Fins and Fingers in One Fishy Fossil

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It was six feet long, and swam in the swampy waters of what is now north-central Pennsylvania, some 370 million years ago. Like fish today, it had fins. But unlike fish today, those fins contained something very much like fingers — eight of them in each fin — even though the creature was not amphibious and could not have spent time on land. It was an evolutionary dead-end, but one with considerable evolutionary implications. 
   It was one of those fin-fingered fossils that Ted Daeschler, a graduate student in geology, and his fossil-hunting partner, Dr. Neil Shubin, associate professor of biology, discovered in a basketball-sized boulder alongside Route 15 north of Williamsport, Pa. And that Sauripterus fossil is so clearly defined — unlike a similar one found in the same part of the state back in 1840 — that it appears to have confirmed an important but hitherto-unproved theory. 
   “It’s a debate that’s been going on for quite a while,” says Daeschler. “‘Did digits arise when animals began to exploit land, or did digits arise in water?’ And this is sort of closing that debate.” The debate is significant enough to have attracted the producers of Nightline and reporters from USA Today, among other places, after Daeschler and Shubin published an article on their find in the January 1998 issue of Nature.
   “Sometimes evolution is more gradual and intermediate than we can ever have imagined,” explains Shubin. “Here you have, in water, the ability to develop digits. The main step here is not the evolution of a whole new set of structures but changing functions of structures that already existed.” While the fish was clearly an evolutionary dead-end, he says, think of it “as a cousin of the lineage that gave rise to us, that actually invaded land.” It may be “an odd cousin, sort of distantly removed,” he acknowledges, “yet it shows enough similarities to tell us what these antecedents of limbs looked like.” 
   Exactly what function the “fingers” had is still debatable. “It could have been used for a lot of things,” says Shubin. “It could have been used to walk on the bottom of the [swamp] — to push the front part of the body up to breathe, to swim around in plant-choked streams. We do know what it wasn’t doing, and that’s the important thing. It wasn’t being used to walk on land, because those things are almost paper-thin.” 
   Daeschler and Shubin, both in their late thirties, discovered the fossil about 20 miles from the site of their last big find: a 365-million-year-old tetrapod fossil, the oldest ever found in North America. In both instances, they have the Pennsylvania Department of Transportation to thank for widening the road and uncovering the rock in question. 
   Northern Pennsylvania, says Shubin with a laugh, “has been very, very good to us.”

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