Жоғалған Марелла тұқымдасы: Камбрий фаунасының өкілі

Extinct genus of Arthropods

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

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.

Specimens of Marrella range from in length. The head shield had two pairs of long posteriorly curved projections/spines, the posterior pair of which had a serrated keel. There is no evidence of eyes. On the underside of the head was a pair of long and sweeping flexible antennae, composed of about total 30 segments, projecting forward at an angle of 15 to 30 degrees away from the midline. On part of the antennae, the joints between segments bear setae. Behind and slightly above the antennae attached a pair of short and stout paddle like swimming appendages, composed of one long basal segment and five shorter segments, the edges of the latter of which were fringed with setae. However the conclusions of the paper regarding other animals with supposed iridescent diffraction gratings have been questioned by other authors. Dark stains are often present at the posterior regions of specimens, probably representing extruded waste matter or hemolymph. A single specimen caught in the act of ecdysis (moulting) is known, which shows that the exoskeleton split at the front of the shield.

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.

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.