Fossils fill evolutionary gap between fish and 4-legged beasts
Adam Rifkin stashed this in Science!
How fish evolved into 4-legged beasts now has some fossil evidence:
Tetrapods first developed flipper-like limbs and started crawling out of the water about 370 million years ago, Anderson told CBC's Quirks & Quarks. At that time, they were quite large, about the size of a person, and very fish-like, with tail fins and internal gills. They also had variable numbers of fingers and toes — "up to eight," Anderson said in an interview that airs Saturday.
Thirty-million years later, tetrapods had become well-adapted to moving around on land and had the five fingers and toes that most amphibians, reptiles, birds and mammals have today. They had lost their gills and tail fins, and developed the muscles for holding their bodies up from the ground, as shown by their footprints.
"There are no trail marks for either tails or stomachs," Anderson said.
The apparent gap in the fossil record between the fish-like and more advanced tetrapods was first written about by U.S. paleontologist Al Romer and has since been named "Romer's gap." To explain the gap, some paleontologists proposed that low oxygen during that period prevented tetrapods from evolving.
Now, Anderson, Chris Mansky of the Blue Beach Fossil Museum in Nova Scotia, and their collaborators have found and described fossils from at least four different kinds of tetrapods that lived during the early part of Romer's gap.
The fossils show that both fish-like and advanced tetrapods lived alongside one another at that time.