N. Strausfeld/University of Arizona
This close-up of the head region of the Alalcomenaeus fossil specimen includes superimposed colors of a microscopy technique that reveal the distribution of chemical elements in the fossil.
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.
The discovery of a fossilized brain in the preserved remains of an extinct "mega-clawed" creature has revealed an ancient nervous system that is remarkably similar to that of modern-day spiders and scorpions, according to a new study.
The fossilized Alalcomenaeus is a type of arthropod known as a megacheiran (Greek for "large claws") that lived approximately 520 million years ago, during a period known as the Lower Cambrian. The creature was unearthed in the fossil-rich Chengjiang formation in southwest China.
Researchers studied the fossilized brain, the earliest known complete nervous system, and found similarities between the extinct creature's nervous system and the nervous systems of several modern arthropods, which suggest they may be ancestrally related. [Photos of Clawed Arthropod & Other Strange Cambrian Creatures]
Living arthropods are commonly separated into two major groups: chelicerates, which include spiders, horseshoe crabs and scorpions, and a group that includes insects, crustaceans and millipedes. The new findings shed light on the evolutionary processes that may have given rise to modern arthropods, and also provide clues about where these extinct mega-clawed creatures fit in the tree of life.
"We now know that the megacheirans had central nervous systems very similar to today's horseshoe crabs and scorpions," senior author Nicholas Strausfeld, a professor in the department of neuroscience at the University of Arizona in Tucson, said in a statement. "This means the ancestors of spiders and their kin lived side by side with the ancestors of crustaceans in the Lower Cambrian."
The newly identified creature measures a little over an inch long (3 centimeters), and has a segmented body with about a dozen pairs of attached limbs that enabled it to swim or crawl.
"Up front, it has a long pair of appendages that have scissorlike components — basically an elbow with scissors on the end," Strausfeld told LiveScience. "These are really weird appendages, and there has been a long debate about what they are and what they correspond to in modern animals."
Previously, researchers suggested megacheirans were related to chelicerates, since the extinct creature's scissorlike claws and the fangs of spiders and scorpions have similar structures, said Greg Edgecombe, a researcher at the Natural History Museum in London, England.
"They both have an 'elbow joint' in the same place, and they both have a similar arrangement of a fixed and movable finger at the tip," Edgecombe told LiveScience. "Because of these similarities, one of the main theories for what 'great appendage arthropods' are is that they were related to chelicerates. Thus, our findings from the nervous system gave an injection of new data to support an existing theory."
This close-up of the head region of the Alalcomenaeus fossil specimen includes superimposed colors of a microscopy technique that reveal the distribution of chemical elements in the fossil.N. Strausfeld/University of Arizona
The researchers used CT scans to make 3D reconstructions of features of the fossilized nervous system. The scientists also used laser-scanning technology to map the distribution of chemical elements, such as iron and copper, in the specimen in order to outline different neural structures.
Though finding a well-preserved ancient nervous system is rare, the new study highlights the potential for similar discoveries, the researchers said.
"Finding ancient preservation of neural tissue allows us to analyze extinct animals using the same tools we use for living animals," Edgecombe said. "It suggests there should be more examples out there."
About a year ago, Edgecombe and his colleagues found a different fossilized brain that revealed unexpected similarity to the brains of modern crustaceans.
"Our new find is exciting because it shows that mandibulates (to which crustaceans belong) and chelicerates were already present as two distinct evolutionary trajectories 520 million years ago, which means their common ancestor must have existed much deeper in time," Strausfeld said in a statement. "We expect to find fossils of animals that have persisted from more ancient times, and I'm hopeful we will one day find the ancestral type of both the mandibulate and chelicerate nervous system ground patterns. They had to come from somewhere. Now the search is on."
The detailed findings of the study were published online today (Oct. 16) in the journal Nature.
More from LiveScience:
Five Fossil Hotspots: National Parks to Visit
Wipe Out: History's Most Mysterious Extinctions
6 Extinct Animals That Could Be Brought Back to Life