Saturday, January 12, 2008

Amazing Paleozoic Annelid Fossil

Paleoblog points to an amazing fossil find, Prehistoric Armored Worms. The fossil in question is very lifelike, showing extensive soft tissue preservation. Check out the Press Release, which has an excellent photograph of the fossil, and a drawing to explain what you're seeing. From the press release:

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 [from 485 to 305 million years ago], and indicating that they were an important component of ancient seafloor ecosystems....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 armor, 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 ¡Èlimbs¡É carrying bristles, unequivocally identified the machaeridians as annelid worms, say the scientists.

Here's the abstract to the actual article:

Machaeridians are Palaeozoic armoured annelids. Jakob Vinther, Peter Van Roy and Derek E. G. Briggs. Nature 451, 185-188 (10 January 2008)

The systematic affinities of several Palaeozoic skeletal taxa were only resolved when their soft-tissue morphology was revealed by the discovery of exceptionally preserved specimens. The conodonts provide a classic example, their tooth-like elements having been assigned to various invertebrate and vertebrate groups for more than 125 years until the discovery of their soft tissues revealed them to be crown-group vertebrates. Machaeridians, which are virtually ubiquitous as shell plates in benthic marine shelly assemblages ranging from Early Ordovician (Late Tremadoc) to Carboniferous, have proved no less enigmatic. The Machaeridia comprise three distinct families of worm-like animals, united by the possession of a dorsal skeleton of calcite plates that is rarely found articulated. Since they were first described 150 years ago machaeridians have been allied with barnacles, echinoderms, molluscs, or annelids. Here we describe a new machaeridian with preserved soft parts, including parapodia and chaetae, from the Upper Tremadoc of Morocco, demonstrating the annelid affinity of the group. This discovery shows that a lineage of annelids evolved a dorsal skeleton of calcareous plates early in their history; it also resolves the affinities of a group of problematic Palaeozoic invertebrates previously known only from isolated elements and occasional skeletal assemblages.

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