Gemelloides, Carvalho & Edgecombe & Smith, 2003

Carvalho, Maria Da Gloria Pires De, Edgecombe, Gregory D. & Smith, Legrand, 2003, New Calmoniid Trilobites (Phacopina: Acastoidea) from the Devonian of Bolivia, American Museum Novitates 3407, pp. 1-18 : 2-5

GEMELLOIDES , NEW GENUS

DERIVATION OF NAME: Literally, ‘‘like Gemellus ’’, in reference to the name Dalmanites gemellus Clarke, 1890 ; gemellus (Latin) , a twin.

TYPE SPECIES: Gemelloides delasernai , n. gen. and sp .

REFERRED SPECIES: Dalmanites gemellus Clarke, 1890 (= ‘‘ Vogesina ’’ gemellus (Clarke) fide Lieberman et al., 1991) is provisionally assigned.

DIAGNOSIS: Cephalon twice wider than long, gently convex (tr. and sag.). All lateral glabellar furrows well incised; apodemal part of S1 abruptly shallowing, S1 faintly confluent with axial furrow; S2 effaced abaxially; S3 steeply inclined exsagittally. Pygidium triangular in outline, relatively wide, lacking marginal spines or lappets; pygidial axis comprises 11 rings (first 5 separated by groovelike impression of distal part of ring furrows, posterior rings progressively weak­ er); pleural field moderately convex (tr.), with at least six ribs.

Gemelloides delasernai , new species

Figure 2A, B

DERIVATION OF NAME: After Salvador de la Serna, field companion to L. Smith for many years, who helped to collect the material.

DIAGNOSIS: Lateral glabellar furrows S2 and S3 sharply defined; apodemal part of S1 wedge­shaped; sculpture consisting of small, strong pits widely distributed on cephalon, thorax, and pygidium, without coarse granules or tubercles.

TYPES: Holotype : MHNC 8130 , external mold of almost complete specimen (fig. 2A, B), from the lower part of the Upper Member of the Belén Formation, layer of Wolfartaspis cornutus ( Wolfart, 1968) , late Emsian, Belén area, La Paz Department, Bolivia (latex cast AMNH 48073 ) . Paratype: AMNH 48074 , internal mold of part of thorax and external mold of frontal glabellar lobe, from type locality.

DESCRIPTION: The cephalon is wider than long, with length (sag.) of cranidium 7.2 mm and width (tr.) across posterior border ap­ proximately 16 mm. Axial furrows are very distinct, narrow, weakly divergent, slightly curved externally at L2. The widest point of the glabella (across L3) is approximately 7 mm, and its posterior width is 4.9 mm. The occipital furrow (S0) is deep, moderately wide, curving anteriorly (sag.). S1 is a deep, wedge­shaped groove, widest mesially, sharply narrowing and shallowing distally, faintly confluent with the axial furrow, posteromedian part positioned close to S0; S2 is longer and narrower than S1 (tr.), slightly convex forward, with weak posteromedial orientation, effaced well inward of axial furrow; S3 is straight, narrow, widest distally, strongly divergent toward the anterior glabellar margin (this margin is not preserved, and it is uncertain whether S3 reaches it); the strongly divergent arrangement of S3 gives rise to a pattern in which the glabellar furrows appear to radiate away from the center of the glabella. The glabellar furrows do not cross the median region of the glabella, which is not inflated. The glabellar lobes likewise lack independent inflation; L1 is narrow mesially, becoming wider near the axial furrow; L2 is wider than L1 and is trap­ ezoidal, becoming wider laterally; L3 is also trapezoidal in outline and is larger than L2. The posterior median impression is a short (sag.), elongate pit on the frontal lobe, anterior to the tips of S3. The frontal lobe is not inflated or distinguished from the rest of the glabella. L0 is moderately arched (tr.), longest sagittally, distinctly wider than L1, with a width of 5.5 mm. The posterior border fur­ row is straight and moderately wide across most of its extent, gradually narrowing near the genal angle. The posterior border is very narrow adjacent to the axial furrow but is considerably wider (exsag.) abaxially, with an evenly convex posterior margin and a rounded genal angle. The palpebral lobe and surrounding area of the fixigena are somewhat swollen, sloping down laterally behind and around the eye. The palpebral lobe is narrow, bounded by a faint palpebral furrow. The cephalon (glabella, L0, fixigenal field, and posterior border) is ornamented with small, widely distributed pits, but lacks any granular or tuberculate sculpture.

The thorax comprises 11 segments, all weakly convex (tr.) with the axial region somewhat higher than the pleurae. Maximum axial width is about one­ third that of the thorax. The first four axial rings are faintly convex forward sagittally; this convexity is gradually accentuated in more posterior rings. Axial furrows are shallow and narrow but well defined. Thoracic pleurae are transverse and horizontal proximally, becoming weakly flexed posteriorly in the distal portion; this flexure is also more accentuated posteriorly. Pleural furrows are deep, narrow, and run straight across the pleurae onto the articulating facets. The margin of the anterior pleural band is convex proximally; the posterior band is longer than the anterior (tr.) and is broader distally (exsag.).

The pygidium is moderately large (9.4 mm maximum width) and triangular in outline. The axis is only slightly higher than the pleural field, defined by shallow, narrow axial furrows, and includes 11 discernible rings. The first four rings are gently flexed anteriorly and are separated by broad ring furrows, which are deepest distally and impressed as transverse grooves; the following rings are progressively fainter. The axial terminus is indistinctly defined, but a postaxial field, if present, is short (sag.). The pleural field is gently arched (tr.) and comprises at least six ribs. The first three are gently curved posteriorly, but the remaining ribs are more distinctly flexed. Pleural furrows are narrow but more distinct than the interpleural furrows, which are only weakly developed. Little of the pygidial margin is visible, but its exposed part (at the ends of the anterior three ribs) lacks marginal spines or lappets. The axial rings and pleurae of the thorax and pygidium are ornamented with small pits like those covering the cephalon.

DISCUSSION: The close similarity between Gemelloides and Vogesina Wolfart, 1968 indicates membership in the Malvinella group (sensu Lieberman et al., 1991), and the genus possesses more general apomorphies of that group, such as depression of the abaxial part of L1 beneath L2 (character 23 of Lieberman et al, 1991). Vogesina is united with Palpebrops Lieberman et al., 1991 by a convex (sag.) glabella ( Lieberman et al., 1991, node 9, character 8), which is shared to a lesser degree by Gemelloides . None of the three characters supporting node 10 in their phylogeny (which pertain to the anterior part of the cephalon and the hypostome) can be determined in our material, but both characters supporting their node 11 ( Vogesina s.l., characters 5, 11) are present (weakly divergent cephalic axial furrows and steep inclination of S3). The latter feature gives rise to a distinctive radial pattern of glabellar furrows which is also weakly evident in Palpebrops , and may thus represent a synapomorphy of Vogesina , Gemelloides , and perhaps Palpebrops .

Within Vogesina as recognized by Lieberman et al. (1991), V. aspera and V. lacunafera are separated from ‘‘ Vogesina ’’ gemellus (= Gemelloides gemellus here) by 12 characters. Three of these cannot be observed in the specimens of Gemelloides delasernai , n. sp., but this taxon definitely lacks two other characters (8, 18; strongly convex glabella; elongate but nearly effaced S2 and S3) and shares seven apomorphies with V. aspera and V. lacunafera (12, 20, 27, 29, 32, 45, 48; S3 straight; absence of coarse spines on lateral glabellar lobe L2; L0 not elevated above posterior glabellar region; absence of spines on L0; palpebral furrow weak and palpebral rim low; absence of spines on thoracic axial rings and on pygidial axis). The well­developed lateral glabellar furrows in Gemelloides delasernai , n. sp. clearly distinguish this form from all Vogesina spp. , which have S2 and S3 as shallow to indistinct, typically resembling thin pencil lines ( Wolfart, 1968). This difference between Gemelloides and Vogesina is marked even on external molds of Vo­ gesina of equivalent size to the holotype of G. delasernai . The glabella is considerably less convex (sag.) in Gemelloides delasernai than in Vogesina and is not significantly elevated above L0. The pygidium of Gemelloides delasernai resembles that of Vogesina in lacking marginal lappets, but has a relatively broader outline, with the distal part of the pleurae less steeply turned down than in Vogesina . Collectively, these findings suggest that G. delasernai is the sister taxon to Vogesina but can be differentiated as a distinct taxon on the basis of cephalic as well as pygidial characters.

‘‘ Vogesina ’’ gemellus (Clarke) , from the Maecuru Formation in the Amazon Basin, is known only from an isolated, poorly preserved glabella ( Lieberman et al., 1991: fig. 8.5, 8.6), and the state of only one of the apomorphic characters uniting G. delasernai with V. aspera and V. lacunafera ( Lieberman et al., 1991) could be determined. ‘‘ Vogesina ’’ gemellus has a similar glabellar profile (sag.) and arrangement of lateral glabellar furrows to G. delasernai . We provisionally assign it to Gemelloides based on these characters, although the generic diagnosis emphasizes the better known G. delasernai . Specific distinction between G. gemellus and G. delasernai is made based on the wider glabellar furrows, more elongated longitudinal median groove on the glabella, and tuberculate sculpture of the former species.