National Geographic : 2018 Mar
When liFe got coMplicated 105 descendants in different forms? And if the end wasn’t so abrupt and complete, what finished the Ediacarans as Ediacarans, dying off species by species in obscurity? Laflamme’s colleague Simon Darroch has offered one possible answer. On the afternoon of our visit to Mistaken Point, Darroch reached into his day pack and produced a surprise: small pieces of flat brown stone from the late Ediacaran beds he studies in Namibia. He had brought them from his lab at Vanderbilt to show me some trace fossils. A trace fossil, as distinct from a body fossil, records traces of animal activity—moving, chewing, defecating—as preserved in rock. It’s a record of behavior, not of bodily shape. Any such traces are notable in the Ediacaran period, be- cause most Ediacarans couldn’t do those things: move, chew, or defecate. “ This is a very static, sessile ecosystem,” Dar- roch said, referring to a famously rich early Edia- caran fossil bed on which we stood. The later Ediacaran, as revealed in Namibian rocks, was much different. One big difference, he said, was that “for the first time we have complex burrowing.” Experts disagree about just when the intricate patterns of burrowing creatures first ap- peared, but by any judgment those traces signaled a big change from the Ediacaran to the Cambrian. Wormy creatures had long been wriggling along on the sea bottom; now they were tunneling down into it as well. Darroch showed me a little slab marked with dotted-line traces. “They’re on the surface, and they disappear, then they come to the surface again.” That was evidence of an organ- ism with complicated musculature, allowing it to move about in three dimensions. If it moved that way, it had a front and a rear end. On its front end, probably a mouth. In the mouth, maybe teeth. These were extraordinary new tools and capaci- ties at the time. The worms crawled in, the worms crawled out, disrupting the microbial mats, possibly munching directly on Ediacarans. In a recent paper, Darroch and his co-authors (led by James Schiffbauer, and including Laflamme) have called this early Cambrian time the “Worm- world.” It was no place for Ediacarans. Worminess wasn’t the only factor that brought oblivion to the Ediacarans and triggered the Cambrian explosion—there also were changes in ocean chemistry that allowed animals to ac- quire hard parts (calcium-rich skeletons, teeth, and shells), a generalized increase in modes of mobility (not just burrowing), and the rise of predatory habits, among other things. But the worminess of that transitional time, in the late Ediacaran period, may have played a crucial role. A few weeks after our Mistaken Point outing, I talked with James Gehling, a leading Ediacaran researcher. Go up to the Flinders Ranges in South Australia, near the Ediacara Hills, he told me by phone from his office in Adelaide, and look at the first formation of Cambrian sedimentary layers. “It’s just Swiss cheese.” Burrowed all through by wormy creatures that had churned the sand and “recycled” the soft-bodied Ediacarans. “ That’s where the Cambrian begins,” Gehling said. “The advent of the musculature to burrow.” Guy Narbonne, at Queen’s University in On- tario, largely agrees with the importance of bur- rowing. But together with his graduate student Calla Carbone, he has taken Wormworld a step further. Based on careful analysis of trace fossils from the late Ediacaran and the early Cambrian, Narbonne and Carbone noticed a significant dif- ference in how those wormy creatures turned. By the early Cambrian, burrowing animals became more systematic in their searches for food, as well as more muscled. They ranged more efficiently, tracking the resources better and crossing their own tracks less. “It reflects the evolution of brain- iness,” Narbonne told me. “Our interpretation,” he added, “is that the Cambrian explosion is when behavior became coded on the genome.” They titled that paper, “When Life Got Smart.” THE RISE OF ‘WORMWORLD’ CREATURES WAS BAD NEWS FOR THE EDIACARANS.