Diplocaulus

Diplocaulus, whose name translates to "double stalk," was a genus of lepospondyl amphibians that existed from the Late Carboniferous through the Late Permian in what is now North America and Africa. It stands out as the largest and most thoroughly studied lepospondyl, famous for its boomerang-shaped skull. Fossils from the Late Permian of Morocco, attributed to this genus, mark the most recent known appearance of any lepospondyl.

The animal had a stout, salamander-like build and could grow to about one meter (3.3 feet) in length. No complete tail has been found, but a nearly whole skeleton described in 1917 included a row of tail vertebrae positioned near the skull, suggesting that when curled up, the tail was long and thin enough to reach the head. Since that discovery, most researchers have concluded that Diplocaulus and its relatives moved primarily through eel-like undulations of the tail.

The most striking feature of Diplocaulus and its close kin was a pair of long, horn-like projections at the back of the skull, giving the head its boomerang shape. The outer and front edges of each horn were formed by an elongated, blade-like squamosal bone, while the rear edge consisted of the postparietal bones (once called dermosupraoccipitals). The main bone of each horn, including the tips, has been debated. Early sources often identified it as a tabular bone—typically a small bone at the rear of the skull in other early tetrapods. In 1951, Olson argued it could not be a tabular because it contacted the parietals, suggesting instead that it was an enlarged supratemporal bone that had shifted backward. Beerbower countered in 1963 by pointing out that in *Urocordylus*, a newt-like relative, both a supratemporal and a tabular bone are present, and the tabular contacts the parietals, undermining Olson's objection. This evidence makes it more likely that the horn's primary bone is a tabular, and many later studies (including a later paper by Olson) refer to them as tabular horns.

Several species are recognized. *D. salamandroides* was the first discovered, based on fossils found near Danville, Illinois, by geologists William Gurley and J.C. Winslow. Edward Drinker Cope described these remains in 1877, noting that the vertebrae resembled those of salamanders (hence the name *salamandroides*), though he hesitated to assign them to any known group. A large jawbone with labyrinthodont teeth was found with some vertebrae, but it was too big for them and likely belonged to *Eryops* or another large amphibian. This species was smaller than *D. magnicornis*—about one-fifth to one-sixth its size—and had less pronounced accessory articular processes. The fossils came from rocks informally called the "Clepsydrops shales," named after a local synapsid genus. Initially thought to be Permian or Triassic, the site was later considered Permian by Cope in 1878. In 1908, E.C. Case noted fish remains from the late Carboniferous and early Permian, arguing the shales could be Carboniferous. Today, these shales are assigned to the McLeansboro or Mattoon formations, and *D. salamandroides* fossils have also been found in Pennsylvania, with these formations now dated to the Missourian (late Carboniferous).

*D. magnicornis*, described by Cope in 1882, is the most common and well-known species. It was the first species known from more than just vertebrae, revealing the bizarre, long-horned nature of *Diplocaulus*. Most modern knowledge of the genus comes from this species, which outnumbers all other *Diplocaulus* remains by hundreds of specimens. Its fossils are widespread in the red beds of Texas and Oklahoma.

*D. brevirostris* resembled *D. magnicornis* but was much rarer, known from a few specimens in a narrow part of the Texas red beds, specifically the Arroyo Formation of the Clear Fork Group. It differs from *D. magnicornis* in having a shorter, blunter snout relative to skull length, more elongated horns, convex upper surfaces on the parietals, and a more strongly and smoothly curved rear skull edge. Although juvenile *D. magnicornis* also have a smoothly curved rear skull edge, all known *D. brevirostris* specimens are clearly adults, based on robust skull ornamentation, long horns, and large size, making this trait a valid distinguishing feature for adults of this species.