Weird Life on the Mats
|Scientists think that trilobites were the first multiple-celled animal to exist on earth. Some crawled on the sea bed, some floated and some swam. The lived all over the world. Crabs and lobsters are modern relatives.
More than a half-billion years ago, as early animal life began evolving and diversifying in the oceans, some strange-looking seafloor critters became scarce or went extinct. The conventional view has been that they represented failed evolutionary experiments — unfit dead ends on life’s inexorable path to greater complexity.
David Bottjer, however, offers a different interpretation. The University of Southern California earth sciences professor says the now-extinct animals were well-adapted to their Precambrian environment. They lived atop microbial mats, which were firm yet slimy layers of archaeans, bacteria and other single-celled microorganisms that covered the seafloor like scum on week-old pudding.
Then, during the Cambrian, a period of about 40 million years that began some 543 million years ago, the environment changed. Worms, early beetle-shaped multi-legged arthropods known as trilobites, and other "bioturbators" evolved and started churning up the seafloor and destroying microbial mats, turning the once-firm seabed into soft, mucky sediment.
Some animal species that lived on microbial mats went extinct; others persisted in limited areas where mats survived; and yet others evolved into new forms able to thrive atop rocks or shells.
Bottjer calls these changes in the seafloor sediment ecosystem "the Cambrian substrate revolution," and compares what trilobites and worms did to the ocean floor to the radical agricultural change wrought by human settlers who spread across the American Great Plains in the 1800s.
"You think of it like settlers sod-busting the prairie," he said. "They changed soil properties tremendously."
Bottjer says that if many early animals or metazoans look strange to us today, it is not because they were strange, but because they were well-adapted to life on a strange ocean seafloor environment: the microbial mat ecosystem.
He notes that to modern people, early motorists look odd, wearing dusters and riding in automobiles with tall wheels. "Yet the unusual clothes and wheels were just adaptations to a mushy road surface dirt roads. Today everybody drives on paved roads a hard surface. Cars can drive low to the road and we have enclosed cabs and don’t have to wear special clothing."
While dirt roads gave way to pavement, Bottjer says the opposite happened to the Cambrian seafloor: relatively firm, mat-covered sediments were replaced by soft, mushy seafloors.
|A well-preserved helicoplacoid specimen. This specimen is partially overlain by another specimen in its uppermost portion. (lower left corner = portion of US one cent coin for scale)
Credit: Palaios, Indiana University
"Today, microbial mats typically are found only in shallow near-shoreline environments where you have [inflowing] freshwater, or where evaporation creates more or less salt than normal seawater. That keeps out the snails and other organisms that chew on and destroy these mats."
In a published study and presentations over the past two years, Bottjer and colleagues have described how the change from firm to mushy seafloor affected two categories of animals: early echinoderms, which later evolved into today’s sand dollars, sea urchins, starfish and sea cucumbers; and early mollusks, which Bottjer says later evolved into "things you find at your seafood restaurant: squid, snails, clams, oysters, scallops."
The radical change in the seafloor environment caused the extinction of echinoderms known as helicoplacoids, which were plate-covered organisms shaped like tops and the size of a human thumb. Helicoplacoids stuck atop microbial mats while filtering microscopic nutrients from ocean water.
Bottjer says helicoplacoids were like "big trees that needed to have a stable surface to sit on." But once seafloor mats were replaced by muck, helicoplacoids probably were unable to live on the unstable sediment. "They only survived for maybe 10 million years after the beginning of the Cambrian. They never were seen again."
Two other types of echinoderms adapted to the change by finding other firm footing. Cupcake-shaped edrioasteroids also filtered seawater for food, but by the late Cambrian, the only known edrioasteroid fossils "were sitting on a shell or rock," Bottjer says. Weird, peanut-shaped eocrinoids, which used arm-like appendages to collect food from water, "evolved a stem. It allowed them to attach to hard surfaces like rocks or shells. They moved up onto these surfaces which projected above the soft sediment."
Several early mollusks also were restricted to life on rocks and other firm surfaces when seafloor mats were churned up. For example, the ancestors of mollusks such as polyplacophorans (or chitons), lacked shells, so "the earliest ones probably looked like slugs crawling across the mat-covered seafloor," Bottjer says. These several-centimeter-long creatures left scratch marks on microbial mat fossils, indicating "these guys were munching on the mat like the snail does [to algae growing on glass walls] in your fish tank. They were grazing on their environment, but they were not destroying it."
Other mollusks, known as monoplacophorans, typically were shaped "like a baseball cap without the bill" and measured up to several centimeters wide, says Bottjer. "They used to be all over the shallow seafloor [until mats were wiped out], but now they are stuck in deeper-water environments living on rocks and other firm surfaces."
Bottjer believes the Cambrian change in the seafloor also caused the extinction of certain lobopods, which looked like worms but moved around with foot-like pods projecting underneath them. Their modern relatives, called velvet worms, live in forest litter. Bottjer says one of the first lobopods discovered had spines and was named Hallucigenia "because it looked pretty darn weird." It may have walked on microbial mats, but went extinct when the seafloor turned mucky.
Bottjer says understanding odd early animal life on Earth is important for astrobiology because life on other planets might look strange to us, but might be perfectly well-adapted."
It also is conceivable that if any animals evolved in Mars’ now-vanished oceans, maybe they would be adapted to microbial mat surfaces and look more like Cambrian things than modern-day organisms" on Earth, he says.
Other scientists agree with Bottjer that the seafloor changed at the dawn of the Cambrian, but they say it will be hard to prove how that affected early animal life.
|Bruce Runnegar, a paleontology professor at UCLA, notes fossil preservation "is something we need to know more about if we’re going to search for life on other planets."
"The basic substrate revolution story is correct in that you see it in the rocks," says Bruce Runnegar, a paleontology professor at the University of California, Los Angeles. "But I’m not sure about what he speculates about the behavior of animals in the Cambrian. This is a nice story. But how would you find out if it was true?"
Runnegar notes that many early animals had soft bodies not likely to be preserved, and that their limited fossils do not necessarily reveal how the creatures lived. While impressions of some soft-bodied early animals can be found where mat materials preserve sediment layers, it is difficult to know if such organisms later went extinct or simply were not preserved after mats were destroyed.
Runnegar notes fossil preservation "is something we need to know more about if we’re going to search for life on other planets."
"The difficulty with Dave’s idea [on how early animals were affected by the change in seafloor sediments] is that while it may be correct, it is awfully difficult to test given the nature of the fossil record in the Cambrian," says paleobiologist Doug Erwin, interim director of the National Museum of Natural History in Washington.
"Reconstructing ecological interactions is one of the great challenges of paleobiology," Erwin says, "while a major challenge for astrobiology is to learn, Did the origin of a new group correlate to some physical change in the environment?"
He says that "if life is found on other planets, the next question is: How has the history of that planet influenced the history of life on that planet? The place we have to start answering it is on this planet. We have a lot more data here than we’re ever going to have from Mars."
Bottjer wants to compare China’s early Cambrian Chengjiang site with Canada’s middle Cambrian Burgess shale for more evidence of "how Cambrian organisms adapted to changing seafloor conditions through time."
Bottjer and USC doctoral student Steve Dornbos have made several trips to southwest China to study the Chengjiang fauna, in conjunction with Junyuan Chen of the Nanjing Institute of Geology and Paleontology, and more studies are planned, particularly on sponges and relatives of corals from the Cambrian.