Ossinodus is an extinct genus of stem tetrapod. Fossils have been found from the Ducabrook Formation in Queensland, Australia dating back to the middle Visean stage of the Early Carboniferous (Mississippian). It was originally placed within the family Whatcheeriidae, but the absence of an intertemporal bone as suggested by a recent reconstruction of the skull based on fragmentary material may prove it to be stemward of all whatcheeriids.
The oldest known pathological bone of a tetrapod, a fractured right radius, has been referred to Ossinodus.
History and discovery
Ossinodus is one of the first large tetrapods discovered during the time known as Romer’s Gap. In 1996, initial inspection of scraps and materials postulated three possible types of tetrapods. A premaxilla with a tusk was originally attributed to a colosteid, then an ilium with a postiliac process linked it to an anthracosaur, or possibly a temnospondyl. Described by Turner and Warren, all Ossinodus bones were discovered together at one site, ranging in all sizes. The recovered remains were deemed to belong in the same group O. pueri due to common ornamentation on the dermal cranial bones. The specimens were found at the Drummond Basin of Queensland, Australia, in the Middle Paddock site of the Duckabrook Formation.
Estimated to be about 333 million years old, the Middle Paddock site is representative of “a shallow fluviatile or lacustrine environment,” which is a large body of water with sedimentary deposits made of slits, clays, and carbonates. The tetrapodal remains found here date back to Late Visean (early Carboniferous), towards the end of Romer’s Gap - a gap in the tetrapod fossil record. The excavation began in 1996 and ended in 2004, concluding with an extensive collection dis-articulated bones as well as half a skull. The remains found during the time period are significant in suggesting that terrestrial tetrapods first inhabited Gondwana, and were not of small European forms.
According to Warren, there were at least five specimens collected at the site, ranging in all sizes. The diversity of sizes indicated that the collection consisted of adults as well as juveniles.
Description
Ossinodus are comparable in mass to modern Chinese giant salamanders. Compared to many other tetrapods of the late Devonian, it was estimated that they were 1–2 meters in length. Dentition and habitat suggests that Ossinodus’ were piscivores considering habitat and teeth shape.
Skull
Ossinodus skulls are broad and heavily ornamented. Ornamentations include deep pits and ridges all over the skull, on the parietal bones around a raised parietal foramen, as well as the jaw. The pits found on the maxilla and premaxilla are smaller and finer than those found on the skull table. Ossinodus differ from sister taxon Pederpes and Whatcheeria as their pits are larger than the pits on Pederpes, with Whatcheeriidae mostly lacking dermal pitting.
On the outer surface of the skull, there is no apparent external sensory sulci. But upon closer inspection, the sensory canal is enclosed in the quadratojugal and only visible posteriorly along the inner surface. The choana is seen on the anterior inner surface of the maxilla. Orbits are small and placed anteriorly. Nostrils are located fairly low on the lateral side of the skull.
The presence of an elongate supratemporal-postparietal contact played a role in eliminating anthracosaur as a possibility. As shared with other tetrapods, Ossinodus tabulars protrude slightly posteriorly and are also quite short. Sutures between the tabular-supraorbital area and squamosal suggest some motility to the posterior cheek.
Dentition
Ossinodus have around 34 teeth along the maxilla as well as an inner row of teeth found on the palatine, vomer, and ectopterygoid. The maxillary fangs are the largest teeth for Ossinodus. The teeth are distally-keeled in the shape of the head of a lance and contain grooves indicative of dentine infolds. From the broken tooth, the tooth were polyplocodont - free of a pulp cavity with "ortho-dentine" folded simply and irregularly to the first degree. Ossinodus teeth are also characterized as having dark dentine along the first fold.
Postcranial skeleton
Most of the postcranial skeleton, other than the neural arches, are smaller in size compared to the skull.