With the 200th anniversary of the birth of Charles Darwin this week, people around the world are celebrating his role as the father of evolutionary theory.
Events and press releases are geared, in part, to combat false claims made by some who would discredit the theory.
One frequently cited "hole" in the theory: Creationists claim there are no transitional fossils, aka "missing links."
Biologists and paleontologists, among others, know this claim is false.
As key evidence for evolution and species' gradual change over time, transitional creatures should resemble intermediate species, having skeletal and other body features in common with two distinct groups of animals, such as reptiles and mammals, or fish and amphibians.
These animals sound wild, but the fossil record — which is far from complete — is full of them nonetheless, as documented by Occidental College geologist Donald Prothero in his book "Evolution: What the Fossils Say and Why It Matters" (Columbia University Press, 2007).
Prothero discussed those fossils last month at the American Museum of Natural History in New York, along with transitional fossils that were announced since the book was published, including the "fishibian" and the "frogamander."
At least hundreds, possibly thousands, of transitional fossils have been found so far by researchers. The exact count is unclear because some lineages of organisms are continuously evolving.
Here is a short list of transitional fossils documented by Prothero and that add to the mountain of evidence for Charles Darwin's theory.
A lot of us relate most to fossils of life closely related to humans, so the list focuses on mammals and other vertebrates, including dinosaurs.
Mammals, including us
-- It is now clear that the evolutionary tree for early and modern humans looks more like a bush than the line represented in cartoons.
All the hominid fossils found to date form a complex nexus of specimens, Prothero says, but Sahelanthropus tchadensis, found in 2001 and 2002, threw everyone for a loop because it walked upright 7 million years ago on two feet but is quite chimp-like in its skull size, teeth, brow ridges and face.
It could be a common ancestor of humans and chimpanzees, but many paleoanthropologists will remain unsure until more fossils are found. Previously, the earliest ancestor of our Homo genus found in the fossil record dated back 6 million years.
-- Most fossil giraffes have short necks and today's have long necks, but anatomist Nikos Solounias of the New York Institute of Technology's New York College of Osteopathic Medicine is preparing a description of a giraffe fossil, Bohlinia, with a neck that is intermediate in length.
-- Manatees, also called sea cows, are marine mammals that have flippers and a down-turned snout for grazing in warm shallow waters.
In 2001, scientists discovered the fossil of a "walking manatee," Pezosiren portelli, which had feet rather than flippers and walked on land during the Eocene epoch (54.8 million years ago to 33.7 million years ago) in what is now Jamaica.
Along with skull features like manatees (such as horizontal tooth replacement, like a conveyor belt), it also had heavy ribs for ballast, showing that it also had an aquatic lifestyle, like hippos.
-- Scientists know that mastodons, mammoths and elephants all share a common ancestor, but it gets hard to tell apart some of the earliest members of this group, called proboscideans, going back to fossils from the Oligocene epoch (33.7 million years ago to 23.8 million years ago).
The primitive members of this group can be traced back to what Prothero calls "the ultimate transitional fossil," Moeritherium, from the late Eocene of Egypt.
It looked more like a small hippo than an elephant and probably lacked a long trunk, but it had short upper and lower tusks, the teeth of a primitive mastodon and ear features found only in other proboscideans.
-- The Dimetrodon was a big predatory reptile with a tail and a large sail or fin-back. It is often mistaken for a dinosaur, but it's actually part of our mammalian lineage and more closely related to mammals than reptiles, which is seen in its specialized teeth for stabbing meat and skull features that only mammals and their ancestors had. It probably moved around like a lizard and had a jawbone made of multiple bones, like a reptile.
Dinosaurs and birds
-- The classic fossil of Archaeopteryx, sometimes called the first bird, has a wishbone (fully fused clavicle) which is only found in modern birds and some dinosaurs. But it also shows impressions from feathers on its body, as seen on many of the theropod dinosaurs from which it evolved.
Its body, capable of flight or gliding, also had many of dinosaur features — teeth (no birds alive today have teeth), a long bony tail (tails on modern birds are entirely feathers, not bony), long hind legs and toes, and a specialized hand with long bony fingers (unlike modern bird wings in which the fingers are fused into a single element), Prothero said.
-- Sinornis was a bird that also has long bony fingers and teeth, like those seen in dinosaurs and not seen in modern birds.
-- Yinlong is a small bipedal dinosaur which shares features with two groups of dinosaurs known to many kids — ceratopsians, the beaked dinosaurs like Triceratops, and pachycephalosaurs, known for having a thick dome of bone in their skulls protecting their brains. Yinlong has the thick rostral bone that is otherwise unique to ceratopsians dinosaurs, and the thick skull roof found in the pachycephalosaurs.
-- Anchisaurus is a primitive sauropod dinosaur that has a lot of lizard-like features. It was only 8 feet long (the classic sauropods later on could be more than 100-feet long), had a short neck (sauropods are known for their long necks, while lizards are not), and delicate limbs and feet, unlike dinosaurs. Its spine was like that of a sauropod.
The early sauropods were bipedal, while the latter were stood on all fours. Anchisaurus was probably capable of both stances, Prothero wrote.
Fish, frogs, reptiles
-- Tiktaalik, aka the fishibian or the fishapod, is a large scaled fish that shows a perfect transition between fins and feet, aquatic and land animals.
It had fish-like scales, as well as fish-like fin rays and jaw and mouth elements, but it had a shortened skull roof and mobile neck to catch prey, an ear that could hear in both land and water, and a wrist joint that is like those seen in land animals.
-- Last year, scientists announced the discovery of Gerobatrachus hottorni, aka the frogamander. Technically, it's a toothed amphibian, but it shows the common origins of frogs and salamanders, scientists say, with a wide skull and large ear drum (like frogs) and two fused ankle bones as seen in salamanders.
-- A turtle on the way to becoming a turtle, Odontochelys semistestacea, swam around in China's coastal waters 200 million years ago. It had a belly shell, but its back was basically bare of armor. Odontochelys had an elongated, pointed snout. Most modern turtles have short snouts.
In addition, the roof of its mouth, along with the upper and lower jaws, was equipped with teeth, which the researchers said is a primitive feature for turtles whose mugs are now tipped with beaks but contain no teeth.
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