Tadpole shrimp, the tiny creatures that comprise the order Notostraca, are branchiopod crustaceans that have been around for about 400 million years. They have left us an abundant fossil record that indicates their body plan has remained more or less consistent over the course of the past 250 million years. As such, scientists have dubbed them “living fossils”—species like ferns and horseshoe crabs that appear to have limited evolution and diversification since ancient times. But the lack of a changing outward appearance doesn’t necessarily mean on a molecular level, the “living fossil” species hasn’t evolved. New research is showing that tadpole shrimp did indeed evolve over the past 200 million plus years.
Tadpole shrimp are difficult to study not only because they are deemed to be living fossils, but also due to their label as cryptic species. A cryptic species is not a legitimate species in itself—it refers to members of different species that look the same and are mistaken for one species. Most Notostraca specimens, both living and fossil, appear to have very similar morphology, and so are lumped into one species. Thhis common mistake has furhtered the belief that tadpole shrimp ceased evolving long ago.
A new study, published in the journal PeerJ, conducted by researchers from the United Kingdom–Thomas Mathers, Bernd Hanfling, and Africa Gomez, from the University of Hull; Robert Hammond from the University of Leicester, and Ronald Jenner from the Natural History Museum in London–found that there are 38 living species of tadpole shrimp, and possibly even more.
The researchers found that many of the tadpole shrimp species evolved and diversified much more recently than the fossil record indicates. The two extant genera of tadpole shrimp—Triops and Lepidurus–diverged 184 million years ago. 73 million years ago, another Notostraca species radiation occurred. This more recent divergence event spread Triops and Lepidurus across the globe, resulting in the broad distribution we see in the two species today. The study offers a likely reason for diversification and distribution: at the same time as tadpole shrimp were diversifying, so were birds. Birds moved into different habitats, and were instrumental in dispersing tadpole shrimp between continents.
Gomez and colleagues sequenced the DNA of a number of tadpole shrimp, most thought to be European tadpole shrimp, Triops cancriformis. They discovered that, although the morphology of many of the specimens appeared identical, the gene sequences were quite different. Triops cancriformis, thought to be 250 million years old; had a significantly different genome than the oldest tadpole shrimp, indicating that it in fact evolved probably around 25 million years ago.
Extant species of tadpole shrimp diverged much more recently than fossil evidence has previously shown. But mistaking similar mosphrology for evolutionary stagnancy is not an uncommon mistake, nor is it one made only for tadpole shrimp. This study has broader implications for paleontologists: when it comes to living fossils, take caution. “Morphological stasis,” the researchers write, “can potentially lead to diversification rates being underestimated.” Studies like this one, which examine molecular as well as fossil data, can give us a clearer image of the evolutionary timeline of a “living fossil.” The paper concludes with one last wise warning: “We therefore caution against drawing conclusions about patterns of diversification based on fossil data alone in groups where widespread morphological conservatism may obscure rampant cryptic speciation.”
Find this study published in the journal PeerJ.