What is the Oldest Living Animal Species on Earth?
The oldest living animal species on Earth cannot be known for sure, because not every animal species or fossil has yet been discovered, but our 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. Because of 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 (sea scorpions), which are even older (yet extinct), living as far back as 510 million years ago. As far as
