Machaeridia is an extinct group of armoured, segmented annelid worms, known from the Early Ordovician (Late Tremadoc) to Carboniferous. It consists of three distinct families: the plumulitids, turrilepadids and lepidocoleids.
Fossils
Only the calcitic sclerites ("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.
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. Scales are ridged with growth lines, implying that they grew episodically.
Their scales almost certainly performed a defensive role.
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. 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. In 2008, the discovery of a fossil preserving soft tissue (including chaetae and parapodia) established an annelid affinity. Machaeridians represent the only instance of this group developing calcitic armour (notwithstanding certain polychaetes that integrate calcite into their chaetae). The exact position with annelids remains unresolved, though some characters indicate a relationship to Aphroditacean annelids (Vinther et al. 2008). In an accompanying commentary, Jean-Bernard Caron suggested that machaeridians must be a stem group based on number of specialised features. However, one cannot assess crown group/stem group affinities based on autapomorphies, but on shared morphological traits or the lack thereof. He also suggested that machaeridians might be polyphyletic, but machaerdians are a well defined group with a number of shared characters and morphological gradations among all three families.
A study in 2019 recognized machaeridians as phyllodocids based on their jaws.
Articulated specimens
Articulated machaeridians are known from:
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! Taxon !! Location !! Date !! Reference
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| Lepidocoleus sarlei (Lepidocoleidae) || Rochester Shale Lagerstätten, New York State || Wenlock, Silurian ||
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| Lepidocoleus hohensteini (Lepidocoleidae) (with soft tissue) || Hunsrück Slate, Germany || Lower Emsian, Devonian ||
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| Plumulites bengtsoni (Plumulitidae) || Fezouata Formation, Morocco || Lower Ordovician ||
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| Turrilepas wrightiana (Turrilepidae) || Gotland, Sweden || Hemse, Silurian||
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| Lepidocoleus sp. || Bois d’Arc Formation, Cravat Member. Coal County, Oklahoma || Helderbergian, Lower Devonian ||
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| Deltacoleus crassus (Turrelipidae) || Balclatchie Formation, Scotland || Upper Ordovician ||
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| Turrilepas modzalevskae (Turrelipidae) || Podolia, Ukraine|| Lower Ludlow, Silurian ||