Marrellomorpha are an extinct group of

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s known from the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to the

Early Devonian The Early Devonian is the first of three Epoch (geology), epochs comprising the Devonian period, corresponding to the Lower Devonian Series (stratigraphy), series. It lasted from and began with the Lochkovian Stage , which was followed by the Pr ...

. 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 The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

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

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

s 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 The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, ...

(divided into two branches) appendages. Recent research has suggested the previously enigmatic Cambrian trilobite-like arthropods ''

Skania ''Skania'' is a Cambrian fossil arthropod. The type species, ''S. fragillis'', is known from the Burgess Shale of British Columbia, Canada. A second possible species ''"S." sundbergi'' is known from the Kaili Formation of China, but its place ...

'' and '' Primicaris'' belong to this group. Their phylogenetic position is uncertain, various studies have alternatively placed them in the Arachnomorpha as relatives of

Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

(

trilobites Trilobites (; meaning "three-lobed entities") are extinct marine arthropods that form the class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most successful of all early animals, ...

and kin), as related to

Mandibulata The clade Mandibulata constitutes one of the major subdivisions of the phylum Arthropoda, alongside Chelicerata. Mandibulates include the crustaceans, myriapods (centipedes and millipedes, among others), and all true insects. The name "Mandibul ...

(the group containing crustaceans, insects and

myriapods Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. Although molecular evidence and similar fossils suggests a diversifi ...

), or as

stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

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 The subphylum Chelicerata (from Neo-Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. Chelicerates include the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, tic ...

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 Isoxyids are members of the order Isoxyida and the family Isoxyidae, a group of basal arthropods that existed during the Cambrian period. It contains two genera, ''Isoxys'', with 20 species found worldwide, and ''Surusicaris'' known from a single ...

as stem-mandibulates. Internal taxonomy of Marrellomorpha after Moysiuk ''et al.'', 2022. * Acercostraca ** ''

, Canada,

(

Miaolingian The Miaolingian is the third Series of the Cambrian Period, and was formally named in 2018. It lasted from about to million years ago and is divided in ascending order into 3 stages: the Wuliuan, Drumian, and Guzhangian. The Miaolingian is prec ...

) ?

Kaili Formation The Kaili Formation () is a Formation (stratigraphy), stratigraphic formation which was deposited during the Lower and Middle Cambrian (~513 to 506 million years ago). The formation is approximately thick and was named after the city Kaili City, ...

, China, Cambrian (

Wuliuan The Wuliuan stage is the fifth stage of the Cambrian, and the first stage of the Miaolingian Series of the Cambrian. It was formally defined by the International Commission on Stratigraphy in 2018. Its base is defined by the first appearance of t ...

) ** '' Primicaris'' Burgess Shale, Canada, Cambrian (Miaolingian),

Maotianshan Shales The Maotianshan Shales () are a series of Early Cambrian sedimentary deposits in the Chiungchussu Formation or Heilinpu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized orga ...

, China,

Cambrian Stage 3 Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approxima ...

** Vachonisiidae *** '' Xylokorys'' Coalbrookdale Formation, England,

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

( Wenlock) *** '' Vachonisia'' Hunsrück Slate, Germany, Lower

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

(

Emsian The Emsian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.62 ±1.95 million years ago to 393.47 ±0.99 million years ago. It was preceded by the Pragian Stage and followed by the Eifelian Stage. It is named after ...

) *** '' Enosiaspis''

Fezouata Formation The Fezouata Formation or Fezouata Shale is a geological formation in Morocco which dates to the Ordovician, Early Ordovician.

, Morocco, Early

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

(

Floian The Floian is the second stage of the Ordovician Period. It succeeds the Tremadocian with which it forms the Lower Ordovician series. It precedes the Dapingian Stage of the Middle Ordovician. The Floian extended from to million years ago. The ...

) * Marrellida ** '' Marrella'' Balang Formation, China, Cambrian Stage 4 Kaili Formation, China, Cambrian (Wuliuan), Burgess Shale, Canada, Cambrian (Miaolingian) ** '' Furca''

Letná Formation The Letná Formation is a Late Ordovician ( Sandbian, or in the regional stratigraphy Berounian) geologic formation of the Prague Basin, Bohemian Massif in the Czech Republic. The formation crops out in the Czech capital, more specifically at ...

'','' Czech Republic, Upper Ordovician (

Sandbian The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species '' Nemagraptus gracilis'' around million ...

) ** "''Furca''" ''mauretanica'' (''

nomen nudum In Taxonomy (biology), taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published ...

'') Fezouata Formation, Morocco, Early Ordovician (Floian) ** '' "Mimetaster" florestaensis''

Floresta Formation The Floresta Formation (, Df) is a geological formation of the Altiplano Cundiboyacense in the Eastern Ranges of the Colombian Andes. The sequence of siltstones, shales, coquinas and sandstone beds dates to the Devonian period; Late Emsian, Eif ...

Argentina, Lower Ordovician (

Tremadocian The Tremadocian is the lowest stage of Ordovician. Together with the later Floian Stage it forms the Lower Ordovician Epoch. The Tremadocian lasted from to million years ago. The base of the Tremadocian is defined as the First appearance datum ...

) ** Mimetasteridae *** '' Tomlinsonus'' Kirkfield Formation, Canada, Late Ordovician ( Katian) *** '' Mimetaster'' Hunsrück Slate, Germany, Lower

(

) 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.

Taxonomy

Phylogeny

Gallery

References