Diplocaulus facts-Jurassic Kratts

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## Facts

***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.

*Diplocaulus* had a stocky, salamander-like body, but was relatively large, reaching up to 1 m (3.3 ft) 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[edit]

### *D. salamandroides*[edit]

*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.

## Paleobiology

### Function of the tabular horns

Various hypotheses have been put forth to the purpose of these horns. One of the earliest suggestions, provided by S.W. Williston in 1909, was that they protected external gills, but in 1911 E.C. Case pointed out that there was slim evidence for this idea. Another hypothesis was provided in a dissertation, published by University of Kansas professor Herman Douthitt in 1917, which focused entirely on the anatomy of *Diplocaulus*. Douthitt argued that the most undisputed function was that the horns acted as a counterweight to offset the heavily-built forward part of the head which would have been difficult to lift otherwise. However, he also noted that this was probably not their primary function, and that they may have been maladaptive developments "as the result of some internal metabolic derangement".

In 1951, E.C. Olson suggested that the horns could have supported skin flaps capable of assisting the animal in skate- or stingray-like locomotion. However, he admitted that his suggestion was entirely conjectural considering a lack of soft tissue evidence. He also briefly proposed other possible functions, such as the use of the broad head as a burrowing tool to escape predators or survive droughts. J.R. Beerbower revived the hypothesis that the horns were involved in respiration during his 1963 description of *Diploceraspis*, which was a close relative of *Diplocaulus*. His argument relied on the possibility that the horns supported operculum-like vertical pouches protecting external or internal gills. One possibility is that the shape was defensive, since even a large predator would have a hard time trying to swallow a creature with such a wide head.

#### Lift

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.

The results showed that the horns generated significant lift, which would have allowed the animal to rise in the water column of a river or stream quite quickly and easily. Lift was present when the head was parallel to the flow of water (modeled by air), with lift increasing at a higher attack angle (angle above the horizontal) and only dropping once the head reached a high stall angle of 22 degrees. Lift and pitching moment was minimized at 1.5 degrees below the horizontal, which may have been the natural resting angle of the head.

When the "mouth" of the model was opened, lift was barely affected, the pitching moment decreased, and drag only slightly increased. This indicates that *Diplocaulus* would not have been seriously disadvantaged if they chose to attack prey items while rising through the water. Cruickshank & Skews also glued numerous small spheres to the model in order to test how an irregular texture would affect the mechanics of the head. The highly irregular spheres drastically reduced lift and increased drag, but when they were rubbed off (leaving only the slightly irregular glue layer), the only major reduction in aerodynamic quality (compared to the smooth model) was that the stall angle decreased to 16 degrees. The study also inquired about the hydrodynamics of *Diploceraspis*, which lacked a flange on the underside of the horns which was present in *Diplocaulus*. When the flange was removed from the smooth model, the resulting lift forces started being generated at a lower angle, 6 degrees below the horizontal rather than 1.5. This may indicate that *Diploceraspis* was better adapted for slower streams, where immediate lift was prioritized over the more gradual lift created by the *Diplocaulus* model, which would have been able to take advantage of a swifter current.

## Paleoecology

A trio of three juvenile *Diplocaulus* in a burrow of eight (plus one juvenile *Eryops*) were found to have been partially eaten by the sail-backed synapsid *Dimetrodon*, which likely unearthed the amphibians during a drought. One of the three was killed with a bite to the head, taking part of its skull and portions of the brain, a fatal injury that the animal could not defend against.