Triops longicaudatusWIKIPEDIA, STEVE JURVETSONToday’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.”
Laurenti has found the same to be true in his own research on coelacanths—two species of fish that are perhaps the most cited example of living fossils. The entire group was thought to have been extinct since the Cretaceous, until South African fisherman caught a live one in 1939. It was perhaps inevitable that the coelacanth came to symbolize stagnant relicts of a bygone age, but Laurenti points out that their genomes have evolved at comparable rates to other species, and that modern coelacanths are actually physically distinct from prehistoric ones in terms of their skulls, spines, size, and more. And in 2011, researchers found that one of the living species is actually two distinct species, which diverged from each other 200,000 years ago.
The same message applies to other living fossils. Cycads, for example, are a type of tree that first appeared around 280 million years ago and dominated the land during the time of the dinosaurs. Today, just 300 species survive of the many more that once flourished. But Nathalie Nagalingum from Harvard University found that they are just 12 million years old. They aren’t ancient survivors of the Jurassic and Cretaceous, but rather the descendants of a more recent comeback, which happened after most of their ancient relatives had disappeared.
The tuatara—a single lizard-like species from New Zealand—has also been branded as a living fossil. But a recent study showed that it differs significantly from its Triassic and Jurassic relatives in the number of its teeth, subtle features of its skull, and probably in its diet. And a 2008 analysis of its genome revealed an even bigger surprise—tuatara DNA has changed faster than that of any other vertebrate, confirming that molecular and physical changes do not necessarily reflect one another.
“The body plan [of so-called living fossils] is effective at what they do, although they might change in minor anatomical features such as their leg hairs or details of their jaws,” said Gomez. And though they may superficially resemble their extinct relatives, they may differ in traits that do not fossilize, such as songs, pheromones, lifespan, or sexual systems. The notostracans, for example, have evolved to become hermaphrodites several times over, something that wouldn’t necessarily be reflected in the paleontological record.
Although Darwin himself coined the term in On the Origin of Species, Gomez said, “I think he would have been pleased if he had found out that living fossils evolve as every organism does and we no longer had to use the term.”