FROG FOSSILS

The evolutionary and creationism models both make predictions about frog fossils? Which is supported by the actual fossil evidence?

EVOLUTIONARY MODEL

If evolution has occurred, then the types of frogs alive today are not expected to be present throughout the early part of life's history. Prior to the appearance of modern frogs, amphibians would have existed which were not frogs but which had anatomical features which link them to frogs. The earliest frogs should be more primitive than those alive today. Fossil groups would have appeared and disappeared at diverse points in the history of amphibians.

CREATIONISM MODEL

If the creationism model is correct, then every "kind" of modern frog has always existed. There should be no period of the fossil record and certainly no period during the fossil record of life on land, which lacks frog fossils. If there were any extinct forms of frog, they all would have become extinct in the same global catastrophe, only a few thousand years ago. Since frogs did not evolve from non-frogs, there is no reason to suspect that amphibians with a mix of frog and non-frog traits or frogs with more primitive traits than modern frogs ever even existed. There is certainly no reason to believe that even if such animals existed, their fossils would predate those of modern frogs.

INTELLIGENT DESIGN

If intelligent design has occurred, then some/substantial evolution could have taken place in frog lineages. However, aspects of frog anatomy or physiology which require the interaction of multiple parts could have been impossible to have develop in a stepwise fashion. As a result, only direct divine intervention could account for certain "irreducibly complex" features in frogs.

WHICH OF THESE TWO PREDICTIONS IS SUPPORTED BY FOSSIL EVIDENCE?

Not only does the fossil evidence strongly support the evolutionary model, it contradicts the predictions of the creationism model.

There are more than 4,100 modern species of frogs and toads. They can be found on every continent except Antarctica and ange from the tropics to North of the Artic circle. They have adapted to a diversity of habitats including tropical forests and the arid deserts, treetops, the forest floor, and aquatic environments. The largest are almost a foot in length (Pugh, 1998). If they had always existed on earth, it is reasonable to suppose that creationists could find fossils of frogs from the eras which make up the majority of earth's history. They haven't.

FROGS WERE ABSENT THROUGHOUT MOST OF THE HISTORY OF LIFE ON EARTH

The vast majority of the life's history occurred without the fossil of a single frog. The majority of the history of life on land and the majority of the history of amphibians also passed without the fossil of a single frog.

ANCESTRAL AMPHIBIANS HAD PRODUCED TRANSITIONAL LINEAGES RELATED TO FROGS BY THE TRIASSIC

The first froglike animal, Triadobatrachus, dates from the Triassic Period is not a frog but rather is classified in the related group Salienta/Proanura. This transitional form possesses a number of traits which are only found in frogs today but it was not a true frog (Pough, 1998). This genus may have evolved from Doleserpeton from the Paleozoic (Carroll, 1988).

Triadobatrachus

PRIMITIVE FROGS ARE KNOWN BY THE JURASSIC

Frogs of the Order Anura date back to the Jurassic Period. Vieralla, a possible ancestor of modern families, possessed unfused ribs (unlike modern frogs) and its legs were short. Prosalirus certainly could jump but the primitive positions of muscle attachments indicate that it did not have same the ability seen in modern frogs (Abrams, 1996). Notobatrachus possessed a number of primitive traits such as a short pelvis and a continuous notochord through its vertebrae. A few other Jurassic frogs with primitive traits are known as well (Estes, 1973). Some fossil frogs belong to Archaeobatrachia, although not to any modern family (such as the extinct family Paleobatrachidae), and have a number of primitive features not present in modern frogs such as the parahyoid and unfused lower leg bones. Lower leg bones are not fused in the fossil species Prosalirus, Notobatrachus, and Yizhoubatrachus (Gao, 2004).

MOST MODERN FAMILIES OF FROGS ARE KNOWN FROM ONLY THE MOST RECENT FOSSILS

Triadobatrachus is classified in the Order Proanura, but is its only member. Of the 24 families of frogs, only 6 or 7 are represented by fossils in the Jurassic or Cretaceous. These Mesozoic families Discoglossidae, Leiopelmatidae, Pipidae, Pelobatidae, and Rhinophrynidae are all classified in the more primitive of the two suborders of frogs, Archaeobatrachia while the family Leptodactylidae is classified in the more derived suborder, Neobatrachia. The families Leiopelmatidae and Discoglossidae of the suborder Archaeobatrachia are the most primitive groups of frogs alive today.(Gao, 2004). All other families of frogs date from the Cenozoic during the “Age of Mammals” (Pough, 1998; Mattison, 1998). Modern genera are known from recent fossils, if at all. For example, the toad genus Bufo is known from the Miocene (McGowan, 1995).

FROG "DESIGN"

If parts of a frog are irreducibly complex, then they could not have evolved from more primitive form. Frogs, however, possess the same organs and systems found in fish; they simply possess modified forms of these structures. Fossils indicate that the skeletal features of frogs evolved in stages. Salientans (frogs and Triadobatrachus) share a fusion of the frontal and parietal skull bones, the shape of the jaw, and elongated iliac bones which project forward (Pough, 1998). Although some of the adaptations of frog anatomy represent adaptations to a leaping lifestyle (such as a shorter back to better absorb shock and the loss of an interfering tail), the ancestors of frogs had already reduced the length of the back and tail before the legs lengthened. Triadobatrachus was not fully a frog however: it possessed more vertebrae than modern frogs, a short tail, the radius and ulna are shorter than in modern frogs, and the bones of the forearm and lower leg had not yet fused (O’Connor, 2003; Rage, 1989; Estes, 1973). After splitting from the genus Triadobatrachus, frog ancestors completely lost the tail which had already been reduced, underwent fusion of the bones of the forearm and lower leg (although several extinct frogs had not yet undergone fusion of the leg bones), elongated their leg and tarsal bones and formed a urostyle from fused vertebrae (Pough, 1998).

Divine intervention does not necessarily need to be invoked to explain these changes. Bones may fuse as part of the normal variation within a species. For example, the bones of the human forearm may even fuse on occasion. The calcaneus and astragalus are fused in the two species of the family Pelodytidae. Some bones can be reduced or lost as a part of normal variation within a biological group. The absence of a sternum is known in two frog families and another frog genus. Pseudid frogs possess an extra bone in their digits (an extra phalanx). Burrowing toads have evolved an extra bony stump on their feet (the prehallux) which helps them to burrow. Many tree-living frogs have evolved additional bony or cartilaginous features in their fingers for a better grip (Mattison, 1998; Pough, 1998).