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 team of Pakistani researchers claims to have unearthed a 1.1 million-year-old stegodon tusk in the central province of Punjab, potentially shedding new light on the mammal’s evolutionary journey.
Stegodonts, distant cousins of modern elephants, are thought to have been present on earth from around 11 million years ago until the late Pleistocene period, which lasted until the end of the last Ice Age around 11,700 years ago.
The tusk measures some eight feet (2.44 metres) in length and is around eight inches (20.3 cm) in diameter, making it the the largest ever discovered in the country, according to the team.
It was found by researchers from the zoology department of the University of Punjab during an expedition in the Padri village of Jhelum district, said Khurram Shahzad, a spokesman for the university.
Professor Muhammad Akhtar, who led the research trip, told AFP: “This discovery adds to our knowledge about the evolution of the stegodon, particularly in this region.
“It also sheds light on what the environment was like at the time of the animal’s life.”
Dr Gerrit Van Den Bergh, a paleontologist at the University of Wollongong in Australia who has done extensive research on the ancient mammals including in Pakistan, said: “If you have a complete tusk, that’s quite special — they are quite rare.”
He cautioned however that further verification, including of the dating, would be required.
Akhtar said the fossil belonged to the late Pleistocene period and its age was determined using a uranium-lead radioactive dating technique.
Stegodonts were known for their long, nearly straight tusks and low-crowned teeth with peaked ridges.
This indicated they were browsers or mixed feeders in a forested environment, in contrast to the high-crowned plated molars of mammoths and elephants which allowed them to graze.
They were strong swimmers and are thought to have originated in Africa but to have quickly spread to Asia, where most remains have been found.
“Around 1.2 million years ago they were still thriving,” said Van Den Bergh. “They are mostly an Asian species but remains have been found further afield. Recently a molar fragment was discovered in Greece.”
He added that the species’ extinction coincided with the emergence of modern humans, though it was difficult to say with certainty that men hunted stegodonts.