Mesolenellus

''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 ( el, Βορέας), 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 but almost parallel to the midline. The back of the eye-ridges is opposite the most backward ring of the glabella (L0 or occipital ring). Genal spines are 6-8 times as long as L0. The outer furrows of the glabella are parallel to the midline between the back of the cephalon and the furrow between side lobes L2 and L3. The thorax is 3 times as wide as the axis at the 3rd segment.
''Mesolenellus'' can be distinguished from the sister-genus ''Mesonacis'', where the back of the eye-ridge extends only to the most backward side lobes (L1), genal spines are only 1-5 times as long as L0, and the glabella widens forwards along L1 and L2. Except for in ''Mesonacis fremonti'' the curved eye-ridges are at an angle of 15°-20° with the midline.
A third genus ''Olenellus'', that constitutes the second subfamily of the family Olenellidae differs from ''Mesolenellus'' in having genal spines 4-5 times as long as L0, the glabella widens forwards along L1 and L2, and the thorax 4-4½ times wider that the axis at the 3rd segment.

Key to the species

References

External links

'' Trilobite info''

{{Taxonbar, from=Q16985426

Olenellidae
Redlichiida genera
Cambrian trilobites
Cambrian trilobites of Europe
Cambrian genus extinctions