Four Legs, a Tail, and Tracks a Third of a Billion Years Old

"Four-legged Creature's Footprints Force Evolution Rethink"
LiveScience (January 6, 2010)

"Four-legged creatures were mucking around a muddy basin in what is now Poland about 397 million years ago. And they left behind distinctive footprints, which have turned back the clock on the evolution of these landlubbers.

"Scientists discovered the fossilized prints, which included various trackways and isolated prints, in the Holy Cross Mountains in southeastern Poland. Analyses suggest most if not all of them came from different tetrapod species — which are four-legged animals that had backbones, such as amphibians — with some possibly belonging to juveniles and adults of the same species.

"The land creatures likely had bodies shaped somewhat like crocodiles, with fin-like tails and stumpy legs. And some of them were pretty big, reaching up to about 10 feet (3 meters) in length, the researchers said...."

I suspect that, once details of how these critters were put together are filled in, they'll be amphibians. At least, not quite. Those tracks are old, and the amphibians we know about come from a (relatively) more recent time.

So, how can the scientists know what they probably looked like? The tracks. Critters with four legs make tracks that are different from critters with two - and a lizard's long, narrow tail makes a different mark from a beaver's broad, relatively short one.

Adapt or Perish: Now There's an Incentive!

There's an interesting detail, about midway through the article:

"Since scientists have used modern amphibians and such as models for the earliest tetrapods, some have assumed the earliest four-limbed creatures emerged from a freshwater environment, Ahlberg said.

"Not so, according to the new prints.

" 'It seems like it was a very extensive muddy basin, marine basin, that was very shallow and very wide, hundreds of kilometers wide,' said study scientist Marek Narkiewicz of the Polish Geological Institute, adding that the basin likely dried out every few years or so...."

That would give critters that could operate in water or on land a big advantage over ones that had the option of staying in water or dying. Which, from what we've pieced together about how the universe works, means that the optional-land-dwellers left more descendants each generation.

Then, again assuming that the folks who study such things haven't been completely addled for the last century or so, eventually some of those tetrapods' really, really distant descendants came back and found some of the tracks they left.¹

"Tetrapod," by the way, isn't a species of animal. It's a critter with four feet: "a vertebrate animal having four feet or legs or leglike appendages" (Princeton's WordNet)

Things Happened Earlier and were Different

I've been following what paleontologists have been piecing together, about the story of life on Earth, for maybe a half-century now. One of the constants I've noticed is that, as we uncover new evidence - or somebody takes a fresh look at something that's been lying on a shelf for decades - the scientists have discovered that whatever they're studying happened earlier than they though it had. And that some of their earlier assumptions simply won't fit with the new data.

For me, it's as much fun as reading a well-crafted detective story.

Vaguely-related posts:

• "Something Was Crawling on the Precambrian Mud"
  (February 19, 2010)
• "Hurdia Victoria: Cambrian Supercritter of the Burgess Shale"
  (March 20, 2009)

More, about new evidence for really early muscles:

• "First evidence for locomotion in the Ediacara biota from the 565 Ma Mistaken Point Formation, Newfoundland"
  Alexander G. Liu, Duncan Mcllroy and Martin D. Brasier1, GeoScienceWorld (February 2010)
  • Alex Liu bio, with a short definition of Ediacara biota

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Related posts, at

• "Change, American Culture, Trilobites, Humanity's History, and the Big Picture"

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¹ I'm a devout Catholic, so the idea that the universe is somewhat orderly and follows consistent rules isn't all that hard for me to accept. I've written about the West's odd notions about faith and reason before, in another blog.