Marrella splendens Walcott, 1912

Marrella splendens Walcott, 1912

Middle Cambrian

Burgess Shale

British Columbia

Canada

The Marrellomorphs are a clade of strange looking stem-group arthropods known from the Cambrian Burgess Shale (Marrella), 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 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.

Marrella 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 Marrella 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.