Marrellomorpha

Marrellomorpha are an extinct group of arthropods known from the Cambrian to the Early Devonian. They lacked mineralised hard parts, so are only known from areas of exceptional preservation, limiting their fossil distribution. The best known member is Marrella, with thousands of specimens found in the Cambrian aged Burgess Shale of Canada.

Taxonomy

The group is divided up into two major orders, Marrellida and Acercostraca. Marrellida is recognised by the possession of head shields with two or three pairs of elongate spine-like projections, and three attached pairs of uniramous (single-branched) appendages, while Acercostraca generally have large ovoid carapaces that cover the entire upper half of the body, and five pairs of uniramous appendages attached to the head section. Both groups have unbranched antennae and a segmented trunk with biramous (divided into two branches) appendages. Recent research has suggested the previously enigmatic Cambrian trilobite-like arthropods Skania and Primicaris belong to this group. Their phylogenetic position is uncertain, various studies have alternatively placed them in the Arachnomorpha as relatives of Artiopoda (trilobites and kin), as related to Mandibulata (the group containing crustaceans, insects and myriapods), or as stem group euarthropods. The engimatic Cambrian arthropod Burgessia may be closely related to marrellomorphs. Some authors have proposed that they may be closely related to sea spiders (Pycnogonida) within Chelicerata though the cladistical support for such a relationship is relatively weak. A 2025 paper yet again found Marrellomorpha to be related to Mandibulata, although unusually their phylogeny also placed the very basal Isoxyida as stem-mandibulates.

Internal taxonomy of Marrellomorpha after Moysiuk et al., 2022.

• Acercostraca
  • Skania Burgess Shale, Canada, Cambrian (Miaolingian) ? Kaili Formation, China, Cambrian (Wuliuan)
  • Primicaris Burgess Shale, Canada, Cambrian (Miaolingian), Maotianshan Shales, China, Cambrian Stage 3
  • Vachonisiidae
    • Xylokorys Coalbrookdale Formation, England, Silurian (Wenlock)
    • Vachonisia Hunsrück Slate, Germany, Lower Devonian (Emsian)
    • Enosiaspis Fezouata Formation, Morocco, Early Ordovician (Floian)
• Marrellida
  • Marrella Balang Formation, China, Cambrian Stage 4 Kaili Formation, China, Cambrian (Wuliuan), Burgess Shale, Canada, Cambrian (Miaolingian)
  • Furca Letná Formation, Czech Republic, Upper Ordovician (Sandbian)
  • "Furca" mauretanica (nomen nudum) Fezouata Formation, Morocco, Early Ordovician (Floian)
  • "Mimetaster" florestaensis Floresta Formation Argentina, Lower Ordovician (Tremadocian)
  • Mimetasteridae
    • Tomlinsonus Kirkfield Formation, Canada, Late Ordovician (Katian)
    • Mimetaster Hunsrück Slate, Germany, Lower Devonian (Emsian)

Fragmentary taxa assigned to Marrellomorpha include Austromarrella from Cambrian Series 3 aged deposits in Australia, and Dyrnwynia from the Ordovician (Darriwilian) aged Llanfallteg Formation of Wales, which in its original description was assigned to Marrelida.

Phylogeny

After Legg, 2016. After Moysiuk et al., 2022. Template:Barlabel

References

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