On beaches spanning the Atlantic Coast from Maine down to the gulf of Mexico, thousands of murky brown shells carpet the sand. Spawning season is just beginning, and swarms of horseshoe crabs dot the intertidal zone to lay their eggs—an unfailing, ancient phenomenon that stretches back millions of years. Horseshoe crabs dominated the seas as far back as the Ordovician, a geological epoch that took place 488 to 443 million years ago.
Known as “living fossils,” organisms like horseshoe crabs are descendents of ancient lineages and look nearly identical to their fossilized ancestors from hundreds of millions of years ago.
However, the term “living fossil” is imperfect—how these species are defined varies among scientists and paleontologists. Though living fossils seem outwardly identical to their ancestors, their DNA has indeed changed, tumbling through multiple cycles of evolution.
Living fossils are also scarce, often the last of their kind, with no close relatives alive today, and they tend to thrive in marine environments because it is easier to sidestep extinction events deep in the ocean.
The mystery of why some species have coasted through evolutionary history and why some species have extinguished mayboil down to chance.
1. Horseshoe crab
This primordial, helmetlike arthropod scuttled across the sandy ocean floors from as early as the Paleozoic (540-248 million years ago), sharing the seas with long gone icons of prehistory such as Trilobites, a hard shelled, insect-like creature and Orthoceras, a strange, conically shelled cephalopod.
Despite their name, horseshoe crabs aren’t actually crabs. They are arthropods, and share more similarities with spiders and scorpions. Dodging multiple mass extinctions and ice ages, they flourished when many of their fellow marine organisms were wiped out. Their survival is credited to theirtolerance for environmental conditions.They can survive in waters that are salty or fresh and low in oxygen.
Today, rows of live horseshoe crabs are hooked up to biomedical labs, their blood drainedinto containers. The horseshoe crab’s copper blue blood is highly coveted, used invaccine and drug testing.
2. Tuatara
Endemic to a scattering of New Zealand’s offshore islands, the Tuatara used to freely roam the supercontinent Gondwana. An unusual looking creature with a third eye in the middle of its head and two rows of top teeth, the Tuatara loosely resembles a lizard. However, the Tuatara is not a lizard, but the last surviving species of an archaic order of reptilians called Rhynchocephalia, belonging to the group Sphenodontia.
The Sphenodontia lineage stretches back at least 230 million years. Previously, all fossils of the reptiles belonging to Sphenodontia were fragmented, consisting of isolated jaws and teeth. These fragments provided an insufficient picture of the origins of the modern day Tuatara until 2022, when scientists discovered a near complete fossil of Navajosphenodon sani or N. Sani, an extremely ancient Sphenodon ancestor.
With an almost fully articulated skeleton and jaw structures, the discovery suggests that physical features on the modern day Tuatara are largely unchanged from 190 million years ago.
3. Nautilus
Nautiluses, near mythic marine mollusks characterized by their multi-chambered shell and pinholed gaze, float through the inky twilight of the deep pelagic.
Humans cannot reach this zone due to the impossibly high pressure of the deep sea, so scientists rely on crab traps to bring them to the surface. With just a tiny snip of shell and a piece of tentacle flesh, scientists can reconstruct some aspects of the nautilus’s life, from where it lived to what it consumed.
Appearing on the fossil record half a billion years ago during the Upper Cambrian, they use chemical cues to detect their main food sources like fish and crustaceans, and have a connective tube called a siphuncle that controls their buoyancy by siphoning water through their internal gas chambers.
“Nautiluses can live with almost zero oxygen. They can go weeks without food. They really have this tough shell, and it’s really hard to break them or kill them. They’re like armored behemoths down there,” says Peter Ward, paleontologist at the University of Washington.
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“They are also superb at finding dead food," continues Ward. “These are obligate scavengers. They have no vision and stay out of the light.”
Despite the Nautilus enduring 500 million years of life on earth, their spiraled beauty imperils populations today, landing them on coffee tables, in personal collections, or in jewelry stores. Several populations in the Philippines are in decline.
As of 2018, the Nautilus has been listed as threatenedunder the U.S. Endangered Species Act.
4. Coelacanth
Coelacanths are an elusive, deep-dwelling species of fish. They are physically robust, with eight powerful, limblike fins and a three-lobed tailfin. Coelacanths were found in the fossil record from the Early Devonian to the Late Cretaceous, then disappeared along with the dinosaurs during their mass extinction.
However, to the world’s surprise, the “coelacanth was dredged up in the 1930s off South Africa, and known as a fossil before it was discovered alive” says Scott Lidgard, emeritus curator of fossil invertebrates at the Field Museum. “Starting in the 1990s, molecular geneticists began examining the two species of living coelacanths, eventually finding that there are parts of the genome that appear to be rapidly evolving.”
Yet despite this evolution, the living fish and its fossil ancestor have a similar outward form.
Coelacanths may be integral to understanding how fish evolved into four legged creatures.
5. Ginkgo tree
The memory of the Ginkgo Biloba is expansive, dating back to the Permian era 270 million years ago. The fan leaved tree has grown alongside primeval ferns and cycads, provided shade for dinosaurs, inspired Song Dynasty poets, and survived the 1945 atom bomb in Hiroshima.
Ginkgos are classified as gymnosperms, meaning they have exposed seeds. The last surviving species of the Ginkgoaceae family, Ginkgo Biloba once grew on nearly all continents, until climate instability brought by Pleistocene ice ages greatly reduced their range. Though they disappeared from the fossil record in other regions, a few straggling populations held on in eastern and south central China.
Though human interference has had a hand in driving many species to extinction, it is mainly thanks to ancient Chinese cultivation that the Ginkgo avoided complete decline.
Ginkgo were valued for their edible nuts, and “the earliest historical reference to ginkgo is a famous exchange of poems written during the Song Dynasty in the 11th century,” according to an article by plant scientist Peter Crane.
In both human and ecological history, the Ginkgo tree has been a fixture. Today, the Ginkgo Biloba grows on with quiet resolve in streets and temples around the world.
