Tiny Scottish Fossil Rewrites the Story of Early Land Vertebrates

Deep in the coastal cliffs at Burnmouth in Berwickshire, Scotland, a fossil no bigger than a human hand is quietly rewriting one of the most consequential chapters in vertebrate evolution. The creature, named Ossirarus kierani, lived roughly 350 million years ago, just after the end-Devonian extinction had wiped out most of the world’s early land vertebrates. Now, a full anatomical description published in the journal Fossil Record reveals that this diminutive animal carries an extraordinary mosaic of primitive and advanced traits, and that its place on the family tree of early tetrapods hints at hidden lineages stretching back into the Devonian, before the great extinction.

Ossirarus is known from a single specimen, catalogued as UMZC 2016.3 in the University Museum of Zoology, Cambridge. It was collected in 2010 from the Ross end of Burnmouth, 340.5 metres above the base of the Ballagan Formation, a rock unit laid down during the Tournaisian stage of the earliest Carboniferous. The bones lie scattered across the uneven surface of an ancient floodplain soil, preserved in a thin layer of clay deposited by a flooding event. The skeleton was evidently well rotted before burial: the skull bones have drifted apart, exposing the delicate areas of sutural overlap that are usually hidden in articulated fossils. That unusual preservation is a gift for anatomists, because the exposed overlap surfaces reveal exactly how each bone fitted against its neighbours.

The specimen comprises the disarticulated bones of the posterior half of the skull, the anterior portion of the axial skeleton, and parts of the pectoral girdle and forelimbs. The animal was small. Its estimated skull length is just 54 millimetres, and the whole creature was probably little more than 300 millimetres long. That makes Ossirarus strikingly different from many of its contemporaries. The whatcheeriid Pederpes, found in the same formation near Dumbarton, reached about a metre in length, and the monstrous Crassigyrinus, possibly represented at Burnmouth by an incomplete lower jaw, attained nearly two metres. Upper Devonian tetrapods were typically large animals too, some exceeding a metre. Ossirarus instead resembles in size the later Viséan tetrapods of East Kirkton, such as the temnospondyl Balanerpeton and the stem amniotes Silvanerpeton and Eldeeceon.

The skull itself preserves a wealth of technical detail. Much of the right cheek is intact, including a large jugal roughly 25 millimetres long that most closely resembles those of Acanthostega and the colosteids. Behind the jugal lies a roughly triangular preopercular, about 7 millimetres long, identified by the presence of a lateral line pore and the pattern of sutural overlap. The skull table preserves the parietal, postparietal, postfrontal, intertemporal, supratemporal and tabular. The tabular is a large triangular bone whose posterolateral edge extends well beyond the corner of the skull to form a conspicuous tabular horn with a convex lateral margin, the feature that serves as the defining autapomorphy of the species.

One of the most significant discoveries in the new description concerns the relationships among the skull roofing bones. The pattern of sutural overlap along the medial edge of the tabular shows that Ossirarus possessed a tabular-parietal suture, a contact between these two bones that is shared with embolomeres, a group of large later Carboniferous tetrapods. The authors report that this is the earliest record of the feature in early tetrapods. Ossirarus also retains an intertemporal, a bone lost in many more derived lineages, and its lateral line system is represented by fine pores in the jugal, postorbital, postfrontal and preopercular rather than open canal grooves. The pores suggest the animal was aquatic or amphibious, an interpretation reinforced by one notable difference from the East Kirkton tetrapods: its orbits are much smaller, implying it relied less on vision for prey capture.

The postcranial skeleton is equally revealing. The vertebrae are multipartite, built from two central elements and neural arches in unfused bilateral halves, surrounding a persistent notochord about 6 millimetres in diameter. The centra form thin crescentic husks of bone less than a millimetre thick, and two centra fit beneath each neural arch, one occupying the pleurocentral position and the other the intercentral position. This gastrocentrous arrangement, in which the intercentrum contributes to the ventral centrum, resembles the vertebrae of Caerorhachis, and the authors describe the Ossirarus centra as probably the earliest known example of this condition in early tetrapods.

The pectoral girdle and forelimbs carry further phylogenetic signals. The cleithrum is a long narrow bone about 30 millimetres long, with a curved stem making up two thirds of its length and a small angular dorsal blade, unlike the broader cleithra of most other early tetrapods. The interclavicle is diamond-shaped, about 36 millimetres long, and lacks a parasternal process. Both humeri, each about 17 millimetres long, show the characteristic L-shape of early tetrapods with a torsion of 20 to 25 degrees along the shaft. Crucially, the brachial foramen pierces the posterior edge of the humerus at the base of the entepicondyle, exactly as it does in Mesanerpeton, another Ballagan Formation tetrapod. The humerus also bears an elongate oblique pectoralis process comparable with the ventral humeral ridge of elpistostegalians and Acanthostega, a vestige of the fin-to-limb transition. The left radius is about 10 millimetres long, roughly 60 per cent the length of the humerus, a ratio comparable with Pederpes and Whatcheeria.

To determine where Ossirarus sits on the tetrapod family tree, the team coded the specimen into a data matrix of 64 taxa and 275 characters and ran maximum parsimony analyses in PAUP*, using equal weights, reweighted characters based on rescaled consistency indices, and Goloboff’s implied weighting with a concavity constant of 12. The equal-weights analysis produced 1140 most parsimonious trees of 1362 steps. Across all analyses, Ossirarus branches from the tetrapod stem as the most plesiomorphic of all the Ballagan Formation taxa, sitting immediately crownward of the Devonian Ventastega but anti-crownward of a diverse array of groups that includes all major post-Devonian clades as well as the Devonian Ymeria, Brittagnathus and Tulerpeton. In the equal-weights and reweighted analyses, Ossirarus forms the sister taxon of Ossinodus, an early Carboniferous tetrapod from Australia. The authors stress that the results should be regarded as provisional, since a comprehensive reassessment of character codings across early tetrapod phylogenies is still under way.

The broader topology of the tree carries a headline implication of its own. The six tetrapods named so far from the Ballagan Formation, including Ossirarus, Aytonerpeton, Koilops, Perittodus, Diploradus and Mesanerpeton, represent distinct and unrelated levels of morphological organisation, confirming that bodyplan diversity among the earliest Carboniferous tetrapods was far greater than formerly surmised. Moreover, the branching patterns suggest that numerous lineages of Carboniferous tetrapods extended back into the Devonian. Groups such as colosteids, baphetids and the clade comprising Aytonerpeton, Acherontiscus, adelospondyls, aïstopods and nectrideans may have originated in the latest Devonian or earliest Carboniferous, with the latter clade possibly dating back some 360 million years. Independent fossil evidence supports this: the baphetoid Spathicephalus pushes back baphetoid diversification by about three million years, and Crassigyrinus-like jaws and fibulae from Burnmouth and Blue Beach, Nova Scotia, may extend that genus from the late Viséan into the mid Tournaisian.

Ossirarus remains an enigma in some respects. Its seemingly reptiliomorph features, such as the tabular-parietal contact, failed to retrieve a phylogenetically more derived position, yet the animal is also primitive in retaining an elongate suspensorium, a preopercular, an intertemporal and a diamond-shaped interclavicle without a posterior process. The formal cladistic analysis backs its stem-tetrapod affinities, but the authors acknowledge that its placement requires additional scrutiny as they continue examining other Ballagan Formation tetrapods. What is already clear is that the recovery of vertebrates after the end-Devonian extinction was rapid, and that the early Carboniferous of the Scottish Borders was a period of innovation, diversification and evolutionary change, with small, strange creatures like Ossirarus preserving the echoes of lineages that had survived the extinction deep in time.

Subject of Research: On Ossirarus kierani, a stem tetrapod from the Tournaisian of Burnmouth, Berwickshire, Scotland, and the phylogeny of early tetrapods

Article Title: On Ossirarus kierani , a stem tetrapod from the Tournaisian of Burnmouth, Berwickshire, Scotland, and the phylogeny of early tetrapods

Article References: Smithson, T. R., Ruta, M., & Clack, J. A. (2024). On Ossirarus kierani, a stem tetrapod from the Tournaisian of Burnmouth, Berwickshire, Scotland, and the phylogeny of early tetrapods. Fossil Record, 27(3), 333-352. https://doi.org/10.3897/fr.27.126410

Image Credits: AI Generated

DOI: 10.3897/fr.27.126410

Keywords: Ossirarus, kierani, stem, tetrapod, Tournaisian, Burnmouth, Berwickshire, Scotland, phylogeny, early, tetrapods, scientific research