PBDB Taxon

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Mesonacis

Trilobita - Redlichiida - Olenellidae

Taxonomy

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 (1890), Walcott (1891) and Palmer and Repina (1993); it was synonymized subjectively with Olenellus by Riccio (1952) and Palmer and Halley (1979).

It was assigned to Mesonacidae by Walcott (1910) and Walcott (1916); to Olenellus by Walcott (1890), Walcott (1891) and Palmer and Repina (1993); to Mesonacinae by Lieberman (1999); and to Olenellida by Sepkoski (2002).

Species

Fremontia canadensis, M. bonnensis (syn. Olenellus brevoculus, Olenellus terranovicus), M. cylindricus, M. eagerensis, M. fremonti, M. hamoculus, M. vermontanus, M. wileyi

Synonyms

Fremontia was named by Raw (1936). It is not extant.

It was reranked as Olenellus (Fremontia) by Lochman (1952); it was synonymized subjectively with Olenellus by Palmer and Halley (1979); it was synonymized subjectively with Mesonacis by Lieberman (1999).

It was assigned to Olenellus by Lochman (1952); to Olenellinae by Harrington (1956); and to Olenellidae by Jell and Adrain (2002).
Schmidtia was named by Moberg (1899).

It was reranked as Elliptocephalus (Schmidtia) by Marcou (1890); it was synonymized subjectively with Mesonacis by Walcott (1910).

It was assigned to Elliptocephalus by Marcou (1890).

Synonymy list

Year	Name and author
1885	Mesonacis Walcott p. 330 figs. 1, 2
1890	Elliptocephalus (Schmidtia) Marcou p. 363
1890	Olenellus (Mesonacis) Walcott p. 41
1891	Olenellus (Mesonacis) Walcott p. 634 figs. pi. 93, fig. I
1899	Schmidtia Moberg p. 319
1910	Mesonacis Walcott p. 261
1916	Mesonacis Walcott p. 406
1936	Fremontia Raw
1952	Olenellus (Fremontia) Lochman p. 91
1956	Fremontia Harrington p. 57 figs. Pl. 15, fig. 6; Text-fig 1B
1993	Olenellus (Mesonacis) Palmer and Repina p. 22
1999	Mesonacis Lieberman p. 38
2002	Fremontia Jell and Adrain p. 377
2002	Mesonacis Sepkoski

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Rank	Name	Author
kingdom	Animalia	()
—	Bilateria
—	Eubilateria	Ax 1987
—	Protostomia
—	Ecdysozoa
—	Panarthropoda
phylum	Arthropoda	Latreille 1829
subphylum	Artiopoda	(Hou and Bergstrom)

Rank	Name	Author
class	Trilobita	Walch 1771
order	Redlichiida	Richter 1932
suborder	Olenellina
superfamily	Olenelloidea
family	Olenellidae
subfamily	Mesonacinae	(Walcott 1891)
genus	Mesonacis

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.

G. †Mesonacis Walcott 1885

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†Mesonacis bonnensis Resser and Howell 1938

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Invalid names: Olenellus brevoculus Resser and Howell 1938 [synonym], Olenellus terranovicus Resser and Howell 1938 [synonym]

†Mesonacis cylindricus Palmer and Halley 1979

†Mesonacis eagerensis Best 1952

†Mesonacis fremonti Walcott 1910

†Mesonacis hamoculus Cowie and McNamara 1978

†Mesonacis vermontanus Hall 1859

†Mesonacis wileyi Gapp and Lieberman 2014

Invalid names: Angustolenellus Palmer and Repina 1993 [synonym], Fremontia Raw 1936 [synonym], Schmidtia Moberg 1899 [synonym]

Diagnosis

Reference	Diagnosis
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 1993	Species 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 1999	Anterolateral 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.