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What Is the Oldest Living Animal Species on Earth?

A horseshoe crab, one of the oldest living animal species.
The horseshoe crab is more closely related to spiders than crabs.
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  • Originally Written By: Michael Anissimov
  • Revised By: Bott
  • Edited By: Bronwyn Harris
  • Last Modified Date: 03 September 2014
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The oldest living animal species on Earth cannot be known for sure, because not every animal species or fossil has yet been discovered, but the current best guess is the horseshoe crab, which has remained pretty much unchanged since the Ordovician period, 445 million years ago. To put this in perspective, multicellular animals only appear in the fossil record about 600 million years ago, and the typical duration of an animal species is just a few million years. For instance, Tyrannosaurus rex lived for only about three million years. In contrast, the horseshoe crab has existed for about 74% of the time that animals in general have, and was one of the few animals to survive the mass extinction of the Ordovician period.

The Horseshoe Crab

Due to its status as one of the oldest living animals, the horseshoe crab, which is actually more closely related to spiders, ticks, and scorpions than crabs, has been dubbed a living fossil. Horseshoe crabs are thought to descend from eurypterids, also known as sea scorpions, which are even older, but extinct, living as far back as 510 million years ago. As far as we know, no animals from the Cambrian (542 - 488 million years ago) or Ediacaran (635 - 542 million years ago) periods are alive today, although there are numerous fossils showing they did exist in the past.

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The horseshoe crab is notable for its simple but effective immune system. When invaded by a foreign organism, such as bacteria, the blood in the local area of the invasion immediately clots into a gel-like substance. This substance, called limulus amebocyte lysate, has been used in modern medicine to test for the presence of bacteria on a surface, exceeding the capabilities of purely artificial detectors. When bacteria is present, it immediately clots. This excellent immune system has surely contributed to the horseshoe crab becoming the oldest living animal.

Surviving Species of the Ordovician Period

Along with the horseshoe crab, several other species have survived since the Ordovician period such as brachiopods, bryozoa, and crinoids. All of these organisms live in the water and all have evolved and adapted in order to survive the changes of earth's climates, making them some of the oldest species on earth. Perhaps these water-dwelling species are not as well-known as the horsehoe crab because of their small size and strange appearances; not only are the difficult to spot, but they are not as recognizable to the general population as a type of crab.

Other Ancient Animal Species

There have been groups of bacteria, such as cyanobacteria, that have been around for not only hundreds of millions of years, but actually several billion — far exceeding the age of the oldest living animal — but bacteria are not animals. The oldest living plant is thought to be gingko biloba, with fossils found dated around 270 million years ago. The long duration of this plant species is thought to be due to a combination of insect-resistant wood and the ability to form aerial roots and sprouts. A fish, the coelacanth, is a runner-up for the oldest living animal, appearing in the fossil record about 410 million years ago.

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anon942030
Post 26

Tardigrades date back over 510 million years and predate tadpole shrimp, lungfish, frilled sharks, and horseshoe crabs. Technically, the answer is sponges, but they're not as cute/freaky-looking.

anon354431
Post 24

Dinosaurs are the oldest animals. Bacteria and horseshoe crabs are animals in the loosest sense. Bacteria don't even have legs and horseshoe crabs can just about twitch. Hardly animals.

anon294590
Post 19

The fossil record is ultimately incomplete. Evolution has serious problems.

anon293957
Post 17

I think that all animals have existed since the beginning of time. All of them were created approximately 8,000-6,000 years ago. The Earth is relatively young. The chance of evolution happening and all of us being here today is 1 to 1,000 to the 400th power, or, in other words, one thousand with 400 zeros at the end.

anon218518
Post 14

the dinosaurs were the oldest.

anon171612
Post 13

Well i think elephants are the oldest species of animals.

anon168760
Post 12

I think a species of the peripatus velvet worm has lived more than 500 million years on this planet.

anon160840
Post 10

The true record of living species is in the earth. Although not all have been found and cataloged, most scientists agree that the horseshoe crab has lived 'unchanged' for millions of years. Only 1 percent of life has been fossilized. Can you imagine what we have missed!

anon149381
Post 9

the oldest living animal species is the sponge, with fossil records dating back 580 million years (but it is rightly believed that they are older). they are also the earliest branching phylum form the common ancestor of all animal species.

anon113234
Post 8

Most scientists agree that the first animals emerged about 600 million years ago, during the Ediacaran period. A simple oval fossil, just 200 microns in diameter, named Vernanimalcula ("small spring animal") represents the first known body fossil of an animal (although some scientists believe the fossil is a relic created through inorganic processes, or a fossil of a giant bacterium).

anon105701
Post 7

the oldest living really species is the Earth is the "Triops cancriformis" also known as horseshoe shrimp.(not to be confused with horseshoe crabs).

They have been alive since the triassic period

anon86061
Post 5

Well, I can't post a URL, but look-up the newly discovered (April 2010) oxygen-free multicellular animals. I bet they have everything beat.

anon78915
Post 4

sharks have been around long before dinosaurs. Horseshoe crabs the same. So i think that horseshoe crabs are a bit longer. The reason i think that is because there are not a lot around anymore. most are near Delaware. That is just my theory.

And same thing with jellyfish, longer than dinosaurs. First Earth was all fire, then water came, then earth was a big sphere of water, then granite came and made our continents.

i am also thinking that jellyfish evolved from the nautilus. just some of my thoughts.

anon77026
Post 3

nautilus are over 500 million years old and were around during the cambrian period.

anon76093
Post 2

I know sharks aren't the oldest species in the world, but they are still very old, being over 400 million years old. I just would like to know where they would be on this list.

anon74222
Post 1

I think the jellyfish is older than the horseshoe crab.

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