Gerrothorax was about long, and had a remarkably flattened body. It probably hid under sand or mud on river and lake bottoms, scanning for prey with its large, upward-facing eyes. Gerrothorax had an unusually shaped skull with angular protrusions on the sides. This looked vaguely similar to the skull of the earlier, unrelated, amphibian Diplocaulus, but was not so developed. Some Gerrothorax fossils preserved and near the neck. This shows that Gerrothorax was pedomorphic, retaining its larval gills as an adult. When originally described in 1946, these bones were considered to correspond to feather-like external gills similar to those of modern-day neotenic salamanders, such as the mudpuppy, the axolotl, and the olm. However, a 2011 paper found that it was more likely that plagiosaurids such as Gerrothorax had internal gills, like those of fish, rather than salamander-like external gills. The authors of that study noted that plagiosaurids and other ancient amphibians which retained gills as adults had grooves on their ceratobranchials. Grooved ceratobranchials are present in both modern and ancient fish, but unknown in modern amphibians. Therefore, they were indicative of internal gills. This would have also been advantageous for survival in large animals, as internal gills would have been protected by a large skin fold and were less likely to have been damaged by the environment. A 2008 study showed that Gerrothorax lifted its head rather than dropping its jaw when catching prey, which has been compared to how a toilet seat opens. In 2011 the skull of Gerrothorax was scanned using microtomography, revealing that the braincase and palatoquadrate regions are highly ossified. A 2013 study argued that Gerrothorax consumed prey using suction feeding. Gerrothorax had strong muscles capable of both raising the cranium and lowering the jaw rapidly. The robust internal gill apparatus would have expelled water through the gills during this motion, creating intense pressure in the throat that would suck in small prey items. The gill arches were also covered in small denticles, prohibiting any prey from escaping once devoured. Although suction feeding is common in fish and modern larval amphibians, Gerrothorax differs from these animals by its lack of cranial kinesis, meaning that its cranial bones could not flex against each other to envelop prey. The fossil record of Gerrothorax pulcherrimus extends 35 million years from the Ladinian stage of the Middle Triassic to the Rhaetian stage of the Late Triassic. Throughout this time span, specimens of the species show few morphologic differences, making G. pulcherrimus an extreme example of evolutionary stasis. G. pulcherrimus may have remained unchanged for so long because it could tolerate a wide range of ecological conditions. Although it always needed to live in an aquatic habitat, G. pulcherrimus may have been able to live in a variety of different water bodies with a wide range of salinity.
