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.
“Living” molecules, meaning intact cellular structures that haven’t fossilized, were recently retrieved from 350-million-year-old remains of aquatic sea creatures uncovered in Ohio, Indiana, and Iowa, according to a study that will appear in the March issue of the journal Geology.
The animals- crinoids- were spindly and had feathered arms. Their relatives today are called by the plant-like name “sea lily.”
The retrieved molecules are quinones, which function as pigments or toxins (to help ward off predators) and are still found in modern sea lilies. The molecules aren’t DNA, unfortunately, but they can reveal other things about past life, such as the color of long gone animals.
“There are lots of fragmented biological molecules — we call them biomarkers — scattered in the rock everywhere,” William Ausich, professor in the School of Earth Sciences at Ohio State and co-author of the paper, said in a press release. “They’re the remains of ancient plant and animal life, all broken up and mixed together. But this is the oldest example where anyone has found biomarkers inside a particular complete fossil. We can say with confidence that these organic molecules came from the individual animals whose remains we tested.”
The ultra ancient crinoids appear to have been buried alive in storms during the Carboniferous Period. At that time, North America was covered with vast inland seas. The skeletal remains of the buried-alive crinoids filled with minerals over time, but some of the pores containing organic molecules were miraculously sealed intact.
This finding helps to negate the prior belief that complex organic molecules cannot survive fossilization.
Lead author Christina O’Malley, from Ohio State too, began the study when she noticed something strange about crinoids that had perished side by side and became preserved in the same piece of rock. She observed that the various species were preserved in different colors.
In one rock sample used in the study, one crinoid species appears a light bluish-gray, while another appears dark gray and yet another more of a creamy white. All stand out from the color of the rock they were buried in. The researchers have since found similar fossil deposits from around the Midwest.
“People noticed the color differences 100 years ago, but no one ever investigated it,” O’Malley said. “The analytical tools were not available to do this kind of work as they are today.”
She and her team employed a high tech machine called a gas chromatograph mass spectrometer to vaporize a liquid mixture that contained small bits of the ground up fossils. Computer software identified some of the resulting molecules as quinones.
The researchers next compared these molecules with ones from living sea lilies. As the scientists suspected, quinone-like molecules occur in both living sea lilies and their fossilized ancestors.
While “mummified” dinosaurs have yielded 66-million-year-old organic material, this level of preservation is exceedingly rare. And consider that these prehistoric sea lilies lived long before the first dinosaurs.
Crinoids tend to preserve really well because, like modern sand dollars, they possess a skin on top of their hard shells, which consist of stacked calcite rings. Calcite is a mineral made up of calcium carbonate. It is stable over geologic time, so organic matter may be protected by it when sealed whole.
“We think that rock fills in the skeleton according to how the crystals are oriented,” Ausich said. “So it’s possible to find large crystals filled in such a way that they have organic matter still trapped inside.”
“These molecules are not DNA,” he added, “and they’ll never be as good as DNA as a means to define evolutionary relationships, but they could still be useful. We suspect that there’s some kind of biological signal there—we just need to figure out how specific it is before we can use it as a means to track different species.”
Image: A modern crinoid, NURC/UNCW and NOAA/FGBNMS