When we think about the history of life on earth and the vast changes that have transpired over millions and millions of years—as single-celled organisms evolved into species as disparate as redwood trees, dragonflies and humans—are wonderfully apparent. But, among all that evolutionary change, some organisms have little modified from their distant ancestors. Creatures such as sharks and crocodiles are often viewed as evolutionary sluggards or “living fossils.” While the rest of nature was caught up in life’s race, the coelacanth and duck-billed platypus sat things out.
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This perception isn’t quite right. Many species of these living fossils differ significantly from their prehistoric counterparts, and often the apparently archaic creatures are the remaining representatives of lineages that were once more varied and diverse. Still, many of these organisms look as if they belong to another era. Charles Darwin explained why in his famous book On the Origin of Species: Natural selection may have vastly modified other branches in the tree of life over time, but, among organisms like the lungfish, the quirks and contingencies of their habitats and lifestyles remained so stable that there was little evolutionary pressure to change. By chance, these lineages occupied an evolutionary sweet spot. The great Victorian naturalist Thomas Henry Huxley called these creatures “persistent types,” but there is an even simpler name for them—survivors.
Watch any documentary about crocodiles and you’re almost certain to hear the line “They have gone unchanged since the time of the dinosaurs.” That isn’t exactly true. While crocodylians as we know them today—the alligators, gharials and crocodiles that live at the water’s edge—have been around for about 85 million years, they belong to a much more diverse and disparate group of creatures that goes back to the Triassic.
Crocodylians are the last living representatives of the crocodylomorpha, an even bigger group that originated over 205 million years ago. They shared the world with the dinosaurs and came in a startling array of forms. Some—like the 112-million-year-old, approximately 40-foot-long giant Sarcosuchus—looked quite similar to their modern cousins, but there were also formidable ocean-going predators such as Dakosaurus; small forms with mammal-like teeth such as Pakasuchus; crocs with tusks and extra armor such as Armadillosuchus; and lithe, land-dwelling carnivores such as Sebecus. Modern crocs do look ancient, but they are just the remainders of an even older and stranger lineage.
2. Velvet worm
“Velvet worm” is something of a misnomer. Stretching a quarter of an inch to eight inches long, and flanked by rows of stubby legs along their smooth bodies, these invertebrates aren’t worms at all. They belong to their own group, which is more closely related to arthropods, and these inhabitants of the forest undergrowth are part of a much, much older lineage that goes back to one of the greatest evolutionary explosions of all time.
In 1909, secretary of the Smithsonian Institution Charles Doolittle Walcott discovered the fauna of the Burgess Shale—exquisitely preserved creatures from a 505-million-year-old sea. Many of these animals were unlike anything seen before, and the true affinities of many of the weird creatures from these deposits are still being debated. Even so, at least one creature looked familiar. Aysheaia, an invertebrate named by Walcott in 1911, closely resembles velvet worms and may be close to the group’s ancestry. Even though this form lacks some of the specialties seen in modern velvet worms, such as a unique nozzle system that squirts an instant web over prey, the Cambrian creature shared the segmented, stubby-legged body plans with living forms. Frustratingly, the soft bodies of velvet worms don’t fossilize very well so no one is entirely sure when they emerged onto land for the first time. But, if you know what to look for, you can still find them crawling through the leaf litter of tropical forests from Australia to South America.
3. Cow sharks
Most living sharks, from nurse sharks to great whites, have five gill slits on a side. But there are four species of cow sharks that have six or seven gills, a feature thought to be retained for millions of years from some of the earliest sharks. These deepwater, six- and seven-gill sharks are considered some of the most archaic of all shark species.