Your Inner Fish In layers of rock that are 385 million years old, you find fossils of fish. These are easily recognizable fish with scales, fins, and no neck. There are no fossils of amphibians or other land-dwelling animals in these rocks. You can look anywhere in the world, and there will be no land-dwelling animals 385 million years ago. No exceptions. In rocks that are 365 million years old, you find fossils of amphibians and reptiles. These creatures closely resemble extant land-living animals with necks, ears and four limbs. The limbs are organized in the manner of all vertebrate tetrapods with a single bone in the upper limb connected to two parallel bones in the lower limb. In the human arm, for example, these are the humerus and the radius/ulna bones, respectively. In the leg, they are the femur and the tibia/fibula. If you are interested in finding fossils of animals as they transitioned from aquatic life to land, it doesn’t take a genius to figure out that you should look in rocks that are about 375 million years old. You will be looking for the first four-legged creatures. In 1999, three paleontologists set out to do just that. Finding the right rocks is not trivial. The rocks had to be 375 million years old, they had to be exposed so that fossils were accessible, and the rocks had to have been part of shallow, freshwater streams during that same period. This last criterion arises because previous work suggested that shallow, freshwater streams were the most likely place for fish to make the transition to land. All of these criteria are met in only a few places on earth. The scientists ended up on the arctic islands of Canada. In the arctic, you work for a few months and then go home and wait until the next year. On their first trip in 1999, they found nothing but fossils of deep sea fish. Failure. On their return in 2000, they moved their site. Near the end of that year, the researchers found small fragments of bones resembling those they sought. The challenge at that point became finding the layer of rock that gave rise to the fragments. They needed to find larger pieces of the animals. This took four years. In 2004, they found Tiktaalik, a 7-9 foot creature that shared characteristics of both fish and land-living creatures. Like a fish, Tiktaalik had scales on its back and fins. Like a land-living animal, Tiktaalik had a flat head and a neck. The bones that normally connect a fish’s shoulder and head were missing, allowing the head to move independent of the body. In addition, unlike fish, the fins contained limb bones with the standard one bone/two bone structure. Primitive beginnings of wrist bones were also evident. Tiktaalik could bear its own weight. It could do a pushup. The results of this work were published in Nature in 2006. Not only was Tiktaalik an intermediate between two kinds of animal, it was found in the right period in earth’s history and in the right ancient environment. Tiktaalik is not the only transitional creature in the fossil record, but the move from aquatic life to land is of particular evolutionary significance for humans. In addition, it is an example of the sophistication and predictive abilities of modern paleontology. If you are unlucky enough to be in the presence of someone who says there are no transitional forms in the fossil record, direct them to Daeschler et al., (2006) A Devonian tetrapod-like fish and the origin of the tetrapod body plan, Nature 757:757-763. Or you could just burst out laughing. Depends on your mood and whether you think the person is worth any effort. *Details of this story are from: Your Inner Fish by Neil Shubin, 2008, Random House. I recommend the book highly. I am poorly versed in paleontology; this book was a welcome education and a wonderful read.
Posted on: Sun, 01 Jun 2014 14:19:57 +0000
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