Search

Sagmatocythere rectum (Brady) (Loxoconchidae) comb. nov.—A case of homeomorphy ? Sagmatocythere rectum (Brady, 1868) is a Recent (presumed living) rare species, whose original description, based on the examination of only one carapace, was brief and without illustration.The analysed specimen was from Lerwick Bay, Shetland, dredged from a depth of c. 21–25 m, and the species was first attributed to the genus Cytheropteron, subfamily Cytheropterinae. Later, the species was found in other localities (west of Ireland, Bay of Biscay), and figured with some drawings in Brady & Robertson (1869, pl. XX, figs 6–8) and in Brady & Norman (1889, pl. XIX, figs 13, 14). Brady & Norman (1889) considered the species as belonging to the genus Bythocythere Sars, 1866, family Bythocytheridae. Athersuch & Horne (1983), studying specimens of Nannocythere pavo (Malcomson, 1886) and of Cytheropteron rectum Brady, 1868, two species considered by Brady & Norman (1889) as only one, with N. pavo being the juveniles of C. rectum, moved both species, completely distinct, to the family Loxoconchidae. Our attention was initially drawn to this species by its resemblance to members of the genus Microceratina, however, closer examination and comparison with Microceratina species especially from the Mediterranean demonstrated that we are dealing with an example of homeomorphy that merits clarification. The illustrations in Brady & Norman (1889), together with SEM images of one specimen from St Magnus Bay, Shetland (Norman Collection 1911.11.8 M3820), allow us to identify some Portuguese material (loose valves) collected in modern sediments of the Eastern Algarve continental shelf, off Tavira, and of the Mira estuary, SW Portugal, as belonging to the same species. The Portuguese specimens have a gongylodont hinge, NPC of two types and StPC (description below), and are considered as Loxoconchidae, in accordance with Athersuch & Horne (1983). We tentatively place the rectum species in the genus Sagmatocythere, described by Athersuch (1976), due to its dimensions> 0.35 mm (L = 0.43–0.45 mm), the sub-quadrangular to sub-rectangular outline, the pitted/reticulate ornamentation, the presence of few and simple marginal pore canals and of the gongylodont hinge with the terminal elements “comma-shaped” and lobate. However, the genus was described using morphological features of the valves and soft parts, the latter not available in the Portuguese material with only loose valves found. The Loxoconchidae genus Nannocythere Schäfer, 1953 possesses some characteristics in common with rectum species but has a smaller size (Microceratina, is presented below. The carapace is small, sub-rectangular in lateral outline. L S. rectum are never located in the cells. Such StPC have a diameter of about 3.2–3.5 µm with the central pore having a diameter of 1–1.5 µm. The sensillum emerging from inside the pore is slender, e.g., those from Fig. 15F have a diameter of about 0.5 µm. The area around the central pore is perforated with 10–15 small pores opening within tubuli which penetrate the valve. There is one, sometimes two rows of such pores within the area surrounding the central pore (Figs 15B, C, F). These minute pores have a size of: M = 0.3 µm (r = 0.18–0.43, N = 11). Internally the selvage of the RV extends up to the OM of the valve (Fig. 15P). The diameter of pores apparently emerging from a StPC (Fig. 15Q) is 3.33 µm; two tubules with a diameter of about 0.6–0.8 µm are visible inside the larger cavity. Other pores on the internal side of the valve are visible in Figs 15T, V, W; their diameter is approximately 5 µm. Hinge gongylodont (Figs 15P, R, S, U–W) with smooth median element and “comma-shaped” and lobate terminal elements (clearer in the posterior part, Figs 15S, V). Marginal zone rather wide anteriorly, relatively narrow posteriorly, both with vestibules, the posterior being very narrow; MPC straight and simple, about seven in the posterior zone, number anteriorly could not be determined; four slightly elongated adductor muscle scars, arranged in a curved vertical row, seen externally in the subcentral depression (Figs 15K, L)., Published as part of 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), pp. 301-340 in Zootaxa 5244 (4) on pages 328-330, DOI: 10.11646/zootaxa.5244.4.1, http://zenodo.org/record/7663518, {"references":["Brady, G. S. (1868) A monograph of the Recent British Ostracoda. Transactions of the Linnean Society of London, 26 (2), 353 - 495. https: // doi. org / 10.1111 / j. 1096.3642.1968. tb 00199","Brady, G. S. & Robertson, D. (1869) Dredging West of Ireland. Annals and Magazine of Natural History, Series 4, 3, 353 - 374. https: // doi. org / 10.1080 / 00222936908695961","Brady, G. S. & Norman, A. M. (1889) A monograph of the marine and freshwater Ostracoda of the North Atlantic and of North- Western Europe. Section I. Podocopa. Scientific Transactions of the Royal Dublin Society, Series II, IV, 63 - 270. https: // doi. org / 10.1080 / 00222938909460549","Sars, G. O. (1866) Oversigt af Norges Marine Ostracoder. Forhandlinger i Videnskabs-Selskabet i Christiania, 1865, 1 - 130.","Athersuch, J. & Horne, D. J. (1983) On Nannocythere pavo (Malcomson). A Stereo-Atlas of Ostracod Shells, 10, 29 - 38.","Malcomson, S. M. (1886) Recent Ostracoda of Belfast Lough. Report of Proceedings of the Belfast Natural Field Club, Appendix 9 (1884 - 1885), 259 - 264.","Athersuch, J. (1976) On Sagmatocythere napoliana (Puri). A Stereo-Atlas of Ostracod Shells, 3, 117 - 124.","Schafer, H. W. (1953) Uber Meeres- und Brackwasser-Ostracoden aus dem Deutschen K ¸ stengebiet. Mit 2: Mitteilung ¸ ber die Ostracodenfauna Griechenlands. Hydrobiologia, 5 (4), 351 - 389. https: // doi. org / 10.1007 / BF 00018667"]}

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 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.