Counting on Toes

First to Conquest Land

pentadactyl
Progression across species of the five fingers and toes

Our four-legged, five-toed ancestors conquered the land earlier and more independently than expected, say paleontologists studying newfound 345 to 359-million-year-old tracks at an eroding beach in eastern Canada.

At least six different kinds of four-limbed reptile-like animals – a.k.a. tetrapods – with five digits on their feet left their tromping prints in the mud of what was once a tropical swamp at Blue Beach, Nova Scotia. The five-digit tracks range in size from four inches to less than an inch and seem to contradict the prevailing idea that the first tetrapods had wide variety of other-than-five-toed (polydactyl) feet, out of which five-toed (pentadactyl) animals later won out as the most efficient on land.

"We’re talking about a fundamental question in the history of life," said paleontologist Spencer Lucas of the New Mexico Museum of Natural History and Science in Albuquerque. "We are talking about the origin of terrestriality. It’s from this conquest (of land) that we are here."

It’s also the evolutionary step that set the stage for our ten fingers and toes.

Research on the Blue Beach track discoveries by Lucas, Adrian Hunt, Chris Mansky and John Calder will be presented at the annual meeting of the Geological Society of America in Denver.

The tracks found in Blue Beach’s Horton Bluff formation appear to be from animals belonging to groups better known from fossil remains in later deposits, explained Lucas. "It shows that some of these groups of animals probably were around a lot earlier," he said.

darwin
Progression from land to sea, legs to fins, walking to swimming come full circle

The fact that all of the tracks at Blue Beach are five-toed suggests that pentadactyl tetrapods developed as a group quite early in the Carboniferous, tens of millions of years earlier than thought. They actually moved ahead of the pack, and were not survivors of the winnowing of polydactyl tetrapods. In other words, all those other-toed tetrapods might have been an evolutionary sideshow rather than the main stage out of which our pentadactyl ancestors evolved, says Lucas.

The discovery also highlights the great value of what has long been an underappreciated track site. "Nova Scotia is the Rosetta Stone for Carboniferous footprints," Lucas said.

It’s also a great opportunity to see how bringing together information from fossil tracks and later fossil bones can illuminate critical evolutionary stages. Unlike fossil skeletons, tracks tell a lot about how an animal behaved and moved – as well as hint at their anatomy.

"I think what’s exciting is that (tracks) show things that the bone record doesn’t," Lucas said. "I’ve come to see how much more you get when you consider both records."

Also making the Blue Beach site unique is how it is excavated, Lucas pointed out. Instead of being a slow, controlled excavation of layers of rock, the ocean is in control. Researchers have to time their work around tides and surf which are rapidly eroding the bluffs and continually exposing new tracks. Most of the fossil tracks have been collected by Blue Beach local Chris Mansky who is able to walk the beach regularly and collect specimens as Nature offers them up.


Related Web Pages

Synchronizing Molecular Clocks
The Pull of the Recent
Dinner with Darwin
Walking Before Flying?

Evolution’s Sweet Tooth
The Paleobiology Database
NASA Workshop on Biodiversity