Caseidae


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, from various parts of Europe, 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 having the largest stratigraphic range. They also represent one of only two basal synapsid groups to survive in therapsid-dominated terrestrial communities. Thus, the last known caseids come from the strata of the middle Permian 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, 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 and dicynodonts.

Description

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, 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. The teeth, simply conical and pointed in insectivorous species, adopt in herbivorous species a leafy or spatulate morphology and are provided with more or less numerous cuspules. Numerous small teeth also adorned several bones of the palate. Herbivorous species do not show a simple evolutionary trend towards increasing tooth complexity. Thus, the teeth of the basal taxa Casea and Arisierpeton have three cuspules just like in the more derived forms Cotylorhynchus and Caseopsis. Ennatosaurus and Euromycter, which occupy an intermediate phylogenetic position, have teeth bearing 5 to 7 cuspules and 5 to 8 cuspules respectively. Angelosaurus, one of the most derived caseids, has teeth with 5 cuspules. In Angelosaurus the teeth have a bulbous morphology with very short and wide crowns. Their sturdiness and the significant wear they show indicate that Angelosaurus must have fed on tougher plants than those on which most other herbivorous caseids fed. Herbivorous caseids also show very different dietary adaptations from those seen in another group of basal synapsids, the Edaphosauridae. The latter had, in addition to the marginal dentition, a dental battery made up of numerous teeth located both on the palate and on the inner surface of the lower jaws. In herbivorous caseids, the palatal teeth are smaller, and the inner surface of the lower jaws bears no teeth. Instead of a dental battery, they had a massive tongue perhaps rough, with which they had to compress food against the palatal teeth.
The forelimbs of caseids are often more robust than the hindlimbs. It has indeed been observed that the bones of the forelimbs gained in robustness from the beginning of the evolution of the group, before the appearance of large species, while the hindlimbs remained slenderer. These characteristics suggest that the initial strengthening of the forelimbs was probably related to a particular function such as digging, and that this trait was later exapted by more derived and larger caseids to support their weights of up to several hundred kilograms. During their evolutionary history, caseids also show a reduction in their phalangeal formula. The most basal caseids like Eoasea, Callibrachion, and Martensius possessed the plesiomorphic condition of early amniotes with a phalangeal formula of 2-3-4-5-3 for manus and 2-3-4-5-4 for pes. In Euromycter the manus has a formula of 2-3-4-4-3. In the more derived forms like Cotylorhynchus the manus and pes show a phalangeal formula of 2-2-3-3-2. 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.

Footprints

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 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. According to Sacchi and colleagues, some footprints from the Lodève Basin are similar to those from the Wellington Formation and these authors designate them as Dimetropus cf. osageorum. The study by Sacchi et al. also demonstrates that the ichnogenus Dimetropus exhibits great morphological variation and that its producers can be assigned to different zoological groups among non-therapsid synapsids and not just Sphenacodontidae as once thought. In 2021, Rafel Matamales-Andreu and colleagues assigned footprints found in the Lower Permian Port des Canonge Formation in Mallorca to a caseid. These footprints resemble those of D. osageorum but they also have differences. These footprints are left in open nomenclature as cf. Dimetropus sp.. They are, however, smaller than those of D. osageorum and would have been produced by a caseid of modest size with proportions comparable to Ennatosaurus.

Paleobiology

Diets

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.