''Mimetaster'' is an extinct genus of

marrellomorph 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 me ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

. The type species, ''Mimetaster hexagonalis'' is known from the Lower

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

(

Pragian The Pragian is one of three faunal stages in the Early Devonian Epoch. It lasted from 410.8 ± 2.8 million years ago to 407.6 ± 2.8 million years ago. It was preceded by the Lochkovian Stage and followed by the Emsian Stage. The most important La ...

- Emsian) Hunsrück Slate, and amongst the most common arthropods from the locality, with over 120 specimens including three juveniles.

Description

The head shield of ''M. hexagonalis'' is oval shaped and raised on its upper surface, with three pairs of elongate straight projections radiating outwards, which have pairs of spines. On the upper surface was attached pair of probable eyes on segmented stalks. Attached on the underside was a pair of forward-projecting elongate segmented antennae, the first 10 segments of which were elongate, while the subsequent 14 were short and

flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

-like, as well as two pairs of large uniramous leg-like appendages. The first of the two pairs was substantially larger than the second. Also present on the underside of the head was a

hypostome In zoology, the hypostome can refer to structures in distinct animal groups: *Hypostome (trilobite), the ventral mouthpart plate in trilobites *Hypostome (tick) The hypostome (also called the maxilla, radula, or labium) is a calcified harpoo ...

, as well as a "ventral organ" of unclear function. The trunk consisted of up to 32 short segments, each of which except the last bore a pair of biramous appendages, which gradually decreased in size posteriorly. The endopods of the biramous appendages had 7 segments/

podomere 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, pl ...

s, each with

endite 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, plur ...

s at the end of each segment except the last. The

exopod 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, plur ...

had up to 40 segments, each of which had an individual

seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. Th ...

projecting downwards.

Ecology

''M. hexagonalis'' probably lived in small groups on the seafloor, walking in a tilted, upright posture propped up on its two uniramous legs. The first six trunk endopods are much larger than the remaining pairs, and were likely also used in locomotion, albeit with less power than the two main legs. They are thought to have been

deposit feeders Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

. Many specimens have been found associated with tentaculitoids and

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

remains, which suggests that these taxa grew on the surface of ''Mimetaster'' as epibionts, which likely acted as camouflage.

"''Mimetaster''" ''florestaensis''

"''Mimetaster''" ''florestaensis'' is only known from a head shield, which is characterized by three pairs of curved principal spines, as well as strong secondary spines in the proximal two-thirds of the anterolateral spines. It was found in the 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 of the ...

)

Floresta Formation The Floresta Formation ( es, Formación Floresta, 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 per ...

of Argentina, the first occurrence of this group in South America. However the taxon is quite different from the type species, and was found in a phylogenetic analysis to be more closely related to '' Furca'', and thus probably warrants being placed in a new genus, with the genus ''

Tomlinsonus ''Tomlinsonus'' is an extinct genus of marellomorph arthropod known from the Late Ordovician ( Katian) Kirkfield Formation of Ontario, Canada. It is a member of Marellida, and closely related to '' Mimetaster''. Description Only a single pa ...

'' being the closest known relative of ''M. hexagonalis.''

References

{{Taxonbar, from=Q19911385 Devonian arthropods Fossils of Germany Hunsrück Slate fossils Devonian Argentina Fossils of Argentina Fossil taxa described in 1931 Marrellomorpha

Description

Ecology

"''Mimetaster''" ''florestaensis''

See also

References