Mesonacis

{{Short description|Extinct genus of trilobites}} {{Automatic taxobox | fossil_range = {{fossilrange|516|513}}Botomian | image = Mesonacis NMNH.jpg | image_upright = 0.8 | image_caption = ''Mesonacis vermontanus'', late lower Cambrian, Vermont, USA, courtesy of the Smithsonian Museum of Natural History. | taxon = Mesonacis | authority = Walcott, 1885 | subdivision_ranks = Species | subdivision = * ''Mesonacis vermontanus'' (Hall, 1859) type species, synonyms ''Olenellus vermontanus'', ''O. georgiensis''<ref name='Lieberman 1999'>{{cite journal|last1= Lieberman|first1=B.S.|year=1999|title=Systematic Revision of the Olenelloidea (Trilobita, Cambrian)|journal=Bulletin of the Peabody Museum of Natural History|volume=45|url= http://peabody.yale.edu/sites/default/files/documents/scientific-publications/ypmB45_1999.pdf |archive-url=https://web.archive.org/web/20190509161341/http://peabody.yale.edu/sites/default/files/documents/scientific-publications/ypmB45_1999.pdf |archive-date=2019-05-09}}</ref> * ''Mesonacis bonnensis'' (Resser and Howell, 1938), synonyms ''Olenellus bonnensis'', ''O. brevioculus'', ''O. terranovicus''<ref name='Lieberman 1999'/> * ''Mesonacis cylindricus'' (Palmer, 1979), synonyms ''Olenellus cylindricus''<ref name='Lieberman 1999'/> * ''Mesonacis eagerensis'' (Best, 1952), synonym ''Olenellus eagerensis''<ref name='Lieberman 1999'/> * ''Mesonacis fremonti'' (Walcott, 1910), synonyms ''Olenellus fremonti'', ''Fremontia fremonti''<ref name='Lieberman 1999'/> * ''Mesonacis hamoculus'' (Cowie & McNamara, 1978), synonym ''Olenellus hamoculus''<ref name='Lieberman 1999'/><ref name='Cowie&McNamara'>{{cite journal|last1=Cowie |first1=J. |last2=McNamara |first2=K.J. |year=1978 |title=''Olenellus'' (Trilobita) from the Lower Cambrian Strata of North-West Scotland |journal=Palaeontology |volume=21 |issue=3 |pages=615–634 |url=http://palaeontology.palass-pubs.org/pdf/Vol%2021/Pages%20615-634.pdf |url-status=usurped |archiveurl=https://web.archive.org/web/20120526170847/http://palaeontology.palass-pubs.org/pdf/Vol%2021/Pages%20615-634.pdf |archivedate=2012-05-26 }}</ref> }}

'''''Mesonacis''''' is an extinct genus of trilobite that lived during the Botomian, 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)''.<ref name="Palmer&Repina"/><ref name='Whittington'>{{cite book |author=H. B. Whittington|title=Treatise on Invertebrate Paleontology. Part O, Revised. Trilobita |chapter=Introduction, Order Agnostida, Order Redlichiida |year=1997|display-authors=etal}}</ref> ''Angustolenellus'' is now regarded a junior synonym of ''Mesonacis''.<ref name='Lieberman 1999'/>

== 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.<ref name='Cowie&McNamara'/>

== Distribution == ''Mesonacis vermontanus'' occurs in the middle Upper ''Olenellus''-zone of Vermont (Parker Slate, Georgia).<ref name='Lieberman 1999'/>

''Mesonacis bonnensis'' has been found in the ''Olenellus''-zone of Newfoundland, Canada (Forteau Formation, East shore of the Bonne Bay East Arm).<ref name='Lieberman 1999'/>

''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).<ref name='Lieberman 1999'/>

''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).<ref name='Lieberman 1999'/>

''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).<ref name='Lieberman 1999'/>

''Mesonacis hamoculus'' occurs in the ''Olenellus-zone'' of Scotland (Loch Awe).<ref name='Cowie&McNamara'/>

== 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 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) angles forwards from the tip of the pleural spine to the genal angle. The central area of the cephalon (or glabella) 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 (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.<ref name="Palmer&Repina">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. 1-35.[http://kuscholarworks.ku.edu/dspace/handle/1808/3760]</ref><ref name="Whittington"/>

== Key to the species == {| |- valign="top" align="left" ! scope="row" width="3%" | '''1''' | width="97%" |The curved eye-ridges are at an angle of 15°-20° with the midline. Except for the third thorax segment, the spine on the side lobes (or pleural spine) is approximately a quarter of the length of the segment, halfway between the spine and the axis. → 2

|- valign="top" align="left" ! scope="row" width="3%" | '''-''' | width="97%" |The curved eye-ridges are at an angle of 0°-5° with the midline. The pleural spines of T1-T2 and T4-T14 are at least half as long as the segment. → '''''Mesonacis fremonti'''''

|- valign="top" align="left" ! scope="row" width="3%" | '''2''' | width="97%" |The furrows between the lobes of the glabella are faintly incised. The frontal lobe of the glabella (L4) is connected with the border surrounding the cephalon with a short ridge on the midline (the so-called “plectrum”). → '''''Mesonacis bonnensis'''''

|- valign="top" align="left" ! scope="row" width="3%" | '''-''' | width="97%" |The furrows between the lobes of the glabella are prominently incised. A plectrum is absent or very faint. → 3

|- valign="top" align="left" ! scope="row" width="3%" | '''3''' | width="97%" |The frontal lobe (L4) of the glabella intersects the anterior border furrow. The furrow between the second (L2) and third (L3) side lobes (S2) is transverse. The furrow between the occipital ring (or posterior lobe, L0) and the first pair of side lobes (L1), indicated as S0, is joint at midline. → 4

|- valign="top" align="left" ! scope="row" width="3%" | '''-''' | width="97%" |L4 does not intersect the anterior border furrow. S2 curves, so that it is further to the back at midline. S0 consist of two furrows that do not joint at midline. → 5

|- valign="top" align="left" ! scope="row" width="3%" | '''4''' | width="97%" |S2 contacts the furrow that defines the glabella (or axial furrow). The posterior rim of the cephalon (or intergenal angle) curves forwards midway between the posterior tip of the eye ridge and the genal angle. Frontal glabellar lobe (L4) as wide as the occipital ring (L0). → '''''Mesonacis hamoculus'''''

|- valign="top" align="left" ! scope="row" width="3%" | '''-''' | width="97%" |S2 does not contact the axial furrow. The intergenal angle is located at three quarters the tip of the eye lobe and the genal angle. L4 wider than L0. → '''''Mesonacis ''cylindricus'''

|- valign="top" align="left" ! scope="row" width="3%" | '''5''' | width="97%" |The frontal margin of the macropleural segment (T3) flexes forwards at at least 110° with the midline. The second (L2) and third (L3) pair of glabellar side lobes are not divided by a furrow at midline (or S2 does not join at midline). → '''''Mesonacis vermontanus'''''

|- valign="top" align="left" ! scope="row" width="3%" | '''-''' | width="97%" |The frontal margin of T3 flexes forwards no more than 95°. S2 is joined at midline. → '''''Mesonacis eagerensis''''' |}

==References== {{Reflist}}

== Sources == *{{cite web|url=http://www.trilobites.info/galredlichiida.htm|title=Photo of ''M. vermontanus''|website=Trilobite info|archiveurl=https://web.archive.org/web/20230820011857/https://www.trilobites.info/galredlichiida.htm|archivedate=August 20, 2023|url-status=live}}

==External links== *{{cite web|url=http://www.trilobites.info|title=Trilobite info}}

Category:Olenellidae Category:Redlichiida genera Category:Cambrian trilobites Category:Fossil taxa described in 1885 Category:Extinct animals of North America