TRIADOBATRACHUS AS AN EVOLUTIONARY TRANSITIONAL
by Lenny Flank
(c) 1995
An excellent but little-known transitional fossil is Traidobatrachus (formerly known as Protobatrachus), which dates from the Triassic era about 200 million years ago. The Triadobatrachus fossil is a perfect transition from the Labrynthodont amphibians to the modern frogs.
Frogs, which are members of the amphibian order known as Anura, have several unique characteristics, nearly all of which are based around their unusual leaping form of locomotion. The most noticeable characteristic is the unique construction of the frog pelvic girdle. The ilium is greatly elongated, and the post-sacral vertebrae are fused into one solid piece known as the urostyle (frogs thus do not have any tails). The ilium and the urostyle together form a three-pronged pelvic structure that is resistant to bending and breaking.
The spine itself is very short (in some species, the entire spinal column is only six vertebrae long), and ribs are entirely absent (although some species have moderately long riblike projections from the transverse spinal processes). The vertebral bones are unique in lacking any pleurocentra and hypocentra, and consist of only neural arches.
The tibia and fibula bones in the lower leg have become fused together into one bone, to withstand the enormous stresses of leaping, and the hind legs themselves have become very long and muscular to provide power for the frog's jumping mode of locomotion.
The skull is very large, but also very light. Both the number and size of the cranial bones are less than other typical amphibians. The frontal and parietal bones have become fused into one single bone, and the pineal opening between the parietal bones is entirely absent. The eye sockets are open and larger than the eyeball, allowing the eyeball itself to float freely in the skull (the frog is able to extend its eyeballs down through the roof of its mouth to help push food items into its stomach). Frogs have a large sphenethmoid bone which protects the front of the braincase.
In many characteristics, the Triadobatrachus fossil is intermediate between the modern frogs and their Labrynthodont ancestors. The skull is very froglike, with a large sphenethmoid bone, fused frontal and parietal bones (although these are longer than they are in any modern frog), and large open orbital sockets. The ilium is much longer than in the earlier Labrynthodonts, but not as long as it is in modern frogs. The skull structure and elongated ilium, along with the large back legs and the lack of ribs, immediately mark Triadobatrachus as a very primitive frog.
Many of these features, however, are only poorly developed in Triadobatrachus, and seem to be in the process of evolving towards the modern Anuran condition. The ilium in Triadobatrachus lacks a strong articulation with the vertebral column, unlike modern frogs. The urostyle is also absent, and Triadobatrachus has a short tail, something no modern frog has. The spine, though somewhat shorter than in Labrynthodonts, is still almost twice as long as any modern frog, and the rear legs are proportionately still very small, though somewhat larger than the front legs. The tibia and fibula bones are unfused and separate, making it probable that Triadobatrachus was not a very efficient leaper. The bones of the pectoral girdle are relatively small and light, unlike the heavy pectoral bones of the mdoern frogs, which are built to withstand the enormous jarring forces encountered upon landing after a long leap.
Thus, Triadobatrachus, far from exhibiting the typical Anuran body structure, instead seems to possess a very primitive version of it, and still retains several of the characteristics of its Labrynthodont ancestors. The tail vertebrae, the lack of a urostyle, the free ilium, and the un-fused tibia and fibula are all characteristics possessed by the Labrynthodonts but not possessed by any other frog species, living or extinct. On the other hand, the large spenethmoid, the elongated ilium, the lack of ribs and the large flat skull structure are typical of frogs but not of Labrynthodont amphibians. Triadobatrachus thus seems to combine traits from two very different groups of animals, and can only be viewed as an evolutionary transition between these two groups.
It should also be noted that a new fossil frog, discovered in Arizona and called called Prosalirus bitis, was uncovered in 1985, and dates from roughly the same time as Triadobatrachus. Like Triadobatrachus, Prosalirus did not have greatly enlarged legs, but possessed the typical three-pronged pelvic structure. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail. Without further fossil discoveries, it is impossible to tell whether Triadobatrachus or Prosalirus was the ultimate ancestor of modern frogs (or indeed whether these represent two distinct lineages within modern frogs). But it does demonstrate that already in the Triassic, the frogs had diversified, with the more primitive Triadobatrachus living alongside the more "modern" Prosalirus, just as the relatively primitive Discoglossid frogs still live alongside th more advanced Ranids.