Mimetaster hexagonalis

Mimetaster hexagonalis Gürich 1931

Lower devonian

Emsian

Bundenbach

Hunsrück

Germany

The Marrellomorphs are a clade of strange looking stem-group arthropods known from the from the Cambrian Burgess Shale and the slightly older Kaili Fauna in China (Marella), the Silurian Herefordshire Lagerstätte in England (Xylokorys), the Ordovician Basal Upper Fezouata Formation (lower Arenig, or lower Floian), north of Zagora in southeastern Morocco and the Caradoc (Upper Ordovician) in Bohemia (Furca) and the Devonian Bundenbach Shale in Germany (Mimetaster and Vachonisia). They lacked mineralised hard parts, so are only known from areas of exceptional preservation, limiting their fossil distribution.

The oldest known Marrellomorph is Marrella sp. from the Cambrian Burgess Shale and the even slightly older Kaili Fauna in China. Marella splendens is an unusual arthropod known only from a single bed in the Middle Cambrian Burgess Shale of British Columbia. Nevertheless, Marrella is the most abundant genus in the Burgess Shale. Informally, Marrella was described by Walcott as a "lace crab". It is a small animal, 2 cm or less in length. Whittington did a thorough redescription of the animal in 1971, concluding on the basis of its legs, gills and the appendages on the head that it was not a trilobite, not a chelicerate ,and not a crustacean.

The head shield has two pairs of long rearward directed spikes. Marrella posessed two pairs of antennae, one long and sweeping, the second shorter and stouter. The two dozen segments each have a pair of six segmented leg / feathery gill structures. There is a tiny, button like telson at the end of the thorax. The best modern guest is that Marella is a moderately evolved primitive arthropod descended from a common ancestor of the major later arthropod groups.

The overall form of Marrella and other Marrellomorphs suggests that it was a soft-bottom dweller. The wide carapace border would have prevented sinking into unconsolidated sediment.

A thorough re-investigation of Marrella splendens based on over 1000 specimens was recently published by D. García-Bellido and D. Collins: "A new study of Marrella splendens (Arthropoda, Marrellomorpha) from the Middle Cambrian Burgess Shale, British Columbia, Canada" in Can. J. Earth Sci. 43(6): 721–742 (2006).

A slightly younger Marrellomorph is from the Silurian Herefordshire Lagerstätte in England.

The Ordovician Marrellomorph Furca is known from only about 50 specimens. Very recently, the discovery of numerous diverse soft-bodied assemblages in the Lower and Upper Fezouata Formations (Lower Ordovician) of Morocco yielded important new material. The fossils itself were once pyritized, but are now oxidized to iron oxy-hydroxides. An overview of the fauna and its significance is available in a paper by Peter van Roy and colleagues (Nature 465).

Another strange looking Marrellomorph is Mimetaster from the Lower Devonian Hunsrück Slate (Bundenbach slate) in Germany. Mimetaster hexagonalis is the most abundant non-trilobite arthropod from this Lagerstätte. They are considered to live in groups of several individuals; two, three or even more species on one slab are not uncommon. A thorough reinvestigation of Mimetaster based on 123 specimens was recently published by G. Kühl and J. Rust in Paläontologische Zeitschrift, volume 84, number 3, 397-411.