3D animation of Marrella splendens.
ANIMATION BY PHLESCH BUBBLE © ROYAL ONTARIO MUSEUM
Marrella has been grouped in the Order Marrellida with various Ordovician to Devonian taxa, which share two to three pairs of extremely elongate spines on the headshield, single-branched head limbs, and a body with greater than 25 ring-shaped body segments (Raymond 1920; Rak et al. 2013; Moysiuk et al. 2022). The broader placement of this group in arthropod evolution has been difficult to resolve. Marrella was historically thought to be allied with trilobites and their relatives (Raymond 1920; Whittington 1971), a result which was later supported by some phylogenetic analyses (Aria and Caron 2017). Other analyses have found affinities with the arthropod stem group (Briggs and Fortey 1989; Wills et al. 1998), or mandibulate arthropods (Legg et al. 2013; Legg 2015). The most recent studies support a placement within the broad grouping of “arachnomorph” arthropods (including trilobites and chelicerates), and possibly close to sea spiders (Vannier et al. 2018; Moysiuk et al. 2022) owing to shared loss of compound eyes and similarities in the head appendages.
Genus – after Dr. John Marr, palaeontologist at Cambridge University and friend of Walcott.
species – from the Latin splendens, “beautiful, or brilliant.”
Burgess Shale and vicinity: None
Other deposits: Marrella sp. from the Kaili and Balang Formations of south China (Zhao et al. 2006; Liu 2013).
The Walcott and Raymond Quarries on Fossil Ridge. Smaller collections from Mount Field, the Tulip Beds (S7) on Mount Stephen, Mount Odaray, and Mount Whymper/Tokumm Creek.
Marrella was one of the first fossils found by Walcott, and sketches appear in his notebook as early as August 31st, 1909. Walcott informally named them “lace crabs” at the time. The next summer, on August 9, 1910, Walcott and son Stuart found the “lace crab beds” in situ, marking the discovery of the fossil-bearing beds of the Walcott Quarry of the Burgess Shale. Walcott (1912) formally described the “lace crabs” as Marrella splendens, but a reconstruction was not attempted until Raymond (1920). Marrella was examined again by Simonetta (1962) and in a major study by Whittington (1971). New specimens collected by the Royal Ontario Museum allowed for the description of a specimen showing Marrella in the act of moulting (García-Bellido and Collins 2004), and another re-description of the taxon (García-Bellido and Collins 2006). The decay stains associated with Marrella specimens were found to contain the copper-rich mineral chalcopyrite, initially interpreted as remnants of the arthropod blood protein hemocyanin (Pushie et al. 2014), but more recent work demonstrates that this mineral in fact formed millions of years after fossilization, due to metamorphic alteration (Gaines et al. 2019).
Marrella is a small arthropod with a wedge-shaped head shield bearing two pairs of prominent spines that project from the sides and dorsal margin and extend back along most of the length of the body. There is also a pair of smaller posteroventral spines. The head bears a pair of long, thin antennae with as many as 30 segments, and a pair of paddle-like appendages with six segments and numerous bushy setae along the edges. Behind the head, the body consists of 26 segments that are small and subcircular, each bearing a pair of biramous appendages. The walking branch of this appendage has six segments, and the second branch is made of tapering gills with long, slim filaments that attach near the base of the legs. The last twelve appendages have conspicuous, blunt inner spines that form a net below the body. There is no developed tailpiece. The stomach is located in the head near the ventral mouth, and the intestine stretches most of the length of the body. Dark stains found around the body are suggested to be the gut contents that were squeezed out during preservation. García-Bellido and Collins (2006) also identified remains of the circulatory system, which, in light of more recent research (Aria and Caron 2015; Mayers et al. 2018), may correspond to hemocoelic cavities around the gut.
Marrella is one of the most common species in the Burgess Shale. Over 25,000 specimens have been collected (García-Bellido and Collins 2006), and it is the second most common arthropod species in Walcott Quarry, comprising 7.3% of the specimens counted (Caron and Jackson 2008). Marrella is commonly present in some other Burgess Shale localities, such as Marble Canyon (Nanglu et al. 2020), but conspicuously absent from others, like the Tulip Beds (O’Brien and Caron 2016). Marrella species have been reported from the Cambrian Kaili and Balang Formations in China (Zhao et al. 2006; Liu 2013), but the taxon is otherwise absent from major early Cambrian fossil assemblages of Burgess Shale type, such as the famous Chengjiang biota.
Marrella has been interpreted as an active swimmer that roamed above the sea floor while feeding on organic particles in the water or the sediment, and using its antennae as its main sensory apparatus. Swimming was achieved by undulating the second pair of paddle-like appendages on the head. The net of inner spines on the last twelve appendages could have been used to trap food particles located in water currents and to pass them along the underside of the animal. Food particles trapped in the net would be moved towards the mouth using the tips of the anterior limbs.
http://paleobiology.si.edu/burgess/marrella.html