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. 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. Scales are ridged with growth lines, implying that they grew episodically. A few taxa experimented with different approaches to scale formation; some were only very weakly calcified and may have mainly been organic in nature. They were never moulted, and each scale could be moved with an attached muscle.
The front two segments of the machaeridians were commonly different from the rest, bearing fewer spiny projections.
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.
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.
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. While most possessed bilateral symmetry, the scales on the right and left side of Turrilepas wrightiana are different in shape and form.
The group had been variously assigned to the echinoderms, barnacles, annelids and mollusks, before the discovery of a fossil preserving soft tissue allowed a firm classification to the annelids, in 2008. This annelid affinity came as some surprise, as it is the only instance of this group developing calcitic armour. The exact relationship to crown group annelids are still unresolved, but some characters indicate a relationship to Aphroditacean annelids (Vinther et al. 2008). Caron (2008) suggest 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 between all three families.
ABNORMALITIES IN THE LOWER ORDOVICIAN CORNUTE PHYLLOCYSTIS THORAL, 1935 (ECHINODERMATA: STYLOPHORA) FROM MONTAGNE NOIRE (SOUTHERN FRANCE): IMPLICATIONS FOR PLATE HOMOLOGY AND PHYLOGENY
Jul 01, 2007; ABSTRACT- Two specimens of the cornute Phyllocystis crassimarginata Thoral, 1935 from the Lower Ordovician of Southern...