Mesonacis is an extinct genus of trilobite that lived during the Botomian, found in North-America, and the United Kingdom. Some of the species now regarded part of Mesonacis, have previously been assigned to Angustolenellus or Olenellus . Angustolenellus is now regarded a junior synonym of Mesonacis.
Etymology
M. vermontanus is named after the State of Vermont, where it was collected. M. bonnensis is called after the Bonne Bay, Newfoundland, where the species is found. M. eagerensis refers to the Eager Formation, British Columbia, in which it occurs. M. hamaculus is derived from the Latin hamus, meaning hooked, and oculus, meaning eye.
Distribution
M. vermontanus occurs in the middle Upper Olenellus-zone of Vermont. M. bonnensis has been found in the Olenellus-zone of Newfoundland, Canada. M. cylindricus was collected in the Upper Olenellus-zone of California. M. eagerensis is present in the Olenellus-zone of British Columbia, Canada. M. fremonti was excavated in California from the Olenellus-zone, the Lower Olenellus-zone and the Upper Olenellus-zone, and in Nevada. M. hamoculus occurs in the Olenellus-zone of Scotland.
Description
As with most early trilobites, Mesonacis has an almost flat exoskeleton that is only thinly calcified, and has crescent-shaped eye ridges with a total length up to 2.8 inches in some fossils from the early cambrian. As part of the suborder Olenellina, Mesonacis lacks dorsal sutures. Like all other members of the superfamily Olenelloidea, the eye-ridges spring from the back of the frontal lobe of the central area of the cephalon, that is called glabella. Mesonacis also shares the typical character of whole family Olenellidae in that the frontal and middle pair of lateral lobes of the glabella are partially merged. This creates two very typical, isolated slits. The exoskeleton of Mesonacis is about 2⅓× as long as wide, measured between the genal angles. The outer ⅓ of the back of the headshield angles forwards from the tip of the pleural spine to the genal angle. The central area of the cephalon and the frontal margin touch or the distance is as long as the margin at most. The thorax has approximately 25 segments, the pleura about 1½× as wide as the axis, excluding the genal spines. The 3rd segment carries extra large pleural spines that reach back only to the tip of the 5th pleural spines. The segments look degenerated behind the 15th. The tailshield is very small and subquadrate in shape, and carries one or two pairs of small marginal spines.
