Microceratina andreui Cabral & Lord, 2023

Danielopol, Dan L., Cabral, M. Cristina, Horne, David J., Namiotko, Tadeusz & Lord, Alan R., 2023, Reconciling diagnostic traits in living and fossil taxa: The taxonomy and evolution of the genus Microceratina (Crustacea, Ostracoda, Cytheruridae), Zootaxa 5244 (4), pp. 301-340 : 325-327

Microceratina andreui Cabral & Lord sp. nov.

Figure 14

Etymology: named after Bernard Andreu (University Paul Sabatier, Toulouse, France) who first noticed the similarity with Microceratina .

Holotype: 1C, SMF Xe 23990, L = 0.39 mm, H = 0.19 mm, W = 0.19 mm, sample BM52 B ( Figs 14A–F )

Paratypes: 2C, SMF Xe 23991, L = 0.39 mm, H = 0.19 mm, W = 0.20 mm, sample BM52 B ( Fig. 14Q ) ; SMF Xe 23992, L = 0.38 mm, H = 0.18 mm W = 0.19 mm, sample BM57 ( Figs 14P, R ) .

Material: 8C, 3 Toarcian, 5 Aalenian (3 C figured: SMF Xe 23993, 23994, 23995).

Type locality: Boca da Mata, Alvaiázere, c. 170 km NE of Lisbon, Portugal; 39º 48’ N; 8º 24’ W.

Type level: S. Gi„o Formation; Upper Toarcian; Speciosum Zone; sample BM 52B.

Diagnosis: A species of Microceratina with an inflated and elongated carapace; shallow dorso-median sulcus; reticulate ornamentation with polygonal cells, whose sola are covered by round fossae; two short and thin, slightly curved antero-ventral ridges.

Description: Carapace small, subrectangular in lateral outline, with valves of almost the same size. L <0.5 mm, H/L% <50% located in the anterior third of L (Holotype, Fig. 14A ). Greatest L above mid-H. Greatest W approximately at three-fifths of L from anterior, less than half of length. AMZ well-developed on both valves ( Figs 14A, B, D, T, U ); it represents 2.5% of valve L (in Fig. 14A , with L = 0.39 mm).

Lateral surface with a weak dorso-median sulcus, ending in an enlarged subcentral depression where the adductor muscle scars can be seen externally ( Fig. 14C ). Mid and postero-ventral zones inflated, forming a longitudinal expansion with a curved outline, which ends in a round protuberance ( Figs 14A, E, R ). Eye tubercle present, represented by a subtle elongated swelling with an associated shallow postocular sulcus. In dorsal view ( Fig. 14Q ) the carapace is inflated, mostly posteriorly, with anterior and posterior ends pointed and dorso-median sulcus marked; in ventral view ( Fig. 14P ) it has a sagittate outline, i.e., anterior pointed and the posterior margin extends beyond the two ventro-lateral expansions resembling an arrowhead. Sexual dimorphism not evident.

Dorsal margin straight, with obtuse cardinal angles. Ventral margin weakly sinuous, tapering towards the posterior, partially covered by the postero-ventral longitudinal expansion. Anterior margin rounded, infracurvate. Posterior margin rounded to slightly pointed, with extremity supracurvate, without forming a caudal process. Both anterior and posterior margins flattened.

Reticulate ornamentation, with mostly pentagonal cells; in the flattened anterior and posterior zones the reticulation has a square mesh ( Figs 14A, B, E, F ). The sola of the reticula are covered with fossae (c. 17 fossae per mesh), generally round and shallow. The median diameter for several fossae visible in Figs 14L, M, O is 2.95 µm (r = 2.65–3.4 µm, N =10). Minute elongated grooves are visible on the floors of some fossae ( Fig. 14N ). In one of these latter fossae with a diameter of 2.74 µm, the size of one of the elongated grooves is 0.7 x 0.1 µm. We presume that it represents the preserved remains of a foveola (= LStPC) similar to those illustrated for M. pseudoamfibola (e.g., Figs 8E, F ). In the anterior zone, two short, thin and slightly curved ridges are present, the first one oblique, ending near the flattened anterior margin, the second one slightly longer, vertical, ending in the ventral margin ( Fig. 14B ); in the Aalenian forms, these ridges are less clear.

Normal pore canals (conuli type A”) can be seen adjacent or on the tectum of muri. A NPC placed on a conulus adjacent to a murus ( Figs 14G–I ) has a diameter of 1 µm. Other NPCs are visible on the muri ( Figs 14J, K, U, V ) .

Internal features not seen; four slightly elongated adductor muscle scars, arranged in a vertical row, can be seen externally in the subcentral depression ( Figs 14B, C ).

Remarks: Microceratina andreui sp. nov. is the oldest known species of a long-lasting evolutionary lineage ranging from the Early Jurassic to living species. However, we are at present unable to recognise other close relatives of Early Jurassic age and it may be that ancestral forms are of Triassic age.

Microceratina andreui sp. nov. differs from all the other known fossil and living species of Microceratina by the presence of the two short antero-ventral ridges in surface ornamentation. Moreover, the presence of the ocular structures (tubercle and postocular sulcus), uncommon in the genus, also allows the easy distinction of M. andreui sp. nov. from most other species of its genus. However, in some fossil species these structures, though very weak, also appear. In M. bhannesensis Colin, Dejax & Gèze, 2005 (in Colin et al., 2005) from the Kimmeridgian (Late Jurassic) of Lebanon, a very shallow postocular sulcus seems to exist ( Fig. 9 ); this species is close to M. andreui sp. nov., but the posterior extremity is located more ventrally, and the anterior and posterior margins denticulate. In M. azazoulensis Andreu & Colin, 2005 (in Colin et al., 2005), from the midlate Cenomanian (Upper Cretaceous) of Morocco, a weak postocular sulcus can be seen, although the authors say that there is no ocular tubercle ( Colin et al. 2005, p. 21—“absence de tubercule oculaire”); this species is much more quadrate than M. andreui sp. nov., with a round posterior margin and a less developed ventro-lateral expansion; the late Albian-early Cenomanian Brazilian specimens of M. azazoulensis , figured in Vázquez-García et al. (2021), almost certainly belong to another species (see Discussion below); they are longer than the Portuguese species, with a less marked dorso-median sulcus and ventro-lateral expansion, and without ocular structures.

Microceratina aequabilis Herrig, 1991 , from the Maastrichtian (Upper Cretaceous) of Germany, is very similar to the Portuguese species, but is larger (L = 0.44 mm, H = 0.21 mm, Holotype), with a less projecting posterior margin.

Occurrence and stratigraphic distribution: A rare species found in the Upper Toarcian, Lower Jurassic (S. Gi„o Formation) and Lower Aalenian, Middle Jurassic (Póvoa da Lomba Formation) of Portugal.

Figures

FIGURE 8. Microceratina pseudoamfibola (Barbeito-Gonzàlez, 1971), valve morphology: A–G—RV-1; H–K—LV-1; L–N— RV-2 (Calvi, Corsica, France); A–G—RV, external; A—general view; B—detail of A, posterior half of valve (light gray arrow indicates angular ventro-posterior ridge); C—detail of B, intrareticular solum with fossae and LStPC; D—detail of A, anterior half of valve; E, F—detail of fossae with LStPC (white arrows indicate aperture of tubuli with a loophole pore); G—detail of A (rotated 90º), intramural fossae within a reticular structure; H–K—LV; H—external, detail of the ventral side (white arrow points to anterior); I–K—internal; I–J—details of pores of LStPC; K—central area with adductor muscle scars (indicated by central white arrow) and apertures of two NPC (lateral white arrows); L–N—RV, internal; L—general view; M, N—detail of L, anterior (M) and posterior (N) hinge (black arrows indicate the positive terminal elements). (M. Peeters sample deposited at Royal Belgian Institute of Natural Sciences, Brussels (RBINS), Inv. Nr. 12703/0C2075).

FIGURE 9. General shape of Microceratina morphotypes:A—Microceratina bhannesensis Colin, Dejax & Gèze, 2005, C right view (holotype); B, C—Microceratina azazoulensis Andreu & Colin, 2005, silhouettes of (B) C, right side and (C) RV, lateral view, standardised for length (bounding box method); B—Tarhzoute, Morocco, Colin et al. (2005, pl. 2, fig. 4); C—Sergipe- Alagoas Basin, Brazil, valve nr. ULVG-12791 also figured in Vázquez-García et al. (2021, fig. 4L).

FIGURE 14. Microceratina andreui Cabral & Lord sp. nov.: A–F—C, holotype, SMF Xe 23990; L = 0.39 mm, H = 0.19 mm; Boca da Mata section, Portugal; Upper Toarcian, Speciosum Zone; sample BM52B; A—left view; B—detail of A, AMZ (white arrow) and ornamentation (antero-ventral ridges); C—detail of B, adductor muscle scars; D—detail of the AMZ from B; E—detail of A, posterior, round postero-ventral protuberance; F—detail of E, posterior extremity and reticulation; G–O—C, SMF Xe 23993; L = 0.41 mm, H = 0.20 mm; Boca da Mata section, Portugal; Lower Aalenian, Opalinum Zone; sample BM161; G—right view; H—detail of G, anterior, NPC (white arrow indicates pore conulus, A”-type); I—detail of H, pore conulus; J— detail of G, reticulate cells with sola covered by fossae and intramural NPC, A”-type (white arrow); K—detail of J, pentagonal cells with fossae and intramural NPC (white arrow); L—detail of G, several fossae on sola; M—detail of L, fossae; N—detail of G, fossae; O—detail of G, fossae; P—C, ventral view, paratype, SMF Xe 23992; L = 0.38 mm, H = 0.18 mm; Boca da Mata section, Portugal; Upper Toarcian, Speciosum Zone; sample BM57; Q—C, dorsal view, paratype, SMF Xe 23991; L = 0.39 mm, H = 0.19 mm; Boca da Mata section, Portugal; Upper Toarcian, Speciosum Zone; sample BM52B; R—C, right view, paratype, same as P; S—C, left view, SMF Xe 23994; L = 0.40 mm, H = 0.19 mm; Boca da Mata section, Portugal; Lower Aalenian, Opalinum Zone; sample BM169B; T–V—C, SMF Xe 23995; L = 0.40 mm, H = 0.19 mm; Boca da Mata section, Portugal; Lower Aalenian, Opalinum Zone; sample BM162T; T—right view (white arrow indicates the AMZ); U—detail of T; V—detail of U, intramural NPC, A”-type (pore conulus).