Machaeridia (annelid)

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.<ref name=Vinther2008>Template:Cite journal</ref>

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.<ref name=Caron2008>Template:Cite journal</ref>

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.<ref name=Dzik1986/> 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.<ref name=Hogstrom2003>Template:Cite conference</ref> Scales are ridged with growth lines, implying that they grew episodically.<ref name=Dzik1986/> A few taxa experimented with different approaches to scale formation; some were only very weakly calcified and may have mainly been organic in nature.<ref name=Dzik1986/> They were never moulted, and each scale could be moved with an attached muscle.<ref name=Dzik1986/>

The front two segments of the machaeridians were commonly different from the rest, bearing fewer spiny projections.<ref name=Dzik1986/>

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

Plumulites bengstoni, a plumulitid from the Fezouata Shale of Morocco.

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.<ref>Template:Cite journal</ref>

Their scales almost certainly performed a defensive role.<ref name=Dzik1986>Template:Cite book</ref>

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.<ref name=Dzik1986/> While most possessed bilateral symmetry, the scales on the right and left side of Turrilepas wrightiana are different in shape and form.<ref name=Dzik1986/> 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.<ref name='Vinther2009'>Template:Cite journal</ref> 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.<ref name=Vinther2009/>

Taxonomic affinity

Template:See also Historically the group has been assigned to the echinoderms, barnacles, annelids and mollusks. Relationships to other Cambrian forms (such as the Halkieriids) have been proposed and discounted.<ref>Template:Cite journal</ref> 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.Template:Citation needed

A study in 2019 recognized machaeridians as phyllodocids based on their jaws.<ref>Template:Cite journal</ref>

Articulated specimens

Articulated machaeridians are known from:

Taxon	Location	Date	Reference
Lepidocoleus sarlei (Lepidocoleidae)	Rochester Shale Lagerstätten, New York State	Wenlock, Silurian	<ref>Template:Cite journal</ref>
Lepidocoleus hohensteini (Lepidocoleidae) (with soft tissue)	Hunsrück Slate, Germany	Lower Emsian, Devonian	<ref>Template:Cite journal</ref>
Plumulites bengtsoni (Plumulitidae)	Fezouata Formation, Morocco	Lower Ordovician	<ref>Template:Cite journal</ref>
Turrilepas wrightiana (Turrilepidae)	Gotland, Sweden	Hemse, Silurian	<ref name="auto">Template:Cite journal</ref>
Lepidocoleus sp.	Bois d’Arc Formation, Cravat Member. Coal County, Oklahoma	Helderbergian, Lower Devonian	<ref name="auto"/>
Plumulites richorum (Plumulitidae)	Humevale Formation, Victoria, Australia	Lower Devonian	<ref>Template:Cite journal</ref>
Deltacoleus crassus (Turrelipidae)	Balclatchie Formation, Scotland	Upper Ordovician	<ref name="auto1"/>
Turrilepas wrightiana (Turrelipidae)	Welsh borderlands, UK	Wenlock, Middle Silurian	<ref name="auto1">Adrain et al. 1991: A new species of machaeridian from the Silurian of Podolia, USSR, with a review of the Turrilepadidae JM Adrain, BDE Chatterton, LRM Cocks - Palaeontology, 1991</ref>
Turrilepas modzalevskae (Turrelipidae)	Podolia, Ukraine	Lower Ludlow, Silurian	<ref name="auto1"/>

... and possibly elsewhere