Spathicephalus

Spathicephalus is an extinct genus of stem tetrapods (early four-limbed vertebrates) that lived during the middle of the Carboniferous Period. The genus includes two species: the type species S. mirus from Scotland, which is known from two mostly complete skulls and other cranial material, and the species S. pereger from Nova Scotia, which is known from a single fragment of the skull table. Based on the S. mirus material, the appearance of Spathicephalus is unlike that of any other early tetrapod, with a flattened, square-shaped skull and jaws lined with hundreds of very small chisel-like teeth. However, Spathicephalus shares several anatomical features with a family of stem tetrapods called Baphetidae, leading most paleontologists who have studied the genus to place it within a larger group called Baphetoidea, often as part of its own monotypic family Spathicephalidae. Spathicephalus is thought to have fed on aquatic invertebrates through a combination of suction feeding and filter feeding.

History of study
The type species of Spathicephalus, S. mirus, was named by paleontologist D. M. S. Watson in 1929. Watson described seven fossil specimens from an outcrop of the Rumbles Ironstone in the town of Loanhead in Midlothian, Scotland. The ironstone dates to the late Namurian stage (earliest Upper Carboniferous) and is part of the Limestone Coal Group. These specimens were discovered in the 1880s and include a mostly complete skull with the palate exposed, an impression of the underside of a skull roof, a right portion of the back of a skull, and various jaw fragments.[1] At the time, Spathicephalus and other tetrapods from the Namurian of Scotland were some of the oldest tetrapods known, predating the better-known Late Carboniferous tetrapod assemblages of the British Coal Measures. In November 1974, Scottish paleontologist Stanley P. Wood discovered additional skull and jaw fragments of Spathicephalus in an open-pit mine (the Dora Open Cast Mine) near the town of Cowdenbeath in Fife. Wood found these fossils in a layer of siltstone that is the same age as the ironstone in Loanhead.

American paleontologist Donald Baird named a second species of Spathicephalus, S. pereger, from Nova Scotia in 1962.[1] Baird named S. pereger on the basis of an impression of the right half of a skull table that collectors from the Harvard Museum of Comparative Zoology found on a beach between Point Edward and Keating Cove on Cape Breton Island. The impression was preserved in siltstone from the Point Edward Formation, which dates to the latest Upper Carboniferous (equivalent to the early Namurian in Europe), meaning that S. pereger predates S. mirus by several million years.

Description
Unlike most other early tetrapods which have rounded or pointed snouts, Spathicephalus mirus has a flattened, almost perfectly square-shaped skull up to 22 centimetres (8.7 in) in both width and length.[1] The squared shape is caused primarily by a widening of the paired nasal bones along the midline of the snout. The premaxilla bones at the tip of the snout form the entire front edge of the square, while the maxilla bones form the side edges. The maxillae of Spathicephalus are unusual in being thin (no more than 1 centimetre (0.39 in) in thickness) along their entire length. The orbits or eye sockets are large, facing directly upward and positioned close together near the back of the skull. Unlike the roughly circular orbits of most tetrapods, the orbits of Spathicephalus are kidney-shaped because they have fused with another pair of holes called the antorbital fenestrae (in other early tetrapods, the antorbital fenestrae lie in front of the orbits and are separate from them). The skull table behind the orbits is very small in comparison to those of other Carboniferous tetrapods, but like those of other tetrapods, it bears a small hole in its center called a pineal foramen. Small tabular "horns" extend laterally from the back of the skull table, forming the temporal notches. The surface of the skull roof is covered by raised bumps and ridges, a condition paleontologists E. H. Beaumont and T. R. Smithson describe as "pustular ornamentation".[1]

Another distinguishing feature of S. mirus is its dentition; it jaws are lined with hundreds of small, chisel-shaped, closely spaced teeth. These marginal teeth are each about 3 millimetres (0.12 in) in cross-sectional diameter and form an unbroken row along both the upper and lower jaws. While they point directly downward in the upper jaw, the marginal teeth slant slightly inward (mesially) in the lower jaw. Teeth on the vomer bones form a second parallel row on the palate that is not as extensive as the marginal tooth row. At about 1 millimetre (0.10 cm) in diameter, these teeth are smaller than the marginal teeth. The rest of the palate has subtle pustular ornamentation at a finer scale than on the skull roof. This is a unique condition among early tetrapods, many of which have more extensive ornamentation on the palate including bony denticles, additional tooth rows, and palatal tusks. The dentition of Spathicephalus is so unusual that isolated jaw fragments with teeth in them can easily be identified.[1]

Although the species S. pereger is only known by a partial skull roof, paleontologist Donald Baird assigned it to Spathicephalus with confidence because the shapes of its bones closely matched the corresponding bones in S. mirus. One of the few differences that separate S. pereger from S. mirus is the reticulate or "honeycomb" ornamentation on its skull roof, characterized by pits and grooves. The pustular ornamentation seen in S. mirus is in fact rare among early tetrapods (plagiosaurid temnospondyls and the very early tetrapod Ichthyostega are some of the few to have them), while the reticulate ornamentation of S. pereger is more typical.