Rough-Hewn Land by Keith Heyer Meldahl

Author:Keith Heyer Meldahl
Language: eng
Format: epub
Publisher: University of California Press

FIGURE 10.3

Selected fossils from the Cambrian Explosion. Top image and ink drawing: Marrella, a primitive arthropod with long antennae and an elegant head shield that covered a segmented body (specimen three-quarters of an inch long). Middle: Hallucigenia, a creature once thought to be as otherworldly as its name but now thought to be an early lobopod—a phylum represented today by caterpillar-like animals that live in tropical rainforests (specimen three-quarters of an inch long). Bottom: Opabinia, an arthropod with five compound eyes on its head and a long, flexible proboscis tipped with a spike-filled claw (specimen 2.5 inches long). The ink drawing shows Opabinia clutching a polychaete worm. All images courtesy of the Smithsonian Institution.

Brachiopods dot the seabed around our landing site. Unable to flee, they snap shut their clam-like shells in our presence. Their similarity to clams is only shell-deep. When we pry one open, we see no fleshy mantle like in clams and other molluscs. Instead, we find a horseshoe-shaped, feathery organ called a lophophore, used for gathering oxygen and food from the sea. The lophophore—another Cambrian innovation—serves brachiopods well to this day.

We kick off and swim slowly across the seabed. Sponges lie scattered about, their spherical or branching forms reminiscent of bushes. They are the simplest of animals, lacking organs or even distinct tissues. They strain seawater for food particles using thousands of flagellated cells, each with a tiny tail that beats like a snapping whip to pump seawater through pores in the body wall. Any sponge that you see today uses the same Cambrian innovation.

Drifting along a few feet above the seabed, we see holes everywhere. Now and then, a column of mud spurts out of one. These are fecal coils, ejected by deposit-feeders—creatures that eat seafloor mud and digest the organic goodies it contains. We plunge our hands into the muck and feel things wriggling away. We catch one and pull it out. The worm's body is segmented, with bundles of bristles called setae projecting from each segment. The bristle-like setae make the worm a very efficient burrower; it uses them for traction as it pushes its way through the mud. The next time you dig up an earthworm, run your fingers along its body; you'll feel a slight roughness. These are its setae, clearly visible under magnification. Your fingers are touching a Cambrian innovation, for setae are the signature feature of the annelids, represented today by earthworms, leeches, and a host of marine polychaete worms.

Plunging my hands into the mud in search of more creatures, suddenly—OWW &#*%#*!! A hideous thing, like a mutant penis, has seized my finger with its circular mouth. It is a priapulid, a unique type of predatory worm with a mouth filled with sharp, backward-pointing hooks. With each gulp, the hooks pull my finger deeper into the priapulid's throat. We can't help but admire the cruel efficiency of this predatory system—once again, a Cambrian innovation that persists to this day.

I cut the priapulid off my finger with a knife. The scent of blood and fresh meat attracts a swarm of visitors.

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