Caseidae

Extinct family of synapsids

Caseidae are an extinct family of basal synapsids that lived from the Late Carboniferous to Middle Permian between about 300 and 265 million years ago. Fossils of these animals come from the south central part of the United States (Texas, Oklahoma, and Kansas), from various parts of Europe (European Russia, France, Germany, Sardinia, and Poland), and possibly from South Africa if the genus Eunotosaurus is indeed a caseid as some authors proposed in 2021. Caseids show great taxonomic and morphological diversity. The most basal taxa were small insectivorous and omnivorous forms that lived mainly in the Upper Carboniferous and Lower Permian, such as Eocasea, Callibrachion, and Martensius. This type of caseid persists until the middle Permian with Phreatophasma and may be Eunotosaurus. During the early Permian, the clade is mainly represented by many species that adopted a herbivorous diet. Some have evolved into gigantic forms that can reach in length, such as Cotylorhynchus hancocki and Alierasaurus ronchii, making them the largest Permian synapsids. Caseids are considered important components of early terrestrial ecosystems in vertebrate history because the numerous herbivorous species in this family are among the first terrestrial tetrapods to occupy the role of primary consumer. The caseids experienced a significant evolutionary radiation at the end of the early Permian, becoming, with the captorhinid eureptiles, the dominant herbivores of terrestrial ecosystems in place of the edaphosaurids and diadectids. In 2016 and 2022, paleontologists proposed a semiaquatic lifestyle for the most derived genera like Cotylorhynchus and Lalieudorhynchus, but this hypothesis is disputed by other researchers. With a fossil record spanning from the Late Carboniferous to the Middle Permian, caseids are one of the basal synapsids groups (formerly known as “pelycosaur”) having the largest stratigraphic range. They also represent one of only two basal synapsid groups (along with the Varanopidae) to survive in therapsid dominated terrestrial communities. Thus, the last known caseids come from the strata of the middle Permian (Upper Roadian Wordian) of France and European Russia, where they cohabited notably with dinocephalians. These last caseids still show a certain morphological diversity with medium to large herbivores (Ennatosaurus and Lalieudorhynchus), as well as small predatory or omnivorous forms such as Phreatophasma and possibly Eunotosaurus. Caseids are so far unknown in Upper Permian strata and probably disappeared at the end of the Middle Permian. They were replaced by pareiasaurs (with similar dentition) and dicynodonts.

Caseids measured from less than to in length. They had a small head wider than high and with a forward inclined snout, a very short neck, a long tail, robust forelimbs, and a body of variable proportions depending on their diet. Small insectivorous species like Eocasea had an unexpanded trunk. Others with an omnivorous diet like Martensius had a barely enlarged rib cage, a more elongated skull, smaller nostrils, and a snout less inclined forward than in herbivorous caseids. The latter were characterized by their disproportionately small skull compared to the size of the body. The postcranial skeleton indeed shows a spectacular increase in the volume of the rib cage, which becomes very wide and barrel shaped, probably to accommodate a particularly developed intestine, necessary for the digestion of high fiber rich plants. In these forms, the skull has very large external nostrils and a very short facial region with a strong forward inclination of the end of the snout which clearly overhangs the dental row. The temporal fenestrae are also relatively large (especially in Ennatosaurus), the supratemporals are large in size, and, on the occipital surface, the paroccipital processes are massively developed, establishing strong supporting contacts with the squamosals. The dorsal surface of the skull is covered with numerous small pits. These suggest the presence of large scales on the head of these animals. Numerous labial foramina running parallel to the ventral edge of the premaxilla and maxilla, as well as along the dorsal edge of the dentary, suggest the presence of scaly "lips" which must have concealed the dentition when the jaws were closed. Thus, the teeth of the basal taxa Casea and Arisierpeton have three cuspules just like in the more derived forms Cotylorhynchus and Caseopsis. Angelosaurus, one of the most derived caseids, has teeth with 5 cuspules. Along with this reduction in the number of phalanges, the proportions of the autopods also change in derived caseids with metacarpals, metatarsals, and phalanges becoming shorter and broader. At the extreme of this specialization the genus Angelosaurus has short, broad, and smooth ungual phalanges which resemble hooves rather than claws.

Many vertebrate tracks have been proposed as belonging to Caseidae. In the early 2000s, large footprints known as Brontopus giganteus from the Permian Lodève basin in southern France were considered to belong either to a caseid or dinocephalian therapsid. In 2019, Lorenzo Marchetti and colleagues, however, determined that dinocephalians were most likely the trackmakers of the ichnogenus Brontopus. In 2012, Rafael Costa da Silva and colleagues proposed that the ichnogenus Chelichnus, widely distributed in Permian desert facies (fossil dunes) of Europe, North America and South America, could represent caseids footprints. In 2019, Marchetti and colleagues, however, reinterpreted Chelichnus as a nomen dubium and a taphotaxon, this type of track showing false anatomical features generated by locomotion on sandy inclined paleosurface. In 2014, Eva Sacchi and colleagues described the ichnotaxon Dimetropus osageorum from about a hundred isolated footprints and several trackways from the early Permian Wellington Formation in Oklahoma. The morphological study of these footprints and their comparison with tetrapod skeletons revealed that they probably belong to a large caseid comparable in size to Cotylorhynchus romeri (the latter from more recent strata). According to Sacchi and colleagues, some footprints from the Lodève Basin (unspecified geological formation) are similar to those from the Wellington Formation and these authors designate them as Dimetropus cf. osageorum.

During their evolutionary history, caseids shifted from faunivorous to herbivorous diet, a pattern that also occurred independently in other Carboniferous and Permian tetrapod groups such as Captorhinidae and Edaphosauridae. Earliest and most basal caseids, such as the late Carboniferous Eocasea and the early Permian Callibrachion had an unexpanded rib cage and dentition composed of very small conical teeth suggesting an insectivorous diet. Another basal caseid, Martensius, has a slightly enlarged barrel shaped trunk and dentition in which teeth indicative of an insectivorous diet in juveniles have been ontogenetically replaced in adults by teeth suggesting an omnivorous diet. In Martensius, the adult was still able to feed on insects, but it also possesses a draft of the herbivorous diet specializations present in later caseids, such as a relatively short, slightly forward inclined snout, and a dentition that is almost homodont in the upper jaws and completely homodont in lower jaws. The sequence of dental trait acquisition in Martensius suggests that intestinal vegetation processing preceded oral processing in the evolution of caseid herbivory. A juvenile insectivorous diet would have provided the opportunity for successful introduction into the intestine of microorganisms capable of endosymbiotic cellulolysis, particularly if the prey ingested were herbivorous insects which harbor such microorganisms in their viscera. Subsequently, the caseids adopted a strictly herbivorous diet and evolved into gigantic forms. These herbivorous caseids had spatulate teeth equipped with more or less numerous cuspules and a very enlarged and barrel shaped rib cage which must have housed highly developed intestines necessary for the digestion of plants with low nutritional value. This adaptation would partly explain the diversification and expansion of the group at the end of the Lower Permian and during the Middle Permian, because it allowed them to exploit a fiber rich plant resource that had by then become abundant and widespread. Nevertheless, small probably faunivorous caseids like Phreatophasma seem to have persisted until the Middle Permian. Another hypothesis suggests that the caseids could have used their powerful forelimbs to fold large plants towards them, which they would have torn off with their powerful claws.

Caseids first appear in the fossil record at the end of the Carboniferous and are already present in both North America and Europe with small predatory forms like Eocasea and Datheosaurus. The last caseids date from the Middle Permian and are mainly known in Europe with Phreatophasma and Ennatosaurus from the Roadian Wordian of Russia, Alierasaurus from the Roadian (Wordian?) of Sardinia,

Caseid fossils have been found in the southern and central United States (Texas, Oklahoma, Kansas), France, Germany, Poland, Sardinia, and European Russia. This geographical distribution corresponds to the paleoequatorial belt of Pangaea during the Carboniferous and the Permian, with the exception of the Russian localities which were located at the level of the 30th parallel north where a more temperate climate prevailed. The absence in Russia of terrestrial vertebrate localities prior to the Kungurian does not allow us to know the precise moment of the arrival of caseids in this region of the world. Between these two dates, other researchers classified Eunotosaurus as a parareptile or a pantestudines. If the first three authors are correct, Eunotosaurus would be the first caseid found in the Southern Hemisphere and the last known representative of the family with a latest Capitanian age for the most recent specimens. Elsewhere in Gondwana, the presence of the ichnotaxon Dimetropus in the Permian of Morocco suggests the future discovery of basal synapsids (including caseids) in the Permian basins of North Africa, which were also part of the paleoequatorial belt and constituted one of the main migratory routes between the Northern and Southern Hemispheres.

The family Caseidae was erected by Samuel Wendell Williston in 1911. In 1940, Alfred Sherwood Romer and L. W. Price placed Caseidae and Edaphosauridae in the same suborder, which they named Edaphosauria. This group is now abandoned, the edaphosaurids being closer to the Sphenacodontia (with which they form the clade Sphenacomorpha) than to the caseids. The first phylogenetic analysis of caseids was published in 2008 by Hillary C. Maddin and colleagues. In this analysis, the genus Oromycter occupies the most basal position within the clade. Ennatosaurus is the sister group of a clade containing Cotylorhynchus romeri and Angelosaurus dolani. This analysis reveals for the first time the paraphyly of the genus Casea, the species “Casea” rutena representing a distinct genus which will be named in 2011 Euromycter. Below is the first cladogram of caseids published by Maddin et al. in 2008. Below is the cladogram of Caseasauria published by Benson in 2012. Below is the phylogenetic analysis published by Romano and colleagues in 2017. In describing the species Martensius bromackerensis in 2020, Berman and colleagues published two cladograms in which Martensius is identified as one of the most basal caseids, and is positioned just above the Late Carboniferous Eocasea martini. In the first cladogram, the position of the remaining caseids is poorly resolved. In the second cladogram, Casea broilii is positioned above Martensius bromackerensis, and is followed by a polytomy between Oromycter dolesorum, Trichasaurus texensis, and a clade of remaining caseid. Within these remaining taxa, a sequence of branching taxa begins with Casea nicholsi, followed by Euromycter rutenus, then Ennatosaurus tecton, then Angelosaurus romeri, then an apical clade of the three Cotylorhynchus species plus Alierasaurus ronchii. In this final clade, Cotylorhynchus hancocki and C. bransoni are sister taxa and the clade forms a polytomy with C. romeri and Alierasaurus ronchii. Below are the two caseid cladograms published by Berman and colleagues in 2020. In 2022, Werneburg and colleagues described the genus Lalieudorhynchus and published a phylogenetic analysis which concluded that the genera Angelosaurus and Cotylorhynchus are paraphyletic and are represented only by their type species. In this analysis, Cotylorhynchus romeri is positioned just above the genus Angelosaurus, and forms a polytomy with a clade containing Ruthenosaurus and Caseopsis and another clade containing Alierasaurus, the two other species of Cotylorhynchus and Lalieudorhynchus. Within this latter clade, Alierasaurus is the sister group of “Cotylorhynchus” bransoni and a more derived clade including Lalieudorhynchus and “Cotylorhynchus” hancocki. Below is the cladogram published by Werneburg and colleagues in 2022.