The first vertebrates to walk the Earth emerged from the sea almost 20 million years earlier than previously thought, say scientists who have discovered footprints from an 8-foot-long (2.4-meter-long) prehistoric creature.
Dozens of the 395-million-year-old fossil footprints were recently discovered on a former marine tidal flat or lagoon in southeastern Poland (prehistoric time line).
The prints were made by tetrapods—animals with backbones and four limbs—and could rewrite the history of when, where, and why fish evolved limbs and first walked onto land, the study says.
Because they are thought to have evolved from such creatures, reptiles, birds, and mammals—including humans—are today classified as tetrapods.
(Related: "Before tetrapods: Fishy Ancestors of Humans Surprisingly Diverse.")
"These are the oldest tetrapod tracks and also the oldest evidence of true tetrapods," study co-author Grzegorz Niedƃwiedzki, a paleontologist at Warsaw University, commented via e-mail.
The tracks were made by several individuals of a four-limbed species that had digits, or toes, on each foot, according to the research.
"We are dealing with creatures that were walking," said Marek Narkiewicz, a geologist at the Polish Geological Institut and co-author of the study, to be published tomorrow in the journal Nature.
The footprints vary in size, some as wide as 10 inches (26 centimeters). The track sizes and shapes indicate flat-bodied, lizard-like creatures up to 8.2 feet (2.5 meters) long with stout legs, the researchers said.
Oldest Tetrapod Tracks
Discovered in an abandoned mountain quarry, the tracks suggest that tetrapods were traipsing the planet 18 million years earlier than previously indicated by the fossil record.
The tracks are also ten million years older than the oldest known fossils of lobe-finned fishes called elpistostegids, which are widely considered to be transitional forms between fish and tetrapods. (See a surviving lobe-finned fish, the coelacanth.)
The age of the newfound tracks suggest that "these transitional fish continued to exist alongside the tetrapods for quite some period of time," said Per Ahlberg, a paleontologist at Uppsala University in Sweden, who led the new research.
It's not so strange for one type of animal to live alongside its evolutionary successors, Ahlberg noted. Several feathered dinosaurs, for example, "continued to exist alongside the birds for millions of years."
The finding also suggests the fins-to-limbs evolution occurred in an intertidal or lagoon environment rather than a seasonally flooded forest, as indicated by earlier finds.
The tidal "scenario has considerable explanatory power," the researchers write in Nature.
Due to the regular coming and going of the tides, marine ancestors of tetrapods, for example, would have had easy access twice a day to marine animals stranded at low tide.
This reliable smorgasbord would have helped tetrapod ancestors find their legs, so to speak.
"If you're picking off dead and moribund animals in the strand land—those things left behind by the receding tide—well then you don't need to be terribly good at moving around," Ahlberg noted.
"You just need to be able to haul your way out, eat what you want to eat, and then haul your way back into the water again."
More Tetrapod Footprints Fossils Needed
The new tetrapod finding "could lead to significant shifts in our knowledge of the timing and ecological setting of early tetrapod evolution," said paleontologist Ted Daeschler via email. Daeschler studies fish-to-tetrapod evolution at the Academy of Natural Sciences in Philadelphia, Pennsylvania, but was not involved in the Nature study.
Daeschler notes, however, that tracks and trackways are notoriously difficult to interpret "with full confidence," and he's awaiting more evidence before abandoning existing explanations for the transition.
"No doubt that I will keep an open mind and keen eye on future developments," he said.