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Crocodylians are the last living representatives of the crocodylomorpha, an even bigger group that originated over 205 million years ago. (David Ponton / Design Pics / Corbis)

The Top 10 Greatest Survivors of Evolution

Travel back millions of years in your time machine and you’d find some of these species thriving and looking much as they do today

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(Continued from page 1)

The evolutionary story of sharks is primarily one of teeth. With the exception of rare fossils that preserve remnants of soft parts, teeth are usually all that is preserved from cartilaginous shark bodies. An articulated specimen of the early shark Doliodus problematicus pushes the shark’s existence back to at least 409 million years ago, and they are probably even older than that. The lineage to which today’s six- and seven-gill sharks belong, however, is more recent. Based upon isolated, saw-blade fossil teeth, paleontologists think cow sharks have existed for at least 175 million years. These deepwater sharks are opportunistic feeders—taking whatever they can—and may have had a stable role as a deep-sea cleanup crew, scavenging on the bodies of marine reptiles during the Mesozoic and shifting to marine mammals after the time of the dinosaurs. We know very little about the appearances of these ancient sharks, but their roughly bladed teeth hint that they have been consummate deep-sea carrion feeders for millions of years.

4. Horsetails

Long-lived lineages of animals often get most of the attention, but there are some survivors among the plants, too. Horsetails must be some of the greatest. These archaic plants are often found growing in patches along streamsides and other wet habitats. Place a dinosaur toy among them, and the prehistoric model will look quite at home.

The reason why horsetails are considered so ancient comes from two lines of evidence. Living horsetails are unique among plants in that they reproduce via spores rather than seeds. Other plants likely gave up this method of reproducing millions and millions of years ago, but, old though it may be, the spore technique makes horsetails resilient and very difficult to remove from places where they are considered weeds. Horsetails also have a very deep fossil record. Though they make up small parts of forests now, enormous horsetails once made up entire forests in the days before modern trees evolved. In fact, much of the world’s coal, which originates from 360- to 300-million-year-old Carboniferous deposits, are the remnants of horsetails such as Calamites that could have grown to be over 100 feet tall.

5. Lice

Not all the great survivors are charismatic. Some of evolution’s greatest success stories are parasites, but few have stuck in there longer than lice.

Although louse fossils are rare, in 2004 paleontologists announced that they had found a 44-million-year-old feather louse that was strikingly similar to lice that live on the plumage of waterbirds today. The record of lice probably goes back even further. Last year, researchers used the few known louse fossils along with genetic comparisons between living lice to determine when major lice lineages evolved. Feather lice, in particular, seem to have split from their hitchhiking relatives sometime between 115 and 130 million years ago—right when little mammals were scurrying through the Cretaceous undergrowth and feathered dinosaurs were flocking around on land. Since feather lice evolved to feed on early birds and feather-covered, non-avian dinosaurs, they have had to change little to keep up with their hosts.

6. Brachiopods

Pick up a brachiopod and you might think you’re looking at an ordinary clam. A shell split into two halves, called valves, protects the invertebrate, but in the case of the brachiopod, these two halves are unequal in size. That’s how they got their common name—the unequal proportions of the shells make some of the creatures look like old oil lanterns, hence the name “lamp shells.”

Whether found in gravel, attached to kelp or clinging to the rock of a continental shelf, brachiopods are relatively rare today. There may be around 100 different genera now living, but over 5,000 are known from a fossil record spanning 530 million years. By about 488 million years ago, brachiopods had become the dominant shelled animals in the seas—they were so thick in some places that their shells compose most of the sediment other fossils are found in—but that all changed with the worst mass extinction of all time. This was the Permian mass extinction, which some paleontologists rightly call the “Great Dying” for its catastrophic effect on the planet’s fauna. Though the exact triggers are still debated, about 251 million years ago a huge amount of greenhouse gases were dumped into the atmosphere, and the oceans became highly acidic. Brachiopods suffered, giving a foothold to the mollusk ancestors and cousins of modern clams and cockles. Brachiopods have hung on in whatever crevices they could attach to but never managed to regain their dominance.

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About Brian Switek
Brian Switek

Brian Switek is a freelance science writer specializing in evolution, paleontology, and natural history. He writes regularly for National Geographic's Phenomena blog as Laelaps.

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