Aug. 30, 2011 --
Evolution and natural selection have played a role in the ever-changing landscape of plants, animals, bacteria and fungi. Although species evolve as they find their niche and adapt to new opportunities, some animals have remained relatively unchanged over the course of history. These animals are known as living fossils. Compared to the animals on this list, humans are relative newcomers to this planet. Homo sapiens emerged out of Africa a mere 200,000 years ago. Many living fossils are considerably older than humans and other mammals; some have even outlasted the dinosaurs. In this slideshow, take an up-close look at animals that have persevered virtually unchanged through the ages and continue to thrive today. We begin with the platypus, an unusual egg-laying animal with fur, a bill and a venomous bite. Charles Darwin himself coined the term "living fossil" while observing the platypus. Native to eastern Australia, the animal is the only surviving example of its family, Ornithorhynchidae. This group of animals is believed to have split from mammals some 166 million years ago.
The horseshoe crab could hold the distinction of being the oldest animal species still in existence. Dating back to the Paleozoic era, the horseshoe crab existed on Earth before the dinosaurs and soldiered on through several mass extinction events. In 2008, a horseshoe crab fossil, the oldest in existence found so far, dated back to around 445 million years ago, according to a report by LiveScience.
The tadpole shrimp, Triops cancriformis, is another contender for the title of oldest living animal species. This shrimp is related to the horseshoe crab so its longevity should come as no surprise. According to a report by The Telegraph, the tadpole shrimp as it appears today is virtually identical to a fossil of a specimen that lived some 200 million years ago just as dinosaurs rose to prominence. Despite the animal's remarkable endurance, the tadpole shrimp is currently listed as an endangered species.
Once thought to be extinct in the same event that killed off the dinosaurs some 65 million years ago, the coelacanth is a lobe-finned fish that sparked a debate over whether this species represented a missing link between aquatic animals and four-legged terrestrial creatures, according to National Geographic. The animal was rediscovered in 1938 and only two species of coelacanth still exist today. In 2007, a fossilized coelacanth fin was found dating back roughly 400 million years.
Snapping turtles as we know them first walked the earth some 40 million years ago, but they have been virtually unchanged over the past 215 million years of their evolution, according to Tortoise Trust. Although not among the most endangered tortoises and turtles according to the Turtle Conservation Coalition, the snapping turtle is listed as threatened.
The more than 20 species of alligators and crocodiles living today have evolved beyond their more primitive ancestors. But the basic physical design of these reptiles has remained essentially the same for the past 320 million years or so. Alligators and crocodiles share a common ancestry, though the two groups separated from each other some 60 million years ago.
The nautilus is the most primitive cephalopod in existence, a group that includes the most complex squid and octopus. Dating back to more than half a billion years ago, the nautilus reached the high point in its evolution during the Paleozoic era about 505 million to 408 million years ago. Several species of nautilus still survive today -- relatively unchanged from their ancestral counterparts.
Goblin sharks are rare, deep-sea dwellers with a unique elongated nose that distinguishes them from other sharks. They're also ancient, and are between 112 million to 124 million years old as a species. Around 2,000 different species of fossil sharks have been discovered, according to the ReefQuest Centre for Shark Research. The earliest sharks predate the dinosaurs by more than 200 million years.
The cockroach is famous for being a survivor. These insects can survive for weeks without their heads and even withstand the fallout following a nuclear blast. Cockroaches are also an especially long-surviving animal. Roaches have thrived on Earth for some 320 million years, with an estimated 5 million to 10 million individual species ranging in shape, size and habitat. This photo shows Blaberus giganteus, one of the largest species of cockroach on Earth.
Hagfish may have had to endure a less-than-flattering name since scientists first described them in the 18th century. However, these famously ugly marine animals have existed for about half a billion years. The hagfish also represents an important evolutionary step in the development of vision. These ancient fish may have been among the earliest animals to evolve more complex, camera-like eyes as opposed to the strictly photosensitive vision possessed by more primitive species. As such, the hagfish represents a kind of missing link in the evolution of the eye.
A fossilized creature found in Canada probably didn't go to the gym, but it may be the oldest animal known to have muscles, a new study finds.
The specimen, which is approximately 560 million years old, is thought to be a relative of sea anemones and jellyfish, and contains fibrous bundles that appear to be muscle tissue, an important adaptation in the evolution of animals, according to a new study detailed today (Aug. 26) in the journal Proceedings of the Royal Society B.
"It's confirmation that muscular organisms were present roughly 560 million years ago," said study co-author Alex Liu, a paleontologist at the University of Cambridge in England. [Marine Marvels: Spectacular Photos of Sea Creatures]
Historically, scientists believed animal evolution began 540 million years ago during the Cambrian Explosion, a period of rapid evolution when most major animal groups first appear in the fossil record.
But in recent decades, researchers have uncovered large, complex fossils and traces of animal activity from the late Ediacaran Period, which spanned from about 580 million to 541 million years ago.
The idea that animals first appeared before the Cambrian is also supported by studies that compare genetic data from different animals to determine how far back in time they diverged, in order to calibrate "molecular clocks," Liu told Live Science. Those clocks have gradually become more and more accurate, with some estimates suggesting animals first arose 700 million years ago, in the Cryogenian Period, he said.
In addition, the presence of chemical compounds, called biomarkers, serve as telltale signs that animals existed before the Cambrian, Liu said.
Most of the fossils found in the Ediacaran Period don't have features that clearly identify them as animals rather than plants, fungi or other life forms. But the new fossil, found in Newfoundland, Canada, differs from all other fossils of that time, Liu said.
The creature's body consists of a circular disc that it likely used to anchor itself to the ocean floor. The disc is connected by a short stalk to a sheetlike body made of fibrous bundles — believed to be muscles — arranged in a four-fold symmetry.
The fossil represents a new genus and species called Haootia quadriformis. Liu and his colleagues classified it as a type of cnidarian — a phylum of aquatic animals that includes corals, sea anemones and jellyfish — because of its resemblance to modern cnidarians, especially the stalked jellyfish Lucernaria quadricornis.
The researchers ruled out other possible explanations for the fiber bundles, such as the movement of tectonic plates or sediments, or the conditions of fossilization. The complexity and arrangement of the bundles, and the fact that some of them appear to be contracted, all suggest that the structures are indeed muscles, Liu said. The researchers compared the bundles to muscle tissue in modern cnidarians and found them to be similar, he added.
In addition to being the oldest fossil with muscles, the newfound cnidarian also serves as a calibration point for the molecular clocks that show when different species diverged from each other, Liu said.
The development of muscles was a critical event in animal evolution. Except for sponges, all animals rely on muscles to move from place to place, to escape from predators, to feed and to reproduce. Vertebrates are the most extreme example: "Everything is dependent on muscle tissue being able to contract and extend," from breathing to digestion, Liu said.
More from LiveScience:
Image Gallery: 25 Amazing Ancient Beasts
Image Gallery: Jellyfish Rule!
Dangers in the Deep: 10 Scariest Sea Creatures