''Ossirarus'' is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of four-limbed

stem-tetrapod The Stem Tetrapoda are a cladistically defined group, consisting of all animals more closely related to extant four-legged vertebrates than to their closest extant relatives (the lungfish), but excluding the crown group Tetrapoda. They are thus ...

from the Mississippian (mid-

Tournaisian The Tournaisian is in the ICS geologic timescale the lowest stage or oldest age of the Mississippian, the oldest subsystem of the Carboniferous. The Tournaisian age lasted from Ma to Ma. It is preceded by the Famennian (the uppermost st ...

) of

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

. It contains a single species, ''Ossirarus kierani'', based on a disarticulated skull and postcranial bones from the

Ballagan Formation The Ballagan Formation is a Formation (geology), geologic formation in Scotland and England. It preserves fossils dating back to the early part of the Carboniferous Period (geology), period (Tournaisian – early Visean). Its name comes from the ...

at Burnmouth. It would have had a large, pointed tabular horn and several minor traits shared with

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

stem-tetrapods. ''Ossirarus'' was described in a 2016 study which was devised to fill in the tetrapod and stem-tetrapod faunas of

Romer's gap Romer's gap is an apparent gap in the Paleozoic tetrapod fossil record noted in the studies of paleontology and evolutionary biology, which represent periods in the Early Carboniferous from which excavators have not yet found relevant transition ...

, an interval of the early

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

with few tetrapod fossils. It was one of five new genera named in this study, along with ''

Aytonerpeton ''Aytonerpeton'' is an extinct genus of stem-tetrapod from the Ballagan Formation of Scotland. It was one of five new genera of early limbed vertebrates from the Ballagan Formation described by Clack ''et al.'' in 2016. These vertebrates were amo ...

'', ''

Diploradus ''Diploradus'' is an extinct genus of four-limbed stem-tetrapod from the Mississippian (mid-Tournaisian) of Scotland. It contains a single species, ''Diploradus austiumensis'', based on an incomplete skull and jaw fragments from the Ballagan F ...

'', '' Koilops'', and ''

Perittodus ''Perittodus'' is an extinct genus of four-limbed stem-tetrapod from the Mississippian (mid-Tournaisian) of Scotland. It contains a single species, ''Perittodus apsconditus'', based on disarticulated skull and postcranial bones from the Ballagan ...

''.

Discovery

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

and only known specimen of ''Ossirarus'', UMZC2016.3, was collected in 2010 from a mid-Tournaisian layer of cliffs in Burnmouth, in Scotland's

. The strata belong to

Romer's Gap Romer's gap is an apparent gap in the Paleozoic tetrapod fossil record noted in the studies of paleontology and evolutionary biology, which represent periods in the Early Carboniferous from which excavators have not yet found relevant transition ...

, a hiatus between the

Late Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at million years ago ( Ma), to the beginning of the succeeding ...

and the

Viséan The Visean, Viséan or Visian is an age in the ICS geologic timescale or a stage in the stratigraphic column. It is the second stage of the Mississippian, the lower subsystem of the Carboniferous. The Visean lasted from to Ma. It follows ...

where comparatively few known tetrapod fossils are known. The specimen was found in a layer of light gray sandy

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

immediately above a gleyed palaeosol, located above the base of the formation. 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 the

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

. Postcranial material includes

vertebral centra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

neural arch Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

and

rib In vertebrate anatomy, ribs () are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the thoracic cavity, enabling the lungs to expand and thus facilitate breathing by expanding the ...

fragments, the

pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists o ...

, both humeri and the left

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

. Fragmentary scales are also preserved. The specimen was presumed to be an adult from the presence of interdigitated sutures. The species was formally described in 2016. The generic name ''Ossirarus'' comes from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

, and , while the specific epithet ''kierani'' honors two members of the local Burnmouth community who supported the study.

Description

A unique characteristic of ''Ossirarus'' is the presence of a large, pointed tabular horn. ''Ossirarus'' also exhibits features similar to stem-amniotes, such as a contact between the tabular and parietal bones, and separation between the exoccipital and basioccipital bones. At the same time, more primitive features are also present, like an

intertemporal bone The intertemporal bone is a paired Skull, cranial bone present in certain Sarcopterygii, sarcopterygians (lobe-finned fish) and extinct amphibian-Evolutionary grade, grade tetrapods. It lies in the rear part of the skull, behind the eyes. Many lin ...

At an estimated of skull length and of total length, ''Ossirarus'' was much smaller than Late Devonian and Tournaisian tetrapods like '' Pederpes'', and more comparable in size to later

genera from East Kirkton Quarry such as '' Balanerpeton''. Unlike the terrestrial East Kirkton tetrapods, ''Ossirarus'' was more adapted for an aquatic or amphibious life and less reliant on vision, having

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelia ...

canals in the skull as well as proportionally smaller orbits.

Ecology

The deposit in which ''Ossirarus'' was found belonged to a

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

environment, more specifically a

marsh In ecology, a marsh is a wetland that is dominated by herbaceous plants rather than by woody plants.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p More in genera ...

or seasonal water body evaporating during the dry season. ''Ossirarus'' shared its environment with the stem-tetrapod ''

'', as well as rhizodonts,

actinopterygians Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin ...

chondrichthyans Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeletons ...

and

lungfish Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...

. Atmospheric oxygen was stable above 16%.

Phylogeny

A provisional 2024 phylogenetic study recovered ''Ossirarus'' as a stem-tetrapod, placing it crownward of most

genera, but branching before the Devonian '' Ymeria'', '' Tulerpeton'' and '' Brittagnathus''.

References

Works cited

* * {{Taxonbar, from1=Q113518022, from2=Q97165631 Stegocephali Tournaisian life Mississippian sarcopterygians of Europe Fossil taxa described in 2016 Taxa named by Jenny Clack