Westlothiana

Westlothiana ("animal from West Lothian") is a genus of reptile-like tetrapods that lived about 341 million years ago during the latest part of the Viséan age of the Carboniferous. The genus is known from a single species, Westlothiana lizziae. It is the oldest known uncontroversial tetrapod, closely related to but not an amniote.

The type specimen was discovered in the East Kirkton Limestone at the East Kirkton Quarry, West Lothian, Scotland in 1984. In a study published in 2025 it was suggested that the specimen is around 341 million years old. This specimen was nicknamed "Lizzie the lizard" by fossil hunter Stan Wood, and this name was quickly adopted by other paleontologists and the press. When the specimen was formally named in 1990, it was given the specific name "lizziae" in homage to this nickname. However, despite its similar body shape, Westlothiana is not considered a true lizard. The anatomy of Westlothiana contained a mixture of both "labyrinthodont" and reptilian features, and was originally regarded as the oldest known reptile or amniote.

However, updated studies have shown that this identification is not entirely accurate. Instead of being one of the first amniotes (tetrapods laying hard-shelled eggs, including synapsids, reptiles, and their descendants), Westlothiana was rather a close relative of Amniota. As a result, most paleontologists since the original description place the genus within the group Reptiliomorpha, among other amniote relatives such as diadectomorphs and seymouriamorphs. Later analyses usually place the genus as the earliest diverging member of Lepospondyli, a collection of unusual tetrapods which may be close to amniotes or lissamphibians (modern amphibians like frogs and salamanders), or potentially both at the same time.

This species probably lived near a freshwater lake, and probably hunted for other small creatures that lived in the same habitat. It was a slender animal, with rather small legs and a long tail. Together with Casineria, another transitional fossil found in Scotland, it is one of the smallest reptile-like amphibians known, being a mere 20 cm in adult length. The small size has made it a key fossil in the search for the earliest amniote, as amniote eggs are thought to have evolved in very small animals. It shares many features with reptiles and other early amniotes rather than most amphibious tetrapods of its age. These include a lower number of ankle bones, no labyrinthodont infolding of the dentin, the lack of an otic notch, and a generally small skull. Many of these features are also present in lepospondyls. Ruta et al. (2003) interpreted the long body and small legs as a possible adaption to burrowing, similar to that seen in modern skinks.

Members of Westlothiana were heavily scaled, with thin scales on the belly and many rows of thick, overlapping scales on the back.

The skull was small compared to the overall body length, and although flattened in both known specimens, many components are visible. Components that were initially hidden by the crushing, such as the palate (roof of the mouth), were later revealed by further preparation and X-ray scans. The orbits (eye sockets) were quite large, each filled with a scleral ring. The skull was fairly broad, but not as shallow as in more basal tetrapods. The teeth are all the same size, unlike the case in many early amniotes which have larger fang-like teeth in the middle of the mouth. In addition, they lack "labyrinthodont" internal folding, as with lepospondyls and amniotes, but unlike the case with larger reptiliomorphs. However, this trait may be correlated with size, and not necessarily relations. The lower jaw was similar to that of amniotes in terms of the pattern of component bones. The quadrate bone of the jaw joint is vertical, rather than slanted as in basal reptiliomorphs such as seymouriamorphs and embolomeres.

The bones of the skull roof (the upper part of the skull, behind the eyes) were characteristic in several ways. They were loosely attached to those of the cheek region (namely the squamosal bones), as with amniotes and most reptiliomorphs, except for basal groups in which this area retains a large otic notch rather than contact. The skull roof was primarily composed of the characteristically large and wide parietal bones, which extended to the outer edge of the skull roof, an area known as the temporal region of the skull. The rear edge of the skull roof is formed by three pairs of bones, the postparietals, tabulars, and supratemporals (listed from inwards to outwards). The postparietals are wide, the tabulars are small and square-shaped, and the supratemporals are narrow. This inward-to-outward series is similar to protorothyridids but very different from the condition in microsaurs. The possession of supratemporals is rare among lepospondyls, but it does occur in urocordylids and a few early aistopods, in which the shape of this bone is similar to that of Westlothiana. Each supratemporal contacts the elongated postorbital bone behind the orbit, therefore cutting off the parietals from the squamosals. This contrasts with sauropsids (reptiles), but is similar to the condition in diadectomorphs and early synapsids. Microsaurs do not possess supratemporals, but in most members of the group, their large tabular bones disable parietal-squamosal contact. In more basal reptiliomorphs, this issue did not occur because an additional bone known as an intertemporal was present at the intersection of the four bones. Most reptiliomorphs which lost the intertemporal filled the space using a 'lappet' of the parietal bones. However, in Westlothiana, Limnoscelis, and lepospondyls, this space is filled by an expansion of the rear branch of the postorbital bone. This provides evidence for the placement of Westlothiana into lepospondyls.

One of Westlothiana's autapomorphies (unique features) of the skull is the fact that the postfrontal bones, which typically occupy the upper rear corner of the orbits, are very elongated. They stretch forward to form almost the entire upper rim of the eye sockets, and possibly contact the prefrontal bones at the front edge of each socket. This level of contact is uncertain, and may be so slight that it can occur on one side of the skull and not the other. Nevertheless, the possession of any amount of contact between the prefrontal and postfrontal is a primitive feature lost by true amniotes.