Mesonacis wileyi, Gapp, I. Wesley & Lieberman, Bruce S., 2014

Gapp, I. Wesley & Lieberman, Bruce S., 2014, New olenelloid trilobites from the Northwest Territories, Canada, Zootaxa 3866 (4), pp. 479-498 : 485-486

Mesonacis wileyi sp. nov.

Fig. 4

Type material. Holotype KUMIP 355550. Paratypes KUMIP 355551 and PWNHC-2013.23.4-5. Olenellus zone or Waucoban Series, Dyeran stage, sensu Webster (2011a, b) and Webster et al. (2011), early Cambrian, Sekwi Formation, Mackenzie Mountains, Northwest Territories, Canada, Section 4, 430– 435m above the base of section; Section 3, 700– 800m above the base of the section.

Etymology. In honour of Edward O. Wiley, University of Kansas, for his fundamental contributions to the field of phylogenetics.

Diagnosis. LA does not contact anterior cephalic border furrow; width of extraocular area approximately equal to width (tr.) of glabella at L1; posterior border between LO and intergenal angle parallel to a transverse line; intergenal angle 45 to 50 degrees relative to a transverse line; genal spines terminate posteriorly opposite T5 or T6.

Description. Anterior cephalic border developed as a raised ridge, length (sag.) directly anterior of LA is approximately 0.5 times length (sag.) LO; small preglabellar field; anterolateral margin of LA directed posteriorly at approximately 45 degrees; LA length (sag.) equal to the combined length (sag.) of LO and L1; ocular furrow present; ocular lobes extend back to SO; S3 convex anteriorly, conjoined adaxially; abaxial margins of L2 adaxial of abaxial margins of L3; S1 and SO both conjoined adaxially; L1 length (sag.) approximately length (sag.) of LO; abaxial margins of L1 adaxial to abaxial margins of LO; posterior margin of LO convex posteriorly; extraocular area relatively wide, with prominent anastomosing ridges; posterior border between intergenal angle and LO is parallel to a transverse line; intergenal angle deflected anteriorly at 45 to 50 degrees; genal spine narrow, developed opposite L1 or S1, extends back to T5 or T6; prothorax consists of 13(?) segments and opisthothorax about 5 or 6; pleural spines long and directed posteriorly; T3 is macropleural with spines extending posterior of pygidium; axial spine is present, questionably on T13, approximately the length (sag.) of the thorax.

Discussion. This species is assigned to Mesonacis due to the presence of several character states including: the anterolateral margins of LA are not prominently separated from the extraocular area by a furrow; the lateral margins of each prothoracic axial ring converge when proceeding from anterior to posterior. Further, this species can be distinguished from species of Olenellus based on several character states including: the transverse profile of the ocular lobe is convex dorsally; the intergenal angle is directed anteriorly approximately 45 degrees relative to a transverse line; and finally the thoracic pleural spines on all segments but the third are relatively narrow (tr.) compared with those of most species of Olenellus . Lastly, this species is assigned to Mesonacis , rather than Bristolia on the basis of several character states including: the lateral margins of L2, when proceeding anteriorly, do not bulge relative to LO; the surface of the interocular area does not slope evenly from the tip of the ocular lobe to the glabella; the width of the interocular area is approximately equal to the width of the ocular lobe at its midlength; and the posterior margin of the thoracic pleural furrow on the third segment is directed evenly posterolaterally.

Mesonacis vermontanus ( Hall, 1859) has shorter thoracic spines and genal spines and possesses a much longer (sag.) thorax. Mesonacis fremonti ( Walcott, 1910) possesses a genal angle further posterior than that of M. wileyi sp. nov., a posterior border directed posteriorly between LO and the intergenal angle, and a prominent intergenal ridge. Mesonacis bonnensis ( Resser and Howell, 1938) has a narrower extraocular area and a prominent intergenal ridge. Mesonacis eagerensis ( Best, 1952) possesses a narrower extraocular area and narrower pleural lobes. Mesonacis hamoculus ( Cowie & McNamara, 1978) has a prominent occipital spine and a narrower (tr.) posterior border between LO and the intergenal angle, which is directed slightly posteriorly.

Mesonacis cylindricus (Palmer in Palmer & Halley, 1979), which Lieberman (1999) determined phylogenetically to be a more derived species within the genus, appears to be most similar to M. wileyi . Similarities include: extraocular area that is wider than the width of the glabella (tr.); LA does not contact the anterior cephalic border furrow; the adaxial and abaxial tips of S2 are on a transverse line; the lateral border of L2 diverges anteriorly; SO is conjoined adaxially (although this is not observed in the smallest specimen of M. wileyi which represents an earlier ontogenetic stage); and the intergenal angle is 45 to 50 degrees relative to a transverse line. Some key differences between the species observed include: the posterior margin of the ocular lobe is opposite SO in M. wileyi and opposite the adaxial part of L 1 in M. cylindricus ; S 2 in M. wileyi is more prominently incised adaxially; the adaxial part of the cephalic border between the lateral margins of LO and the intergenal angle parallels a transverse line rather than being directed posteriorly (though again this is not observed in the smallest specimen of M. wileyi ); and lastly the direction of the ocular lobes of M. wileyi do not form as large an angle to a sagittal line as those in M. cylindricus .

Figures

FIGURE 4. Mesonacis wileyi sp. nov. 1, holotype KUMIP 355550, x 1.9; 2, KUMIP 355551, x 2.4; 3, PWNHC- 2013.23.4, x 2.6; 4, PWNHC- 2013.23.5, x 2.4.