The Arroyo Formaton (Leonardian: Lower Permian) and Its Vertebrate Fossils

Studies of the Arroyo formation from northern Haskell County south to Runnels County, Texas trace the changes in organisms and environments from the classic terrestrial beds of Baylor and Wilbarger Counties to the fully marine sections to the south. Terrestrial deposits that contain vertebrate remains have been found only as far south as Haskell, Texas. Limestones, sampled and treated by acetic acid, have produced tetrapods to the level of Abilene, Texas. Among these are the commonest genera of the Arroyo, including several types of microsaurs. Notably absent is the highly terrestrial genus Captorhinus. Above the Lueders Limestone, the Arroyo section in the southern area consists of four marine limestones alternating with varied thicknesses of red mudstones, with small increments of sandstones and fine conglomerates. The distributions of the limestones and the elastics indicate four major transgressions of the sea, with a very irregular coastline during the peaks of transgression. The limestones have yielded a wide variety of fish, including xenacanthid sharks, various other Chondrichthyes, and dipnoan and palaeoniscoid Osteichthyes. Special attention is paid to the systematics, distributions, and habitats of the fish, with special emphasis upon the use of scale histology in taxonomic studies.