''Tomlinsonus'' 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 known from the Late

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 ...

(

Katian The Katian is the second stage of the Upper Ordovician. It is preceded by the Sandbian and succeeded by the Hirnantian Stage. The Katian began million years ago and lasted for about 7.6 million years until the beginning of the Hirnantian mill ...

) Kirkfield Formation of

Ontario Ontario is the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...

, Canada. It is a member of Marrellida, and closely related to '' Mimetaster''.

Description

Only a single partial specimen (ROMIP 66233) was recovered during fieldwork in September 2020, by independent researcher Marc Haensel. This specimen preserves a mostly complete head and some appendages. Like other marrellids, the head is composed of a cephalic shield with pairs of spine-like projections, with two large pairs present in ''Tomlinsonus''. These projections curve posteriorly. Like most other marrellids, these projections are covered in second-order spines. There is also a pair of smaller projections without spines extending posteriorly from the back of the head. Attached to the head is a pair of extremely elongate appendages, around in length, with 9

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, with a spine present on the end of the 8th podomere, which is the longest leg segment. There is also another, shorter moderately sized cephalic appendage with at least four podomeres, with fourth and distalmost podomere being triangular in shape. There were pairs of filamentous appendages running along the trunk.

Ecology

The depositional environment of the Kirkfield Formation was an open marine

carbonate platform A carbonate platform is a Sedimentary rock, sedimentary body which possesses topographic relief, and is composed of Autochthon (geology), autochthonic calcareous deposits. Platform growth is mediated by Sessility (zoology), sessile organisms whose ...

''. Tomlinsonus'' was likely a slowly moving animal that lived on the seafloor. It likely used the large first appendage pair to lift itself up off the substrate, and may have rested plantigrade on the 9th podomere to spread its bodyweight, while using its trunk biramous appendages to propel itself forwards.

Taxonomy

In a phylogenetic analysis ''Tomlinsonus'' was found a member of Mimetasteridae, as the closest known relative of ''Mimetaster hexagonalis'', with which it shares an enlarged uniramous first appendage, with the supposed Argentinian ''Mimetaster'' species "''M." florestaensis'' found to be more closely related to '' Furca''. After Moysiuk ''et al.'', 2022:

References

{{Taxonbar, from1=Q111745059, from2=Q111745052 Marrellomorpha Fossil taxa described in 2022 Ordovician Ontario Fossils of Canada Monotypic arthropod genera