Early in this century, paleontologists studying crinoids in the Silurian of Britain and Sweden, and the Devonian of Germany and New York State, noticed an interesting sequence of related crinoids that seemed to make up a clear evolutionary sequence.
Crinoids are "sea lilies". They are echinoderms, a phylum of invertebrates that include sea stars and sea urchins. While these latter two groups are mobile, moving around on little spines and sticky tube feet, crinoids are usually fastened to the sea floor by a stalk, and use their arms and sticky tube feet to catch tiny prey organisms from water currents that flow past them.
Echinoderms often have structures in multiples of 5, so crinoids have 5 (rarely) or 10 (sometimes) or 20 or more arms (often), all standing or waving in the water to catch prey. Anything that is caught is transported down the arms to the mouth, which is on the top side of the body in the center of the arms.
It is most efficient for a crinoid to filter all the water that flows through its arms (otherwise potential food is lost). Most crinoids have branching arms, to multiply the chances that a food organism will be caught. All members of a crinoid species (or family) have a characteristic branching pattern. But there are many possible branching patterns that will produce many arms, and crinoids have probably used them all in one family or another.
Now let's return to the Silurian and Devonian crinoids I mentioned. They evolved from an Ordovician ancestor that had a basic structure of 10 unbranched arms. Its descendants evolved a branching pattern in which each of the 10 arms branched off a sequence of sub-arms that lined up parallel and directed outward. However, one arm would branch only on its left, and the other only on its right, to produce a peculiar and memorable pattern that I showed numerous times on the slides.
The evolutionary sequence that continued for many tens of millions of years gradually made this pattern more rigid, with special fusion of arms, and hooks that prevented the sub-arms fro moving. All this set up a fan-shaped collecting system that had to be fixed and rigid in order to work. The crinoids with this system, the melocrinitids, became very strongly calcified. At the end of the Devonian, they became extinct. Why did they evolve the pattern, and why did they go extinct?
I happened to notice that the melocrinitid collection pattern had been re-invented by a British road engineer who had been asked to design the best possible banana plantation with the best possible road system. In his design, expensive hard-top roads radiated from the center of the plantation, branching into a set of cheaper dirt roads that had a pattern like the sub-arms of the crinoid. The processing plant was in the center of the plantation, corresponding to the mouth (and gut) of the crinoid.
The peculiar "best design" for the plantation is a result of the way that bananas ripen: irregularly. The crop is collected in small handfuls, and the transport costs are a major part of the total cost. And that applies to crinoids, collecting tiny prey from a large fan.
So the analogy explains the evolutionary sequence seen in melocrinitid crinoids. Over time, they slowly evolved the optimum collecting system: and the evolutionary steps in the process make the system more and more efficient along the way.
So that's why the sequence evolved the way it did. But we have not finished. Why did they go extinct? And why is it that other crinoids hardly ever evolved anything like the "optimum" system?
My analysis, for what it's worth, suggests that it is cost. The system is expensive to build, and it would take a long time to get the investment back. Other crinoids built less efficient but cruder systems, and survived to reproduce at least as well as melocrinitids. Whatever crisis finished off the melocrinitids at the end of the Devonian allowed other crinoids to survive (maybe it was a food crisis).
Few crinoids, then, are equipped with the "optimal" and spectacular adaptations of the melocrinitids, yet they are more succesful. And the reason is not to do with "optimal" structures, but with the economics of structures. It's no good setting up a huge and efficient factory if you go bankrupt doing it.
Whan you look at fossils, most of them are "ordinary". There are few brontosaurs, or giant pterodactyls. And when you look at organisms, most of them are "ordinary" too. The economics of adaptation must be very important. It may be better to build cheap "throw-away" bodies, and reproduce soon and often!
All this does not mean adaptation is not important. It does mean that the costs of building a body may be significant, and may be a significant part of natural selection.
[ASIDE: The original research was published as
Cowen, R. 1981. Crinoid arms and banana plantations: an economic harvesting analogy. Paleobiology 7, 331342.] QE721 P3 in Physical Sciences Library.
Return to Topics in Evolution
Return to Geology Department home page