Diplocaulus (meaning "double caul") is an extinct genus of lepospondyl amphibians which lived from the Late Carboniferous to the Late Permian of North America and Africa. Diplocaulus is by far the largest and best-known of the lepospondyls, characterized by a distinctive boomerang-shaped skull. Remains attributed to Diplocaulus have been found from the Late Permian of Morocco and represent the youngest-known occurrence of a lepospondyl.
Description
Diplocaulus had a stocky, salamander-like body, but was relatively large, reaching up to in length. Although a complete tail is unknown for the genus, a nearly complete articulated skeleton described in 1917 preserved a row of tail vertebrae near the head. This was construed as circumstantial evidence for a long, thin tail capable of reaching the head if the animal was curled up. Most studies since this discovery have argued that anguiliform (eel-like) tail movement was the main force of locomotion utilized by Diplocaulus and its relatives.
Horns
The most distinctive features of this genus and its closest relatives were a pair of long protrusions or horns at the rear of the skull, giving the head a boomerang-like shape. Most of the outer/front edge of each horn was formed by the elongated, blade-like squamosal bone. The rear edge of the skull and horns, on the other hand, was formed by the postparietal bones, also known as dermosupraoccipitals in older publications. However, the primary component of each horn (including the tips) is a long bone with a historically controversial identification. Many early sources considered the bone to be a tabular, which in other early tetrapods is a small bone lying at the rear edge of the skull. However, Olson (1951) doubted this, arguing that the bone's contact with the parietals excluded the possibility of it being a tabular. He argued that the bone was the supratemporal bone, which had enlarged and shifted towards the rear tip of the skull. Beerbower (1963) countered Olson's reasoning by pointing out that Urocordylus, a newt-like relative of Diplocaulus, retained both a supratemporal bone and a tabular bone. In Urocordylus, the tabular lies closer to the back of the skull and even contacts the parietals, invalidating Olson's main point. Based on this observation, it is more likely that the primary bone of the horns in Diplocaulus is a tabular. Many studies (even a later publication by Olson) now refer to Diplocaulus horns as tabular horns based on Beerbower's argument.
Species
D. salamandroides
D. salamandroides was the first species of Diplocaulus to be discovered. Remains from this species were discovered near Danville, Illinois by William Gurley and J.C. Winslow, a pair of local geologists. The fossils were later described by renowned paleontologist Edward Drinker Cope in 1877. This species is only known from a small number of vertebrae sent to Cope by Gurley and Winslow. These vertebrae were noted for their similarities to those of salamanders (hence the specific name salamandroides), although Cope was reluctant to refer them to any known group. A large jaw bone with labyrinthodont teeth was associated with some of these vertebrae, but it was much larger than expected for the vertebrae and likely belonged to Eryops or some other larger amphibian. D. salamandroides could be distinguished from D. magnicornis by its small size (from a fifth to a sixth the size of the latter) and less pronounced accessory articular processes (at the time identified as zygosphene-zygantrum articulations).
The rocks in which these fossils were discovered had been informally referred to as the "Clepsydrops shales", named after a local genus of early synapsid by Cope in 1865. The shales were initially believed to be from either the Permian or Triassic periods in age based on the purported presence of reptile and lungfish fossils. By 1878, Cope had decided that the site was Permian. In 1908, E.C. Case noted that the shales also contain remains from fish which were from the late Carboniferous and early Permian periods. He argued that, while the Clepsydrops shales of Illinois and the similar red beds of Texas were evidently formed after the major Carboniferous coal deposits, there was not sufficient evidence to exclude them from the Carboniferous period itself. Nowadays the Clepsydrops shales are typically assigned to the McLeansboro or Mattoon Formations. D. salamandroides fossils have also been found in Pennsylvania. These formations are now believed to be Missourian (late Carboniferous) in age.