Pellucistoma curupira, Gross & Ramos & Piller, 2016

Gross, Martin, Ramos, Maria Ines F. & Piller, Werner E., 2016, A minute ostracod (Crustacea: Cytheromatidae) from the Miocene Solim oes Formation (western Amazonia, Brazil): evidence for marine incursions?, Journal of Systematic Palaeontology 14 (7), pp. 581-602 : 583-589

Pellucistoma curupira sp. nov.

( Figs 2 , 3 )

Holotype. MPEG-503-M, right valve ( Fig. 3H, P, T, V ).

Paratypes. MPEG-504-M to MPEG-513-M ( Figs 2A, B , 3A—G, I—O, Q—S, U ).

Additional material. MPEG-514-M, 26 adult specimens from sample AM10 /30.

Diagnosis. A very small-sized, extremely thin-shelled species of Pellucistoma with subrhomboidal shape, ornamented with wrinkle-like ridges forming anteroventrally a weak reticulum and a unique combination of hinge structures.

Derivation of name. ‘ Curupira ’, the name of a mythic dwarf of Brazilian legends with backward turned feet, which should confuse pursuers; used as a noun in apposition; in reference to the small size of the species and its baffling discovery.

Type locality. Borehole 1AS-10- AM at Sucuriju close to Rio Itúı (04 Ǫ 50, S, 70 Ǫ 22, W, ~ 62 km south-west of Benjamin Constant; municipality Atalaia do Norte, state of Amazonia , Brazil; Fig. 1 ).

Type horizon. Sample AM 10/30 (= depth: 141.2 m, altitude: —56.2 m; Gross et al. 2014).

Description.

Shape. Subrhomboidal in lateral view; anterior margin moderately infracurvate, dorsal margin almost straight and subhorizontal, ventral margin with slight concavity below the mandibular scars, posterior margin with a blunted subdorsal caudal process; valves anterodorsally, posteroventrally and posteriorly laterally flattened; lensshaped in dorsal view with beaked posterior end.

Ornamentation. The very thin-shelled valves are basically smooth except for shallow, wrinkle-like ridges along the free valve margin as well as in the centro- and ventrodorsal area, forming a weak reticulum anteroventrally; at the caudal process an additional, oblique ridge is always present.

Inner lamella. Anterior and posterior wide; anterior vestibulum large, posterior one narrow-elongated, extending up to the caudal process; several inner lists developed; selvage subperipheral, inconspicuous anteriorly, forming at the ventral concavity a bulge, which fits into a groove of the other valve (groove more prominent in right valves).

Marginal pore canals. Widened at their base, leading to an irregular line of concrescence; occasionally bifurcated, some branches developed as false pore canals.

Hinge. Right valve — anterior element with four roundish sockets (the most anterior being the largest, the most posterior one is barely developed) and a spatulate anti-slip tooth below; median element consists of a smooth groove, which is deepened at its posterior end; posterior hinge element with three elongated teeth, succeeded by a fourth, indistinct tooth, which merges backwards into a thin expansion of the posterior margin; left valve — anterior element with a trilobate tooth and a further weak tooth postjacent; median element consisting of a smooth bar, which forms an elongated tooth-like structure at its posterior end; posterior element with three, elongated, shallow sockets followed by a fourth, indefinite, elongated socket fading out towards the posterior end.

Normal pores. Widely scattered, very small (~3—5 µm in diameter); sieve-type.

Central muscle scars. A row of four, slightly posteriorly inclined adductor scars; two oval mandibular scars, one irregularly ovate frontal scar; numerous dorsal muscle scars; well above the row of adductor scars a row of four dorsal scars is developed, probably corresponding to the ‘lucid spot’ ( Morkhoven 1963; Sandberg 1969), which, however, is not a discrete, single spot here.

Eye-spot. Slight eye-spot developed.

Sexual dimorphism. Unclear; a few specimens (e.g. Fig. 3B ) are slightly larger and display a somewhat higher posterior valve proportion, which could be related to sexual dimorphism.

Dimensions. Right valve (number of measured specimens = 9): length = 0.34—0.38 (mean = 0.36) mm, height = 0.17—0.18 (mean = 0.18) mm; left valve (number of measured specimens = 7): length = 0.34—0.38 (mean = 0.36) mm, height = 0.17—0.18 (mean = 0.17) mm.

Remarks.

Generic classification. The current species imitates several genera of different families by its subrhomboidal outline and almost smooth shells.

Amongst the Bythocytheridae Sars, 1866 , some species of Bythocythere Sars, 1866 and Pseudocythere Sars, 1866 superficially resemble Pellucistoma curupira sp. nov. However, both genera differ explicitly due to the development of five adductor scars, and, less clearly, in their hinges ( Pseudocythere : adont; Bythocythere : adont, lophodont or merodont; e.g. Morkhoven 1963; Athersuch et al. 1989; Stepanova 2006; Sciuto 2009).

The loxoconchid genera Palmoconcha Swain & Gilby, 1974 (= syn. Lindisfarnia Horne & Kilenyi, 1981 ; Horne & Whatley 1985; Athersuch et al. 1989), Elofsonia Wagner, 1957 , Pseudoconcha Witte, 1993 and, especially, Phlyctocythere Keij, 1958 , are similar to some degree.

However, Palmoconcha is distinguished by its gongylodont hinge (right valve: anterior socket—tooth—socket sequence; median smooth furrow; posterior tooth—socket—tooth sequence; Swain & Gilby 1974; Horne & Kilenyi 1981; Athersuch & Horne 1984; Horne & Whatley 1985; Athersuch et al. 1989) and to a minor degree by its less prominent caudal process, strong fulcral point and Y-shaped frontal scar.

Pseudoconcha has a bipartite hinge (right valve: anterior element formed by a strong bar, with a groove below; posterior half with groove and bar below), a less developed caudal process, a well-punctate surface and a narrower inner lamella ( Witte 1993; Sarr et al. 2008).

Although some variability in details of the hinge, muscle scar patterns and pore canals seem to be present in Elofsonia ( Aiello & Szczechura 2002) , this genus differs by its less prominent, less pointed caudal process (except Elofsonia sp. in Keyser & Schoning 2000) and its more simple hinge (right valve: dorsally crenulated anterior socket; smooth median groove; weak posterior tooth; Whittaker 1973; Athersuch & Horne 1984; Athersuch et al. 1989).

Originally, Phlyctocythere was characterized by its inflated, almost spherical carapaces with a peripherally compressed zone and an obtuse, subdorsal caudal process. Its surface is smooth, lacks eye-spots and the valves are very thin-shelled. The hinge is adont (right valve: curved, smooth bar), marginal pore canals are simple, and one frontal muscle scar is developed ( Keij 1958; compare also Morkhoven 1963). Subsequently, several species were included in Phlyctocythere , which blur the prime generic diagnosis. For example: (1) outline: Phlyctocythere hamanensis Ikeya & Hanai, 1982 (more elongated, less arched dorsal margin), Phlyctocythere japonica Ishizaki, 1981 (subovate), Phlyctocythere recta Bold, 1988 (straight dorsal margin), Phlyctocythere sicula Sciuto & Pugliese, 2013 (more elongated) and Phlyctocythere stricta Bold, 1988 (straight dorsal margin); (2) ornament: Phlyctocythere curva Bold, 1988 , Phlyctocythere retifera Bonaduce, Masoli & Pugliese, 1978 , P. sicula and P. stricta display a faint reticulation and/or longitudinal ridges/wrinkles; (3) Phlyctocythere curva , P. recta , P. stricta and probably P. fennerae Mostafawi, 1992 have a slight eye-spot; (4) for P. fennerae , P. japonica and P. retifera few (anteroventrally) branched marginal pore canals are described and these structures are often observed to be bifurcated in P. curva ; (5) for P. hamanensis normal pores are ‘presumably’ of sieve-type; (6) in P. hamanensis two frontal scars and one elongated mandibular scar are mentioned; in the illustration of P. retifera a double frontal scar is indicated; in P. sicula the adductor scars are very elongated; and (7) hinge: in Phlyctocythere caudata Hartmann, 1979 , P. curva , Phlyctocythere pellucida ( Muller, 1894) , P. retifera and P. sicula right valves display a smooth median groove (crenulated in P. curva ) and (two) terminal sockets (in P. curva : teeth); P. hamanensis has a reduced gongylodont hinge. (Note: Phlyctocythere hartmanni Omatsola, 1970 is attributed to Elofsonia ( Athersuch & Horne 1984; Schornikov 2011) or to Pseudoconcha ( Witte 1993) . Phlyctocythere pellucida is discussed as belonging to Loxocauda Schornikov, 1969 ( Athersuch & Horne 1984; Schornikov 2011)). Consequently, Phlyctocythere is either quite variable or has turned into a collective genus due to inclusion of profuse species. An in-depth revision is obviously needed but is beyond the scope of the present work. In particular, P. retifera from the Red Sea is similar but it is more ovate (but note sexual dimorphism displayed in Mostafawi (1992) for P. fennerae ), it diverges in details of the hinge (as far as reproducible) and has two frontal scars. However, by following the original diagnosis of Phlyctocythere ( Keij 1958; Schornikov 2011) especially its outline (much more arched dorsal margin), its adont, right-bar hinge and simple marginal pore canals are considered herein to exclude an assignment of the current specimens to that genus.

Some authors ( Bold 1950, 1958; Benson et al. 1961; Morkhoven 1963) have discussed a possible synonymy of Javanella Kingma, 1948 with Pellucistoma Coryell & Fields, 1937 (see also Gou & Chen 1988; Howe & McKenzie 1989; Ayress 1996). Lately, Bergue & Coimbra (2007) revised Javanella , revalidated it and reassigned it into the family Cytheridae Baird, 1850 . According to this work only two species are left in Javanella , which clearly differ in outline (more elongated; caudal process below the middle of valves height) and in details of the terminal hinge elements from the present material.

Amongst the Cytheromatidae Elofson, 1939 , the genus Paracytheroma Juday, 1907 is closely related to Pellucistoma . Nevertheless, Paracytheroma can be differentiated from the latter by lacking strong terminal anti-slip hinge elements, the absence of a caudal process and — to a minor degree — by missing a complex marginal zone with branched marginal canals ( Hartmann 1978; Ayress 1990; compare also Sandberg 1969; Keyser 1976; Garbett & Maddocks 1979). Based on those features, our specimens do not belong to Paracytheroma but fit best with Pellucistoma as originally defined by Coryell & Fields (1937; for genus definition compare also Edwards 1944; Morkhoven 1963; Sanguinetti 1979). A few, minor differences concern the valves’ hinge and surface ornament.

For the hinge of the left valve an “anterior long bladelike triangular tooth” is indicated ( Coryell & Fields 1937, p. 17), being trilobate in the present specimens. The median element is formed by a “serrated bar”, which “terminates at the posterior cardinal angle” ( Coryell & Fields 1937, p. 17). Here, that bar is smooth — at least as preserved. As far as described or perceptible on the provided figures, a smooth median element occurs in Pellucistoma scrippsi Benson, 1959 and Pellucistoma bensoni McKenzie & Swain, 1967 ( Benson & Kaesler 1963; McKenzie & Swain 1967; Swain & Gilby 1967, 1974). For the type species, P. howei , the drawings of Bold (1967) and Teeter (1975) do not show such a crenulation. The crenulation of the median hinge element is in some species of Pellucistoma probably very delicate or indeed not developed.

A posterior hinge element, consisting of four elongated sockets/teeth as in our examples, has not been mentioned for Pellucistoma so far. However, based on the dorsal view of a left valve in Coryell & Fields (1937), behind the thickened, tooth-like terminal end of the bar, a shallow groove may be present which might analogously receive tiny teeth of the right valves. Moreover, the illustration of P. scrippsi in Swain & Gilby (1967) implies the presence of small posterior teeth in the right valve (note that this feature is not indicated in e.g. Benson 1959; Benson & Kaesler 1963; McKenzie & Swain 1967; Swain 1967). The description and illustration of the hinge structure of Pellucistoma spurium Bold, 1963 (p. 406: “In the left valve the selvage curves around the sockets and forms the upper border of the groove”) also hints at the presence of a posterior element. Thus, the subtle posterior sockets/teeth, clearly visible in our species (under the SEM), seem to be present in other Pellucistoma species equally. Garbett & Maddocks (1979, p. 871) carefully described a similar posterior hinge structure for Paracytheroma stephensoni ( Puri, 1954) of which Pellucistoma atkinsi Hall, 1965 is a synonym ( Keyser 1976): “[a] posterior tooth formed by the expanded end of the posterior margin.” That resemblance mirrors the close relation between Pellucistoma and Paracytheroma again (see above).

The surface of Pellucistoma is described as “finely perforated” ( Coryell & Fields 1937, p. 17) and “smooth or finely punctate” ( Morkhoven 1963, p. 436). Here, the valves are basically smooth, but display a weak reticulate pattern anteroventrally, some wrinkle-like ridges dorsocentrally and ventrocentrally, as well as a characteristic, oblique, light ridge on the caudal process. Shallow, posterocentral and posteroventral ridges, which converge towards the caudal process, can be seen on P. magniventra in Garbett & Maddocks (1979). True eye-spots have not been included in the genus definition so far. However, a slight eye-spot — like in P. curupira sp. nov. — is recognized in P. scrippsi (Swain 1967; Swain & Gilby 1974).

Comparison with other Pellucistoma species. To our knowledge, 15 Pellucistoma species have been formally described so far (e.g. Kempf 1986, 1995, 2008; Brand̃ao 2015).

The type species, Pellucistoma howei Coryell & Fields, 1937 (first record: Panama, latest middle—early late Miocene), is quite similar but differs by: its more ovate outline; its more projecting posteroventral margin; a more acuminate caudal process; a narrower anterior vestibulum (which seems to be almost restricted to the lower half of valve height); the ventral snap-mechanism is less developed; and its larger size (holotype: length/height = 0.48/ 0.27 mm; Coryell & Fields 1937; Bold 1967; see also e.g. Teeter 1975; Bold 1988). For differences in hinge and ornamentation see above.

Pellucistoma magniventra Edwards, 1944 (first record: North Carolina, Pliocene) is distinguished by (largely based on the redescription of Garbett & Maddocks 1979): its more arched, upwards rising dorsal margin and its strongly projecting posteroventral margin, respectively (if not aligned to the base line = reversal points in front and backwards of the ventral concavity); its much more infracurvate anterior margin; its more pointed and acuminate caudal process; its ornament (see above); its anterior vestibulum, which is largely restricted to the anteroventral area (for variability see Garbett & Maddocks 1979); details of the hinge (crenulated median element; simple anterior and posterior sockets/teeth; lack of posterior hinge elements); and its larger size (holotype: length/ height = 0.62/0.33 mm; Edwards 1944; Garbett & Maddocks 1979; see also e.g. Puri 1960; Bold 1963; Benson & Coleman 1963; Hall 1965; Morales 1966; Grossman 1967; Sandberg 1969; Cronin 1979; Krutak 1982; King Lyon 1990).

Pellucistoma scrippsi Benson, 1959 (first record: Baja California, Recent) has more convex dorsal and ventral margins and a less oblique anterior margin as well as a smooth surface. The posterior vestibulum is almost absent (but see Swain & Gilby 1967); the hinge is slightly different (simple anterior socket (right valve) and tooth (left valve); posterior teeth are lacking (except illustration of Swain & Gilby 1967)); marginal pore canals are missing on the apex of the caudal process; and it is larger (holotype: length/height = 0.69/0.33 mm; Benson 1959; Benson & Kaesler 1963; McKenzie & Swain 1967; Swain 1967; Swain & Gilby 1967, 1974).

Pellucistoma bensoni McKenzie & Swain, 1967 (first record: Baja California, Recent) differs in its strongly arched dorsal margin resulting in a subtriangular outline and a posterocentrally located caudal process, its smooth surface, its almost absent posterior vestibulum; and it is larger (holotype: length/height = 0.44/0.25 mm) .

Pellucistoma spurium Bold, 1963 (first record: Trinidad, late Miocene) has a more convex dorsal margin; a more accentuated caudal process; and a hinge with a simple knob-like anterior tooth (left valve) and minutely crenulated median elements. The anterior vestibulum is restricted to the anterocentral area, the posterior vestibulum is only developed at the caudal process; and it is larger (holotype: length/height = 0.49/0.25 mm; compare also Pellucistoma? spurium of Bold (1988)).

Pellucistoma santafesinensis Zabert, 1978 (first record: Argentina, middle—late Miocene; correct spelling according to Kempf (2008): P. santafesinense ; note: in the following we refer to correct spellings of species names but retain the original spellings in this work) is similar to Pellucistoma gibosa Sanguinetti, 1979 (see below) and possibly both are synonyms. However, it differs significantly in outline (subtriangular-elongate in lateral view; ventromedian long, pointed caudal process) and hinge structures from the current material. Size of holotype: length/height = 0.53/0.26 mm.

Pellucistoma gibosa Sanguinetti, 1979 (first record: southern Brazil, late Miocene; correct spelling according to Kempf (1986): P. gibosum ) has an extremely humped (right valve) dorsal margin and an acuminate, long caudal process well below the half valves’ height. It is smooth; it has a hinge with a simple, strong anterior tooth (left valve) and a slightly serrated bar; the anterior vestibulum is restricted to lower half of valves height; and it is larger (holotype: length/height = 0.51/0.23 mm).

Pellucistoma elongata Whatley et al., 1997a (first record: Argentina, Recent; correct spelling according to Kempf (2008): P. elongatum ) differs by: its more convex dorsal and posteroventral margin; more acuminate and more ventrally located caudal process; the inner lamella curves inwards posteroventrally; it is smooth (lacks eye-spots); and it is larger (holotype: length/height = 0.52/ 0.24 mm; Whatley et al. 1997a).

Further species of questionable generic classifica tion. Bold (1950) considered his Miocene Venezuelan species Pellucistoma kendengensis (Kingma) to be synonymous with Javanella kendengensis Kingma, 1948 (Pliocene, Java). Later, Bold (1972a) included P. kendengensis of Bold (1950) in his new species Pellucistoma? kingmai Bold, 1972a and assumed J. kendengensis of Kingma (1948) not to be a synonym of P.? kingmai . Bergue & Coimbra (2007) re-examined the type material of Bold (1950), excluded it from Javanella but left the generic status of P. kendengensis (according to Bold 1972a: P.? kingmai ) open. Although there are some features perceptible that are unlike Pellucistoma (anterior margin almost equicurvate (cf. Bold 1950, p. 86: “obliquely rounded”); subventral caudal process; quite heavily punctate surface (as shown in Bergue & Coimbra 2007), as well as short and straight marginal pore canals, it cannot be definitively excluded from Pellucistoma .

Due to their simple marginal pore canals, the species Pellucistoma ? sp. ( Bold 1958, 1972a), Pellucistoma ? compactum Bold, 1972a and Pellucistoma? kingmai Bold, 1972a , that are questionably attributed to Pellucistoma , differ in outline, hinge structure and development of the inner lamella. Only Pellucistoma sp. in Bold (1970, 1972b, 1988) is rather similar in its shape to the current species. Nevertheless, it has a smooth surface and is larger (length/height = 0.48/0.25 mm). Unfortunately, neither the hinge nor inner characters are accessible, which does not enable further comparisons.

Pellucistoma tumida Puri, 1954 (correct spelling according to Kempf (1986): P. tumidum ) from the Pliocene of Florida is poorly described and a re-examination of the type material already failed ( Bold 1988). Although its outline is comparable with the present individuals (except the equicurvate anterior margin), without additional traits (see also Bold 1963; Hulings 1967) a generic assignment or a species-specific identification is unfeasible. Bold (1988) discussed possible congruence with his Phlyctocythere sp. 2 , which again demonstrates the superficial similarity of that loxoconchid genus with Pellucistoma (see above).

Pellucistoma atkinsi Hall, 1965 (see above) is a junior synonym of Paracytheroma stephensoni ( Puri, 1954) ( Keyser 1976; Garbett & Maddocks 1979).

Pellucistoma ovaliphylla Hu, 1981 from the Plio-/Pleistocene of southern Taiwan has been recognized by Hu (1984) to belong to Paradoxostoma Fischer, 1855 . Two further Taiwanese species, Pellucistoma magnolioidea Hu & Tao, 2008 and Pellucistoma chushunshui Hu & Tao, 2008 , differ notably in outline, especially due to the almost lacking caudal process, from both the current as well as from other Pellucistoma species (important internal characters are not accessible because of missing illustrations and descriptions). Most likely, these species belong to another genus ( Paracytheroma ?) but this claim needs additional investigations.

Ayress (1990, 1996) described Pellucistoma coombsi Ayress, 1990 , Pellucistoma fordycei Ayress, 1990 and Pellucistoma punctata Ayress, 1996 (correct spelling according to Kempf (2008): P. punctatum ) from New Zealand and the Tasman and Coral Seas. These species diverge significantly from the present species and the genus Pellucistoma in general (outline: much more elongated-rectangular; hinge structures: e.g. P. coombsi and P. punctata have a right-bar hinge; inner lamella: much wider; ornament: P. punctata ), which shed doubt on their generic allocation. However, those species are not comparable with the material described herein.

The illustration and description of Pellucistoma sp. from Henderson Island ( Pitcairn Islands, S. Pacific; Recent; Whatley & Roberts 1995; Whatley et al. 2004) do not offer enough details (e.g. hinge) for an assured generic attribution. Its laterally inflated valves are rather unlike those of Pellucistoma .

Faug̀eres et al. (1984) mentioned Pellucistoma from the Ghubbet el Kharab ( Djibouti; Holocene) but provided no figure. Presumably, this material belongs to another genus ( Phlyctocythere ?).

Whatley et al. (1997b) recorded — without figure or description — Pellucistoma sp. 1 from the southern Strait of Magellan ( Chile; Recent). Due to extremely low water temperatures at the sampling sites, this record is ecologically very unlikely for Pellucistoma (see below). Thus, we do not consider it subsequently.

The Late Cretaceous Pennyella foveolata Majoran & Widmark, 1998 from the Maud Rise (Southern Ocean, off Antarctica), listed under Pellucistoma in the ‘World Ostracoda Database’ ( Brand̃ao et al. 2015), actually belongs to the former genus ( Yasuhara et al. 2013).

To conclude, all the above compared species can be clearly differentiated from Pellucistoma curupira sp. nov. and are noticeably larger. Most similar are P. howei from the Miocene of Panama and Pellucistoma sp. of Bold (1970, 1972b, 1988; Miocene: Antilles and Panama), however, the latter is little known and a closer examination is not possible. Most likely, the Australasian and Taiwanese species ( Ayress 1990, 1996; Hu & Tao 2008) do not belong to Pellucistoma . The records from the Pitcairn Islands and southern Chile ( Whatley et al. 1997b, 2004) need additional affirmation. Currently, we assume that the genus Pellucistoma is confined to the Americas.

Figures

Figure 1. Location of the studied well 1AS-10-AM in western Amazonia. A, overview map; B, position of exploration wells (after Maia et al. 1977); star = herein investigated core; compare Gross et al. (2014).

Figure 2. Transmitted light photographs (A, B, focus stacked) and schematic drawings (C, D) of Pellucistoma curupira sp. nov. A, MPEG-513-M, left valve, internal view (length = 0.37 mm, height = 0.18 mm); B, MPEG-509-M, right valve, internal view of Figure 3F; C, left valve, internal view, based on A and Figure 3E; D, right valve, internal view, based on B and Figure 3H (compare also Fig. 3F).

Figure 3. Pellucistoma curupira sp. nov. A, MPEG-504-M, left valve, external view (length = 0.36 mm, height = 0.17 mm); B, MPEG-505-M, right valve, external view (length = 0.38 mm, height = 0.18 mm); C, MPEG-506-M, left valve, external view (length = 0.38 mm, height = 0.18 mm); D, MPEG-507-M, right valve, external view (length = 0.36 mm, height = 0.18 mm); E, MPEG-508-M, left valve, internal view (length = 0.35 mm, height = 0.17 mm); F, MPEG-509-M, right valve, internal view (length = 0.34 mm, height = 0.17 mm); G, MPEG-510-M, left valve, internal view (length = 0.37 mm, height = 0.18); H, holotype MPEG-503-M, right valve, internal view (length = 0.36 mm, height = 0.18 mm); I, MPEG-511-M, left valve, dorsal view (length = 0.34 mm, height = 0.17 mm); J, MPEG-512-M, right valve, dorsal view (length = 0.36 mm, height = 0.17 mm); K, anterior hinge element of I; L, anterior hinge element of J; M, posterior hinge element of I; N, posterior hinge element of J; O, ventral concavity of E; P, ventral concavity of H; Q, anti-slip tooth of J (oblique dorsal view); R, normal pore, sieve-type of B; S, hinge of E; T, hinge of H; U, central muscle scars of E; V, central muscle scars of H.