Branchiosauridae is an extinct family of small amphibamiform temnospondyls with external gills and an overall juvenile appearance. The family has been characterized by hundreds of well-preserved specimens from the Permo-Carboniferous of Middle Europe.[2] Specimens represent well defined ontogenetic stages and thus the taxon has been described to display paedomorphy (perennibranchiate). However, more recent work has revealed branchiosaurid taxa that display metamorphosing trajectories.[3] The name Branchiosauridae (“Branchio” in Ancient Greek denoting gills and “saurus” meaning lizard) refers to the retention of gills.
Branchiosaurids mostly inhabited Permo-Carboniferous freshwater mountain-lake habitats of Middle Europe at an altitude of up to 2000 meters.[4] Large quantities of specimens have been collected from the grey shales and limestones of Rotliegend localities of the Variscan orogen. The branchiosaurid fossil record is exceptional due to Lagerstatten conditions of these localities and the preservation of specimens representing various ontogenetic stages.[4] 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.[5] 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.[4] A fine lamination of C(org)-rich grey to black shales indicates a belt of lakes of tropical to subtropical climate and the existence of variable levels of oxygen for aquatic life in the Late Paleozoic. Permo-Carboniferous mass-mortality events are observed in several basins of Germany possibly caused by episodic mixing of the water column resulting in oxygen deficiency.[5]
Although the absolute ages of certain Rotliegend strata have yet to be elucidated the stratigraphically oldest branchiosaurid belonging to the genus Branchiosaurus occurs in Westphalian D. The earliest occurrence of Melanerpeton-clade is the Stephanian C and the first occurrence of the genus Apateon in Upper Carboniferus. The stratigraphic range of named members of Branchiosauridae extends to the Sakmarian (Early Permian).[2] A partial skeleton is known from the Wordian aged Kaş Formation of Southeastern Turkey, making it the only fossil of the group known from the Guadalupian.[6]
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).[5]
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.[2]