Today’s tadpole shrimp, or notostracans, have a shield-shaped body, ending in a forked pair of filaments—a shape that makes them almost indistinguishable from their ancestors in the Triassic period some 265 million years ago. This outward constancy has earned them the description of “living fossils”—a term referring to species with no close living relatives, which seem to have gone unchanged for long spans of time.
But according to a genetic analysis of notostracans published today in PeerJ, these animals have by no means stopped evolving. Indeed, researchers are coming to realize that the term “living fossil” is a misnomer. One by one, the classic examples—horseshoe crabs, coelacanths, cycads, and more—have turned out to be very different from the fossils that they apparently resemble, either at a genetic level or through subtle physical changes. Their recognizable nature is a red herring—these creatures simply did not exist in their current form millions of years ago.
“I would favor retiring the term ‘living fossil’ altogether as it is generally misleading,” said Africa Gomez at the University of Hull who led the study.
In the latest study to disprove the concept of a living fossil, Thomas Mathers, Gomez’s student from the University of Hull, sequenced several genes from 270 tadpole shrimp, and built a family tree charting their evolutionary relationships. Although taxonomists recognize 11 species of tadpole shrimp, based on DNA evidence, Mathers suspects that there are actually 38 distinct species alive. These fall within two living genera—Triops and Lepidurus—which diverged from one another 184 million years ago, during the Jurassic period. The diversification of species within these genera arose around 73 million years ago. “Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions,” said Gomez.
“The article provided more proof, if more were needed, that the term "living fossil" is both poorly defined and misleading,” said Patrick Laurenti, an evolutionary biologist from the CNRS in France. “Calling a given species a living fossil suggests that it has crossed time without evolving—a hypothesis that is in sharp contrast with evolutionary genetics principles.”
Written By: Ed Yong continue to source article at the-scientist.com