Eucritta

Eucritta is known from five specimens. These specimens have only been recovered from Unit 82, a Viséan-epoch layer of black shale found at the East Kirkton Quarry of Scotland. Four of these specimens were described by English paleontologist Jenny Clack in a 1998 edition of Nature. The most well-preserved specimen, UMZC T1347, is a crushed but fairly complete skeleton and skull split between two slabs. This specimen has been designated the holotype of the genus. A second specimen, UMZC T1348, contains only a skull, which is larger than that of the holotype. Both of these specimens were discovered by famed Scottish fossil collector Stan Wood in 1988. Wood also discovered another large and well-preserved skull, NMS 1992.14, in 1989. The smallest specimen, and the earliest to be discovered, was UMZC 1285, a small skull and the front half of a torso. This specimen was discovered by Robert Clack in 1985. By the time of Eucritta's more comprehensive 2001 description courtesy of Jenny Clack, a fifth specimen had been discovered. This fifth specimen, discovered by Stan Wood, was a partial skull and torso designated UMZC 2000.1.[1]

Due to multiple specimens of various sizes preserving skull material, Eucritta was one of only a few Carboniferous stegocephalians known from a growth series. Most of the skull experienced isometric growth (i.e. the proportions stayed relatively consistent), though the orbits experienced negative allometric growth (i.e. they became proportionally smaller in larger specimens). Apparently Eucritta lived in the habitat preserved at East Kirkton during multiple life stages, while its contemporaries (such as Balanerpeton) only lived there as adults. Eucritta had fairly long and well-developed limbs, which may indicate a decent degree of terrestriality. The femur (thigh bone) was 45% the length of the skull, comparable to the ratio in its contemporaries Eldeceeon (47%) and Balanerpeton (~ 50%).[1]

Eucritta has several characteristics which suggest that it utilized buccal pumping. This form of breathing, also utilized by modern amphibians, uses neck muscles to inflate the mouth with air and then pump air into and out of the lungs. A short, broad skull and straight ribs are generally considered to correspond to this form of breathing, and they are features present in Eucritta. Several other East Kirkton stegocephalians, such as Eldeceeon and Silvanerpeton, have taller skulls and more drastically curved ribs. This suggests that they used costal ventilation, a form of breathing based on inflating and deflating the lungs using the ribs and/or a diaphragm. Costal ventilation is utilized by modern amniotes like mammals, reptiles, and birds.[1]

Eucritta may have been a close relative of baphetids like Loxomma

Eucritta was already recognized as a unique animal in its original 1998 description. A phylogenetic analysis provided in the description often found it close to baphetids, but groups at and around the base of crown-Tetrapoda were not stable in their position. The most parsimonious family tree tentatively placed Eucritta and baphetids closer to "anthracosaurs" than to temnospondyls.[2]

The more comprehensive 2001 description added the reptile-like tetrapod Gephyrostegus to the 1998 paper's phylogenetic analysis, and made several more edits and corrections. In response, Eucritta and the baphetids shifted their position, and a connection to temnospondyls became equally parsimonious. Clack commented that most of the connections between Eucritta, baphetids, and "anthracosaurs" were "plesiomorphies, ambiguous, or reversals".[1] Ruta, Coates, and Quicke (2003)'s analysis of the tetrapod family tree placed Eucritta alone at the base of Temnospondyli. However, they also admitted that it would not be unlikely for it to have a variety of other positions scattered around the base of crown-Tetrapoda (the group encompassing the ancestors of all modern tetrapods, typically including temnospondyls and reptiliomorphs). When certain traits are emphasized through implied weighting, Eucritta is connected to baphetids as part of a branch immediately basal to crown-Tetrapoda.[3] A 2009 restudy of Baphetes assigned the name Baphetoidea to the branch incorporating baphetids, Eucritta, and the unusual filter-feeding stem tetrapod Spathicephalus.[4]

Ruta and Coates' 2007 revision of their earlier analysis further supported this position next to baphetids and basal to crown-Tetrapoda.[5] Many subsequent studies have cemented this position, although some instability still exists. In the 2016 description of five new stegocephalians by Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo,Reeves, Ross, Ruta, Smithson, and Walsh, Eucritta was removed from Baphetoidea, and consistently found to be either the most basal reptiliomorph or most basal temnospondyl (placing Lissamphibia somewhere within Temnospondyli; so in other words, the most basal batrachomorph and amphibian).[6] Marjanovic and Laurin's 2019 deconstruction of Ruta and Coates (2007) sometimes agreed with their interpretation, although under quite a few cases they placed the family Colosteidae next to Baphetidae, separating Eucritta from the baphetids. Temnospondyli complicates things further, as some family trees found by the 2019 study place temnospondyls as derived from the Eucritta + Colosteidae + Baphetidae clade, while others had temnospondyls branch off later, after certain supposed "anthracosaurs" (particularly embolomeres) had already evolved. Regardless of its position, Eucritta did not drift far from the common ancestry of baphetids, colosteids, temnospondyls, and "anthracosaurs".[7]