''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 Lagerstätte, areas of exceptional preservation, limiting their fossil distribution. The b ...

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 ...

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

(

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 ...

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 ...

)

Hunsrück Slate The Hunsrück () is a long, triangular, pronounced upland in Rhineland-Palatinate, Germany. It is bounded by the valleys of the Moselle-Saar (north-to-west), the Nahe (south), and the Rhine (east). It is continued by the Taunus mountains, past ...

, 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 (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

-like, as well as two pairs of large

uniramous 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, : ...

(single-branched) 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 (insect), labium) is a Os ...

, 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 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, ...

(two-branched) appendages, which gradually decreased in size posteriorly. The

endopod 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, : ...

s (lower, leg-like branches) 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, : ...

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, : ...

s (structures present on the underside) at the end of each segment except the last. The exopods (upper branches of the limbs) had up to 40 segments, each of which had an individual

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

(hair-like structures) 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. Many specimens have been found associated with tentaculitoids and

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

remains, which suggests that these taxa grew on the surface of ''Mimetaster'' as

epibiont An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An ep ...

s, 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 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 ...

(

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 ...

)

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 ...

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

Phylogeny

After Moysiuk ''et al.'', 2022.

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''

Phylogeny

See also

References