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Found: earliest known animal tracks?

Oct. 5, 2008
Courtesy Ohio State University
and World Science staff

Faint, fossilized tracks of an ancient aquat­ic crea­ture sug­gests an­i­mals walked us­ing legs at least 30 mil­lion years ear­li­er than had been thought, some sci­en­tists say. But they ad­mit the lack of a fos­sil of the crea­ture it­self will probably fos­ter a healthy skep­ti­cism, and that re­search­ers will need to look for ad­di­tion­al ev­i­dence.

Re­search­ers say this rock dis­plays tracks of one of the ear­li­est an­i­mal­s—small, round dots in silt that lat­er be­came stone. (Pho­to by Kev­in Fitzsi­mons, OSU)


The track­s—two par­al­lel rows of small dots, each about two mil­lime­ters wide—are dat­ed to some 570 mil­lion years ago, to a per­i­od called the Edi­a­ca­ran. That pre­ced­ed the Cam­bri­an per­i­od, when most ma­jor groups of an­i­mals evolved.

Sci­en­tists once thought that mainly mi­crobes and sim­ple mul­ti­cel­lu­lar an­i­mals ex­isted be­fore the Cam­bri­an, but that idea is chang­ing, said Lor­en Bab­cock, pro­fes­sor of earth sci­ences at Ohio State Uni­ver­s­ity.

He pro­nounced him­self “rea­sonably cer­tain” a centipede-like ar­thro­pod or a leg­ged worm made the tracks. An ar­thro­pod is an in­ver­te­brate hav­ing joint­ed limbs and a seg­mented bod­y—a group that in­cludes in­sects.

Soo-Yeun Ahn, a doc­tor­al stu­dent at Ohio State and a co-author of the re­search, pre­sented the find­ings at the Ge­o­log­i­cal So­ci­e­ty of Amer­i­ca meet­ing Sun­day in Hous­ton.

Bab­cock said he found the tracks while sur­vey­ing rocks in the moun­tains near Gold­field, Ne­vada in 2000. “We came on an out­crop that looked like it crossed the Pre­cam­brian-Cam­bri­an bound­ary.... We just sat down and started flip­ping rocks over. We were there less than an hour when I saw it.”

The crea­ture must have stepped lightly on­to the soft seabed, be­cause its legs pressed only shal­low pin­points in it, Bab­cock said. But when he flipped over the rock bear­ing the lit­tle pits, the low-angle sun­light cast them in crisp shad­ow, he re­called. He could­n’t be sure of the crea­ture’s length or num­ber of legs, but he guessed it car­ried a centimeter-wide body on many spindly legs.

In 2002, oth­er re­search­ers re­ported a si­m­i­lar fos­sil trail from Can­a­da that dat­ed back to the mid­dle of the Cam­bri­an per­i­od, about 520 mil­lion years ago. Anoth­er set of tracks found in South Chi­na date back to 540 mil­lion years ago.

Bab­cock is an ex­pert in the spe­cial chem­i­cal, phys­i­cal and bi­o­log­i­cal con­di­tions that en­abled some soft-bodied crea­tures to fos­silize, a rare oc­cur­rence with them. He has found a me­nag­er­ie of un­usu­al fos­sils, from un­usu­al echin­o­derms in Ne­vada to sulfur-eating bac­te­ria in Ant­arc­ti­ca.

The shal­low sea over west­ern Ne­vada 570 mil­lion years ago would have been good for pre­serv­ing soft-bodied an­i­mals, Bab­cock said. The sed­i­ment sur­face was probably bound to­geth­er by a mi­cro­bi­al mat—a co­he­sive car­pet of bac­te­ria and sed­i­ment grains, which would readily pre­serve prints.

“I ex­pect that there will be a lot of skep­ti­cism,” he said. “There should be. But I think it will cause some ex­cite­ment. And it will probably cause some peo­ple to look harder at the rocks they al­ready have. Some­times it’s just a mat­ter of think­ing dif­fer­ently about the same spec­i­men.”


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A newfound, fossilized trail of an aquatic creature suggests animals walked using legs at least 30 million years earlier than had been thought, some scientists say. But they admit the lack of a fossil of the creature itself will probably foster a healthy skepticism, and that researchers will need to look for additional evidence. The tracks—two parallel rows of small dots, each about two millimeters wide—are dated to some 570 million years ago, to a period called the Ediacaran. That preceded the Cambrian period, when most major groups of animals evolved. Scientists once thought that mainly microbes and simple multicellular animals existed before the Cambrian, but that idea is changing, said Loren Babcock, professor of earth sciences at Ohio State University. He pronounced himself “reasonably certain” a centipede-like arthropod or a legged worm made the tracks. An arthropod is an invertebrate having jointed limbs and a segmented body—a group that includes insects. Soo-Yeun Ahn, a doctoral student at Ohio State and a co-author of the research, presented the findings at the Geological Society of America meeting Sunday in Houston. Babcock said found the tracks while surveying rocks in the mountains near Goldfield, Nevada in 2000. “We came on an outcrop that looked like it crossed the Precambrian-Cambrian boundary.... We just sat down and started flipping rocks over. We were there less than an hour when I saw it.” The creature must have stepped lightly onto the soft seabed, because its legs pressed only shallow pinpoints in it, Babcock said. But when he flipped over the rock bearing the little pits, the low-angle sunlight cast them in crisp shadow, he recalled. He couldn’t be sure of the creature’s length or number of legs, but he guessed it carried a centimeter-wide body on many spindly legs. In 2002, other researchers reported a similar fossil trail from Canada that dated back to the middle of the Cambrian period, about 520 million years ago. Another set of tracks found in South China date back to 540 million years ago. Babcock is an expert in the special chemical, physical and biological conditions that enabled some soft-bodied creatures to fossilize, a rare occurrence with them. He has found a menagerie of unusual fossils, from unusual echinoderms in Nevada to sulfur-eating bacteria in Antarctica. The shallow sea over western Nevada 570 million years ago would have been good for preserving soft-bodied animals, Babcock said. The sediment surface was probably bound together by a microbial mat—a cohesive carpet of bacteria and sediment grains, which would readily preserve prints. “I expect that there will be a lot of skepticism,” he said. “There should be. But I think it will cause some excitement. And it will probably cause some people to look harder at the rocks they already have. Sometimes it’s just a matter of thinking differently about the same specimen.”