The ankle of Westlothiana was similar (but probably not identical) to that of basal amniotes, rather than amphibians.
In modern amphibians and most non-amniote tetrapods, the ankle is formed by twelve bones.
Five small, rounded ankle bones, known as "distal tarsals", connect to the metatarsal bones which each lead to a toe.
Three somewhat larger ankle bones, known as "proximal tarsals", connect to the bones of the lower leg.
These three are the medium-sized fibulare (which connects to the fibula), the small tibiale (which connects to the tibia), and the quite large intermedium which lies in the middle and contacts both the fibula and tibia.
Four small numbered bones known as "centralia" fill in the gaps between the proximal and distal tarsals.

In basal amniotes, the condition is quite different, with the ankle only formed by eight bones.
The five distal tarsals are typically unchanged, but usually only a single centrale is preserved.
This single remaining centrale, referred to as the "navicular" bone, was either formed by the retention of only the second centrale or the fusion of the first and second.
Amniotes are also very different from amphibians in the fact that they possess only two large proximal tarsals.
The calcaneum, which contacts the fibula, is almost unanimously considered to be identical to the fibulare.
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.
Some lepospondyls, such as the reptile-like microsaur Tuditanus, also may have evolved these modifications.

The ankle of Westlothiana, as well as that of most other "reptiliomorphs", is intermediate between these two conditions.
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.
Yet he also noted that centrales one and two (which formed the navicular bone) were unfused.
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.
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.

The foot was most likely five-toed, with a phalangeal formula (the number of phalanges per toe from the innermost to outermost toe) of 2-3-4-5-4.
This formula is identical to that of early amniotes, but conversely the foot of Westlothiana is shorter and more robust than the long-toed feet of amniotes.
Additionally, the phalanges decrease in relative size towards the tip of the foot in this genus, while the opposite is true of amniotes.
The ankle of Westlothiana was similar (but probably not identical) to that of basal amniotes, rather than amphibians. In modern amphibians and most non-amniote tetrapods, the ankle is formed by twelve bones. Five small, rounded ankle bones, known as "distal tarsals", connect to the metatarsal bones which each lead to a toe. Three somewhat larger ankle bones, known as "proximal tarsals", connect to the bones of the lower leg. These three are the medium-sized fibulare (which connects to the fibula), the small tibiale (which connects to the tibia), and the quite large intermedium which lies in the middle and contacts both the fibula and tibia. Four small numbered bones known as "centralia" fill in the gaps between the proximal and distal tarsals.
In basal amniotes, the condition is quite different, with the ankle only formed by eight bones. The five distal tarsals are typically unchanged, but usually only a single centrale is preserved. This single remaining centrale, referred to as the "navicular" bone, was either formed by the retention of only the second centrale or the fusion of the first and second. Amniotes are also very different from amphibians in the fact that they possess only two large proximal tarsals. The calcaneum, which contacts the fibula, is almost unanimously considered to be identical to the fibulare. 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. Some lepospondyls, such as the reptile-like microsaur Tuditanus, also may have evolved these modifications.
The ankle of Westlothiana, as well as that of most other "reptiliomorphs", is intermediate between these two conditions. 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. Yet he also noted that centrales one and two (which formed the navicular bone) were unfused. 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. 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.
The foot was most likely five-toed, with a phalangeal formula (the number of phalanges per toe from the innermost to outermost toe) of 2-3-4-5-4. This formula is identical to that of early amniotes, but conversely the foot of Westlothiana is shorter and more robust than the long-toed feet of amniotes. Additionally, the phalanges decrease in relative size towards the tip of the foot in this genus, while the opposite is true of amniotes.
Westlothiana