Dromicosuchus
Encyclopedia
Dromicosuchus is an extinct genus
of sphenosuchia
n, a type of basal
crocodylomorph
, the clade
that comprises the crocodilians and their closest kin. It was found in Upper Triassic rocks of North Carolina
, United States
, and is known from a nearly complete skull and partial skeleton. This specimen is unusual in that it was found beneath the skeleton of a larger rauisuchia
n and has apparent bite damage, suggesting that it was attacked by the larger carnivore before both died and were buried together.
of Dromicosuchus is a partial skeleton including a nearly complete skull and lower jaws, articulated vertebrae from the atlas
to the second tail vertebra, bony armor
from the back, ribs and the dermal bones called gastralia
, partial shoulder girdles, most of the left arm and leg, and the right upper arm
, thigh bone
, and shin bone
. It was found with the fossil
s of several other animals in a brick-clay quarry
in Durham County, North Carolina
, in blocks excavated by University of North Carolina at Chapel Hill
students in fall 1994. The fossils came from a mudstone
next to a river channel deposit, in Lithofacies Association II of the Deep River basin, part of the extensive Newark Supergroup
, dated as pertaining to either the late Carnian
or early Norian
faunal stage
s of the Late Triassic
. The type species
is D. grallator, meaning "one who walks on stilts" in Greek, in reference to its long slender limbs. It was named and described by Hans-Dieter Sues and colleagues in 2003.
When the students excavated the blocks, the fossils were tentatively identified remains as those of a large, unnamed new genus of popsaurid
rauisuchian. Upon preparation, however, it was discovered that there was the skeleton of a smaller archosaur under its pelvis, and the bones of several different animals as gut contents that remained in place in the rauisuchian fossil: these included bones and scute
s of the aetosaur
Stegomus, some with tooth marks; bones from the snout, arm, and shoulder of the traversodont
cynodont
Plinthogomphodon
; phalanx bones of a dicynodont
; and a bone fragment from a possible temnospondyl. Additionally, the small archosaur, later named Dromicosuchus, had damage to its neck armor and lower jaw that appears to have been caused by rauisuchian teeth, perhaps those of the animal that was found above it.
, and the animal is estimated as 1.2 to 1.3 meters long (3.9 to 4.3 ft). The age of the animal is not completely clear, because some bones that are ordinarily expected to be fused in adults are fused, but others may not be. The fusion of two shoulder bones suggests that it was mature. The skull was lightly built and triangular, and about 150 millimeters long (5.9 in). The eye sockets were circular and about 30 millimeters in diameter (1.2 in). There were five teeth per premaxilla
(the paired bones that made up the snout), twenty per maxilla
(the paired bones that made up the cheeks), and an unknown number in the lower jaw. The teeth differed slightly
from the tip of the snout to the cheeks, having more flattened cross-sections and stronger curves in the cheek. The third or fourth tooth of the lower jaw was notably enlarged, fitting into a notch in the upper jaw between the premaxilla and maxilla.
There were at least 23 vertebrae from the skull to the hips. Nine or ten may have been neck vertebrae, the rest forming the back. The hip had two vertebrae anchoring it. Two rows of scutes ran along the middle of the back, as in other basal crocodylomorphs. The limbs are more complete on the left side; the left upper arm was 89 millimeters long (3.5 in), and left ulna
was 102 millimeters long (4.0 in), so the forearm was noticeably longer than the upper arm. The hand was small. For the left hindlimb, the thigh bone was 144 millimeters long (5.7 in) and the shin was 130 millimeters long (5.1 in).
and found that their new genus (identified as Dromaeosuchus in the diagrams) grouped as a sphenosuchia
n, possibly closest to Hesperosuchus and Kayentasuchus
. Sphenosuchia is a poorly understood and controversial clade
of early crocodile
relatives, known from the Late Triassic and Jurassic
, and it may be paraphyletic
, not a natural group. Sues and colleagues found a clade of sphenosuchians in their analysis, but it was weakly supported.
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 sphenosuchia
Sphenosuchia
Sphenosuchia is a suborder of basal crocodylomorphs that first appeared in the Triassic and occurred into the Late Jurassic. Most were small, gracile animals with an erect limb posture. They are now thought to be ancestral to crocodyliforms, which include all living crocodilians.-Stratigraphic...
n, a type of basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
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...
, the clade
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
that comprises the crocodilians and their closest kin. It was found in Upper Triassic rocks of North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, and is known from a nearly complete skull and partial skeleton. This specimen is unusual in that it was found beneath the skeleton of a larger rauisuchia
Rauisuchia
Rauisuchia is a group of predatory and mostly large Triassic archosaurs. As a clade, Rauisuchia includes these Triassic forms and all crocodylomorphs, which are descendants of Triassic rauisuchians. The group in its traditional sense is paraphyletic, because it does not include crocodylomorph...
n and has apparent bite damage, suggesting that it was attacked by the larger carnivore before both died and were buried together.
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...
of Dromicosuchus is a partial skeleton including a nearly complete skull and lower jaws, articulated vertebrae from the atlas
Atlas (anatomy)
In anatomy, the atlas is the most superior cervical vertebra of the spine.It is named for the Atlas of Greek mythology, because it supports the globe of the head....
to the second tail vertebra, bony armor
Armour (zoology)
Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
from the back, ribs and the dermal bones called gastralia
Gastralium
Gastralia are dermal bones found in the ventral body wall of crocodilian and Sphenodon species. They are found between the sternum and pelvis, and do not articulate with the vertebrae...
, partial shoulder girdles, most of the left arm and leg, and the right upper arm
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
, thigh bone
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
, and shin bone
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
. It was found with the fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s of several other animals in a brick-clay quarry
Quarry
A quarry is a type of open-pit mine from which rock or minerals are extracted. Quarries are generally used for extracting building materials, such as dimension stone, construction aggregate, riprap, sand, and gravel. They are often collocated with concrete and asphalt plants due to the requirement...
in Durham County, North Carolina
Durham County, North Carolina
-Demographics:As of the census of 2000, there were 223,314 people, 89,015 households, and 54,032 families residing in the county. The population density was 769 people per square mile . There were 95,452 housing units at an average density of 329 per square mile...
, in blocks excavated by University of North Carolina at Chapel Hill
University of North Carolina at Chapel Hill
The University of North Carolina at Chapel Hill is a public research university located in Chapel Hill, North Carolina, United States...
students in fall 1994. The fossils came from a mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
next to a river channel deposit, in Lithofacies Association II of the Deep River basin, part of the extensive Newark Supergroup
Newark Group
The Newark Group, also known as the Newark Supergroup, is an assemblage of Late Triassic and Early Jurassic sedimentary rocks which outcrop intermittently along the United States East Coast; the exposures extend from Massachusetts to North Carolina, with more still in Nova Scotia...
, dated as pertaining to either the late Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...
or early Norian
Norian
The Norian is a division of the Triassic geological period. It has the rank of an age or stage . The Norian lasted from 216.5 ± 2.0 to 203.6 ± 1.5 million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.-Stratigraphic definitions:The Norian was named after the Noric Alps in...
faunal stage
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...
s of the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...
. The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
is D. grallator, meaning "one who walks on stilts" in Greek, in reference to its long slender limbs. It was named and described by Hans-Dieter Sues and colleagues in 2003.
When the students excavated the blocks, the fossils were tentatively identified remains as those of a large, unnamed new genus of popsaurid
Poposauridae
The Poposauridae are a family of large carnivorous archosaurs which lived alongside dinosaurs during the Late Triassic. They are known from fossil remains from North and South America...
rauisuchian. Upon preparation, however, it was discovered that there was the skeleton of a smaller archosaur under its pelvis, and the bones of several different animals as gut contents that remained in place in the rauisuchian fossil: these included bones and scute
Scute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...
s of the 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...
Stegomus, some with tooth marks; bones from the snout, arm, and shoulder of the traversodont
Traversodontidae
Traversodontidae is a family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Traversodonts first appeared in the Middle Triassic, diversified...
cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
Plinthogomphodon
Plinthogomphodon
Plinthogomphodon is a genus of traversodontid cynodont from the Late Triassic of the eastern United States. Fossils have been found from the Deep River basin of North Carolina, part of the larger Newark Supergroup. It is known from a single eroded snout. The type and only species is P....
; phalanx bones of a dicynodont
Dicynodont
Dicynodontia is a taxon of anomodont therapsids or mammal-like reptiles. Dicynodonts were small to large herbivorous animals with two tusks, hence their name, which means 'two dog tooth'...
; and a bone fragment from a possible temnospondyl. Additionally, the small archosaur, later named Dromicosuchus, had damage to its neck armor and lower jaw that appears to have been caused by rauisuchian teeth, perhaps those of the animal that was found above it.
Description of the specimen, UNC 15574
The skeleton of the type and only known individual of Dromicosuchus is similarly proportioned to that of Hesperosuchus agilisHesperosuchus
Hesperosuchus is an extinct genus of crocodylomorph reptile that contains a single species, Hesperosuchus agilis. Remains of this sphenosuchian have been found in Late Triassic strata from Arizona and New Mexico....
, and the animal is estimated as 1.2 to 1.3 meters long (3.9 to 4.3 ft). The age of the animal is not completely clear, because some bones that are ordinarily expected to be fused in adults are fused, but others may not be. The fusion of two shoulder bones suggests that it was mature. The skull was lightly built and triangular, and about 150 millimeters long (5.9 in). The eye sockets were circular and about 30 millimeters in diameter (1.2 in). There were five teeth per premaxilla
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....
(the paired bones that made up the snout), twenty per maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
(the paired bones that made up the cheeks), and an unknown number in the lower jaw. The teeth differed slightly
Heterodont
The anatomical term heterodont refers to animals which possess more than a single tooth morphology. For example, members of the Synapsida generally possess incisors, canines , premolars, and molars. The presence of heterodont dentition is evidence of some degree of feeding/hunting specialization...
from the tip of the snout to the cheeks, having more flattened cross-sections and stronger curves in the cheek. The third or fourth tooth of the lower jaw was notably enlarged, fitting into a notch in the upper jaw between the premaxilla and maxilla.
There were at least 23 vertebrae from the skull to the hips. Nine or ten may have been neck vertebrae, the rest forming the back. The hip had two vertebrae anchoring it. Two rows of scutes ran along the middle of the back, as in other basal crocodylomorphs. The limbs are more complete on the left side; the left upper arm was 89 millimeters long (3.5 in), and left ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
was 102 millimeters long (4.0 in), so the forearm was noticeably longer than the upper arm. The hand was small. For the left hindlimb, the thigh bone was 144 millimeters long (5.7 in) and the shin was 130 millimeters long (5.1 in).
Classification
Sues and his colleagues performed a phylogenetic analysisCladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
and found that their new genus (identified as Dromaeosuchus in the diagrams) grouped as a sphenosuchia
Sphenosuchia
Sphenosuchia is a suborder of basal crocodylomorphs that first appeared in the Triassic and occurred into the Late Jurassic. Most were small, gracile animals with an erect limb posture. They are now thought to be ancestral to crocodyliforms, which include all living crocodilians.-Stratigraphic...
n, possibly closest to Hesperosuchus and Kayentasuchus
Kayentasuchus
Kayentasuchus is a genus of sphenosuchian, a type of basal crocodylomorph, the clade that comprises the crocodilians and their closest kin...
. Sphenosuchia is a poorly understood and controversial clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of early crocodile
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...
relatives, known from the Late Triassic and Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, and it may be paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
, not a natural group. Sues and colleagues found a clade of sphenosuchians in their analysis, but it was weakly supported.