Greenops perijaensis, DE CARVALHO & MOODY, 2000

DE CARVALHO, MARIA DA GLORIA PIRES & MOODY, JOHN, 2000, A Middle Devonian Trilobite Assemblage from Venezuela, American Museum Novitates 3292, pp. 1-15 : 1-15

Greenops perijaensis , new species

Figure 5 , C–E

DIAGNOSIS: Cephalon semicircular in outline, with anterior border pointed; frontal glabellar lobe expended anteriorly and laterally, genal spine flat, smooth, extended to sixth thorax segment; eyes with maximum of seven lenses per dorsoventral file; visual surface in dorsal view lies partly within a sagittal line drawn tangentially to the frontal lobe; pygidium with five pairs of triangular lappets all approximately the same size as their respective pleurae and in contact with each other proximally for at least 30% of their length; tips of first three, pointed; fourth and fifth, somewhat rounded.

HOLOTYPE: MBLUZ P­1448 A and B, part and counterpart of an almost complete specimen with total length of approximately 29 mm, from upper level of the Caño del Oeste Formation , Perijá Mountains, NW Venezuela. Cast of holotype AMNH 46583 .

ETYMOLOGY: Named for Perijá Mountains, of NW Venezuela.

DESCRIPTION: The cephalon is semicircular in outline, moderately convex, a little more than twice as wide as long, with length (sag.) approximately 8.7 mm and an inferred width (across the posterior border of the cephalon) of 19 mm (the right side is incomplete). The anterior cephalic border is pointed medially. The axial furrows are moderately wide; they are more divergent anteriorly from S1. The glabella has three distinct pairs of lateral glabellar furrows. S1 is moderately wide, developed as a smooth convex curve posteriorly, becoming deeper proximally and in contact with the axial furrows. S2 is roughly transverse, with its anterior margin weakly convex, and almost reaching the axial furrows. S3 is widest distally and slightly convex anteriorly. The frontal glabellar lobe is expanded anteriorly and laterally, declines smoothly forward, has a rounded anterior margin, and represents about 70% of total glabellar length. The glabellar lobes are flattened, their height decreases posteriorly (from L3 to L1), with the medial region flat. L3 is wedge­shaped, lengthening distally; L2 is approximately rectangular, and L1 is narrower than L2. S0 is shallower than S1, moderately wide, convex anteriorly (sag.). The occipital lobe (L0) is longest sagittally, high­ er medially, slightly above the median area of glabella; becoming lower and narrower distally, without ornamentation. The eyes are raised slightly above the glabella, with a maximum of seven lenses per dorsoventral file on the visual surface. The anterior edge of the visual surface contacts the axial furrow at the anterolateral corner of L3; the posterior edge of the visual surface is located posterior to a transverse line tangent to the anterior edge of L0. In dorsal view the visual surface lies partly within a sagittal line drawn tangent to the frontal lobe. The fixigena slopes from the axial furrow to the palpebrum; the librigena is moderately declined toward the lateral cephalic border, whose margin forms a broad continuous arch. The posterior cephalic border furrow is nearly transverse and broadest distally, and the posterior cephalic border widens abaxially. Genal spines are flat and smooth, and extend back to the sixth thoracic segment (their exact length is not very clear on the specimen). No ornamentation is observed on the cephalon.

The thorax contains 11 moderately convex segments, with the axis occupying about one third the width of the thorax (maximum thorax width 17.6 mm; axis width 6.1 mm). Axial furrows are shallow but well defined. The axis is somewhat higher than the pleural field. Each axial ring is of uniform length (exsag., sag.) with the medial portion convex anteriorly and higher than the distal extremities, which have an almost straight anterior margin and a convex posterior margin. The articulation furrows are moderately wide and deep. The proximal part of the pleura is transverse, approximately horizontal, whereas the distal part is strongly flexed downward. Pleural furrows are moderately deep, wide, and almost straight.

The pygidium is broad, semicircular in outline, approximately 9.4 mm long (sag.) and 16.3 mm wide (maximum width anteriorly). The axis is raised slightly above the pleural fields, and is well defined by narrow and shallow axial furrows. Only the first seven axial rings are distinct, plus the anterior part of the eighth. Axial rings are delimited by shallow and moderately wide ring furrows. The first five axial rings are gently flexed anteriorly at the midline and are of nearly equal length throughout; the more posterior rings are almost straight. There are five pairs of pleurae; the first three are gently curved backward and the last two are more abruptly flexed; pleurae become slightly wid­ er distally. Interpleural furrows are obsolete; the pleural furrows are shallow and relatively wide. The first three pleural furrows are more conspicuous than the posterior two. The anterior and posterior bands of the pleural segments lie at the same level. The border furrow is well defined. There are five pairs of triangular lappets, all approximately the same size as their respective pleurae. The distal tips of the first three lappets are point­ ed, but the fourth and fifth are somewhat rounded. The lappets make contact with each other proximally, for at least 30% of their length, but are separated distally. The lateral margins of the lappets are curved posteriorly, whereas their inner margins seem to be straight. The lappets are covered by small, dense granules. The terminal lappet is approximately equal in length (sag.) to the fifth lappet, although its end is not very clear in the specimen.

Figures

Fig. 5. (A, B) Rhenops odremani, n. sp. Paratype, MBLUZ P­985; internal mold of pygidium. (A) dorsal view; (B) lateral view. (C­E) Greenops perijaensis, n. sp., latex cast of external mold in the holotype, MBLUZ P­1448. (C) Dorsal view; (D) frontal view; (E) lateral view.