Diadectidae
Diadectidae is an extinct family of early tetrapods that lived in what is now North America and Europe during the Late Carboniferous and Early Permian, and in Asia during the Late Permian. They were the first herbivorous tetrapods, and also the first fully terrestrial animals to attain large sizes. Footprints indicate that diadectids walked with an erect posture.
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.
Description
Diadectids 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 pelycosaurs."Diadectids have a heterodont 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 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.
History of study
The first diadectid to be described was Diadectes. American 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 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 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.
In 2015, the known geographic range of diadectids was expanded with the description of a new genus and species of diadectid from China, Alveusdectes fenestralis. Alveusdectes is also the youngest known diadectid by 16 million years, coming from a unit of the Late Permian Shangshihezi Formation that dates to about 256 million years. However, the assignment of Alveusdectes to Diadectidae has been questioned. A phylogenetic analysis from 2024 recovers Alveusdectes as a non-diadectomorph synapsid, more closely related to the eothyridids Eothyris and Oedaleops.
Classification
Diadectids have long been considered close relatives of the amniotes, tetrapods that lay eggs on land or retain the fertilized egg within the mother. In 1987, the paleontologist D.M.S. Watson placed the family in the larger group Diadectomorpha, which includes another family of large-bodied diadectomorphs, the Limnoscelidae, as well as the monotypic diadectomorph family Tseajaiidae, represented by the genus Tseajaia. Throughout the twentieth century, amniotes and diadectomorphs were often grouped together using the old name Cotylosauria, a name originally used for the most basal grade of what was then thought to be reptiles. In the early part of the century, many paleontologists regarded diadectids, along with other cotylosaurs, to be close relatives of turtles. In most recent studies of early tetrapod phylogeny, Cotylosauria is no longer recognized and Diadectomorpha is placed as the sister taxon of Amniota. However, while the majority of analyses now place diadectids outside Amniota, some have found 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. absitus and D. sanmiguelensis. D. sanmiguelensis, the more basal of the two forms, was placed in the new genus Oradectes. D. absitus was renamed Silvadectes. However, according to the ICZN, a name presented in an initially unpublished thesis such as Kissel's is not valid. Because the names Oradectes and Silvadectes have not yet been formally erected in a published paper, they were not, as of 2010, considered valid. A 2024 study found "D. sanmiguelensis" as more basal than Orobates pabsti, thus rendering the genus Diadectes polyphyletic. To solve this, the authors erected the new genus Kuwavaatakdectes for Kuwavaatakdectes sanmiguelensis.
In a 2013 study, David Berman argued that there wasn't enough evidence to justify Ambedus being in Diadectidae. He stated in his paper that its assignment to Diadectidae is based only on several isolated maxillae and dentaries containing cheek teeth that only exhibited a resemblance in their molar-like morphology to those in diadectids. There are also a number of other characteristics that distinguish Ambedus from all other diadectids, such as a shallow rather than deep deep dentary, and relatively high maxillary and dentary tooth counts, among other characteristics that distinguish them from Diadectidae. Furthermore, the appearance of Ambedus pusillus so late in the fossil record also casts a doubt on the fact that it is supposed to represent the basalmost member of the Diadectidae lineage. In contrast, the first diadectids from the Upper Pennsylvanian were far more developed and had the characteristic dentary and maxillary features of the Diadectidae lineage. This implies that there should be a ghost lineage that goes back all the way back to the Middle Pennsylvanian, which is highly unlikely according to Berman.