Branchiosauridae

Branchiosaurids mostly inhabited Permo-Carboniferous freshwater mountain-lake habitats of Middle Europe at an altitude of up to 2000 meters. In the Late Carboniferous and Early Permian, western Europe was subjected to annual and long-lasting changes between dry and monsoon periods which produced highly variable lake environments and thus rapid diversification and speciation of amphibian populations. Smaller, shallow lakes especially would have experienced unstable environments due to these changing weather conditions whereas large and deep lakes, which yield most of the Branchiosaurid, specimens would have been more buffered. Branchiosaurid gene flow enhanced by periods of wet climate led to successful colonization of roughly 15 basins (known so far). It has been hypothesized that Branchiosaurids originated in the Central Bohemian basin of Czech Republic (WestphalianD), from which they migrated to basins of the Massif Central in France (Stephanian B) and subsequently to several Central German basins including Thuringian Forest, Ilfeld and Saale (Stephanian C). == Historical information/discovery ==

Branchiosaurids were recognized as a distinct group and given the family name by Fritsch (1879). In 1939 Romer hypothesized that branchiosaurids were, instead, larval morphotypes of larger temnospondyl amphibians. However, Boy (1972) rejected this hypothesis by recognizing autapomorphic larval features of branchiosaurids distinguishing them from other temnospondyls. Boy argued for the neotenic state of Branchiosauridae and distinguished Micromelerpeton, another paedomorphic dissorophoid group, as distinct from Branchiosauridae. He concluded that branchiosaurids form a monophyletic clade closely related to other dissorophoids. Branchiosaurids have since been suggested as close, if not immediate, lissamphibian (extant amphibian) relatives. == Classification ==

Branchiosaurids form a clade within dissorophoid temnospondyls (one of the hypothesized origins of Lissamphibia). Dissorophoidea encompasses Micromelerpetidae and Xerodromes (all other taxa). Xerodromes includes the Olsoniformes and salamander-like Amphibamiformes. The latter includes four clades: Micropholidae, Amphibamidae, Branchiosauridae and Lissamphibia. The amphibamids Amphibamus and Platyrhinops are the closest relatives of branchiosaurids and despite sharing homoplasies with amphibamids, Branchiosauridae forms a monophyletic group. == Family description ==

The synapomorphies of Branchiosauridae include a palatine with a prominent process which extends from the center of the bone to contact the maxilla; six rows of isolated, slender and multi-ended branchials; 21-22 presacral vertebrae (reversed in some forms). == Genera descriptions ==

The family Branchiosauridae includes the genera Branchiosaurus, Apateon, Melanerpeton, Leptorophus and Schoenfelderpeton. == Paleobiology ==

The specialized pharyngeal denticles with brush-like branches of Branchiosauridae are indicative of gill clefts and suggest a filter-feeding mechanism focusing on plankton. The jaw-like apparatus may have served to hold back prey items leaving the pharyngeal cavity with the water current or to form a tight closure of gill cleft during feeding. Branchiosauridae diversified partly through adaptations that included the co-evolution of delayed development of the upper jaw and cheek which resulted in a kinetic maxilla and allowed for more efficient suction feeding. This mechanism would have been adaptive for suspension feeding and feeding on single prey. In the Apateon-clade different morphotypes evolved due to heterochronic changes. In some species (A. caducus and A. flagrifer), the maxilla consolidated early in development and the gape size and irregular dentition indicate an early transition in diet from suspension to carnivory. In other species (A. dracyiensis) the maxilla was never consolidated indicating a reinforced suction mechanism. Diagnostics of the Melanerpeton-clade indicate a combination of filter-feeding and occasional capture of larger prey. Although the Melanerpeton-Apateon dichotomy is not correlated with any significant adaptations, the Melanerpeton-clade generally had a larger body size which likely allowed them to occupy new niches in lake ecosystems. Most Apateon species did not appear to have competitors and thus were successful invaders. == Ontogeny ==

Both neoteny (retention of larval somatic features into adulthood) and metamorphosis have been reported ontogenic pathways in branchiosaurids. Certain terrestrial branchiosaurid adaptations, such as the short trunk and long limbs, suggest that it was an initially terrestrial clade and thus reversals to aquatic life and metamorphosing trajectories occurred within the clade. == Histology ==

Histological observations confirm anatomical evidence of neoteny in branchiosaurids. Skeletochronological analysis allows for the identification of sexual maturity (i.e. when the distance between lines of arrested growth (LAGs) suddenly decreases). In Apateon specimens determined to be sexually mature, the cartilaginous Katschenko's line can be observed when perichondral ossification is complete. The diaphyseal and epiphyseal ossification patterns of Apateon specimens (i.e. persistence of histological larval features into adulthood) are suggestive of paedomorphy and similar to those of urodeles (extant neotenic amphibians). ==References==