Mesolenellus is an extinct genus of trilobites that lived during the lower Cambrian (Botomian), found in Greenland and Spitsbergen. Etymology Mesolenellus is a contraction of meso - middle - and Olenellus, the genus from which it was split off. The epithet hyperborea is a contraction of hyper (Greek ὑπέρ "excess") and Boreas (), god of the North Wind in Greek mythology, referencing the current geographical position of the deposits of its type location. M. svalbardensis has been named for the Norwegian island group Svalbard or Spitsbergen, where it was originally collected. Taxonomy It was considered a subgenus of Olenellus before. The nearest relatives of Mesolenellus are the species of the genus Mesonacis, with which it constitutes the subfamily Mesonacinae. Distribution Mesolenellus hyperborea occurs in the Botomian (Nevadella-zone) of the paleocontinent Laurentia, now Greenland (Upper Buen Formation, above Brillesø, 1.5 km east of Borglum Elv, Peary Land, North-Eastern end of the island). The occurrence of M. hyperborea has been specified as the early mudstone layer of the Upper Buen Formation. The famous Sirius Passet fauna (e.g. Kleptothule and Aaveqaspis) occurs in the lowest (sandy) member of the Buen Formation and thus predates Mesolenellus. M. svalbardensis has been found in the Botomian (Nevadella-zone) of Laurentia, now Svalbard/Spitsbergen (lower Slakli Series, Sofiekammen Formation, northern side of the Hornsund, West-Spitsbergen). A specimen that probably belongs to M. svalbardensis or is closely related was found in Greenland (Schley Fjord Formation, Peary land). There is a further mention of the occurrence in Greenland for the late mudstone layer of the Upper Buen Formation, indicating that M. hyperborea predated M. svalbardensis. Habitat The mudstone in which both species were found indicates deeper, quiet waters. Ecology Mesolenellus hyperborea occurs together with Limniphacos perspiculum, Serrodiscus, hyoliths, Petrianna fulmenta (Bradoriida), and inarticulate brachiopods. Description As with most early trilobites, Mesolenellus has an almost flat exoskeleton, that is only thinly calcified, and has crescent-shaped eye ridges. As part of the suborder Olenellina, Olenellus lacks dorsal sutures. Like all other members of the superfamily Olenelloidea, the eye-ridges spring from the back of the frontal lobe (L4) of the central area of the cephalon, that is called glabella. Mesolenellus also shares the typical character of the whole family Olenellidae that the frontal (L3) and middle pair (L2) of lateral lobes of the glabella are partially merged. This creates two very typical, isolated slits. Mesolenellus is a genus within the Mesonacinae, with eye-ridges curved