The Evolution of Frogs

While they are the most common of living amphibians, frogs are among the oddest amphibians of all time with their short torso and long hind legs. The skull has undergone great changes from the primitive state with many bones reduced (Fig. 1).

Frog skull evolution. Here, starting with the Permian Utegenia (with relatives back to the Visean) you can see stages in the evolution of the frog skull through Doleserpeton and Gerobatrachus to Rana, the bull frog.

Figure 1. Frog skull evolution. Here, starting with the Permian Utegenia (with relatives back to the Visean) you can see stages in the evolution of the frog skull through Doleserpeton and Gerobatrachus to Rana, the bull frog.

The wide flat skull of Utegenia (Earliest Permian with roots in the Visean, Laurin 1996) is our starting point. It represents a basic seymouriamorph skull with a full complement of skull bones, palatal fangs and sharp marginal teeth.

In Doleserpeton (Early Permian, Sigurdsen and Bolt 2010) the palate changes the most as the interpterygoid vacuity greatly expands. The palatal fangs are gone. The marginal teeth are tiny and continue behind the orbit. The vomer expands with a shagreen of tiny teeth. The intertemporal fuses to the postfrontal. The otic notch deepens. Note the palatines, now form part of the ventral orbit in lateral view.

In Gerobatrachus (Early Permian, Anderson et al. 2008) these trends continue as the skull widens, the orbits enlarge and the palatal bones become more gracile. Here the intertemporal bones may not have fused to the postfrontal, but the specimen has many cracks and is exposed ventrally.  Here the otic notch is not so deep and the voters are tiny.

In Rana (extant) the skull bones are reduced to struts (no more post parietal shelf) to accommodate the giant orbit and narial opening. The otic notch is angular here, not a smooth curve. It appears as if the intertemporal were still present, but sources available at present don’t delineate the cranial bones. Send a good reference if you have it and I’ll make changes as necessary.

The evolution of frogs from Utegenia, through Doleserpeton, Gerobatrachus, Triadobatrachus and Rana.

Figure 2. The evolution of frogs from Utegenia, through Doleserpeton, Gerobatrachus, Triadobatrachus and Rana. Click to enlarge. It wasn’t until Triadobatrachus in the Early Triassic that frogs developed their long hind limbs, and long ankle bones, but they still retained a short pelvis and long torso.

We skipped one
In Triadobatrachus (Fig. 2, Early Triassic, Piveteau 1936, Rage and Rocek 1989) the hind limbs and ankle bones begin to elongate and the vertebral count is reduced. Distinct from Gerobatrachus, the skull of Triadobatrachus was relatively smaller with a narrower skull roof and a larger orbit that extended into the cheek where the bones were reduced. The frontal and parietal were fused together, but note the parietals were not completely fused to each other! The intertemporal was fused to the parietal. A large postparietal shelf was absent. The lower jaw was toothless. The presacral vertebral count was reduced to 14. The tail was reduced to a nub of at least six vertebrae. The ilium included a larger anterior process, but note the thigh muscles did not extend to the anterior tip of the ilium as in reptiles and mammals. The femur, tibia and fibula were elongated as were two proximal ankle bones, the tibiale and fibulare. Unfortunately the hands and feet remain unknown. Despite the much longer hind legs, Triadobatrachus was considered an ineffective jumper.

References
Anderson JS et al. 2008.  A stem batrachian from the Early Permian of Texas
and the origin of frogs and salamanders. Nature 453:
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Laurin M 1996. A reappraisal of Utegenia, a Permo-Carboniferous seymouriamorph (Tetrapoda: Batrachosauria) from Kazakhstan. Journal of Vertebrate Paleontology 16(3):374-383.
Piveteau J. 1936. Une forme ancestrale des amphibiens anoures dans le Trias inférieur de Madagascar. Comptes Rendus hebdomadaires des séances de l’Académie des Sciences 202:1607–1608.
Rage J-C and Rocek Z 1989. Redescription of Triadobatrachus massinoti (Piveteau, 1936) an anuran amphibian from the early Triassic. Palaeontographica Abt. A 206(1-3):1-16. online pdf
Sigurdsen T and Bolt JR 2010. The Lower Permian amphibamid Doleserpeton (Temnospondyli: Dissorophoidea), the interrelationships of amphibamids, and the origin of modern amphibians. Journal of Vertebrate Paleontology 30(5):1360-1377.

wiki/Doleserpeton
wiki/Gerobatrachus
wiki/Utegenia
wiki/Triadobatrachus

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