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 fossil of a 520-million-year-old animal is so well preserved that its individual nerve fibers are still visible, according to a new study on the crustacean-like creature that once lived in southern China.
The fossil represents the oldest and most detailed central nervous system ever found, reports the study, which is published in the journal Proceedings of the National Academy of Sciences.
The vast majority of fossils only reveal bone and other hard body parts, such as teeth, since the nervous system and soft tissues are essentially made of fatty-like substances that degrade over time.
"This is a unique glimpse into what the ancestral nervous system looked like," study co-author Javier Ortega-Hernández of the University of Cambridge's Department of Zoology said of the fossil in a press release. "It's the most complete example of a central nervous system from the Cambrian period."
The animal, Chengjiangocaris kunmingensis, lived during a key part of Earth’s history known as the Cambrian explosion, a time during which rapid evolutionary development took place. At this time about a half billion years ago, most major animal groups first appear in the fossil record.
C. kunmingensis was a type of animal known as a fuxianhuiid, and was an early ancestor of modern insects, spiders and crustaceans.
Its nervous system featured a nerve cord running throughout its body. The nerve cord was similar to the spinal cord that we and many other organisms have today. Bead-like ganglia, or bundles of nerve cells, controlled the animal's single pair of walking legs.
Ortega-Hernández and his team closely examined the exceptionally preserved ganglia and discovered dozens of spindly fibers, each measuring about five thousandths of a millimeter in length.
“These delicate fibers displayed a highly regular distribution pattern, and so we wanted to figure out if they were made of the same material as the ganglia that form the nerve cord,” he explained. “Using fluorescence microscopy, we confirmed that the fibers were in fact individual nerves, fossilized as carbon films, offering an unprecedented level of detail. These fossils greatly improve our understanding of how the nervous system evolved.”
The researchers found that some aspects of the creature's nervous system are similar to that of modern velvet worms and penis worms, which really do resemble the male sexual organ. All of these animals have regularly spaced nerves that emanate from the central nerve cord.
Conversely, dozens of these nerves were independently lost over evolutionary time in many living animals including water bears, which are eight-legged segmented water-dwelling organisms that are so tiny that scientists refer to them as "micro-animals." The loss of the nerves over time in many species suggests that simplification played an important role in the evolution of the nervous system.
While the nerve cord is similar to today's spinal cord, it was a unique structure that is otherwise unknown in living organisms.
Ortega-Hernández said, "The more of these fossils we find, the more we will be able to understand how the nervous system — and how early animals — evolved."
Photo: Complete specimen of Chengjiangocaris kunmingensis from South China. Credit: Jie Yang, Yunnan University