|
|
|
|
|
Snake fossil found in Patagonia, 90 million years old. First
snake with a sacrum and two small rear legs; most primitive snake
fossil ever found; apparently lived on land. See article below. Ethiopian skull could represent clearest example of transitional form between Homo erectus and Homo sapiens. Posted 27 March 2006 on the BBC News Online, <http://news.bbc.co.uk/1/hi/sci/tech/4849320.stm>, the new, nearly complete skull dates to between 500,000 and 250,000 years ago, and was found in the context of stone tools and the fossils of many other animal types. There are also many links to earlier BBC announcements in human evolution. New "walking-fish" fossils found in northern Canada reveal clearest example of fore-arm and wrist bones articulating with fins, and other features predicted for fish-to-tetrapod transitionals. Posted on the "World Science" website at <http://www.world-science.net/othernews/060405_tiktaalikfrm.htm>. Useful graphics are included (can click and drag to desktop for classroom display) to show features of this 375 million year old fossil in context with other early vertebrates. See DISCUSSION OF SIGNIFICANCE of this fossil BELOW. I would encourage signing up for either one or both of these sites for regular news announcements in science. |
|
9 April 2006: Walking Fish Fossil: Discussion of Significance: With the Tiktaalik fossil recently reported (April, 2006), teachers may wonder what is so important about this new find. How is it different from the Coelacanth, or a lungfish, or a mudskipper? How does it compare with other fossils of early amphibians and fishes? The following comes from Physicist Pim van Meurs' efforts to cobble the research bearing on this issue, courtesy of Dr. van Meurs. It has long been clear that limbed vertebrates (tetrapods)
evolved from osteolepiform lobe-finned fishes: <http://www.nature.com/nature/journal/v440/n7085/full/440747a.html#B3>
, Ichthyostega Panderichthys Into this gap drops Tiktaalik. The fossils are earliest
Late Devonian in age, making them at most 2 million or 3 million
years younger than Panderichthys. With its crocodile-shaped
skull, and paired fins with fin rays but strong internal limb
skeletons, Tiktaalik also resembles Panderichthys
quite closely. The closest match, however, is not to Panderichthys
but to another animal, Elpistostege, from the early Late
Devonian of Canada. Elpistostege is known only from two
partial skulls and a length of backbone, but it has long been
recognized as a fish - tetrapod intermediate, So, if Tiktaalik is in effect a better-preserved version of Elpistostege, why is it important? First, it demonstrates the predictive capacity of paleontology. The Nunavut field project had the express aim of finding an intermediate between Panderichthys and tetrapods, by searching in sediments from the most probable environment (rivers) and time (early Late Devonian). Second, Tiktaalik adds enormously to our understanding of the fish-tetrapod transition because of its position on the tree and the combination of characters it displays. So why are lungfish or mudskippers not relevant? Because hundreds of millions of years have passed. An impediment to understanding the fin-limb transition has been the nature of available evidence from the sister group of tetrapods. The closest living relatives of tetrapods -- lungfishes and coelacanths -- either lack homologous elements to distal limb bones or are so specialized that comparisons with tetrapods are uncertain. Mudskippers belong to a different branch. Sarcopterygians are the lobe finned fishes (e.g., the Coelacanth); the four-legged vertebrates include lungish, mammals, reptiles, amphibians, birds etc. Actinopterygians are the ray finned fishes (including the mudskipper). See also <http://en.wikipedia.org/wiki/Perciformes>. |
|
A fossil find in Argentina has revealed a two-legged creature that's the most primitive snake known, a discovery that promises to fire up the scientific debate about whether snakes evolved on land or in the sea. The snake's anatomy and the location of the fossil show it lived on land, researchers said, adding evidence to the argument that snakes evolved on land. Snakes are thought to have evolved from four-legged lizards, losing their legs over time. But scientists have long debated whether those ancestral lizards were land-based or marine creatures. The newly found snake lived in Patagonia some 90 million years ago. Its size is unknown, but it wasn't more than 3 feet long, said Hussam Zaher of the University of Sao Paulo in Brazil. He and an Argentine colleague report the find in Thursday's issue of the journal Nature. It's the first time scientists have found a snake with a sacrum, a bony feature supporting the pelvis, Zaher said. That feature was lost as snakes evolved from lizards, he said, and since this is the only known snake that hasn't lost it, it must be the most primitive known. The creature clearly lived on land, both because its anatomy suggests it lived in burrows and because the deposits where the fossils were found came from a terrestrial environment, said Zaher. So, if the earliest known snake lived on land, that suggests snakes evolved on land, he said. There has been little new evidence in recent years in the land-versus-sea debate, and "we needed something new," said Zaher. "We needed a new start. And this snake is definitely a new start for this debate." He said that although the creature had two small rear legs, it crawled like a modern-day snake and probably used its legs only on occasion, though for what purpose is unclear. The creature, named Najash rionegrina, is "a fantastic animal," said Jack Conrad, a researcher at the American Museum of Natural History in New York and co-curator of an upcoming exhibit on lizards and snakes. "It's really going to help put to rest some of the controversy that's been going around snake evolution and origins," he said. Conrad said he never took sides in the land-versus-sea debate, but "but this is starting to convince me." Olivier Rieppel, a fossil reptile expert at the Field Museum in Chicago, called the find important and said Najash is clearly the most primitive known snake. If snakes did evolve on land rather than the sea, their fossil record might be less complete because early fossils would have been better preserved in a marine environment, he said. That, in turn, suggests "we may not know all the lineages of early snake evolution," he said. Maybe several snake lineages lost the legs of their lizard ancestors independently, he said. The creature's name comes from a Hebrew word for snake and the Rio Negro province of Argentina, where the discovery was made. On the Net: Nature: http://www.nature.com/nature |