From coelacanths to crinoids, these 9 “living fossils” haven’t changed in millions of years

We see evolution all around us, constantly, in every living thing. However, in the deep oceans we find a number of “living fossils” reminiscent of creatures from prehistoric times.

In his 1859 book On the Origin of Species, the eminent naturalist Charles Darwin coined the term “living fossil” to describe living organisms that appeared unchanged from their extinct fossil relatives. The term has since been used to describe long-standing lineages, relict populations, low-diversity groups, and groups with DNA that has changed little over millions of years.

The ocean depths appear to be a good place for “living fossils,” with cartilaginous fish like sharks and rays generally being two to four times more evolutionary than land animals. In other words, while each species is unique, these species are unique particularly in contrast to their closest relatives.

Let’s take a look at some of these relics from the past.

1. Coelacanth

Coelacanths are fish that live deep off the coasts of Africa and Indonesia. They have unusually shaped, paired body fins that they move alternately, almost as if they were “walking” underwater. Their ancestry dates back to the Devonian period at least 410 million years ago.

It was once thought that coelacanths went extinct along with (non-avian) dinosaurs around 70 million years ago, as they disappeared from the fossil record around that time.

So imagine the surprise when a live specimen was dredged from the deep sea in 1938! This fish became known as the “old quadruped” and was thought to be the direct fishy ancestor of all land animals (although we now know this is not entirely accurate).

Today there are two living species of coelacanths, known as Latimeriawhich have remained essentially unchanged over the past 100 million years.

2. Horseshoe crab

Horseshoe crabs are ancient creatures that first appeared at least 480 million years ago during the Ordovician period and don’t seem to have changed much since then. Not crabs at all, they are “chelirats” and therefore more closely related to spiders and sea scorpions.

Today there are four species, all within the family Limulidae, which are found in waters off Asia and North America. They migrate to shallow coastal waters to reproduce in massive “orgies,” with females laying tens of thousands of eggs in the sand.

They also have strange blue blood, so colored due to a high copper content. Horseshoe crabs are harvested by the pharmaceutical industry for their blood as they are used in biomedical tests.

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Read more: ‘Living Fossils’: We Mapped Horseshoe Crabs Half a Billion Years Ago to Save Them From Blood Draws

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3. Elephant Shark

Similar to horseshoe crabs, “elephant shark” (Callorhinchus milii) is a misnomer. Also known as the Australian ghost shark, this species is not a shark at all. It’s a related species of cartilaginous fish known as the “chimera,” belonging to a subclass called the Holocephali that split from the shark lineage more than 450 million years ago.

Named for their bizarrely shaped snout, these “plow-like” chimeras live on the continental shelves of Australia and New Zealand.

Analysis of its genome has shown that the species is changing at a snail’s pace. In fact, it has the slowest evolving genome of any vertebrate, with its DNA changing almost imperceptibly over hundreds of millions of years.

4. Nautilus

Nautilus are a species of marine cephalopod molluscs and are therefore related to squid and cuttlefish. Unlike other cephalopods, however, they are housed in a distinctive smooth, hard shell.

Nautilus live in open water in and around coral reefs in the Indo-Pacific. They are hunted for their beautiful shells to make art and jewelry, but international trade is now regulated to protect them from over-exploitation.

Members of the family Nautilidae are known to have existed from the Late Triassic and appear to have remained relatively unchanged for more than 200 million years. Darwin himself referred to these creatures as “living fossils.”

5. Goblin Shark

The Goblin Shark (Mitsukurina owstoni) is a bizarre animal with a long, flat snout and toothy jaws that protrude in front of the face to catch unsuspecting prey. It is a relatively rare deep-sea shark that lives in all major oceans. With a face only a mother could love, it was described as “grotesque” when first encountered in 1910.

The goblin shark is the only living member of its family, the Mitsukurinidae, and the most evolutionarily pronounced shark known; its ancestry goes back about 125 million years.

6. Mantis Shrimp

Mantis shrimp, also called stomatopods, are distinctive crustaceans found in tropical and subtropical coastal waters around the world. They are fearsome sea carnivores known for delivering a dizzyingly fast and painful punch.

They also live colorful lives. They fluoresce (emit light) during mating season and have complex eyes for observing these displays. In fact, they have up to 16 color receptors while humans only have three.

The mantis shrimp lineage branched off from other crustaceans in the Malacostraca class (such as crabs, lobsters, and krill) during the Carboniferous, about 340 million years ago. So these fabulous, lively critters have been thriving for a long time. Today there are hundreds of species belonging to the suborder Unipeltata, which appeared about 190 million years ago.

7. Striped Panray

Many cartilaginous fish tend to vary widely evolutionarily, but at the forefront is the striped panray (Zanobatus schoenleinii). This fish has a mean “evolutionary age” of 188 million years.

Today, the striped panray lives in tropical waters in the eastern Atlantic (and possibly the Indian Ocean), feeding on small seafloor invertebrates. It belongs to the order Rhinopristiformes and is oviparous, meaning it gives birth to live young. It is classified as “Vulnerable” by the International Union for Conservation of Nature.

8. Brachiopods

Brachiopods are shell-like sea creatures with long, fleshy stalks that live in burrows on the sea floor. They act as reef-building organisms that filter themselves from the water around them. Existing brachiopods such as lingula, look more or less the same as their Cambrian counterparts about 500 million years ago! They are considered to be the oldest known animal (genus) that still contains living representatives.

In The Origin of Species, Darwin noted “some of the oldest […] animals as […] Nautilus, lingulaetc. are not very different from living species”. These observations led him to propose the term “living fossil”.

9. Crinoids

Crinoids are known from at least the Devonian (359-419 million years ago), but may have existed as early as the Ordovician (more than 445 million years ago). Also known as “sea lilies,” these sea creatures once lived on the ocean floor in a symbiotic relationship with coral. Corals grew from the stalks of crinoids to get higher in the water column for better feeding opportunities.

This association was very common until the supposed extinction of sea lilies 273 million years ago. In 2021, however, these two sea creatures have been rediscovered in Japanese waters and are thriving in a blissful aquatic partnership. It remains a mystery why no fossil evidence of this happy marriage has been found in the meantime.

How are living fossils formed?

While animals called “living fossils” typically evolve, many of these changes are imperceptible to the human eye. To track how animals change over time, we look at molecular changes visible in genes, or “morphological” changes in physical form.

Internal (or molecular) drivers include gene drift, which is the random change in frequency of gene variants in a population over time. External forces include natural selection, particularly sexual selection, which causes certain traits to be inherited in a population over time.

All of the sea creatures in this list appear to be going through morphological stasis (slowing down or standing still). Some may also have molecular stasis. Their slowed evolutionary rates are likely a result of the relatively stable underwater environment, particularly in the deep sea. These remote havens are some of the least affected by direct human impact and weather and climate changes.

On the other hand, these animals are not immune. And if we’re not careful, we could lose some of these strange creatures forever.

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Read more: “Sea Monsters” Were Real Millions of Years Ago. New fossils tell of their rise and fall

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