phys.org

Fish fossils reveal how tails evolved

Despite their obvious physical differences, elephants, lizards and trout all have something in common. They possess elongated, flexible structures at the rear of their bodies that we call tails. But a new study by a University of Pennsylvania paleobiologist reveals that the tails of fish and the tails of tetrapods, or four-limbed animals, are in fact entirely different structures, with different evolutionary histories.

[typhlonectes]

With an analysis of 350-million-year-old fossil fish hatchlings, Lauren Sallan, an assistant professor in the School of Arts & Science’s Department of Earth and Environmental Science, showed that these ancient juvenile fish had both a scaly, fleshy tail and a flexible fin, one sitting atop the other. A similar dual tail structure is seen in the embryos of modern teleosts, a group of ray-finned fish that make up more than 95 percent of living fish species.

Over evolutionary time, to adapt to their environments, adult teleosts and tetrapods each lost one of these tails…

Image:

Aetheretmon (facing right), an early ray-finned fish, swims in a 348-million-year-old river in Foulden, Scotland. Aetheretmon exhibits the ancestral state of two distinct ‘tails’, fleshy tail above and caudal fin below.

The 32,000 species of living teleost fishes (the pufferfish, center facing left) have lost the upper tail, while early tetrapods (upper left) lost the lower caudal fin. Thus, living fish and tetrapod tails are entirely distinct structures