Marrella

''Marrella'' is an extinct genus of marrellomorph arthropod known from the middle Cambrian Burgess Shale of British Columbia. It is the most common animal represented in the Burgess Shale, with tens of thousands of specimens collected. Much rarer remains are also known from deposits in China.

History

''Marrella'' was the first fossil collected by Charles Doolittle Walcott from the Burgess Shale, in 1909. Walcott described ''Marrella'' 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 undertook 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.

''Marrella'' is one of several unique arthropod-like organisms found in the Burgess Shale. Other examples are '' Opabinia'' and ''Yohoia''. The unusual and varied characteristics of these creatures were startling at the time of discovery. The fossils, when described, helped to demonstrate that the soft-bodied Burgess fauna was more complex and diverse than had previously been anticipated.

Morphology

''Marrella'' itself is was small animal, with known specimens ranging from 2.4 mm to 24.5 mm in length. The head shield had two pairs of long posteriorly curved projections/spines, the posterior pair of which had a serrated margin. On the underside of the head was a pair of long and sweeping antennae, along with a pair of short and stout paddle-like swimming appendages, which were covered with setae. ''Marrella'' has a body composed of 24–26 body segments ( tagma), each with a pair of branched (biramous) appendages. The lower branch of each appendage (the endopod) is elongate and leg-like, while the upper branch (the exopod) is a long, feathery gill. There is a tiny, button-like telson at the end of the thorax.García-Bellido, Diego & Collins, Desmond. (2006). A new study of Marrella splendens (Arthropoda, Marrellomorpha) from the Middle Cambrian Burgess Shale, British Columbia, Canada. Canadian Journal of Earth Sciences. 43. 721-742. 10.1139/e06-012. A 1998 paper suggested that striations present on the front projection of well-preserved specimens of ''Marrella'' represented a diffraction grating pattern, that in life would have resulted in an iridescent sheen. Dark stains are often present at the posterior regions of specimens, probably representing extruded waste matter or hemolymph.

Ecology

''Marrella'' is likely to have been an active swimmer that swam close to the seafloor (nektobenthic) with its swimming appendages used in a backstroke motion, with the large spines acting as stabilizers, as well as possibly also having a defensive function. They have been suggested to be filter feeders, with food particles sifted out of the water column by the posterior appendages during swimming before being passed forward by the appendages towards the mouth.

Taxonomy

''Marrella'' is placed within the Marrellida clade of the Marrellomorpha, a group of arthropods with uncertain affinities known from the Cambrian to Devonian. Within the Marrelliida, is it placed as most basal known member of the group.

Occurrence

Marrella is the most abundant genus in the Burgess Shale. Most ''Marrella'' specimens herald from the 'Marrella bed', a thin horizon, but it is common in most other outcrops of the shale. Over 25,000 specimens have been collected;5028 specimens of ''Marrella'' are known from the Greater Phyllopod bed, where they comprise 9.56% of the community.

A few dozen specimens of an indeterminate species of ''Marrella'' have been reported from the Kaili Formation of Yunnan, China, dating to the Wuliuan stage of the Cambrian. A single fragmentary specimen of an indeterminate species is also known from the Balang Formation of Yunnan, China, dating to Cambrian Stage 4. Both deposits are earlier than the Burgess Shale.

References

External links

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{{Taxonbar, from=Q12015622

Marrellomorpha
Burgess Shale fossils
Cambrian arthropods of North America
Cambrian genus extinctions