The Loom

i-8ae670cb20d35f2e60ac32356e7ac0ba-petallid150.jpgHere 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…

i-93756e4f9761db9fa531c0f32737a678-petallidnow500.jpg


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.

i-e14d373cd8dbb056c053b27b5fdacc94-petallidthen500.jpg

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.)

Update: I’ve also posted the article on my web site.

Comments

  1. #1 Kevin Z
    August 28, 2007

    Christopher Taylor illuminates exactly What is a daddy-longs? Apparently, there are three taxa that fit the bill.

  2. #2 Kevin Z
    August 28, 2007

    Christopher Taylor illuminates exactly What is a daddy-longs? Apparently, there are three taxa that fit the bill.

  3. #3 derek
    August 28, 2007

    I recently saw a similar illustration at Mike Dickison’s blog Pictures of Numbers, where he’s giving tips on finding and using map files for scientific illustrations.

    One of the examples he gives is a polar projection with the emu, ostrich, and other ratites plotted on the continents. Because it’s centered on Antarctica, there’s no puzzle for the viewers to solve, as there is for your equatorial projection: the reason for their distribution is obvious.

  4. #4 Ben D
    August 28, 2007

    So that’s why I was so confused – I consider daddy longlegs to be crane flies. Maybe that’s a UK thing?

  5. #5 xebecs
    August 28, 2007

    Isn’t it odd, though, that the range on each continent is restricted to the shore of the ocean? Or is that observation just an artifact of a very large scale map?

    Also, aren’t there documented cases of tiny organisms such as spiders being carried long distances in the upper atmosphere? Is there anything about the distribution of Petallids that weighs against such an explanation?

  6. #6 Carl Zimmer
    August 28, 2007

    Xebecs [5]: The maps (which are from the Cladistics journal paper) just use modern coastlines to make it easier to recognize the landmasses in their previous arrangement. Of course, the precise outline of the coasts were different. (They were different just 20,000 years ago as well, thanks to the lower sea level during the ice age.)

    As for drifting spiders–indeed, many animals and plants have managed to travel long distances. I discuss that fact in the article (which I’ve also posted on my web site if you want to read it). This group of daddy longlegs shows no sign of having dispersed by air. As I mentioned in the post, it is only found on ancient landmasses, but not young islands. A particularly striking case can be found in New Caledonia, an island near New Zealand. It’s a tiny fragment of an ancient continent, and it’s got these animals on it. But young volcanic islands right next door have none of these animals. If these daddy longlegs could disperse much at all, you’d expect them to have made that short journey.

  7. #7 xebecs
    August 28, 2007

    Carl: Thank you for your response. However, my (admittedly unspecific) observation about distribution following the coastlines was with regard to the modern map, not the ancient map; does that change your response at all?

  8. #8 Carl Zimmer
    August 28, 2007

    Whoops–sorry, xebecs, not enough caffeine in the head when I read your first comment. The petallids are not particularly coastal species–I think you get that impression because of the scale of the maps. They can live far inland. Obviously, you’re not going to find them in the middle of a place like Australia, however, because there’s no dense forest there for them. And while you can be many miles inland on the island of Sumatra, on a map like this, it may look like it’s on the coast.

  9. #9 xebecs
    August 28, 2007

    Carl: Got it, thanks.

  10. #10 Brian Baker
    August 28, 2007

    Great article.

  11. #11 Terry Collmann
    August 29, 2007

    Looks as if, from the 150m-year-old map, one could surmise the poor little sods originally lived in the middle of what is now Antarctica, since their current distribution, plotted back, rings what became that landmass – must have been a nasty, chilly shock for them as their home drifted further and further south …

  12. #12 bug_girl
    August 29, 2007

    Carl, someone on another blog brought this to my attention–the photo in the NYT on the right isn’t actually a daddy long legs (opiliones), it’s a spider.

    :(

  13. #13 Christopher Taylor
    August 29, 2007

    Also, aren’t there documented cases of tiny organisms such as spiders being carried long distances in the upper atmosphere? Is there anything about the distribution of Petallids that weighs against such an explanation?

    The spiders that disperse long distances are generally those that have a ballooning stage in their life cyle, where the young disperse by letting out a line of silk that gets caught by the wind. Harvestmen don’t produce silk and so don’t have ballooning dispersal – in fact, harvestmen are notoriously poor dispersers. There are a great many harvestmen species with very restricted distributions.

    Pettalidae are not particularly coastal animals. Harvestmen are very vulnerable to dessication, so they aren’t generally found in particularly arid environments, which is probably why they aren’t found from the centre of Australia. There is also an issue of collection error – most pettalids have probably not yet been described, and coastal areas have probably been more often collected from than central continental areas.

  14. #14 Anil
    August 30, 2007

    This is fascinating. Always have been intrigued by how life got distributed by the movement of the land masses.

  15. #15 chemistgnat
    September 3, 2007

    Great article – That is very interesting. I’ve always been intrigued by how animals got where they are now. My first introduction to that was when I was younger with horses…

The site is currently under maintenance and will be back shortly. New comments have been disabled during this time, please check back soon.