Wednesday, January 23, 2008

How Old Is That Platypus In The Window?

The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. 2008. T. Rowe et al. PNAS, published online before print January 23, 2008.

CT scans of a fossil jawbone reveal a large jaw canal in a creature once thought to be a forbearer of the platypus and related species. Images: T. Rowe
From National Geographic News:

Platypuses and their closest evolutionary relatives, the four echidna species, were thought to have split from a common ancestor sometime in the past 17 million to 65 million years. But remains of what was believed to be a distant forebear of both the platypus and the echidna—the fossil species Teinolophos—actually belong to an early platypus, according to scientists who performed an x-ray analysis of a Teinolophos jawbone. The finding means the two animals must have separated sometime earlier than the age of the fossil—at least 112 million years ago.

Platypus bills are complex sensory organs loaded with electrical receptors. In murky waters the animals hunt by tracking the weak electrical fields generated by muscle activity in fish and other prey.

All mammals have some type of canal that conducts nerve fibers to the teeth, Rowe noted. But in the platypus, this canal is greatly enlarged to accommodate a massive network of fibers that carry sensory information from the bill. The claim that Teinolophos is a very ancient platypus rests largely on this feature.

But Matt Phillips, of the Australian National University in Canberra, said more evidence may be needed. The research "does not confirm that the platypuses and echidnas diverged more than 112 million years ago," Phillips said. Read his counter argument HERE.

This is another Open Access article so you should be able to download the PDF HERE.