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 world’s only animal, past or present, with a complete 360-degree spiral of teeth was Helicoprion, which sliced into prey like a buzz saw.
This shark-like fish, which lived 270 million years ago, is described in the latest issue of Biology Letters. It had one of the most unusual mouths and sets of teeth in the animal kingdom.
"When the animal closed its mouth on prey, the spiral of sharp teeth rotated backwards, like a circular saw, and slashed through the meat,” lead author Leif Tapanila, an associate professor in the Department of Geosciences at Idaho State University, told Discovery News.
Tapanila is also the research curator and head of the Earth Sciences Division at the Idaho Museum of Natural History. For the study, he and his colleagues took the first ever 3D images of Helicoprion remains.
Scientists have puzzled over this animal for more than a century, given its highly unusual "tooth whorl.” The new research sheds light on what this prehistoric marine species looked like, what its ancestry was and how it behaved.
"Helicoprion looked a lot like a big-bodied modern shark, but it had a very unusual mouth,” Tapanila said. "An arc of 15 to 18 serrated teeth were exposed in the center of its lower jaw, and it had no protruding teeth in the upper jaw.”
The buzz saw-looking tooth whorl had two functions, the researchers determined. The outermost part anchored the teeth for biting, while the rest of the inner spiral was designed to house the old and previously used teeth from when the animal was younger.
Leif Tapanila with Ray Troll
The scientists did not see much wear, tear and breakage, so they suspect Helicoprion primarily sliced into squid or other ancient relatively soft and somewhat chewy sea life. Aside from squid and their early relatives, armored and cartilaginous fish lived in Helicoprion’s ecosystem, along with brachiopods, bivalves and snails. "Cartilaginous” refers to fish made up of cartilage, a firm yet flexible connective tissue.
While Helicoprion looked and acted like a shark, the researchers determined that it’s at the base of the family tree that today includes chimaera (aka "ghost sharks”) and ratfish. Ghost sharks are not technically sharks, but they look and act a lot like them.
Tapanila explained that cartilaginous fish are divided into two main categories: sharks and rays on one side, ratfish and chimaera on the other. They are all marine predators.
No living land or sea animal directly resembles Helicoprion -- especially it’s buzz-saw tooth whorl.
"It was really an improbable animal, and maybe one of the best examples of a successful ‘Hopeful Monster,’” Tapanila said, explaining that this refers to evolutionary processes that can result in very unusual body types, with most doomed to failure.
While Helicoprion eventually went extinct, it used to have a nearly global distribution and existed over a period of 10 million years or more, proving that even some eccentric body designs can be successful if they meet the particular needs influenced by the animal’s environment, food sources and more.
John Long, a professor of paleontology at Flinders University, told Discovery News that he fully supports the new findings about Helicoprion and its kin.
"This study ends a century old mystery about this iconic fossil (species) and highlights the unexpected diverse body form that holocephalans occupied,” Long said.
Tapanila and his team would love to find a fossilized prey animal in the mouth of such a prehistoric shark-like animal, to better determine which exact species they were hunting and eating. Given that they lived even before the dinosaurs, Tapanila isn’t "holding his breath” for such a rare find.