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The discovery of a 365-million-year-old forelimb is helping scientists better understand how ancient creatures made the transition from water to land. A report published today in the journal Science describes the fossil, which represents an intermediate stage in the evolution of fish fins into vertebrate limbs.
Neil H. Shubin of the University of Chicago and his colleagues recovered the bone, which was encased in a brick-size piece of red sandstone, from the side of a highway in north central Pennsylvania. The layered rocks are the remains of a stream system dating back to the Late Devonian Period. "We knew it was a humerus," study co-author Michael I. Coates of the University of Chicago recalls, "but it was an entirely different kind. We had never seen one like it before." For example, compared with the anatomy of other tetrapods of the same age there is a large space for chest muscle attachment, the scientists report. This added brawn would have enabled a motion similar to a benchpress or push-up. Based on the apparent size and extent of the muscles, the authors posit that the humerus played a significant role in the support and movement of the animal.
The findings indicate that the ability to prop up the body is more ancient than previously believed. Says Coates, "This means that many of the features that we thought evolved to enable life on land originally evolved in fish living in aquatic ecosystems." Other fossils recovered from the same site indicate that the waterway was home to a variety of plant and animal life. The forelimb would have allowed the animal to propel itself along the bottom of the riverbed or elevate its head out of the water. In an accompanying commentary Jennifer A. Clack of the University of Cambridge notes that Devonian tetrapods "probably did not walk efficiently, but their modes of locomotion certainly varied, as they adapted skeletons and sensory organs for the challenges posed by emergence from the water."