from France, Poland, Slovakia, Italy, Spain, Australia and the USA...
, and Grallator
Grallator
Grallator is an ichnogenus which covers a common type of small, three-toed print made by a variety of bipedal theropod dinosaurs. Grallator-type footprints have been found in formations dating from the late Triassic through to the early Cretaceous periods...
, the prosauropod tracks Otozoum
Otozoum
Otozoum tracks are Jurassic age, fossilized footprints and other markings in sandstones. They were made by heavy, bipedal animals with a short stride that walked on four toes directed forward....
, and the ornithischia
Ornithischia
Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...
n dinosaur tracks Anomoepus
Anomoepus
Anomoepus gracillimus is the name assigned to fossil footprints first reported from Early Jurassic beds of the Connecticut River Valley, Massachusetts, USA in 1802....
). By comparison to other protosuchids, Stegomosuchus was probably a terrestrial carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
.
Tracks
In the same issue of The American Journal of Science that contained the description of Stegomus longipes, Richard Swann LullR. S. Lull
Richard Swann Lull was an American paleontologist from the early 20th century, active at Yale University, who is largely remembered now for championing a Pre-Neo-Darwinian Synthesis view of evolution, whereby mutation could unlock mysterious genetic drives that, over time, would lead populations...
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.
External links
- Stegomosuchus in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...