''Mesonacis'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

that lived during the

Botomian Cambrian Stage 4 is the still unnamed fourth stage of the Cambrian and the upper stage of Cambrian Series 2. It follows Cambrian Stage 3 and lies below the Wuliuan. The lower boundary has not been formally defined by the International Commission ...

, found in North-America (excluding Greenland), and the United Kingdom (North-Western Scotland). Some of the species now regarded part of ''Mesonacis'', have previously been assigned to ''Angustolenellus'' or ''Olenellus (Angustolenellus)''. ''Angustolenellus'' is now regarded a junior synonym of ''Mesonacis''.

Etymology

''Mesonacis vermontanus'' is named after the State of Vermont, where it was collected. ''Mesonacis bonnensis'' is called after the Bonne Bay, Newfoundland, where the species is found. ''Mesonacis eagerensis'' refers to the Eager Formation, British Columbia, in which it occurs. ''Mesonacis hamoculus'' is derived from the Latin hamus, meaning hooked, and oculus, meaning eye.

Distribution

''Mesonacis vermontanus'' occurs in the middle Upper ''Olenellus''-zone of Vermont (Parker Slate, Georgia). ''Mesonacis bonnensis'' has been found in the ''Olenellus''-zone of Newfoundland, Canada (Forteau Formation, East shore of the Bonne Bay East Arm). ''Mesonacis cylindricus'' was collected in the Upper ''Olenellus''-zone of California (Eagle Mountain Shale, Carrara Formation, Grapeville Mountains, White/Inyo Mountains, South end of the Marble Mountains, and in San Bernardino County). ''Mesonacis eagerensis'' is present in the ''Olenellus''-zone of British Columbia, Canada (Eager Formation, just South of the Fort Steel-St. Eugene Mission road, 6 miles North-East of Cranbrook). ''Mesonacis fremonti'' was excavated in California from the ''Olenellus''-zone (Pyramid Shale Member, Carrara Formation, Funeral Mountains, Resting Spring Range, and Salt Spring Hills), the Lower ''Olenellus''-zone (Upper Poleta and lower Harkless Formations) and the Upper ''Olenellus''-zone (middle part of the Latham Shale, at the Southern end of the Marble Mountains; Latham Shale, Marble Mountains, ½ mile East of Cadiz; the Mohave Desert portion of San Bernardino County; the Mule Spring Limestone, White/Inyo Mountains region), and in Nevada (arenaceous shales at the summit of Prospect Mountain, Eureka County). ''Mesonacis hamoculus'' occurs in the ''Olenellus-zone'' of Scotland (Loch Awe).

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 (L4) 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 (L3) and middle pair (L2) of lateral lobes of the glabella are partially merged. This creates two very typical, isolated slits. The

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

of ''Mesonacis'' is about 2⅓× as long as wide, measured between the genal angles. The outer ⅓ of the back (or posterior margin) of the headshield (or

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino Jr., Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont ...

) angles forwards from the tip of the pleural spine to the genal angle. The central area of the cephalon (or

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior ...

) and the frontal margin touch or the distance is as long as the margin at most. The

thorax The thorax (: thoraces or thoraxes) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main di ...

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 (or macropleural spines) that reach back only to the tip of the 5th pleural spines. The segments look degenerated behind the 15th (or an opisthothorax can be distinguished). The tailshield (or pygidium) is very small and subquadrate in shape, and carries one or two pairs of small marginal spines.A.R. Palmer and L.N. Repina (1993). Through a Glass Darkly: Taxonomy, Phylogeny and Biostratigraphy of the Olenellina. The University of Kansas Paleontological Contributions 3, pp. 1-3

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Key to the species

References

Sources

External links

* {{Taxonbar, from=Q16985432 Olenellidae Redlichiida genera Cambrian trilobites Fossil taxa described in 1885 Extinct animals of North America