Akanthosuchus
Encyclopedia
Akanthosuchus is an extinct genus
of alligatoroid
crocodilian, possibly an alligatorine
. Its fossil
s are found in the Paleocene
-age Nacimiento Formation
of the San Juan Basin
, New Mexico
, USA
. It is known from a partial skeleton and isolated bony armor
. Its armor was distinctive, with spike-shaped and blade-shaped pieces, and it is the only known crocodilian with a combination of both.
NMMNH
NP-139, a partial skeleton lacking the skull. The specimen was found in a concretion
ary sandstone
lens with the anterior
portion eroding out. It was discovered in Torrejon Wash, northwestern Sandoval County
, New Mexico. The hind legs, numerous back, hip, and tail vertebrae (28), and armor (about 200 scutes) were the primary elements preserved. F. Michael O'Neill et al. described the genus in 1981. The type species
is A. langstoni, honoring paleontologist Wann Langston, Jr.
, known for his work on fossil crocodilians.
Akanthosuchus was a moderately sized crocodilian. The thigh bone
of NMMNH NP-139 was 137 millimetres (5.4 in) long, and the shin
was 102 millimetres (4 in) long. However, this specimen may not have been fully grown. Based on growth rings, Hill and Lucas suggest that it was no younger than eight years old when it died. The tail may have been relatively short, given the reduction of bony projections on known tail vertebrae. The armor was unusual in having elongate spikes or broad blades. Its scutes came in four basic types: square (13%, keeled or unkeeled), oval (27%, keeled), bladed (<1%, similar to oval but with a much exaggerated triangular keel), and spiked (60%, square-based, conical and blunt). The spikes were nearly as tall as the base of their scutes were long, rising up to 29 millimetres (1.1 in). They contacted only one other scute each, and may have been arranged in paired sets down the midline of the back and tail, and along the sides of the animal. Single rows of paired oval scutes may have filled in between the spiked rows. The bladed scutes, an extreme of the oval scute form, may have been found at the ends of the oval scute rows. The square scutes may have been restricted to one area, such as the neck and shoulders, or they may represent the belly armor.
Spiked and bladed scutes are not unknown among crocodilians, although no other known crocodilian had or has both. O'Neill and colleagues compared Akanthosuchus to Pinacosuchus
, a crocodylomorph
with spiked scutes known from the Late Cretaceous
North Horn Formation
of Utah
, but it differed from Akanthosuchus in several ways: it was much smaller (three to four times smaller than NMMNH NP-139), it had less derived
vertebrae, and it lacked bladed scutes.
or Navajosuchus
, contemporaneous crocodilians known primarily from skulls, but elected to name a new genus instead. More recently, Hill and Lucas, using a cladistic analysis
, rejected the hypothesis
that Akanthosuchus is the same as either Ceratosuchus or Navajosuchus, and found that Akanthosuchus is an alligatoroid. It tended to sort out with alligatorines
, but Hill and Lucas found that this was weakly supported in their analysis.
portion of the Nacimiento Formation. Later work identified Akanthosuchus langstoni from the older Puercan
-age portion of the Nacimiento Formation as well, which gives the genus a range including much of the Nacimiento Formation, deposited between approximately 64.5 and 61.0 million years ago. A scute and femur from the De-Na-Zin Member of the late Campanian
-age (Late Cretaceous) Kirtland Formation
are similar to the corresponding elements of Akanthosuchus, but cannot be conclusively assigned to the genus at this time. This material is also from the San Juan Basin.
and lacustrine
depositional setting. O'Neill et al. interpreted Akanthosuchus as either a terrestrial crocodilian or a specialized, modestly sized, heavily armored aquatic crocodilian.
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 alligatoroid
Alligatoroidea
Alligatoroidea is a superfamily of crocodilians that evolved in the Late Cretaceous period. Cladistically, it is defined as Alligator mississippiensis and all crocodylians more closely related to A. mississippiensis than to either Crocodylus niloticus or Gavialis gangeticus ....
crocodilian, possibly an alligatorine
Alligatorinae
Alligatorinae are one of two subfamilies of the family Alligatoridae.-Taxonomy:* Subfamily Alligatorinae** Genus Chrysochampsa ** Genus Hassiacosuchus ** Genus Navajosuchus ** Genus Ceratosuchus...
. Its fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are found in the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
-age Nacimiento Formation
Nacimiento Formation
The Nacimiento Formation is a sedimentary rock formation found in the San Juan Basin of western New Mexico and named for the Nacimiento Mountains...
of the San Juan Basin
San Juan Basin
The San Juan Basin is a geologic structural basin in the Four Corners region of the Southwestern United States; its main portion covers around , encompassing much of northwestern New Mexico, southwest Colorado, and parts of Arizona and Utah....
, New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
, USA
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. It is known from a partial skeleton and isolated bony armor
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...
. Its armor was distinctive, with spike-shaped and blade-shaped pieces, and it is the only known crocodilian with a combination of both.
History and description
Akanthosuchus is based onHolotype
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...
NMMNH
New Mexico Museum of Natural History and Science
The New Mexico Museum of Natural History and Science is a natural history and science museum in Albuquerque, New Mexico near Old Town Albuquerque. The Museum was founded in 1986....
NP-139, a partial skeleton lacking the skull. The specimen was found in a concretion
Concretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...
ary 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,...
lens with the anterior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...
portion eroding out. It was discovered in Torrejon Wash, northwestern Sandoval County
Sandoval County, New Mexico
-Indian reservations:Sandoval County has 12 Indian reservations and two joint-use areas lying within its borders, the second most of any county in the United States -Indian reservations:Sandoval County has 12 Indian reservations and two joint-use areas lying within its borders, the second most of...
, New Mexico. The hind legs, numerous back, hip, and tail vertebrae (28), and armor (about 200 scutes) were the primary elements preserved. F. Michael O'Neill et al. described the genus in 1981. 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 A. langstoni, honoring paleontologist Wann Langston, Jr.
Wann Langston, Jr.
Wann Langston, Jr. is an American paleontologist and professor at the University of Texas at Austin. He has worked on a number of different reptiles and amphibians in his long career, beginning with the 1950 description of the theropod dinosaur Acrocanthosaurus...
, known for his work on fossil crocodilians.
Akanthosuchus was a moderately sized crocodilian. The 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...
of NMMNH NP-139 was 137 millimetres (5.4 in) long, and the shin
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....
was 102 millimetres (4 in) long. However, this specimen may not have been fully grown. Based on growth rings, Hill and Lucas suggest that it was no younger than eight years old when it died. The tail may have been relatively short, given the reduction of bony projections on known tail vertebrae. The armor was unusual in having elongate spikes or broad blades. Its scutes came in four basic types: square (13%, keeled or unkeeled), oval (27%, keeled), bladed (<1%, similar to oval but with a much exaggerated triangular keel), and spiked (60%, square-based, conical and blunt). The spikes were nearly as tall as the base of their scutes were long, rising up to 29 millimetres (1.1 in). They contacted only one other scute each, and may have been arranged in paired sets down the midline of the back and tail, and along the sides of the animal. Single rows of paired oval scutes may have filled in between the spiked rows. The bladed scutes, an extreme of the oval scute form, may have been found at the ends of the oval scute rows. The square scutes may have been restricted to one area, such as the neck and shoulders, or they may represent the belly armor.
Spiked and bladed scutes are not unknown among crocodilians, although no other known crocodilian had or has both. O'Neill and colleagues compared Akanthosuchus to Pinacosuchus
Pinacosuchus
Pinacosuchus is an extinct genus of crocodylomorph. Its fossils were found in the Late Cretaceous-age North Horn Formation of Utah . Pinacosuchus was a diminutive crocodylomorph with spiky armor, and is known only from meagre remains....
, a 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...
with spiked scutes known from the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
North Horn Formation
North Horn Formation
The North Horn Formation is a geological formation in Utah whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation.-Vertebrate paleofauna:...
of Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
, but it differed from Akanthosuchus in several ways: it was much smaller (three to four times smaller than NMMNH NP-139), it had less derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
vertebrae, and it lacked bladed scutes.
Classification
O'Neill et al. suggested that Akanthosuchus was an alligatorine, based on the characteristics of a partial lower jaw bone (retroarticular). They considered the possibility that Akanthosuchus is a postcranial skeleton of either CeratosuchusCeratosuchus
Ceratosuchus is an extinct genus of alligatorine from latest Paleocene rocks of Colorado's Piceance Basin and earliest Eocene rocks of Wyoming's Bighorn Basin in North America, a slice of time known as the Clarkforkian North American Land Mammal Age. Like its modern relatives, Ceratosuchus was a...
or Navajosuchus
Navajosuchus
Navajosuchus is an extinct genus of alligatorine crocodylian. Its fossils have been found in the Paleocene-age Nacimiento Formation of the San Juan Basin, New Mexico . It was named in 1942 by Charles C. Mook, and the original type species was N. novomexicanus. N. novomexicanus was based on...
, contemporaneous crocodilians known primarily from skulls, but elected to name a new genus instead. More recently, Hill and Lucas, using a cladistic analysis
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...
, rejected the hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
that Akanthosuchus is the same as either Ceratosuchus or Navajosuchus, and found that Akanthosuchus is an alligatoroid. It tended to sort out with alligatorines
Alligatorinae
Alligatorinae are one of two subfamilies of the family Alligatoridae.-Taxonomy:* Subfamily Alligatorinae** Genus Chrysochampsa ** Genus Hassiacosuchus ** Genus Navajosuchus ** Genus Ceratosuchus...
, but Hill and Lucas found that this was weakly supported in their analysis.
Distribution
NMMNH NP-139 came from the upper part of the TorrejonianTorrejonian
The Torrejonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 63,300,000 to 60,200,000 years BP lasting . It is usually considered to overlap the Selandian and Thanetian within the...
portion of the Nacimiento Formation. Later work identified Akanthosuchus langstoni from the older Puercan
Puercan
The Puercan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 65,000,000 to 63,300,000 years BP lasting . It is usually considered to be within the Paleocene...
-age portion of the Nacimiento Formation as well, which gives the genus a range including much of the Nacimiento Formation, deposited between approximately 64.5 and 61.0 million years ago. A scute and femur from the De-Na-Zin Member of the late Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
-age (Late Cretaceous) Kirtland Formation
Kirtland Formation
The Kirtland Formation is a sedimentary geological formation. It is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that existed on the inland seashore of North America, in the late Cretaceous period. It overlies the Fruitland Formation...
are similar to the corresponding elements of Akanthosuchus, but cannot be conclusively assigned to the genus at this time. This material is also from the San Juan Basin.
Paleobiology
The Nacimiento Formation is interpreted as having had a fluvialRiver
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
and lacustrine
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
depositional setting. O'Neill et al. interpreted Akanthosuchus as either a terrestrial crocodilian or a specialized, modestly sized, heavily armored aquatic crocodilian.