Discovery of fossil of ancient slug-like animals solves mystery about their origin

Washington, Jan 10 : Paleontologists have discovered a fossil specimen in southeastern Morocco, which has solved the mystery about the origin of an extinct group of slug-like animals that existed 305 to 485 million years back.

Known as a machaeridian, an invertebrate, or animal without a backbone, this creature had rows of mineralized armor plates on its back.

First described over 150 years ago, armor plates of these strange animals have been found in marine fossil deposits worldwide covering the time span of their existence, and indicating that they were an important component of ancient seafloor ecosystems.

Until now, there was little information about their body design or how they might be related to other ancient - or currently living - animals.

But the discovery of the fossil specimen in an area that had earlier been identified as a rich source of exceptionally preserved fossils including sponges, trilobites, echinoderms and other less-familiar Invertebrates, has revealed much information about the animal's origins.

"The new specimen unequivocally identifies machaeridians as annelid worms, an extremely successful and diverse group of animals that includes familiar living animals like the sea mouse, the earthworm and the leech," said Jakob Vinther, graduate student in the Department of Geology and Geophysics at Yale.

"These animals disintegrated quickly after death, so complete fossils of their dorsal armor are rare, and their record until now consisted mostly of isolated armor plates scattered in the sediment," he added.

What helped the researchers to identify the machaeridians as annelid worms, was the observation that below the dorsal armor, the animal had an elongate body with paired, soft, limb-like extensions on each segment, and two bundles of long, stiff bristles on each extension.

According to scientists, the segmented nature of the body, and especially the presence of soft "limbs" carrying bristles, clearly identified the animals as annelid worms.

Although the exact relationship of machaeridians within the annelid worms is still uncertain, the presence of modified scales suggests that they may even belong to a group of marine bristle worms that are still in existence today.

"This exciting discovery has provided important new insights into annelid evolution, showing that some of these worms, which first appeared during the Cambrian radiation, evolved a highly distinctive dorsal, mineralised armor early in their history," said senior author Derek Briggs, the Frederick William Beinecke Professor of Geology and Geophysics at Yale University.

"It also highlights the great importance of the study of exceptional fossil sites, and of palaeobiology in general, for a better understanding of the evolution of our biosphere," he added.

ANI