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Jaw Shows Platypus Goes Way Back

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Science  23 Nov 2007:
Vol. 318, Issue 5854, pp. 1237a
DOI: 10.1126/science.318.5854.1237a

SOCIETY OF VERTEBRATE PALEONTOLOGY, 17–20 OCTOBER 2007, AUSTIN, TEXAS

Ancestors of the duck-billed platypus may have had the same electrosensory bill.

CREDIT: NICOLE DUPLAIX/NATIONAL GEOGRAPHIC/GETTY IMAGES

When scientists first laid eyes on the duckbilled platypus and the echidnas in the late 18th century, they were so baffled by these bizarre egg-laying mammals that some considered the specimens a hoax. Modern researchers have uncovered other implausible features, including 40,000 tiny glands in the broad bill that sense electric currents, which may help the platypus catch prey underwater. The ant-eating echidna has about 100 in its tiny snout. The platypus and echidna are so unusual that they were assigned an order—the Monotremata—separate from the more common marsupial and placental mammals.

The fossil record of monotremes is also sparse. The oldest known specimen is a single tooth from Patagonia, about 62 million years old, with a distinctive compressed shape like that of juvenile platypuses before they lose their teeth. A reanalysis of fossil jaws from Australia, reported at the meeting, suggests it belonged to a platypus that lived at least 112 million years ago. “It's really, really old for a monotreme,” Timothy Rowe of the University of Texas (UT), Austin, told the audience.

Teinolophos trusleri was discovered near Inverloch, Australia, in 1997 and described by Thomas Rich of the Museum Victoria in Melbourne, Pat Vickers-Rich of Monash University, and colleagues. The specimens consist of jaws and teeth.

The dark gray area in the CT scan (above) of a jaw marks the path of nerve fibers.

CREDIT: TIMOTHY ROWE/PNAS

Looking for more anatomical clues to the evolution of mammals, Rich's team took fossil jaws to Rowe, a paleontologist who also runs a computed tomography-scanning facility at UT Austin. Scans of three specimens revealed a large internal canal along the entire length of the jaw, like the canal in a modern platypus that carries nerve fibers from the electrosensory glands in the bill to the brain. “There's no other mammal that has a canal this size,” Rowe said. Even back in the early Cretaceous, it seems, the platypus was using electrosensation. “This is the most compelling evidence to us that Teinolophos is a platypus.”

That would push back the fossil record of the platypus quite a bit; the next youngest fossil is Obdurodon dicksoni from 15-million-year-old rocks in Australia. It is also much older than current estimates from DNA of when platypuses and echidnas diverged from their most recent common ancestor. Molecular clocks put that date somewhere between 17 million and 80 million years ago. Rowe speculated that one reason for the underestimate may be that monotremes evolve at slower rates than other mammals do, an idea that fits with their lower diversity.

Zhe-Xi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, agrees that the canal in Teinolophos resembles that of a modern platypus: “I'm leaning toward accepting Rowe's idea.”

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