A living fossil is a species that has not evolved significantly over millions of years and is very similar to the ancestors found in the fossil record.
Charles Darwin coined the term "living fossils" in 1859 to describe species of organisms that still look like ancestors millions of years ago, and are often the last surviving lineage. Anatomically, these species tend to appear unchanged, although genetically, species are always evolving.
The term "living fossils" has been hotly debated by scientists because the definition of immutability and over what time period varies greatly. But in general, living fossils are ancient species whose anatomy is still very similar to that of related fossil organisms earlier in evolutionary history.
The coelacanth is known as the Lazarus species because it was thought to have gone extinct 65 million years ago – until it was found alive in 1938. （*getty images）
Coelacanth is an elusive, deep-sea-dwelling ancient bony fish found off the coast of Africa and Indonesia. Coelacanths first appeared 400 million years ago in the Devonian period (419.2 billion to 358.9 billion years ago) and stopped appearing when non-avian dinosaurs became extinct.
Scientists believe that this critically endangered creature became extinct 65 million years ago, until 1938, when the western Indian Ocean coelacanth (latimeria chalumnae) living off the coast of South Africa was discovered. As a result of this unexpected reappearance, it became known as the Lazarus species.
Coelacanths can grow up to 66 feet (2 meters) and weighs up to 198 pounds (90 kilograms). One study found that there is evidence that these creatures can live up to 100 years.
Because the primitive species had multiple fleshy lobed fins, somewhat similar to limbs, many scientists believe that coelacanths may have played a role in the evolution of fish to terrestrial animals.
Horseshoe crabs have been around for 300 million years. （*daniela duncan/getty images）
Horseshoe crabs first appeared more than 300 million years ago, older than non-avian dinosaurs. During that time, the species did not evolve much. While horseshoe crabs are very similar to prehistoric crabs, they are more closely related to spiders and scorpions.
Related: Golden horseshoe crab's 'hauntingly beautiful' image wins wildlife photo contest.
Four species of horseshoe crabs (Limulus polyphemus) found on the Atlantic coasts of North and Central America and three species of Indo-Pacific (Tachypleus gigas), trispinous (Tachypleus tridentatus) and mangroves (Carcinoscorpiu rotundicauda) found in the coastal waters of Asia tend to live in areas where rivers meet the ocean.
Crabs have a tough exoskeleton with 10 legs to walk on the ocean floor and a pair of legs known as crayfish that can deliver food into their mouths. They have a copper-based protein in their blood that turns blue when exposed to oxygen. Crab blood is used in medical research as part of vaccine development.
Goblin sharks have been swimming in the Earth's oceans for 12.5 billion years. （*george melin/getty images）
The goblin shark is a rare and somewhat sinister-looking deep-sea dwelling fish. This ancient species is found in the Pacific, Atlantic, and Indian oceans, and is found in 1It first appeared 2.5 billion years ago. The goblin shark has several unique adaptations that make it a deadly predator, such as a long, flat snout filled with electroreceptors that allow it to sense the electric field of its prey. Its lower jaw is also full of teeth attached to ligaments;When it bites, these teeth can stick out of its mouth to grab its prey.
Leprechaun sharks have a flabby body covered in pink** and can grow up to 13 feet (4 meters) long and weigh up to 460 pounds (210 kg). It has small fins and moves slower than other shark species.
The platypus first appeared in the Cretaceous period, alongside the dinosaurs. （*johncarnemolla/getty images）
The platypus is an aquatic adapted mammal that first appeared in 1100 million years ago during the Cretaceous period (145 to 66 million years ago). A 2008 study published in the journal Nature found that the platypus' genetic code is made up of mammals, birds, and reptiles.
Amami rabbits retain the original characteristics of the species that lived hundreds of thousands of years ago. （*tokiomarinelife/getty images）
The Amami rabbit is a dark-haired species that is the last remnant of a primitive rabbit species that became extinct on the Asian continent during the Pleistocene (2.6 million to 11,700 years ago). Now found living on only two small islands off the coast of Japan, it is an endangered species, with only 5,000 remaining. Inhabiting forests and burrows, Amami rabbits are small in size and have a unique appearance with short ears and long claws.
Nautilus is the oldest "living fossil" on Earth, dating back hundreds of millions of years. （*aleksei permiakov/getty images）
Nautilus is a cephalopod or marine mollusk and one of the oldest "living fossil" species on Earth. These spiral-shelled organisms have been around since more than 500 million years ago in the early Paleozoic era (54.1 billion to 25.2 billion years ago) has barely changed since it first appeared. The nautilus inhabits the western Pacific and Indian oceans, perched in large rooms with hard shells, and uses jet propulsion to swim and forage in the ocean.
The Komodo dragon is the largest lizard in the world, growing up to 10 feet long. （*jamie lamb/getty images）
The Komodo dragon is an ancient venomous reptile that has been around for millions of years. It lives on the Lesser Sunda Islands in Indonesia, including Komodo Island. Scientists have found that its similar-looking ancestors appeared in Australia about 100 million years ago. The world's largest lizard can grow up to 10 feet (3 meters) long and weigh up to 330 pounds (150 kilograms). This dominant predator can eat up to 80% of its body weight in a single feed.
The purple frog, also known as the hognose frog, was discovered in 2003 and evolved independently for 100 million years. （*nature picture library/alamy）
The purple frog, also known as the hognose frog, is a rare amphibian belonging to the family Purple Frogidae. It evolved independently for 100 million years. Scientists discovered the species in 2003 in the Western Ghats of India. Purple frogs spend most of their lives underground, appearing to reproduce briefly. Purple frogs have bloated bodies, short legs, and small heads.
Cockroaches belong to one of the oldest insect orders and consist of cockroaches and termites. The fossil record of early cockroaches dates back more than 300 million years to the Upper Carboniferous. There are about 4,000 species of cockroaches worldwide, and they look similar to fossils.
According to the fossil record, the aardvark is a nocturnal and solitary mammal native to Africa that first appeared more than 50 million years ago. As the last member of the ancient order Tuplidentata, the species did not evolve much during that time, making it a living fossil. The aardvark translates to "earth pig" in Afrikaans because of its resemblance to a pig, although this species is most closely related to elephants.