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.
D. magnicornis 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 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.
D. minimus 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.
Dubious species •
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. •
D. parvus, named by E.C. Olson in 1972, was designated as a new species with no connection whatsoever with "
Permoplatyops parvus", which at that point was treated as a synonym of
D. magnicornis.
D. parvus is known from a single specimen from the
Chickasha Formation of
Oklahoma. It was generally very similar to
D. recurvatus, differing primarily due to its smaller size as isolated geographical location. Germain (2010) did not consider these traits sufficient to justify retaining
D. parvus separate from
D. recurvatus. The
D. parvus specimen is potentially the youngest
Diplocaulus fossil recovered from North America, at about 270 million years old. == Paleobiology ==