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.
Trilobites were savvy killers who hunted down their prey and used their many legs to wrestle them into submission, newly discovered fossils suggest.
The fossils come from a site in southeastern Missouri, not far from the city of Desloge. They are trace fossils, which means they preserve not the organisms themselves, but their burrows. The burrows were made by various species of trilobite as well as by unknown, wormlike creatures.
A statistical analysis of these burrows and their intersections shows that they cross one another more than expected, a sign that the trilobites were deliberately hunting down their wormy prey. In a subset of those cases, the trilobites seemed to sidle up to the burrows in parallel, perhaps so they could latch onto the worms lengthwise with their row of legs. [Video: Primitive Sea Creatures Were Advanced Ninja Attackers]
“This is legitimately the moment of interaction between the trilobite and the animal that it ate,” said study researcher James Schiffbauer, a paleobiologist at the University of Missouri.
The discovery of these fossils came about by accident. During a department field trip to visit a local lead mine, the researchers made a side trip to a known fossil spot. While there, study co-author John Huntley, also a professor at the University of Missouri, stumbled across a block of fossilized burrows, frozen in silty shale. The sediment was set down during the Cambrian period, between 540 million and 485 million years ago, when the area was a shallow nearshore environment. The shallow bottom was likely covered with a dense microbial mat, which made for a rich food source for wormy (or “vermiform”) creatures. These worms were, in turn, prey for trilobites.
“It became sort of a small shallow-water hunting ground for the trilobites,” Schiffbauer told Live Science.
Graduate student Tara Selly took on the painstaking task of cataloguing and counting the burrows and their intersections. Her findings revealed that the worm and trilobite tunnels intersected about 30 percent of the time — more than would be expected based on chance alone.
“Likely one-third of [the burrows] were actually capturing predatory events,” Selly told Live Science.
The trilobites known from this area belong to species with particularly large eyes, Schiffbauer said. Those eyes may have made them adept hunters, he said, able to seek out burrow entrances or impressions. The critters would then burrow down to grasp their prey.
“What we’re seeing is really sophisticated behavior fairly early on in what some people would say is a very simple creature,” Schiffbauer said. The trilobites might also have used scent to sniff out their prey, he said.
Predation is important to understand, Huntley told Live Science, but it can be hard to see in the fossil record. Some Cambrian fossils have recorded animals inside the gut tracts of other animals, but it’s not clear whether they were hunted and eaten or scavenged. Other signs of predation in the fossil record are wounds or drill holes in skeletons or shells, Huntley said.
“In this case, what we’re getting is actually impressions of the body,” Huntley said. “It’s a different window into this process that we know is important ecologically and really important evolutionarily as well.”
The research iss detailed online in the Feb. 15 issue of the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
In Images: A Filter-Feeding Cambrian Creature
Cambrian Creatures Gallery: Photos of Primitive Sea Life
Beastly Feasts: Amazing Photos of Animals and Their Prey