Marella
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Marella splendens is an unusual arthropod known from fossils found in only a single stratum of the Middle Cambrian Burgess Shale of British Columbia. It is, however, the most common fossil in the Burgess Shale, with over 15000 specimens cataloged. Historically, Marella was the first fossil collected by Charles Walcott from the Burgess Shale. Walcott described Marella informally as a "lace crab" and described it more formally as an odd trilobite. It was later reassigned to the now defunct class Trilobitoidea in the Treatise on Invertebrate Paleontology. In 1971, Whittington did a thorough redescription of the animal and, on the basis of its legs, gills and head appendages, concluded that it was neither a trilobite, nor a chelicerate, nor a crustacean.
Marella itself is a small animal, 2 cm or less in length. The head shield has two pairs of long rearward directed spikes. On the underside of the head are two pairs of antennae, one long and sweeping, the second shorter and stouter. Marella has a body composed of 24-26 body segments, each with a pair of branched appendages. The lower branch of each appendage is a leg for walking, while the upper branch is a long, feathery gill. There is a tiny, button-like telson at the end of the thorax. It is unclear how the unmineralized head and spines were stiffened. Marella has one too many antennae pair, three too few cephalic leg pairs, and two too few segments per leg to be a trilobite. It lacks the three pairs of legs behind the mouth that are characteristic of crustacea. The legs are also quite un-crustacean.
The best modern guess is that Marella is a moderately-evolved, primitive arthropod descended from a common ancestor of the major later arthropod groups. It is thought to have been a benthic (bottom-dwelling) marine scavenger living on detrial and particulate material. It has no known ancestors, descendants or close relatives.
Taken with two other unexpectedly unique arthropods, Opabinia and Yohoia, Marella has demonstrated that the softbodied Burgess faunas were much more complex and diverse than anyone had previously suspected.