A new study published in the new peer-reviewed open access journal PeerJ sheds light on the origin of tadpole shrimps, a group commonly regarded as ‘living fossils.’ The study reveals that living species of tadpole shrimp are much younger than the fossils they so much resemble, calling into question the term ‘living fossil’.

Charles Darwin introduced the term ‘living fossil’ when talking about the platypus and lungfish, groups that appear to have diversified little and appear not to have changed over millions of years. For him living fossils were odd remnants of formerly more diverse groups, and suggestive of a connection between different extant groups.

Ever since, the term has been widely used to describe organisms such as the coelacanth, the horseshoe crab and the ginkgo tree. The term has been controversial, as it appears to suggest that evolution has stopped altogether for these organisms, and some scientists have argued that it should be abandoned.

Tadpole shrimps are a small group of ancient crustaceans that are often called ‘living fossils’, because the living species look virtually identical to fossils older than the dinosaurs.

Analyzing DNA sequences of all known tadpole shrimps, and using fossils from related crustacean groups, researchers led by Dr Africa Gómez from the University of Hull showed that tadpole shrimps have in fact undergone several periods of radiation and extinction.

Different species of tadpole shrimp often look very similar, and so it is only with the advent of DNA sequencing that scientists have realized that they are a surprisingly diverse group.

The team’s results uncovered a total of 38 species, many of them still undescribed. This abundance of ‘cryptic species’ makes it very difficult for fossils to be assigned to any particular species as they all look remarkably similar. For example, 250-million-year-old fossils have been assigned to the living European species Triops cancriformis whereas the team’s results indicate that the living T. cancriformis evolved less than 25 million years ago.

“In groups like tadpole shrimps where cryptic speciation is common, the fossil record says very little about patterns of evolution and diversification and so the term ‘living fossil’ can be quite misleading. For this reason, we used fossils from related groups to gain an understanding about the evolution of tadpole shrimps,” explained co-author Dr Tom Mathers, also from the University of Hull.

“Living fossils evolve like any other organism, they just happen to have a good body plan that has survived the test of time. A good analogy could be made with cars,” Dr Gómez added.

“For example the Mini has an old design that is still selling, but newly made Minis have electronic windows, GPS and airbags: in that sense, they are still evolving, they are not unchanged but most of the change has been ‘under the hood’ rather than external,” she said.

“By comparison, organisms labeled as ‘living fossils’ such as tadpole shrimps, are constantly fine-tuning their adaptation to their environment. Although outwardly they look very similar to tadpole shrimp fossils from the age of the dinosaurs, their DNA and reproductive strategies are relatively hidden features that are constantly evolving. The flexibility of their reproductive strategies, which our research has revealed, could be the evolutionary trick that has allowed them to persist as a morphologically conservative group for so long.”

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Bibliographic information: Thomas C. Mathers et al. 2013. Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils.’ PeerJ 1: e62; doi: 10.7717/peerj.62