The Mystery of the Wandering Daddy Longlegs

This article was published in 2007 on National Geographic's blog Phenomena.

Here is a lovely little creature from Sri Lanka, Pettalus cf. cimiciformis, a member of the same lineage that includes the daddy longlegs we’re all familiar with. You could call it a daddy longlegs too, but its legs aren’t particularly long (plus it’s tiny–the size of a sesame seed.

It may not seem like much, but it poses a fascinating riddle. It belongs to a family of daddy longlegs called Petallidae. Below is a map of where other species of Petallidae can be found. They seem to be scattered randomly across the world. But petallids are terrible at dispersing. Their ranges are small (usually less than fifty miles). And they live only on ancient continent crust. Petallids live on Sri Lanka and on Madagascar. But they live on none of the young volcanic islands in between–or anywhere else in the world, for that matter. So they couldn’t have swum or flown to their farflung locations. Yet DNA evidence clearly shows that the petallids all descend from a common ancestor. So, how did they get there? The answer lies below the fold…

Here is a picture of the southern hemisphere about 150 million years ago. Ever since the continents formed, they have been drifting slowly across the surface of the planet. Back in the Jurassic Period, South America, Africa, Antarctica, India, and Australia were all welded together. So were a few islands, such as Madagascar and Sri Lanka. And the current ranges of petallids, so fragmented today, appear to have covered the southern part of this vanished supercontinent. The lineage first evolved there, slowly spread out across a continuous landmass, and then rode across the planet on drifting continents.

The journeys of animals and plants are the subject of one of science’s lesser known branches, historical biogeography. I’ve always thought its obscurity was a shame, because it can do such a remarkable job of illuminating the history of the planet through a comparison of living things. Much to my surprise, stumpy daddy longlegs are turning out to be among the best guides to that history. The full story can be found in my article in tomorrow’s New York Times, plus an even more detailed map. (Two papers in press can be found here and here.)

Lead Image: National Geographic

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