The ‘tailed’ frog, Ascaphus, enters the LRT

Everyone knows so-called ‘tailed frogs’ are primitive,
but not more primitive than Triassicus, the long-torso, short-leg frog from the Triassic. Ascaphus (Figs. 1, 2) enters the LRT alongside Rana, the only other extant frog in the large reptile tree.

Figure 1. Male Ascaphus truei in vivo. Not a true tail, that tissue is an extension of the cloaca for internal insemination.

The semicircular skull with enormous orbits
in Ascaphus (Fig. 2) recalls Triassicus. The absence of lacrimals and the reduction of other circumorbital bones is apomorphic, or at least not shared with Rana.

Figure 2. Ascaphus skeletons in dorsal view. Skull and wrist colors applied here. Note five carpals are present but only four fingers were present, the plesiomorphic number for basal tetrapods.

Ascaphus truei
(Stejneger 1899) is the extant tailed frog. The so-called tail is only found in males and is an extension of the cloaca for internal fertilization, the primitive state. More dorsal and lumbar vertebrae are present than in Rana (below).

Figure 3. Rana skull in several views.

Rana catesbeiana
(Linneaus 1758, Shaw 1802) extant, bull frog. Derived from a sister to Triadobatrachus, Rana is the last of this lineage and includes some 90 species.

Distinct from Triadobatrachus, the skull of Rana is more spade-shaped in dorsal view. The nasals extend laterally to the maxilla. The squamosal is reduced to a frame for the eardrum.

The presacral count is further reduced to 9. The tail vertebrae are fused into a single long bone, the urostyle, following the sacral vertebrae, but note that while the urostyle is long, it lies completely between the ilia and does not extend beyond the ischium.

Figure 4. Rana skeleton in several views. Note the fewer vertebrae.

A cartilaginous element,
the suprascapula, extends dorsal to the scapula and roofs over the first few dorsal ribs. The radius and ulna fuse. The hand is larger. The thumb is absent. Only a vestigial stub sometimes remains of metacarpal 1. Digit 2 of the manus has one fewer phalanx probably by fusion. Digits 3 and 4 have two fewer phalanges.

The ilium is elongated to half the torso length. The femur is gracile and elongated. The tibia and fibula are fused. The tibiale (astragalus) and fibulare (calcaneum) further elongate the foot. No trace of pedal 2.1 is present. A small medial carpal spur is present.

References
Stejneger LH 1899. Type species: Ascaphus truei. Proceedings of the US National Museuem 21:899.

wiki/Triadobatrachus
wiki/bull frog
wiki/Rana
wiki/Tailed_frog