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New Fossil Is World's Oldest Plant-Eating Lizard

Ker Than
for National Geographic News
March 24, 2008
 
A rare fossil discovered in Japan is the oldest known plant-eating lizard, which could shed light on an evolutionary puzzle that Charles Darwin described as an "abominable mystery," scientists say.

The 130-million-year-old jaw and skull bones were unearthed in the Ishikawa Prefecture of Japan (see map of Japan).

Based on the size of the skull, the researchers estimate that the lizard measured between 10 to 12 inches (25 to 30 centimeters ) in length.

Prior to the new discovery, the oldest known plant-eating lizard was Dicothodon, which lived in North America about 100 million years ago.

Even today, fully herbivorous, or plant-eating, lizards are rare, with only about 3 percent of modern lizards belonging to the group. Most lizards eat flesh, usually insects, or a combination of flesh and plants.

Modern herbivorous lizards eat flowering plants, or angiosperms, whose buds and leaves are typically softer than nonflowering plants.

Thus the new fossil species, dubbed Kuwajimalla kagaensis, could indicate that angiosperms were already in existence and perhaps widespread millions of years earlier than had been thought, the researchers say.

"By finding this particular fossil from Japan, it might suggest that flowering plants were already there, but we don't have direct evidence yet," said study team member Makoto Manabe of Japan's National Science Museum in Tokyo.

The discovery is detailed in a recent issue of the journal Paleontology.

Darwin's Dilemma

Currently the oldest evidence a flowering plant is a 125-million-year-old fossil from China.

The apparently sudden appearance of angiosperms in the fossil record confounded Darwin, who worried that it might pose a problem for his theory of evolution by natural selection.

(Read related story: "Lizards Help Explain Survival of the Not-So-Fittest" [November 24, 2004].)

Scientists have since uncovered fossils tracing the evolution of angiosperms from nonflowering plants, called gymnosperms.

The K. kagaensis fossil was unearthed in 2001, but it was only with recent analysis by Manabe and Susan Evan, a paleontologist at the University College London, that its significance was realized.

"We noticed the importance by looking at the teeth," Manabe said. "It's so rare, and it's so old."

The lizard's teeth were similar to those of modern day iguanas, which are one of the few fully herbivorous lizards alive today.

The researchers think the animal dined on some of the world's first flowers.

Modern gymnosperms such as ferns and gingko are typically not very nutritious and are tougher to chew than angiosperms.

"If you're a dinosaur, your jaw is bigger, so it's OK," Manabe said.

"But lizards are small, and they have delicate teeth, so it is very difficult for them to eat and chop off very tough material."

Alternatively, K. kagaensis might also have fed on young gymnosperm leaves, the researchers say.

"The big leaves are tough and fibrous," Manabe said. "But the young ones tend to be softer and smaller."

Christopher Austin, a herpetologist and curator at Louisiana State University's Museum of Natural Science, called the fossil discovery a "spectacular" find that challenges long-held views about lizard evolution.

"The ancestral condition for lizards has always been assumed to be insectivorous [insect-eating], so this new fossil provides data that challenges this thinking," Austin said.

He added that the anatomical features of K. kagaensis suggest "that either the ancestral condition for lizard diet was not as restricted as once thought or that diet has been highly labile [easily changed] throughout lizard evolution."

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