| Marrella | |
|---|---|
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| Fossillectotype ofMarrella, USNM PAL 57674 | |
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| Life reconstruction ofMarrella | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | †Marrellomorpha |
| Order: | †Marrellida |
| Family: | †Marrellidae |
| Genus: | †Marrella Walcott, 1912 |
| Species: | †M. splendens |
| Binomial name | |
| †Marrella splendens Walcott, 1912 | |
Marrella is an extinctgenus ofmarrellomorpharthropod known from theMiddle Cambrian of North America and Asia. It is the most common animal represented in theBurgess 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 byCharles Doolittle Walcott from the Burgess Shale, in 1909.[1] Walcott describedMarrella informally as a "lace crab" and described it more formally as an oddtrilobite. It was later reassigned to the now defunctclass Trilobitoidea in theTreatise 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 achelicerate, nor acrustacean.[2]
Marrella is one of several unique arthropod-like organisms found in the Burgess Shale. Other examples areOpabinia andYohoia. 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.[3]
Specimens ofMarrella range from 2.4 to 24.5 millimetres (0.094 to 0.965 in) 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 flexibleantennae, 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 bearsetae. 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 withsetae (hair-like structures).[5][2]
The body had a minimum of 17 segments (tagma), increasing to over 26 segments in larger specimens, each with a pair ofbiramous (two-branched) appendages. The lower branches of each appendage (theendopod) were elongate and leg-like with 5 segments/podomeres excluding the basal segment/basipod, with the terminal segments being tipped with claws. The endopods sequentially decreased in size posteriorly, with the size reduction accelerating beyond the 9th pair. The upper branch (theexopod), which functioned as gill was segmented and bore thin filamentous structures. There is a tiny, button-liketelson at the end of thethorax.[5][2]
A 1998 paper suggested that striations present on the front projection of well-preserved specimens ofMarrella represented adiffraction grating pattern, that in life would have resulted in aniridescent sheen.[6] However the conclusions of the paper regarding other animals with supposed iridescent diffraction gratings have been questioned by other authors.[7][8] Dark stains are often present at the posterior regions of specimens, probably representing extruded waste matter[9] orhemolymph.[10] A single specimen caught in the act ofecdysis (moulting) is known, which shows that the exoskeleton split at the front of the shield.[11][12]
Marrella is likely to have been an active swimmer that swam close to the seafloor (nektobenthic) with its swimming appendages used in abackstroke motion, with the large spines acting as stabilizers, as well as possibly also having a defensive function. They have been suggested to befilter 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.[5]
Marrella is placed within theMarrellida clade of theMarrellomorpha, a group of arthropods with uncertain affinities known from theCambrian toDevonian. Within the Marrellida, is it placed as the mostbasal known member of the group. Cladogram of Marrellida after Moysiuk et al. 2022[13]
| Marrellida |
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Marrella is the most abundant genus in theBurgess Shale.[14] MostMarrella 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.[15] 5028 specimens ofMarrella are known from the GreaterPhyllopod bed, where they comprise 9.56% of the community.[16]
A few dozen specimens of an indeterminate species ofMarrella have been reported from theKaili Formation of Yunnan, China, dating to theWuliuan stage of the Cambrian. A single fragmentary specimen of an indeterminate species is also known from theBalang Formation of Yunnan, China, dating toCambrian Stage 4. Both deposits are earlier than the Burgess Shale.[17]
The full width of each sclerite [ofWiwaxia] is striated by finely spaced longitudinal lineations. Parker (1998) argued that these were superficial – although they are not visible on surfaces imaged under SEM and do not exhibit interference under transmitted light, so might be better interpreted as internal channels indicating microvillar secretion.
In Canadia, longitudinal striations along chaetae, which have previously interpreted as external evidence for iridescence, are concordant with the dimensions of microvilli and represent internal rather than external features.
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