Diplocaulus

Extinct genus of amphibians

Diplocaulus (meaning "double stalk") 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.

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. 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.

This species, described by Cope in 1882, is by far the most common and well described member of the genus. D. magnicornis was the first species known from more than vertebrae, and it allowed Cope and other paleontologists to realize the nature of Diplocaulus as a bizarre long horned "batrachian" (amphibian). Much of modern knowledge on the genus is based on this species, as it outnumbers any other Diplocaulus remains by hundreds of specimens. D. magnicornis had a wide temporal distribution throughout the red beds of Texas and Oklahoma.

D. brevirostris was similar to D. magnicornis, although it was significantly more rare. It is represented by a small number of specimens found in an early strata of the Texas red beds, specifically the Arroyo Formation of the Clear Fork Group. This species can be differentiated from D. magnicornis by the much shorter and blunter snout compared to the length of the skull as a whole. In addition, the horns are more elongated, the parietals have a convex upper surface, and the rear edge of the skull is more strongly and smoothly curved. While juvenile members of D. magnicornis also have a smoothly curved rear edge of the skull, all known D. brevirostris specimens are clearly adults as shown by their robust skull ornamentation, long horns, and large size. Therefore, this trait is a legitimate distinguishing feature of adult specimens of this species. The only specimen known from more than a skull is the type specimen, AM 4470, which preserves some vertebrae similar to those of "D. primigenius". E. C. Olson, the original describer of the species, suggested that it occupied different habitats than D. magnicornis such as mountain streams, accounting for its comparative rarity. However, other studies have suggested that D. magnicornis would have lived in similar environments, invalidating Olson's hypothesis. D. recurvatus differs from D. magnicornis in one specific trait: the tips of the tabular horns are "crooked". The tips are bent relative to the rest of the horns, and abruptly taper. Comparison to a growth series of D. magnicornis indicates that D. recurvatus specimens had developmental pathways which significantly differed from D. magnicornis. For example, skull length and width seem to be inversely correlated in D. recurvatus and directly correlated in D. magnicornis.

Diplocaulus minimus is a species known from the Ikakern Formation of Morocco. It had an unusually asymmetrical skull, with the left prong being long and tapering as in other species but the right prong being much shorter and more rounded. This feature was present in multiple skulls referred to this species, so it is very unlikely to be a result of crushing or distortion. Some studies have suggested that this species is more closely related to Diploceraspis than to Diplocaulus magnicornus. This may suggest that either Diplocaulus is not a true monophyletic genus, that Diploceraspis is a junior synonym of the genus, or that "Diplocaulus" minimus represents a distinct genus.

D. limbatus was the third species of Diplocaulus to be named, and remained the second most well known member of the genus until the 1950s. It was described by E. D. Cope in 1895 based on several incomplete specimens found in the Texas red beds. The type specimen was a poorly preserved skull and partial skeleton designated AM 4471. Cope found that the skull of this specimen had shorter, thinner horns than those of D. magnicornis, as well as a seemingly unique feature: a large notch separating the quadratojugal from the rest of the tabular horn. E. C. Case later provided additional distinctions present in a skull referred to D. limbatus, including smoother edges to the skull, larger eyes, and more pointed horns. However, additional D. limbatus specimens prepared by Douthitt have shown that many of Case's identifications were erroneous, and that only the notch identified by Cope could be used to distinguish it from D. magnicornis. D. copei was known from three Texan specimens, all of which were heavily crushed and incomplete. Broili argued that this species was unique due to its small size and horns which bend inwards. However, E. C. Case could find no way to distinguish between its specimens and those of D. magnicornis and "D. limbatus", and he rejected the species as indeterminate, a decision followed by later sources. He brought up the possibility that the skulls were from an extremely young Diplocaulus, and in response Olson (1951) designated D. pusillus (and therefore Permoplatyops parvus) as a synonym of one of the other red bed Diplocaulus species, such as D. magnicornis. E. C. Olson (1951) noted that the vertebrae were comparable to those of the holotype of D. brevirostris (AM 4470), but also that the skull was much more akin to D. magnicornis instead. While Olson did decide to synonymize D. primigenius with D. magnicornis, he also noted that the specimen remained an interesting conundrum with implications for the disconnect between vertebral and skull development in Diplocaulus. but in 1911 E. C. Case pointed out that there was slim evidence for this idea.

A new hypothesis for the function of the horns was presented by South African paleontologist Arthur Cruickshank & fluid dynamicist B. W. Skews in a 1980 paper. They proposed that the tabular horns acted as a hydrofoil, allowing the animal to more easily control how water flows over its head. In the process of their investigation, Cruickshank & Skews developed a full scale model of the head and a portion of the body of a Diplocaulus, constructed from balsa wood and modelling clay. The model was placed in a wind tunnel, and subjected to several tests to determine drag, lift, and other forces experienced by the head in different situations.