Ancient Fossils Of Tough Fish Shed Light On Evolution Of Four-Footed Animals
March 28, 2013

Species From Ancient ‘Fish-Eat-Fish’ World Sheds Light On Evolution Of Four-Footed Animals

April Flowers for - Your Universe Online

Describing life in the Devonian period in what is now northern Canada, Dr. Ted Daeschler of Drexel University said, "We call it a 'fish-eat-fish world,' an ecosystem where you really needed to escape predation.”

The famous fossil fish species Tiktaalik roseae lived in this environment 375 million years ago. Daeschler, associate professor at Drexel University in the Department of Biodiversity, Earth and Environmental Science, and associate curator and vice president of the Academy of Natural Sciences of Drexel University, discovered the lobe-finned fish with his colleagues Dr. Neil Shubin and Dr. Farish A. Jenkins, Jr.

Tiktaalik roseae was first described in the journal Nature in 2006, and has received scientific and popular acclaim for providing some of the clearest evidence to date of the evolutionary transition from lobe-finned fish to limbed animals known as tetrapods.

The original Tiktaalik research team has now described a new lobe-finned fish species from the same time and location in the Canadian Arctic. The new species, Holoptychius bergmanni, is described in the latest issue of Proceedings of the National Academy of Sciences of Philadelphia.

"We're fleshing out our knowledge of the community of vertebrates that lived at this important location," said Dr. Jason Downs,“¯research associate at the Academy. Downs said the description of species from this very important time and location would aid the scientific community in understanding the transition from finned vertebrates to limbed vertebrates.

"It was a tough world back there in the Devonian. There were a lot of big predatory fish with big teeth and heavy armor of interlocking scales on their bodies," said Daeschler.

Both Holoptychius and Tiktaalik were large predatory fish adapted to life in stream environments. It is possible that the two fish competed for food, although the tetrapod-like skeletal remains of Tiktaalik suggest that they might have specialized in slightly different niches as it would have been especially well suited to living in the shallowest waters.

The fossilized specimens used by the research team to characterize Holoptychis bermanni come from multiple fish remains. They include lower jaws with teeth, skull pieces including the skull roof and braincase, and parts of the shoulder girdles. A complete specimen would have been 2 to 3 feet long when it was alive.

"The three-dimensional preservation of this material is spectacular," Daeschler said. "For something as old as this, we'll really be able to collect some good information about the anatomy of these animals."

Downs, who also teaches at Swarthmore College, led the research team that named the new fossil species in honor of the late Martin Bergmann. Bergmann was the former director of the Polar Continental Shelf Program (PCSP), Natural Resources Canada. For more than a decade, PCSP provided logistical support during the team's Arctic research expeditions. In 2011, shortly after the team's most recent field season in Nunavut, Bergmann died in a plane crash.

"We decided to choose Martin Bergmann to honor him, not ever having met him, but with the understanding that his work with PCSP made great strides in opening the Arctic to researchers," said Downs. "It's an invaluable project happening in the Canadian Arctic that's enabling this type of work to happen."

Many aspects of the expedition's logistics, including difficult flight operations to carry supplies and research personnel to remote research sites on Ellesemere Island, were carried out by Bergmann and PCSP. According to Daeschler, the pilots were capable of landing a Twin Otter aircraft almost anywhere there was solid ground — a claim they tested by touching down the airplane and circling back to see whether the tires left a mark in the mud.

The research team intends to return to Ellesmere Island for another field season in the summer of 2013. They will excavate a more northerly site in hopes of finding fossils in older rock formations.


In the same Bicentennial issue of Proceedings of the National Academy of Sciences, but with a different set of co-authors, Daeschler describes a second Devonian fish fossil, this time found in a 370-million-year-old red sandstone highway roadcut in north-central Pennsylvania.

The new species of armored fish is a phyllolepid placoderm, known for the distinctive ornamentation of ridges on their exterior plates. Scientists often find the remains of these fish as impressions in stone, rather than 3D versions of their skeletons. The process of studying and describing the fish's anatomy, therefore, takes advantage of a technology that looks more suited to a crime scene investigation than a paleontological dig.

The researchers made a rubber cast by pouring latex into the natural impression left by the fossil in the surrounding rock. Once the latex hardens, the scientists peel it out and dust the surface with a fine powder to better show the edges of the bony plates and the shapes of the fine ridges on the armor. This process is akin to dusting for fingerprints to show minute ridges left on a surface. This dusting allowed the researchers to get a clearer view of the fossil's features in order to prepare a detailed scientific description of the species.

The new species has been named Phyllolepis thomsoni. Daeschler and his co-author, Dr. John A. Long, a leading authority on placoderms from Flinders University in Australia, named the species in honor of Dr. Keith S. Thomson. Thompson is the “¯Executive Officer of the American Philosophical Society, and has served as a mentor and colleague to many Devonian fossil researchers.