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Trilobita - Redlichiida - Olenellidae

Mesonacis was named by Walcott (1885) [Sepkoski's age data: Cm Boto-u Cm Tojo]. Its type is Olenus vermontana.

It was reranked as Olenellus (Mesonacis) by Walcott (1891) and Palmer and Repina (1993); it was synonymized subjectively with Olenellus by Riccio (1952).

It was assigned to Mesonacidae by Walcott (1910); to Olenellus by Walcott (1891) and Palmer and Repina (1993); to Mesonacinae by Lieberman (1999); and to Olenellida by Sepkoski (2002).

Synonymy list
YearName and author
1885Mesonacis Walcott p. 330 figs. 1, 2
1890Elliptocephalus (Schmidtia) Marcou p. 363
1891Olenellus (Mesonacis) Walcott p. 634 figs. pi. 93, fig. I
1899Schmidtia Moberg p. 319
1910Mesonacis Walcott p. 261
1936Fremontia Raw
1956Fremontia Harrington p. 57 figs. Pl. 15, fig. 6; Text-fig 1B
1993Olenellus (Mesonacis) Palmer and Repina p. 22
1999Mesonacis Lieberman p. 38
2002Mesonacis Sepkoski, Jr.
2003Fremontia Jell and Adrain p. 377

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EubilateriaAx 1987
phylumArthropodaLatreille 1829
classTrilobitaWalch 1771
orderRedlichiidaRichter 1932
subfamilyMesonacinae(Walcott 1891)

If no rank is listed, the taxon is considered an unranked clade in modern classifications. Ranks may be repeated or presented in the wrong order because authors working on different parts of the classification may disagree about how to rank taxa.

H. J. Harrington 1956 (Fremontia)Olenellidae with moderately advanced genal spines. Metagenal spines absent or minute in the adult. Metagenal angle broadly obtuse. Glabella parallel-sided, with three pairs of lateral glabellar lobes. Anterior (4p) lobes slightly narrower than the remainder, well defined by the second (3p) transglabellar furrow. Frontal lobe of glabella moderately expanded, reaching anterior border of cephalon. Eye lobes short, submedian (slightly posterior), reaching level of anterior third of preoccipital glabellar lobe. Cephalic border raised, narrowing anteriorly.
A. R. Palmer and L. N. Repina 1993Species of Olenellus with posterior margin of
cephalon angled forward distal to intergenal angle. Preglabellar field absent or very short, with sagittal length generally equal to or less than that of anterior border. Posterior tips of ocular lobes directed nearly straight pos- teriorly, situated opposite or anterior to medial part of LI. Interocular area may be extended posteriorly as low, broad ridge. Most species with transverse width of inner part of macropleural third segment, exclusive of spine, less than 1.5 times width of axis. Pygidium, known only for type species, has two pairs of short marginal spines.
B. S. Lieberman 1999Anterolateral margins of frontal lobe of glabella not prominently separated from extraocular area by furrow; posterior margins of frontal lobe expanding where ocular lobes intersect with it; anterior portions of posterior margins of ocular lobes opposite medial part of distal margins of L1; posterodistal margins of L3 formed by ocular lobes; length (exsag.) of genal spine less than or equal to length (sag.) of first four thoracic segments; lateral lobes absent from L0; intergenal angle forms 30 to 50 degree angle with transverse line; lateral margins of each prosomal thoracic axial ring converging when proceeding from anterior to posterior edges; field of thoracic pleural furrows at medial portion long (exsag.), equal to approximately 1.3 times length of posterior pleural band; anterior margin of T3 medially flexes anteriorly; posterior margin of thoracic pleural furrow on T3 directed evenly poste- riorly laterally; width (tr.) of thoracic axis 100% width of pleural field excluding spines.