Machaerid

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Machaeridians are a group of armoured, segmented annelid worms, known from the Early Ordovician (Late Tremadoc) to Carboniferous. The group consist of three distinct families: the plumulitids, turrilepadids and lepidocoleids. [1]

Contents

Fossils

Only the calcitic scleretomes ("armour plates") of these worms tend to be preserved in the fossil record. These are tiny, and usually found disarticulated: articulated specimens reach about a centimeter in length, and are incredibly rare – hence the limited degree of study since their description in 1857.[2] Scleritomes which bear a strong resemblance to the machaeridians are found in the small shelly fauna of the early Cambrian, 530 million years ago, suggesting an early origin of the group.[3]

The machaeridians are characterized by having serialized rows of calcitic shell plates. The dorsal sclerites were convex and almost isometric; lateral sclerites were flatter and longer.[4] The plates comprised two calcite layers: the outer layer is thin and formed by lamellar deposition, whereas new elements were added to the thicker inner layer as it grew.[5] Scales are ridged with growth lines, implying that they grew episodically.[4] A few taxa experimented with different approaches to scale formation; some were only very weakly calcified and may have mainly been organic in nature.[4] They were never moulted, and each scale could be moved with an attached muscle.[4]

The front two segments of the machaeridians were commonly different from the rest, bearing fewer spiny projections.[4]

The plumulitids are flattened from above and looks much like the coat of mail armour of chitons. The two other families are laterally compressed and some lepidocoleids formed a dorsal hinge, which make these machaeridians look like a string of bivalves.

Ecology

Machaeridians are often found in association with stylophorans - the cornutes and mitrates. This suggests that they possessed a similar ecology. They probably fed on organic detritus, perhaps even the faeces of the accompanying stylophorans.[6]

Their scales almost certainly performed a defensive role.[4]

The organisms would have had limited ability to flex to the right and left (in the sagittal plane), but would have been able to roll up.[4] While most possessed bilateral symmetry, the scales on the right and left side of Turrilepas wrightiana are different in shape and form.[4] The Plumulitid machaeridians would have moved across the surface of the sea floor using parapodia, whereas the fully-armoured Turrelepids and Lepidocoelids burrowed in a peristaltic fashion reminiscent of their evolutionary cousins, the earthworms.[7] This burrowing role has subjected them to the same evolutionary pressures which affect burrowing bivalves; convergent evolution as a result of their shared function probably contributed to early suggestions that the machaeridians should be classified with the molluscs.[7]

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