Evolutionary survivors we consider the history of life on Earth, the immense changes that have occurred over millions of years—when single-celled organisms developed into species as diverse as redwood trees, dragonflies, and humans—are astounding. Despite this, some creatures have remained mostly unchanged from their distant forebears. Sharks and crocodiles are frequently seen as evolutionary sluggards or “living fossils.” While the rest of nature was racing through existence, the coelacanth and duck-billed platypus sat things out.
Watch any crocodile documentary and you’ll almost certainly hear the words “They have remained unmodified since the time of the dinosaurs.” This isn’t precisely true. While crocodylians as we know them today—alligators, gharials, and crocodiles that live near water—have been around for around 85 million years, they are part of a much larger and more diverse collection of species that dates back to the Triassic.
The term “velvet worm” is a bit misleading. These invertebrates aren’t worms at all, measuring a quarter of an inch to eight inches long and bordered by rows of stubby legs along their smooth bodies. They are members of their own group, which is more closely linked to arthropods, and these forest undergrowth dwellers are descendants of a much, much older lineage that dates back to one of the largest evolutionary explosions of all time.
In 1909, Smithsonian Institution secretary Charles Doolittle Walcott found the Burgess Shale fauna—exquisitely preserved organisms from a 505-million-year-old sea. Many of these organisms were unlike anything seen before, and the real affinities of many of the strange critters discovered in these deposits are still being contested. Nonetheless, at least one of the creatures appeared to be familiar. Walcott named an invertebrate Achaia in 1911, which resembles velvet worms and may be related to them.
The cow shark
From nurse sharks to big white sharks, most extant sharks have five-gill slits on each side. However, four cow shark species have six or seven gills, a characteristic considered to have survived for millions of years from some of the earliest sharks. These Deepwater, six- and seven-gill sharks are among the most primitive shark species.
Sharks’ evolutionary narrative is essentially one of teeth. Except for exceptional fossils that preserve soft tissue remnants, teeth are frequently the only thing left of cartilaginous shark bodies. An articulated specimen of the early shark Solidus problematics dates the shark’s existence back at least 409 million years, and they are likely even older. The lineage of today’s six- and seven-gill sharks, on the other hand, is more recent.
Long-lived animal lineages often garner the most attention, but there are some plant survivors as well. Horsetails have to be among the best. These extinct plants are frequently observed growing in patches along streamside’s and in other moist areas. If you put a dinosaur toy amid them, the prehistoric model will look right at home.
Two factors explain horsetails’ antiquity. Only horsetails reproduce via spores. Horsetails are resilient and hard to remove from weedy areas due to their spore mechanism, which other plants likely abandoned millions of years ago. Horsetails also fossilize. Before modern trees, horsetails covered entire forests.
Brachiopods resemble clams. The brachiopod’s valves are unevenly sized, protecting the invertebrate. “Lamp shells” are named for their uneven shells, which resemble old oil lamps.
Kelp, continental shelf rock, and gravel lack brachiopods. 530 million-year-old fossils indicate 5,000 genera. Brachiopod shells dominated 488 million-year-old silt. The greatest mass extinction changed that.