Researchers have examined a well-preserved amphibian fossil and found that it fills the gap between frogs and salamanders. Gerobatrachus hottoni, “Hotton’s old frog,” has a mosaic of features such as a salamander’s walk and a tail but a wide, froggy head. It comes from a time, 280 million years ago, before frogs and salamanders became separate branches on the tree of life.
their research, published in Thursday’s issue of the journal Nature, settles a long and hot debate in scientific circles as to just how these species evolved.
“This fossil is the most like the modern amphibian that you find and it’s from incredibly ancient times,” said principal investigator Jason Anderson, an assistant professor of veterinary anatomy at the University of Calgary who specializes in vertebrate paleontology.
“So what this does is provide conclusive evidence that frogs and salamanders have an origin among one particular group of extinct fossil amphibians,” he said Wednesday from Calgary. “This fossil falls right into a gap in the fossil record between one archaic group of amphibians and the earliest examples of the modern amphibians, frogs and salamanders.”
The fossilized remains were discovered in 1995 in the scrub land of north-central Texas by the late Nicholas Hotton of the Smithsonian Institution, the man for whom the long-extinct creature is named.
Hotton, who died two years later, knew he had made a significant find, said Anderson.
“With a slip of paper found with the specimen and in his handwriting is the nickname ‘Froggie.’ So he recognized the specimen for what it was immediately after he found it.”
The slab of silt stone bearing the 12-centimetre-long creature’s imprint had languished in the Smithsonian’s collection for some time before a U.S. colleague brought it to Anderson’s attention; he jumped at the chance to create a team to partially raise it from its rocky grave.
Co-author Robert Reisz, a professor of biology who heads a vertebrate paleontology research lab at the University of Toronto, came on board to lay bare the creature’s skeleton, delicately chipping away at the chalk-like rock in which it was embedded.
It has small teeth, a reduced number of vertebrae, salamander ankle bones, a frog’s ear, and a lightly built skeleton.
The first frog fossils date from about fifty million years later.
Complete specimen in ventral view, photograph (left) and interpretive outline drawing (right). Abbreviations: bc, basale commune; cl, cleithrum; cv, clavicle; dm, digital elements of the manus; dt3, distal tarsal 3; fe, femur; h, humerus; ic, intercentrum; il, ilium; is, ischium; op, olecranon process of ulna; pc, pleurocentrum; r, radius; sr, sacral rib. [Diagram from Pharyngula]