Brachymetopus (Conimetopus) alekseevi Mychko, 2019

Mychko, Eduard V. & Savchuk, Oles V., 2019, A new brachymetopid trilobite from the Early Permian Shakhtau reef complex of the southwestern Urals, Bashkortostan, Russia, Zootaxa 4555 (3), pp. 346-358 : 352-354

Brachymetopus (Conimetopus) alekseevi Mychko sp. nov.

Plate 1 ; Figure 3

urn:lsid:zoobank.org:act:E5970BCA-7D22-4DB2-B3A9-1126698C75C7

Brachymetopus (Brachymetopina) moelleri n. nom. (?): Weber, 1937, p. 84, Pl. 10, figs 10, 11; Fig. 68b.

non Brachymetopus (Conimetopus) ouralicus ouralicus: Hahn and Hahn, 1996, p. 136 , figs 181–183.

Material. Holotype, PIN RAS, no. 5610/2, cephalon (Pl. 1, Fig. 1 a–c), and paratypes (TsNIGR Museum , no. 5217/ 2063, cephalon, Fig. 1 , b; PIN RAS, no. 5610/3, pygidium, Pl. 1, Fig. 4; PIN RAS, no. 5610/4, pygidium, Pl. 1, Fig. 3 ; PIN RAS, no. 5610/5, pygidium, Pl. 1, Fig. 5a,b; PIN RAS, no. 5610/6, pygidium, Pl. 1, Fig. 2a,b ; PIN RAS, no. 5610/7, pygidium, Pl. 1, Fig. 6; TsNIGR, no. 5217/2064, pygidium, Fig. 1 , c.), Lower Permian ( upper Asselian or lower Sakmarian ), Shakhtau quarry, Sterlitamak town , Ishimbay District , Bashkortostan, Russia (specimens from PIN RAS) and Lower Permian , Asselian , “Kazarmenny Kamen” locality, right bank of the Sim River , the town of Asha , Ashinsky District, Chelyabinsk Region (specimens from TsNIGR).

Etymology. In honor of A.S. Alekseev, paleontologist and stratigrapher, professor at the Faculty of Geology, Lomonosov Moscow State University, scientific mentor of the corresponding author of this article.

Description. The cephalon is large (more than 1 cm in width) and of semielliptical shape, stretched in width (L/W=0.6), strongly convex. It terminates with short and narrow genal spines rounded at the edges. The glabella is very convex, conical in shape, short (slightly less than 40% of the length of cephalon) and it narrows down from back end to front. The posterior part of the glabella is 2.6 times as wide as the anterior part. The basal lobes are very weakly defined. The entire surface of the glabella is covered with small round convex tubercles of different sizes, equidistant from each other and staggered. The occipital ring is long, narrow, convex. On its surface there are five or six large convex tubercles aligned in a row. The eyes are small, crescentic, and very prominent, located opposite the posterior half of the glabella. The palpebral lobes cover 1/3 of the eye. On either side of the glabella, a pair of large convex and detached tubercles is located at the top of the eyes. Posterior to them, between the palpebral lobe and the glabella there is a row of three other smaller tubercles. A number of variably sized tubercles are located around the eyes. The preglabellar field is generally flat though close to the anterior part of the glabella it rises steeply. The entire surface of cephalon is tuberculate. The largest, flattened tubercles are located closer to the marginal rim. They are more diffuse than the tubercles on the steep, central part of the cephalon - cranidia and regions of librigenae—on which the tubercles are denser and alternate with very small tubercles. The marginal rim is flat and very narrow, with 5 thin terrace lines on it.

The thorax is not known.

The pygidium is large (up to 4 cm in length), of semielliptical shape, wider than long (L/W=0.9), flattened. The rachis is long, occupying about 80% of the length of the pygidium. It becomes narrower towards the posterior margin (the posterior margin of the rachis is 6 times narrower than the anterior margin) and has a slight lateral narrowing nearer to the posterior end. The anterior margin of the rachis is three times narrower than the overall pygidium. In profile, the rachis is convex and subtriangular. It consists of 25 rings separated by deep and wide furrows. On each ring there is a row of 10 densely distributed large tubercles. The largest tubercles are located on the dorsal part of the rachis, and the tubercles decrease in size proportionally towards the dorsal furrows of the pygidium. In the posterior half of the rachis, on the dorsal parts of its five rings, there are large spine-like tubercles (broken off on all the specimens in the collection). The distance between them from the most rear to the frontal spine-like tubercle increases exponentially. The lateral lobes of the pygidium are nearly flat, and bear nine broad and long pleural ribs that are separated by wide and deep interpleural furrows which gradually widen to the marginal rim. In the anterior part of the pygidium, pleural ribs are almost perpendicular to the dorsal furrows, but as they approach the posterior end of the pygidium, the posterior pleural ribs become parallel to the dorsal furrows. The pleural furrows are distinct, deep, dividing the pleural ribs into a wider anterior and narrower posterior halfribs. The posterior half-rib is decorated with densely spaced, elongated tubercles aligned in a row. The anterior half is covered with chaotically located tubercles of different sizes, among which the largest tubercles are located closer to the dorsal furrows. The pleural ribs smoothly pass into a very narrow marginal rim which is devoid of terrace lines. The marginal furrow is absent. The marginal rim along its edge is densely littered with numerous small tubercles aligned in a single row.

Dimensions of the holotype and paratype specimens are given in Table 1.

Legend: CL —length of cephalon, CW —width of cephalon, GL —length of glabella, ORL —length of occipital ring, AGW —width of glabella in the anterior part, PGW —width of glabella in the posterior part, EL —length of the eye, MRW —width of the marginal rim, L/W is the ratio of the length of the cephalon to its width, CW/GW is the ratio of the width of the cephalon to the width of the glabella in the anterior part, PL —length of pygidium, PW —width of pygidium, WR is width of the rachis in the anterior part, L/W is the ratio of the length of the pygidium to its width, W/WR is the ratio of the width of the pygidium to the width of the rachis in the anterior part, PMR is the distance from the end of the rachis to marginal rim of pygidium.

Remarks. B. (C.) alekseevi is very similar to the type species, B. (C.) ouralicus ( Hahn and Hahn, 1996, figs 181–183) but differs in having 25 versus 20 pygidial rachial rings and nine versus ten pairs of pleural ribs. From the species B. (C.) ingridae (see: Gandl 2011, Taf. 14, figs 205–208) it is distinguished by the presence of a pair of distinct tubercles on both sides of glabella in the anterior part of the eyes and a smaller number of pleural ribs (B. (C.) ingridae has 10 pairs of pleural ribs). From B. (C.) ultimus (see: Hahn, Hahn and Yuan, 1989, Table 2, figs 1– 4; Hahn and Hahn, 1996, Abb. 159–160) it differs in the shape of the tubercles on cephalon (in B. (C.) ultimus they are drop-like and elongated ones), relatively large tubercles on the occipital ring (in B. (C.) ultimus these tubercles are small and scattered chaotically) and a larger number of the rings of the rachis (rachis of B. (C.) ultimus consists of 20 rings). From B. (C.) ornatus (see: Hahn and Hahn, 1996, Abb. 163–168) it differs in the round shape of cephalon (cephalon B. (C.) ornatus has a subtriangular shape), flat marginal rim of cephalon (marginal rim of B. (C.) ornatus is dented) and a greater number of rings of the rachis ( B. (C.) ornatus has 17–20 rings on the rachis). From the species B. (C.) conifrons (see: Gandl, 1987, Taf. 8, Fig. 111a–c; Hahn and Hahn, 1996, Abb. 161–162) it is distinguished by a flat marginal rim of cephalon (marginal rim of B. (C.) conifrons is dented) and a larger number of rings of rachis (in B. (C.) conifrons has 17–20 rachis rings). It differs species B. (C.) arcticus (see: Hahn and Hahn, 1992, Abb. 13–14, Taf. 1, figs 13–15; Hahn and Hahn, 1996, Abb. 157–158) in the following: the presence of a tubercles sculpture on the surface of the entire cephalon (the tubercles of B. (C.) arcticus are located only on the glabella and the eyes), less apparent basal lobes, tubercles on the occipital ring, comparatively large eyes, a smaller number of pleural ribs (lateral lobes of B. (C.) arcticus carry 12 pairs of ribs). It is very different from B. (C.?) psilus (see: Hahn and Hahn 1970, Abb. 15, Taf. 2, Fig. 20; Hahn and Hahn, 1996, Abb. 184): the presence of a tubercles sculpture on the carapace (cephalon of B. (C.?) psilus is smooth and devoid of sculptural formations).

Figures

PLATE 1. Brachymetopus (Conimetopus) alekseevi Mychko, sp. nov., Upper Asselian or Lower Sakmarian, Shakhtau quarry, Sterlitamak town, Ishimbay District, Bashkortostan, Russia. 1a—holotype, spec. PIN RAS, no. 5610/2, external mold; 1b, clatex cast from holotype, dorsal and lateral views. 2a, b—paratype, spec. PIN RAS, no. 5610/6, pygidium, dorsal and posterior views. 3—paratype, spec. PIN RAS, no. 5610/4, pygidium, dorsal view. 4—paratype, spec. PIN RAS, no. 5610/3, pygidium, dorsal view. 5a, b—paratype, spec. PIN RAS, no. 5610/5, pygidium, dorsal and posterior views. 6—paratype, spec. PIN RAS, no. 5610/7, pygidium, external mold. The scale bar is 5 mm.

FIGURE 1. Some members of the genus Brachymetopus from the Permian of Russia. a—B. (Acutimetopus) moelleri (Weber, 1932); reconstruction of cephalon; presumably kept (possibly lost) in the collections of the St. Petersburg Mining University museum, St. Petersburg; Lower Permian, Artinskian, Sarginian (reproduced from Möller [1868, Pl. 1, Fig. 32]). b, c—B. (Conimetopus) alekseevi Mychko, sp. nov.; b—cephalon, spec. TsNIGR no. 5107/2063; c—pygidium, spec. TsNIGR no. 5107/ 2064; Chelyabinsk Region, Ashinsky District, right bank of the Sim River, the town of Asha, “Kazarmenny Kamen” locality; Lower Permian, Asselian; (reproduced from Weber [1937, Pl. 10, figs 10–11]). d, e—B. (Acutimetopus) caucasicus Likharev, 1944; spec. TsNIGR no. 5217/86, holotype, pygidium; Krasnodar Region, Mostovsky District, the Malaya Laba River basin, “Nikitinskaya Balka” locality; Upper Permian, Dzhulfian (~Wuchiapingian), Urushten or Nikitin Formations; d—view from the dorsal side, e—view from the end of the posterior margin (reproduced from Weber [1944, Pl 2, Fig. 15]). The scale bar is 1 cm.

FIGURE 2. Locality map. A—the European part of Russia with administrative division; B—the Republic of Bashkortostan and adjacent regions; C—the area around the Shakhtau locality.

FIGURE 3. Reconstruction of the carapace of Brachymetopus (Conimetopus) alekseevi Mychko, sp. nov. a—cephalon. bpygidium.