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. Yet 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, esteemed 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-enduring lineages, relict populations, groups with low diversity, and groups with DNA that has hardly changed in millions of years.

The marine depths seem to be a good place for “living fossils”, with cartilaginous fish such as sharks and rays generally being 2-4 times more evolutionarily distinct than land animals. In other words, while every species is unique, these species are particularly unlike 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 which they move alternatively, almost as if they’re “walking” underwater. Their lineage stretches back to the Devonian Period, at least 410 million years ago.

It was once thought coelacanths had gone extinct alongside (non-bird) dinosaurs about 70 million years ago, as they disappear from the fossil record around this time.

So imagine the surprise when a living specimen was dredged up from the deep ocean in 1938! This fish became known as “Old Fourlegs” and was thought to be the direct fishy ancestor of all land animals (although we now know this isn’t strictly correct).

Today there are two living coelacanth species, known as Latimeria, which have basically remained 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 appear to have changed much since. They are not crabs at all, but “chelicerates” and therefore more closely related to spiders and sea scorpions.

There are four species alive today, all within the family Limulidae, found in waters off Asia and North America. They migrate to shallow coastal waters to breed in massive “orgy” events, with females laying many tens of thousands of eggs in the sand.

They also have strange blue blood, coloured that way due to a high copper content. Horseshoe crabs are harvested for their blood by the pharmaceutical industry since it has uses in biomedical testing.

Read more: 'Living fossils': we mapped half a billion years of horseshoe crabs to save them from blood harvests

3. Elephant shark

Similar to horseshoe crabs, “elephant shark” (Callorhinchus milii) is a misnomer. This species, also known as the Australian ghost shark, is not a shark at all. It’s a related type of cartilaginous fish known as a “chimaera” and belongs to a subclass called Holocephali which diverged from the shark lineage more than 450 million years ago.

These “plownose” chimaeras take their name from their bizarrely shaped snout and can be found living off the continental shelves of Australia and New Zealand.

Analysis of their genome has shown the species changes at a veritable snail’s pace. In fact, it has the slowest evolving genome of all vertebrates, with its DNA almost imperceptibly altered over hundreds of millions of years.

4. Nautilus

Nautilus are a type of marine cephalopod mollusc, and are therefore related to squid and octopus. However, unlike other cephalopods, they are housed within a distinctive smooth, hard shell.

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

Members of the Nautilidae family are known to have existed from the Late Triassic, and appear to have remained relatively unchanged for more than 200 million years. Darwin himself described 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’s a relatively rare deep-water shark living 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 representative of its family, Mitsukurinidae, and is the most evolutionarily distinct shark we know of; its lineage stretches back some 125 million years.