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Sci Tech
Fossil sheds light on mystery
While evolution has produced great diversity in the body designs of animals, over the course of history several highly distinct groups, such as trilobites and ammonites, have become extinct. Now, a new fossil of an unusual creature known as a machaeridian, an invertebrate, or animal without a backbone, that existed for about 180 million years from 485 to 305 million years ago has been discovered.
First described over 150 years ago, armour 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.
Discovery of this exceptional fossil specimen in southeastern Morocco and preservation of evidence of the animal’s soft tissues has solved a palaeontological puzzle about the origins of an extinct group of bizarre slug-like animals with rows of mineralized armour plates on their backs, according to a paper in Nature.
Annelid worms
“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 & Geophysics at Yale.
The specimen was found 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. “These animals disintegrated quickly after death, so complete fossils of their dorsal armour are rare, and their record until now consisted mostly of isolated armour plates scattered in the sediment,” said Vinther.
The dilemma of studying ancient organisms, he notes, is that the soft body parts, including most internal organs, are unavailable for study because they usually decompose before they can become fossilized, according to Yale University press release.
Much speculation
Previous patchy evidence was insufficient to reveal the relationships of the machaeridians to other animals and there was much speculation about their position in the tree of life. Different authors suggested relationships to groups as varied as mollusks (clams and snails), barnacles (crustaceans — including shrimps, crabs and crayfish), echinoderms (starfish and sea urchins) and annelid worms (aquatic bristle worms and garden earthworms).
This inch-long specimen that was recently discovered shows that, below the dorsal armour, the machaeridians had an elongate body with paired, soft, limb-like extensions on each segment, and two bundles of long, stiff bristles on each extension.
The segmented nature of the body, and especially the presence of soft ‘limbs’ carrying bristles, unequivocally identified the machaeridians as annelid worms, say the scientists. According to the authors, 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, mineralized armor early in their history,” said senior author Derek Briggs, the Frederick William Beinecke Professor of Geology and Geophysics at Yale University. “It highlights the importance of studying exceptional fossil sites for a better understanding of the evolution of our biosphere.” — Our Bureau
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