Westlothiana ("animal from West Lothian") is a genus of reptile-like tetrapods that lived about 338 million years ago during the latest part of the Viséan age of the Carboniferous.

This specimen was nicknamed "Lizzie the lizard" by fossil hunter Stan Wood, and this name was quickly adopted by other paleontologists and the press.

The anatomy of Westlothiana contained a mixture of both "labyrinthodont" and reptilian features, and was originally regarded as the oldest known reptile or amniote.

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.

[4] 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.

[5][6] Later analyses usually place the genus as the earliest diverging member of Lepospondyli,[7] 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.

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.

Ruta et al. (2003) interpreted the long body and small legs as a possible adaption to burrowing, similar to that seen in modern skinks.

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 rear edge of the skull roof is formed by three pairs of bones, the postparietals, tabulars, and supratemporals (listed from inwards to outwards).

[4] 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.

[5] 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.

The palate lacks "labyrinthodont" features such as large fangs, and instead the pterygoids are completely covered with small tooth-like prickles known as denticles.

Further preparation of the skull to reveal this feature of the palate is one of the primary reasons why Westlothiana's classification was reworked to belong outside Amniota.

The pleurocentra are spool-shaped cylinders, pierced by a large canal for the spinal cord and fused to low, triangular neural spines.

The rear portion of the puboischiadic plate is particularly elongated in Westlothiana compared to in other reptiliomorphs, but the other parts of the pelvis are fairly normal by the group's standards.

Also similar to the humerus, the femur's crests and roughened areas for muscle attachment are large considering the small size of the animal.

The other proximal tarsal, the astragalus (or talus), is likely a mass formed by the fusion of the intermedium, tibiale, and the fourth (and possible also the third) centrale.

Smithson (1989) reported that "Lizzie" had nine ankle bones, including an astragalus and calcaneum, and that the third and fourth centrales were already fused or lost.

[3] Smithson et al. (1993) later claimed that there were ten bones, arguing that the components of the astragalus were not completely fused, with the intermedium and tibiale still separate.

[4] Piñeiro, Demarco, & Meneghel (2016) could not determine which of these two interpretations were superior, but did note that the largest bone more closely resembled a fused astragalus rather than an unfused intermedium.

[6] The actual phylogenetic position of Westlothiana is uncertain, reflecting both the fragmentary nature of the find and the uncertainty of "labyrinthodont" phylogeny in general.