Marrellomorpha are an extinct group of
arthropods known from the
Cambrian to the
Early Devonian.[3] 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. 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 pairs of
uniramous appendages on the
cephalon, while Acercostraca generally have large ovoid
carapaces that cover the entire upper half of the body, and five pairs of uniramous cephalic appendages. Both groups have unbranched
antennules and a segmented
trunk with
biramous appendages.[2] Recent research has suggested the previously enigmatic Cambrian trilobite-like arthropods Skania and Primicaris belong to this group.[4][2] 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, or as stem group euarthropods.[2]
Marrellomorpha are an extinct group of
arthropods known from the
Cambrian to the
Early Devonian.[3] 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. 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 pairs of
uniramous appendages on the
cephalon, while Acercostraca generally have large ovoid
carapaces that cover the entire upper half of the body, and five pairs of uniramous cephalic appendages. Both groups have unbranched
antennules and a segmented
trunk with
biramous appendages.[2] Recent research has suggested the previously enigmatic Cambrian trilobite-like arthropods Skania and Primicaris belong to this group.[4][2] 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, or as stem group euarthropods.[2]