Diadectidae
Encyclopedia
Diadectidae is an extinct family
of large diadectomorph
reptiliomorphs. Diadectids lived in North America
and Europe
during the Late Carboniferous and Early Permian. They were the first herbivorous
tetrapod
s, and also the first fully terrestrial animals to attain large sizes. Footprints indicate that diadectids walked with an erect posture. They were the first to exploit plant material in terrestrial food chains, making their appearance an important stage in both vertebrate evolution and terrestrial ecosystem
s.
The best known and largest representative of the family is Diadectes
, a heavily built animal that attained a maximum length of several metres. Several other genera
and various fragmentary fossil remains are also known. Although well known genera like Diadectes first appear in the Late Pennsylvanian, fragmentary remains of possible diadectids are known from much earlier deposits, including a piece of lower jaw found in Mississippian strata from Tennessee
.
Diadectids were some of the first tetrapods, or four-legged vertebrates, to attain large sizes. Diadectids first appear in the Late Carboniferous with the genus Desmatodon
, although recently described bones from Tennessee suggest that they may have appeared even earlier in the Early Carboniferous. They have large bodies with relatively short limbs. The rib cage is barrel-shaped to accommodate a large digestive tract necessary for the digestion of cellulose
in plants. The skulls of diadectids are wide and deep with blunt snouts. The internal nares are also short. Paleontologist E.C. Case
compared diadectids to turtles in 1907, noting their large pectoral girdles, short, strong limbs, and robust skulls. Case described them as "lowly, sluggish, inoffensive herbivorous reptiles, clad in an armor of plate to protect them from the fiercely carnivorous pelycosaur
s."
Diadectids have a hederodont dentition, meaning that their teeth vary in shape along the length of the jaws. The teeth are wide and bear many cusps or projections, an indication that diadectids ate tough plants. Some teeth are leaf-shaped and laterally compressed, another indication that diadectids were able to shred plant material. The procumbent front teeth of the lower jaw project forward. Diadectids likely had strong jaw muscles for processing plant material; the placement of the jaw joints above or below the level of the occlusal plane
s (the planes at which the teeth come together) would have given diadectid jaws mechanical advantage
. The joints themselves give the jaws a complex range of movement suitable for consuming plants. Large holes and cavities in the skull called adductor chambers and temporal openings would have provided room for large jaw-closing muscles. A ridge on the dentary bone of the lower jaw may have provided a surface for chewing or even supported a beak.
paleontologist Edward Drinker Cope
named the genus in 1878 on the basis of several vertebrae and teeth from the Early Permian of Texas
. Cope erected the family Diadectidae in 1880 to include Diadectes and Empedocles
, a genus he named two years earlier. Nothodon, named by Cope's rival Othniel Charles Marsh
in 1878, was soon placed in the family.
Cope named several other diadectids, including Helodectes in 1880, Chilonyx and Empedias in 1883, and Bolbodon in 1896. Paleontologist E.C. Case named four other diadectids: Desmatodon
in 1908, Diasparactus
in 1910, Diadectoides in 1911, and Animasaurus along with paleontologist Samuel Wendell Williston
in 1912. Case and Williston considered Marsh's Nothodon and Cope's Bolbodon to be synonymous with Diadectes. Marsh named Nothodon in the American Journal of Science
only five days before Cope described Diadectes in Proceedings of the American Philosophical Society
. Under rules of the International Code of Zoological Nomenclature
, the name Nothodon would have priority over Diadectes, but because the name Diadectes has been in use since Case and Williston first synonymized the genera, Diadectes remains the accepted name.
In North America, diadectids are known from Texas, Colorado
, Utah
, New Mexico
, Oklahoma
, Ohio
, West Virginia
, Pennsylvania
, and Prince Edward Island
. A possible diadectid has also been found from Tennessee. It is known from a broken lower jaw and several teeth found in Mississippian-age (Chesterian) strata that are likely part of the Bangor Formation. In a detailed review of Diadectidae, paleontologist E.C. Olson placed three North American genera within the family: Diadectes, Diasparactus, and Desmatodon. Chilonyx, Empedias, Diadectoides, and Animasaurus were synonymized with Diadectes, and four species of Diadectes (D. sideropelicus, D. tenuitectes, D. lentus, and D. carinatus) were recognized. A fourth genus, Ambedus
, was named in 2004 from the Early Permian of Ohio.
Diadectids are also known from Germany
. Phanerosaurus
was described from several vertebrae near Zwickau
by German paleontologist Christian Erich Hermann von Meyer
in 1860, but was not recognized as a diadectid until 1925. A second species of Phanerosaurus was identified from some vertebrae and a fragmentary skull in 1882, and was given its own genus, Stephanospondylus
, in 1905. In 1998, a new species of Diadectes, D. absitus, was described from the Bromacker sandstone
quarry
of the Tambach Formation in the Thuringian Forest
of central Germany. A new genus of diadectid called Orobates
was also named from the Bromacker Quarry in 2004.
s, tetrapods that lay eggs on land. The family was placed in the larger group Diadectomorpha
by paleontologist D.M.S. Watson
in 1917. Diadectidae is closely related to another family of large diadectomorphs, the Limnoscelidae
, as well as the monotypic diadectomorph family Tseajaiidae, represented by the genus Tseajaia
. In most studies, Diadectomorpha is placed as the sister taxon of Amniota. Amniotes and Diadectomorpha are sometimes grouped together using the old name Cotylosauria, a name originally used for the most basal grade of what was than thought to be reptiles. While most studies now place diadectids outside Amniota, some have considered them to be true amniotes.
Most phylogenetic studies of the three diadectomorph families – Diadectidae, Limnoscelidae, and Tseajaiidae – have found diadectids and limnoscelids to be more closely related to each other than either is to Tseajaia. In other words, Diadectidae and Limnoscelidae form a clade
within Diadectomorpha and Tseajaia is excluded from the clade. In a 2010 phylogenetic analysis, Diadectidae formed a clade that was characterized by wide cheek teeth with cusps on either side. Unlike previous studies, it was found to be more closely related to Tseajaiidae than Limnoscelidae. The family was defined as Diadectes and all taxa sharing a more recent common ancestor with Diadectes than with Tseajaia. Below is a cladogram
modified from the 2010 analysis:
Diadectes is the best known diadectid with six species named since its initial description. In a 2005 phylogenetic analysis, most species of Diadectes formed a clade with Diasparactus zenos. Two species, Diadectes absitus and Diadectes sanmiguelensis, were placed in more basal
positions. These species possess primitive characteristics found in non-diadectid forms such as Limnoscelis and Tseajaia. Because D. absitus and D. sanmiguelensis were placed far from other species of Diadectes in the analysis, their assignment to the genus was questioned. The same results were found in the 2010 analysis. Two new genera were erected in the study to include D. abstus and D. sanmiguelensis. D. sanmiguelensis, the more basal of the two forms, was placed in the new genus Oradectes
. D. abstus was renamed Silvadectes
.
bones in their ankles, diadectids have a more complex astragalus
formed from the fusion of these bones. Astragali are present in terrestrial amniote
s and are identical in structure to those of diadectids. Therefore, the ankle structure of diadectids bears a closer resemblance to those of advanced terrestrial vertebrates like mammals and reptiles than those of earlier tetrapods. Since diadectids are the only diadectomorphs with astragali, they likely developed the structure independent of amniotes.
Although they bear similarities to those of amniotes, the taral bones of diadectids are poorly ossified and loosely connected. The digits of the foot connect only to the fourth distal tarsal, providing a wide range of movement in the foot. This flexibility enabled diadectids to rotate their feet in a forward position while walking, providing greater force when pushing off. The feet could also be placed closer to the midline of the body to give diadectids an erect stance.
Evidence for an erect stance can be found in trackway
s attributed to diadectids. The most well-preserved of these trackways are present in the Tambach Formation in central Germany. A 2007 study identified two different ichnospecies, Ichniotherium cottae and I. sphaerodactylum, as footprints of the diadectids Silvadectes absitus and Orobates pabsti, respectively. This was the first species-level identification of trackmakers of Paleozoic
-era trackways, making the footprints the oldest yet associated with specific animal species. The close positioning of the footprints attributed to the more advanced diadectides suggests that the animals held their feet almost underneath their bodies, giving them a more efficient gait and to some degree paralleling the stance of mammals more than the sprawling amphibians and reptiles.
Diadectids were the first fully herbivorous tetrapods. Although several other groups of early tetrapods independently acquired herbivory, diadectids were the only Carboniferous tetrapods that were able to process high-fiber terrestrial plants. Diadectids were also the most diverse group of herbivores, representing the first radiation of plant-eating tetrapods. Both Cope and Marsh recognized that diadectids were herbivores in 1878 when they studied their distinctively broad, cusped teeth. In his description of Diadectes, Cope mentioned, "animals belonging to this genus were, in all probability, herbivorous."
Diadectids underwent an evolutionary radiation, diversifying into thirteen species in the Late Carboniferous and Early Permian, outnumbering other diadectomorphs such as the limnoscelids. This radiation was likely the result of diadectids' expansion into a new herbivorous ecological niche
that was previously unfilled. While the distribution of limnoscelids is limited to parts of North America and Tseajaia is restricted to only the southwestern United States, diadectids are present in Europe and much of North America, occupying a much wider geographic range than other diadectids.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of large diadectomorph
Diadectomorpha
Diadectomorpha are a clade of large reptile-like amphibians that lived in Euramerica during the Carboniferous and Early Permian periods, and are very close to the ancestry of the Amniota. They include both large carnivorous and even larger herbivorous forms, some semi-aquatic and others fully...
reptiliomorphs. Diadectids lived in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
during the Late Carboniferous and Early Permian. They were the first herbivorous
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s, and also the first fully terrestrial animals to attain large sizes. Footprints indicate that diadectids walked with an erect posture. They were the first to exploit plant material in terrestrial food chains, making their appearance an important stage in both vertebrate evolution and terrestrial ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s.
The best known and largest representative of the family is Diadectes
Diadectes
Diadectes was a genus of large, very reptile-like amphibians that lived during the early Permian period...
, a heavily built animal that attained a maximum length of several metres. Several other genera
Genera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
and various fragmentary fossil remains are also known. Although well known genera like Diadectes first appear in the Late Pennsylvanian, fragmentary remains of possible diadectids are known from much earlier deposits, including a piece of lower jaw found in Mississippian strata from Tennessee
Tennessee
Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...
.
Description
Desmatodon
Desmatodon is an extinct genus of diadectid reptiliomorph. With fossils found from the Late Carboniferous of Pennsylvania, Colorado, and New Mexico in the United States, Desmatodon is the oldest known diadectid. Two species are currently recognized: the type species D. hollandi and the species D...
, although recently described bones from Tennessee suggest that they may have appeared even earlier in the Early Carboniferous. They have large bodies with relatively short limbs. The rib cage is barrel-shaped to accommodate a large digestive tract necessary for the digestion of cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
in plants. The skulls of diadectids are wide and deep with blunt snouts. The internal nares are also short. Paleontologist E.C. Case
Ermine Cowles Case
Ermine Cowles Case , invariably known as E.C. Case, was a prominent American paleontologist in the second generation that succeeded Othniel Charles Marsh and Edward Drinker Cope...
compared diadectids to turtles in 1907, noting their large pectoral girdles, short, strong limbs, and robust skulls. Case described them as "lowly, sluggish, inoffensive herbivorous reptiles, clad in an armor of plate to protect them from the fiercely carnivorous pelycosaur
Pelycosaur
The pelycosaurs are an informal grouping composed of basal or primitive Late Paleozoic synapsid amniotes. Some species were quite large and could grow up to 3 meters or more, although most species were much smaller...
s."
Diadectids have a hederodont dentition, meaning that their teeth vary in shape along the length of the jaws. The teeth are wide and bear many cusps or projections, an indication that diadectids ate tough plants. Some teeth are leaf-shaped and laterally compressed, another indication that diadectids were able to shred plant material. The procumbent front teeth of the lower jaw project forward. Diadectids likely had strong jaw muscles for processing plant material; the placement of the jaw joints above or below the level of the occlusal plane
Occlusion (dentistry)
Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary and mandibular teeth when they approach each other, as occurs during chewing or at rest....
s (the planes at which the teeth come together) would have given diadectid jaws mechanical advantage
Mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
. The joints themselves give the jaws a complex range of movement suitable for consuming plants. Large holes and cavities in the skull called adductor chambers and temporal openings would have provided room for large jaw-closing muscles. A ridge on the dentary bone of the lower jaw may have provided a surface for chewing or even supported a beak.
History
The first diadectid to be described was Diadectes. AmericanUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
paleontologist Edward Drinker Cope
Edward Drinker Cope
Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...
named the genus in 1878 on the basis of several vertebrae and teeth from the Early Permian of Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
. Cope erected the family Diadectidae in 1880 to include Diadectes and Empedocles
Empedocles
Empedocles was a Greek pre-Socratic philosopher and a citizen of Agrigentum, a Greek city in Sicily. Empedocles' philosophy is best known for being the originator of the cosmogenic theory of the four Classical elements...
, a genus he named two years earlier. Nothodon, named by Cope's rival Othniel Charles Marsh
Othniel Charles Marsh
Othniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
in 1878, was soon placed in the family.
Cope named several other diadectids, including Helodectes in 1880, Chilonyx and Empedias in 1883, and Bolbodon in 1896. Paleontologist E.C. Case named four other diadectids: Desmatodon
Desmatodon
Desmatodon is an extinct genus of diadectid reptiliomorph. With fossils found from the Late Carboniferous of Pennsylvania, Colorado, and New Mexico in the United States, Desmatodon is the oldest known diadectid. Two species are currently recognized: the type species D. hollandi and the species D...
in 1908, Diasparactus
Diasparactus
Diasparactus is an extinct genus of diadectid.In Diasparactus, the spines of the dorsal vertebrae are higher than in other genera in the family.-Diasparactus zenos:...
in 1910, Diadectoides in 1911, and Animasaurus along with paleontologist Samuel Wendell Williston
Samuel Wendell Williston
Samuel Wendell Williston was an American educator and paleontologist who was the first to propose that birds developed flight cursorially , rather than arboreally . He was also an entomologist, specialising in Diptera.-Early life:Williston was born in Boston, Massachusetts to Samuel Williston and...
in 1912. Case and Williston considered Marsh's Nothodon and Cope's Bolbodon to be synonymous with Diadectes. Marsh named Nothodon in the American Journal of Science
American Journal of Science
The American Journal of Science is the United States of America's longest-running scientific journal, having been published continuously since its conception in 1818 by Professor Benjamin Silliman, who edited and financed it himself...
only five days before Cope described Diadectes in Proceedings of the American Philosophical Society
Proceedings of the American Philosophical Society
Proceedings of the American Philosophical Society is a quarterly philosophy peer-reviewed journal published by the American Philosophical Society since 1838.-External links:* , Biodiversity Heritage Library...
. Under rules of the International Code of Zoological Nomenclature
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...
, the name Nothodon would have priority over Diadectes, but because the name Diadectes has been in use since Case and Williston first synonymized the genera, Diadectes remains the accepted name.
In North America, diadectids are known from Texas, Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, 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...
, 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...
, Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
, West Virginia
West Virginia
West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...
, Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
, and Prince Edward Island
Prince Edward Island
Prince Edward Island is a Canadian province consisting of an island of the same name, as well as other islands. The maritime province is the smallest in the nation in both land area and population...
. A possible diadectid has also been found from Tennessee. It is known from a broken lower jaw and several teeth found in Mississippian-age (Chesterian) strata that are likely part of the Bangor Formation. In a detailed review of Diadectidae, paleontologist E.C. Olson placed three North American genera within the family: Diadectes, Diasparactus, and Desmatodon. Chilonyx, Empedias, Diadectoides, and Animasaurus were synonymized with Diadectes, and four species of Diadectes (D. sideropelicus, D. tenuitectes, D. lentus, and D. carinatus) were recognized. A fourth genus, Ambedus
Ambedus
Ambedus is an extinct genus of diadectid reptiliomorph. Fossils have been found from the Early Permian Dunkard Group of Monroe County, Ohio. The type species A. pusillus was named in 2004. The genus name comes from the Latin word ambedo meaning "to nibble", in reference to its herbivorous diet...
, was named in 2004 from the Early Permian of Ohio.
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. Phanerosaurus
Phanerosaurus
Phanerosaurus is an extinct genus of diadectid reptiliomorph from the Early Permian of Germany. Fossils are known from the Leukersdorf Formation near Zwickau. German paleontologist Christian Erich Hermann von Meyer named the type species P. naumanni in 1860 on the basis of several sacral and...
was described from several vertebrae near Zwickau
Zwickau
Zwickau in Germany, former seat of the government of the south-western region of the Free State of Saxony, belongs to an industrial and economical core region. Nowadays it is the capital city of the district of Zwickau...
by German paleontologist Christian Erich Hermann von Meyer
Christian Erich Hermann von Meyer
Christian Erich Hermann von Meyer was a German palaeontologist.He was born at Frankfurt am Main.In 1832 von Meyer issued a work entitled Palaeologica, and in course of time he published a series of memoirs on various fossil organic remains: molluscs, crustaceans, fishes and higher vertebrata,...
in 1860, but was not recognized as a diadectid until 1925. A second species of Phanerosaurus was identified from some vertebrae and a fragmentary skull in 1882, and was given its own genus, Stephanospondylus
Stephanospondylus
Stephanospondylus is an extinct genus of diadectid reptiliomorph from the Early Permian of Germany. Fossils have been found in deposits of the Lower Rotliegend near Dresden. The type species S...
, in 1905. In 1998, a new species of Diadectes, D. absitus, was described from the Bromacker 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,...
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...
of the Tambach Formation in the Thuringian Forest
Thuringian Forest
The Thuringian Forest running northwest to southeast, forms a continuous stretch of ancient rounded mountains posing ample difficulties in transit routing save through a few navigable passes in the southern reaches of the German state of Thuringia. It is about long and wide...
of central Germany. A new genus of diadectid called Orobates
Orobates
Orobates is an extinct genus of diadectid. It lived in the middle Permian, about 260 million years ago. Its remains were found in Germany....
was also named from the Bromacker Quarry in 2004.
Classification
Diadectids have traditionally been considered close relatives of the amnioteAmniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s, tetrapods that lay eggs on land. The family was placed in the larger group Diadectomorpha
Diadectomorpha
Diadectomorpha are a clade of large reptile-like amphibians that lived in Euramerica during the Carboniferous and Early Permian periods, and are very close to the ancestry of the Amniota. They include both large carnivorous and even larger herbivorous forms, some semi-aquatic and others fully...
by paleontologist D.M.S. Watson
D.M.S. Watson
David Meredith Seares Watson FRS was the Jodrell Professor of Zoology and Comparative Anatomy at University College, London from 1921 to 1951.-Early life:...
in 1917. Diadectidae is closely related to another family of large diadectomorphs, the Limnoscelidae
Limnoscelidae
Limnoscelidae is a family of carnivorous Diadectomorphans. They would have been the largest terrestrial carnivores of their day, the other large carnivores being aquatic or semi aquatic labyrinthodont amphibians...
, as well as the monotypic diadectomorph family Tseajaiidae, represented by the genus Tseajaia
Tseajaia
Tseajaia is an extinct genus of tetrapod. It was an anthracosaurian that lived in the Permian of North America. The skeleton is that of a medium sized, rather advanced reptile-like amphibian. In life it was about a meter long and may have looked vaguely like an iguana, though slower and and with...
. In most studies, Diadectomorpha is placed as the sister taxon of Amniota. Amniotes and Diadectomorpha are sometimes grouped together using the old name Cotylosauria, a name originally used for the most basal grade of what was than thought to be reptiles. While most studies now place diadectids outside Amniota, some have considered them to be true amniotes.
Most phylogenetic studies of the three diadectomorph families – Diadectidae, Limnoscelidae, and Tseajaiidae – have found diadectids and limnoscelids to be more closely related to each other than either is to Tseajaia. In other words, Diadectidae and Limnoscelidae form a 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...
within Diadectomorpha and Tseajaia is excluded from the clade. In a 2010 phylogenetic analysis, Diadectidae formed a clade that was characterized by wide cheek teeth with cusps on either side. Unlike previous studies, it was found to be more closely related to Tseajaiidae than Limnoscelidae. The family was defined as Diadectes and all taxa sharing a more recent common ancestor with Diadectes than with Tseajaia. Below is a cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
modified from the 2010 analysis:
Diadectes is the best known diadectid with six species named since its initial description. In a 2005 phylogenetic analysis, most species of Diadectes formed a clade with Diasparactus zenos. Two species, Diadectes absitus and Diadectes sanmiguelensis, were placed in more 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:...
positions. These species possess primitive characteristics found in non-diadectid forms such as Limnoscelis and Tseajaia. Because D. absitus and D. sanmiguelensis were placed far from other species of Diadectes in the analysis, their assignment to the genus was questioned. The same results were found in the 2010 analysis. Two new genera were erected in the study to include D. abstus and D. sanmiguelensis. D. sanmiguelensis, the more basal of the two forms, was placed in the new genus Oradectes
Oradectes
Oradectes is an extinct genus of diadectid reptiliomorph. It is known from a single partial skeleton collected from the Early Permian Cutler Formation of Colorado in the United States. The type species, O. sanmiguelensis, was originally named as a species of Diadectes in 1965...
. D. abstus was renamed Silvadectes
Silvadectes
Silvadectes is an extinct genus of diadectid reptiliomorph. It is known from several nearly complete articulated skeletons collected from the Early Permian Tambach Formation in the Thuringian Forest of central Germany. The type species is S. absitus...
.
Locomotion
Diadectids were once thought to be sprawling animals with their short, robust legs positioned to the sides of their large bodies. Despite this, several lines of evidence including trackways and limb morphology suggest that diadectids moved in a more erect posture. While earlier tetrapods possess several simple tarsalTarsal
Tarsal could refer to:*tarsus *tarsus *superior tarsal muscle...
bones in their ankles, diadectids have a more complex astragalus
Astragalus
Astragalus is a large genus of about 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae, subfamily Faboideae. The genus is native to temperate regions of the Northern Hemisphere...
formed from the fusion of these bones. Astragali are present in terrestrial amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s and are identical in structure to those of diadectids. Therefore, the ankle structure of diadectids bears a closer resemblance to those of advanced terrestrial vertebrates like mammals and reptiles than those of earlier tetrapods. Since diadectids are the only diadectomorphs with astragali, they likely developed the structure independent of amniotes.
Although they bear similarities to those of amniotes, the taral bones of diadectids are poorly ossified and loosely connected. The digits of the foot connect only to the fourth distal tarsal, providing a wide range of movement in the foot. This flexibility enabled diadectids to rotate their feet in a forward position while walking, providing greater force when pushing off. The feet could also be placed closer to the midline of the body to give diadectids an erect stance.
Evidence for an erect stance can be found in trackway
Trackway
A trackway is an ancient route of travel for people or animals. In biology, a trackway can be a set of impressions in the soft earth, usually a set of footprints, left by an animal. A fossil trackway is the fossilized imprint of a trackway. Trackways have been found all over the world...
s attributed to diadectids. The most well-preserved of these trackways are present in the Tambach Formation in central Germany. A 2007 study identified two different ichnospecies, Ichniotherium cottae and I. sphaerodactylum, as footprints of the diadectids Silvadectes absitus and Orobates pabsti, respectively. This was the first species-level identification of trackmakers of Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
-era trackways, making the footprints the oldest yet associated with specific animal species. The close positioning of the footprints attributed to the more advanced diadectides suggests that the animals held their feet almost underneath their bodies, giving them a more efficient gait and to some degree paralleling the stance of mammals more than the sprawling amphibians and reptiles.
Diet
Diadectids underwent an evolutionary radiation, diversifying into thirteen species in the Late Carboniferous and Early Permian, outnumbering other diadectomorphs such as the limnoscelids. This radiation was likely the result of diadectids' expansion into a new herbivorous ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
that was previously unfilled. While the distribution of limnoscelids is limited to parts of North America and Tseajaia is restricted to only the southwestern United States, diadectids are present in Europe and much of North America, occupying a much wider geographic range than other diadectids.