‘Marine’ Fossils May Instead Represent Early Land Dwellers
The fossils of various frondlike and sacklike organisms that supposedly lived at the bottom of ancient oceans may actually represent some of the earliest organisms to dwell on land. That’s the controversial interpretation of a new study, which suggests that rocks long thought to have been formed from sediments deposited on ancient seafloors may actually be the remnants of early soils. If true, the finding would push back life’s transition from sea to land by tens of millions of years—and possibly by 100 million years or more.
Fossils reveal that life on Earth diversified rapidly during the Cambrian period, which began about 542 million years ago and lasted until about 485 million years ago. The so-called Cambrian explosion yielded most of the major groups of animals known today, but fossils of a host of organisms bearing little resemblance to modern life forms are embedded in Precambrian rocks—including those of the Ediacaran period, which began about 635 million years ago and lasted until the onset of the Cambrian. Most researchers have considered these unusual organisms—some resemble segmented worms or fronds, and others look like nothing more than bags of tissue—to have lived in the sea, because the types of rocks that entombed them typically accumulate as sediments in marine environments. Not so fast, says Gregory Retallack, a paleobotanist at the University of Oregon. His new analysis of fossil-bearing Ediacaran rocks from southern Australia suggests that those rocks formed from paleosols, or ancient soils. That makes the fossils found within the rocks terrestrial and not marine, he contends.