| Ossirarus | |
|---|---|
 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Stegocephali |
| Genus: | †Ossirarus Clacket al., 2016 |
| Type species | |
| †Ossirarus kierani Clacket al., 2016 | |
Ossirarus is anextinctgenus of four-limbedstem-tetrapod from theMississippian (mid-Tournaisian) ofScotland. It contains a single species,Ossirarus kierani, based on a disarticulated skull and postcranial bones from theBallagan Formation atBurnmouth. It would have had a large, pointedtabular horn and several minor traits shared withDevonian stem-tetrapods.Ossirarus was described in a 2016 study which was devised to fill in the tetrapod and stem-tetrapod faunas ofRomer's gap, an interval of the earlyCarboniferous with few tetrapod fossils. It was one of five new genera named in this study, along withAytonerpeton,Diploradus,Koilops, andPerittodus.[1]
Theholotype and only known specimen ofOssirarus, UMZC2016.3, was collected in 2010 from a mid-Tournaisian layer of cliffs inBurnmouth, in Scotland'sBallagan Formation.[2] The strata belong toRomer's Gap, a hiatus between theLate Devonian and theViséan where comparatively few known tetrapod fossils are known.[1] The specimen was found in a layer of light gray sandysiltstone immediately above agleyedpalaeosol,[3] located 340.5 metres (1,117 ft) above the base of the formation.[4]
The holotype consists of multiple cranial and postcranial elements. Skull bones are disarticulated, and around two-thirds of the skull length has been preserved. The preorbital region is missing, as well as the jawbones, with the exception of a fragment belonging to either themaxilla orpremaxilla.[5] Postcranial material includesvertebral centra,neural arch andrib fragments, thepectoral girdle, bothhumeri and the leftradius.[6] Fragmentary scales are also preserved.[7] The specimen was presumed to be an adult from the presence of interdigitatedsutures.[4]
The species was formally described in 2016. The generic nameOssirarus comes fromLatinossi, 'bones', andrarus, 'rare, scattered', while the specific epithetkierani honors two members of the local Burnmouth community who supported the study.[4]
A unique characteristic ofOssirarus is the presence of a large, pointedtabular horn.[4]Ossirarus also exhibits features similar to stem-amniotes, such as a contact between the tabular andparietal bones, and separation between theexoccipital and basioccipital bones. At the same time, more primitive features are also present, like anintertemporal bone.[8]
At an estimated 54 millimetres (2.1 in) of skull length[5] and 30 centimetres (12 in) of total length,Ossirarus was much smaller than Late Devonian and Tournaisian tetrapods likePederpes, and more comparable in size to laterViséan genera fromEast Kirkton Quarry such asBalanerpeton. Unlike the terrestrial East Kirkton tetrapods,Ossirarus was more adapted for an aquatic or amphibious life and less reliant on vision, havinglateral line canals in the skull as well as proportionally smaller orbits.[9]
The deposit in whichOssirarus was found belonged to afloodplain environment, more specifically amarsh or seasonal water body evaporating during the dry season.[10]Ossirarus shared its environment with the stem-tetrapodAytonerpeton, as well asrhizodonts,actinopterygians,chondrichthyans andlungfish. Atmospheric oxygen was stable above 16%.[4]
A provisional 2024 phylogenetic study recoveredOssirarus as a stem-tetrapod, placing it crownward of mostDevonian genera, but branching before the DevonianYmeria,Tulerpeton andBrittagnathus.[11]
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