From the seafloor off New Caledonia comes a strange colonial creature that was supposed to have been extinct for 300 million years.
When naturalists in the nineteenth century began to turn their attention to the deep sea, they expected to find it crawling with living fossils. Darwin had taught them that organisms evolve in response to a changing environment, and since the environment in the deep never changed-- or so it was then assumed--there was no reason for organisms there to evolve at all. The assumption was wrong, and for the most part the search for living deep-sea fossils has been a disappointment. But there have been some notable exceptions. In 1938 a South African chemist and amateur ichthyologist named J.L.B. Smith, staring at a sketch of a fish netted off Madagascar, realized he was looking at a coelacanth, a fish that had been thought to be extinct for 100 million years. And in 1992 Noel Dilly, sorting through a pile of unlovely organisms retrieved by French oceanographers from the seafloor off New Caledonia, found himself looking at a graptolite: a strange sea creature last seen alive around 300 million years ago.
I don’t want to sound arrogant, says Dilly, an amateur marine biologist and professional ophthalmologist at St. George’s Hospital Medical School in London, but I feel I must almost be in the same bracket as Dr. Smith. Here I am, a complete outsider, who by virtue of an incredible stroke of good fortune has stumbled across something that is very exciting and very interesting.
Actually Dilly is hardly an outsider in marine biology. He is considered one of the world’s leading authorities on pterobranchs: colonial animals, somewhat like coral, that attach themselves to the seafloor at depths ranging from 2 to 2,000 feet. A pterobranch colony is typically an inch or so wide. It consists of a hardened mass of collagen inhabited by tens of zooids--the individual members of the colony. Each zooid is less than a tenth of an inch long and lives in its own tube or tiny depression, depending on the species. The zooids crawl out of these holes to filter microscopic plankton out of the water with their wavy tentacles. As they’re eating, they’re also secreting collagen from a suckerlike organ near their mouth. Thus the colonial skeleton is built up layer by layer, much like a plaster cast.
For more than 50 years some researchers have suggested that pterobranchs must be related to the graptolites, a group of marine organisms that flourished throughout the world ocean between 570 million and 300 million years ago. Like nearly all fossil organisms, graptolites are known only from their hard parts. But those hard parts look very much like pterobranch hard parts: they are colonial skeletons with individual tubes or holes, called thecae, that presumably housed zooids. And they appear to have been built in the same way as pterobranch skeletons, of layers of material secreted by the zooids. Some graptolites attached to the seafloor, and those looked either like a bushy seaweed or like a flattened bagpipe, with thecae protruding from a skeletal blob. But many graptolite species floated in the water, and their skeletons resembled hacksaw blades, with each tooth a theca.
Those little blades have been beloved of geologists and gold miners for more than a century. Because graptolites were floating organisms, they were widely distributed by ocean currents, explains William Berry, a geologist at the University of California at Berkeley. More important, the shape of the little hacksaw blades changed quickly over geologic time. Sometimes there are cups on one side of the blade, sometimes on both sides; sometimes the blades form a V or a Y. Once you understand the pattern of change, you can date the layers in a geologic structure relative to one another based on the relative positions of graptolite deposits. And then, if you know that a gold deposit always occurs, say, in the youngest layer in a pile, all you have to do is look for the graptolite structure that corresponds to that layer. Back in the last century, it turns out, prospectors in Australia used graptolites extensively to find gold deposits.
In spite of the physical resemblance between graptolites and pterobranchs, though, some paleontologists have balked at putting them in the same class. One part of the graptolite skeleton, a long, thin spine called the nema, could not, these researchers have argued, have been formed the way pterobranch zooids form their skeleton, through successive layering. A zooid, the argument went, could not have left its theca, climbed a spine many times longer than its body length, secreted material at the tip, and still have remained connected to the colony. In fact, in some species of graptolite it seemed the zooids couldn’t leave their thecae at all, because the openings looked to be smaller than their bodies. But the linchpin of the argument against saying pterobranchs were living graptolites was simply that no one had ever found a pterobranch that built a nema.
Enter the French research submersible Cyana and a French marine biologist named Michel Roux, who collected a batch of pterobranchs off the coast of New Caledonia and sent them to Noel Dilly for sorting. I remember the morning vividly, Dilly says of the day the package arrived. I thought, ‘Oh my God, not another boring collection to hack through.’ And then when I took the first one out of the pot I thought, ‘This is like nothing I’ve seen.’ I saw those long needlelike processes and thought, ‘I don’t believe this.’ There was about a 40-minute period when I was looking at these things through the microscope and waiting to wake up from a dream.
The things Dilly spied under his microscope--he called them Cephalodiscus graptolitoides--were orange-colored colonies of pterobranchs consisting of a flattened crust of collagen peppered with saclike cups. Each cup contained a single white zooid, between a twenty-fifth and a tenth of an inch long and, in many cases, fatter than the door to its home. Yet these zooids were clearly capable of squeezing out: near the cups were long, tapered, needlelike spines that looked for all the world like a graptolite’s nema. Some of the spines were more than an inch long--nearly 30 times the body length of a typical zooid and the same length as the impossible-for-a-pterobranch-to-build nema.
When Dilly examined the spines under an electron microscope, he found that their internal structure was the same as that of a nema, with layers of collagen piled one on top of the other, from the bottom of the spine up. I’ve examined electron micrographs of the ultrastructure of floating and bushy kinds of graptolites, says Berry, and the patterns in those 400-million-year-old forms are identical to those of Dilly’s pterobranchs. If I were shown pictures of these spines I’d say it was a graptolite.
The zooids, Dilly suspects, use spines as feeding poles. It’s no good filter-feeding down a deep, dark hole, Dilly explains, or on the bottom in the mud. The place for a filter feeder is out in the current. I think the nema is just a gadget that allows the colony to get off the bottom. Each time a zooid crawls or squeezes out of its hole, Dilly says, it leaves a trail of collagen, much like a snail leaving a trail of mucus. When it stops to feed, the collagen lumps up underneath it and hardens. The next time the zooid crawls out, it sits on that lump and secretes more collagen on top. Gradually the lump gets taller. The next thing you know, Dilly says, you have a tower--and a rather silly-looking headless organism perched precariously on top of it, its tentacles flapping in the current, looking far more like a creation of Dr. Seuss’s than a hardened survivor of natural selection.
Yet pterobranchs are survivors: the one obstacle to linking them to graptolites has now been overcome, and zoologists can study the living organisms knowing they are also learning about the long-dead. Since his discovery, Dilly has videotaped a different species of pterobranch off Bermuda; his footage shows zooids actually crawling out of their holes and making spines, albeit smaller ones than those of C. graptolitoides. In the near future he plans to dive on Australia’s Great Barrier Reef and in the Florida Keys, and he’s hoping to find more living fossils. But he’s not getting greedy. Maybe I’ll never do it again, he says. But history will always have to put me down as the guy who said that Cephalodiscus graptolitoides was a graptolite. Even if it decides to debunk me.