What makes a snake a snake? The oldest snake fossils on record are forcing researchers to reconsider the question.

When it comes to separating snakes from their lizard ancestors, paleontologists have long considered legs to be the deciding factor: lizards have them, snakes do not.

In a report published in the journal Nature Communications, scientists describe four newly discovered fossils 140 million to 167 million years old. Two fossils thought to be lizards have now been classified as snakes because of the structure of their skulls and jawbones.

“Skull evolution and feeding mechanics is most likely what drove the evolution of snakes,” said lead author Michael Caldwell, a paleontologist at the University of Alberta. “The limbless thing comes later.”

The upper jawbones of the fossils did not have “bony constructions” attaching them to the rest of the skull, which are found in lizards, Caldwell said. It is that absence of attachment that allows snakes to open their mouths wider than their heads.

Judging from parts of the vertebrae, however, Caldwell says he is nearly certain that these snake forebears crawled around on four feet.

Mercury levels in tuna on rise

Mercury levels in yellowfin tuna caught in the Pacific Ocean have been rising at a 3.8 percent annual rate since 1998, a new study said.

The findings, published online in the journal Environmental Toxicology and Chemistry, add to evidence that air pollution, particularly from burning coal, is pumping mercury into the ocean food chain, potentially posing a hazard to human health.

“It’s coming from mercury emissions that are falling into the ocean,” said University of Michigan eco-toxicologist Paul Drevnick, lead author of the study.

The levels found in yellowfin, a species that is not at the top of the food chain and could be considered a bellwether, are “concerning,” said co-author Carl Lamborg, who conducted the research while at Woods Hole Oceanographic Institution in Massachusetts, and now is at the University of California, Santa Cruz. “What this number is saying is that the amount of mercury in fish is getting higher and higher all the time.”

None of the measured levels of methyl mercury, the kind that is absorbed by the body, are likely to be a current hazard to health, and they probably don’t outweigh the health benefits of a fish-enriched diet, according to the researchers.

News services