Ostracoda (Arthropoda, Crustacea) from the Riachuelo Formation, Sergipe-Alagoas Basin, Brazil, Upper Aptian-Albian

The Sergipe-Alagoas Basin has one of the most complete, exposed lithological successions of the Cretaceous period in the continental margin of Brazil. It captures several phases of the evolution of the South Atlantic Ocean, including rift, gulf and drift. The upper Aptian–Albian Riachuelo Formation corresponds to the first stages of the southern proto-Atlantic Ocean invasion in that basin. The present study reviews the taxonomic identification and ecology of 39 ostracod species of this formation, proposing a new genus – Gabonorygma gen. nov. – and three new species – Praebythoceratina deltalata sp. nov., Gabonorygma sergipana gen. et sp. nov. and Brachycythere smithsoniana sp. nov. Other taxa include Conchoecia? sp. 1, Cytherella sp. 1, C. besrineensis comb. nov., Cytherelloidea aff. globosa, C. btaterensis, Bairdoppilata sp. 1, Bairdoppilata sp. 2, B. comanchensis comb. nov., B. pseudoseptentrionalis, Robsoniella falklandensis, Cetacella sp. 1, Paracypris eniotmetos, Harbinia sinuata?, H. crepata, Liasina sp. 1, Praebythoceratina amsittenensis comb. nov., P. trinodosa comb. nov., Patellacythere sp. 1, P. shimonensis comb. nov., Xestoleberis? sp. 1, Xestoleberis? sp. 2, Apatocythere? sp. 1, Neocythere? aff. pseudovanveeni, N. (Physocythere) tenuis, Aracajuia antiqua comb. nov., A. benderi, A. fragilis comb. nov., Eocytheropteron sp. 1, Metacytheropteron aff. minuta, Microceratina? sp. 1, M. azazoulensis, Veenia guianensis, Algeriana? sp. 1, Quasihermanites? sp. 1 and Sergipella viviersae. European Journal of Taxonomy 244: 1–57 (2016) 2


Introduction
The Sergipe-Alagoas Basin is a marginal basin located in northeastern Brazil (9-11°30' S, 37-35°30' W). Its mainland portion is between 16 and 50 km long and 170 km wide, covering a total area of approximately 11 000 km 2 (Feijó 1994), bounded by the Pernambuco-Paraíba Basin to the northeast (Maragogi structural high) and the Jacuípe Basin to the southwest (Itapuã Fault) (Fig. 1). It has been the subject of several studies, mainly due to the amount of data acquired through well and outcrop sampling, seismic lines, gravimetric and magnetic surveys, etc. Most of these studies focused on its oil and gas producing strata, i.e., its basement (Proterozoic), the Carmópolis oil field (Late Jurassic-Aptian) and the Calumbi Formation (Santonian-Cainozoic) (Marques 1965;Souza-Lima et al. 2002;Campos Neto et al. 2007;Graddi et al. 2007).

7
The sample contents separated for preparation were treated at the Laboratory of Micropaleontology as follows: first they were submerged in a solution containing 30% hydrogen peroxide (H 2 O 2 ), aiming to fragment rock particles. Ethanol at 92 % was added to interrupt this reaction, and after that, the separation of grains by size took place, using mesh sieves of 630, 250, 160 and 80 μm. For samples MP-1430to MP-1434, MP-1493, MP-1494and MP-1678, the 80 μm sieve was replaced by a 90 μm-56 μm dual set.
Carapaces and valves of several species were studied at the LabMicro-UnB and the Smithsonian National Museum of Natural History (NMNH), Washington D.C., United States. For comparison, specimens of several genera belonging to NMNH collections were also studied. The study of the additional 189 already processed samples from Petrobras was done at the Centro de Pesquisas Leopoldo Américo Miguez de Mello (CENPES), Rio de Janeiro, Brazil. The type material of 37 species from 16 families is housed in the Research Collection of the LabMicro-UnB, under the prefix "CP" ("Coleção de Pesquisa") and numerations from 668 to 735 and from 756 to 769. Additional type material from two other species is at CENPES, totaling three specimens under the prefix "LBP" ("Laboratório de Bioestratigrafia da Petrobras").
Height, length and width measurements for these specimens were mostly taken using an Olympus SZX12 microscope with a Q5 camera, plus QCapture 7 software. Some additional measurements were made with a Heerbrugg Wild M3Z microscope and Leica measuring lenses. Most of the scanning electron microscopy (SEM) was done using a Philips XL-30 scanning electron microscope and palladium-gold coating to enhance the contrast and definition. Additional photos were taken in a Zeiss EVO 40 at CENPES, under palladium-gold, and a JEOL JCM-5000 Neoscope at the LabMicro-UnB, under carbon coating. Table 1. Sampled depths from wells ceded by Petrobras S.A. studied in the present work.

Remarks
The presence of halocypridids in the Early Cretaceous of the Atlantic Ocean was first noticed by Pokorný (1964), when dealing with ostracods from the Coniacian of Czechoslovakia. Since then, several taxa have been tentatively classified in the genus Conchoecia by several authors (e.g., Colin & Andreu 1990), although the only morphological resemblance to this Recent genus has to do with the presence of a rostrum (some of the specimens; certainly not the current ones) and striate ornamentation. More detailed studies on the internal features of those specimens have never been performed, and are also not possible with the present material. Therefore, we opt to maintain the current taxonomic status, but doubtfully, in the hope of collecting better preserved specimens in the future.

Remarks
Cytherella besrineensis (Bischoff, 1964) comb. nov. was initially assigned by Bischoff (1964) to the genus Cytherelloidea, although it did not present any ridge on its lateral surface, which is diagnostic of that genus (Alexander 1929). Therefore, we transfer it to Cytherella, based on ornamentation similarities.

Remarks
The diagnosis follows Bischoff (1964). Cytherelloidea btaterensis Bischoff, 1964 resembles Cytherelloidea circumvallata Bonnema, 1941 (middle Albian-early Cenomanian of the Netherlands) (Bonnema 1941;Gründel 1966), but the latter has an extra central ridge, which is absent in the former. Cytherelloidea knaptonensis Kay, 1963, from the Albian of Lebanon (Bischoff 1964), can be separated from the present species by its ornamentation, which does not include reticulations and punctuations between the ridges; also, the shape of the posterior ridge is remarkably different, especially along the dorso-posterior margin. A similar species, Cytherelloidea kayei Weaver, 1982 from the middle-late Cenomanian of England (Weaver 1982), has thicker ridges overall.

Remarks
The specimens of Cytherelloidea aff. globosa Kaye, 1964 are similar to those of Cytherella globosa Kaye, 1964, from the Albian-Cenomanian of England (Kaye 1964;Weaver 1982). They differ, though, by a more pronouncedly sloping posterior section of the dorsal margin and the absence of the posteroventral tubercle in right view. It is not clear, however, to which extent these differences are merely morphological or caused by diagenetic processes, considering that the present specimens are poorly preserved.

Remarks
The new generic identification for Bairdoppilata comanchensis (Alexander, 1929) comb. nov. follows Maddocks (1969) and Becker et al. (1989). According to both, the genus Bairdia McCoy, 1844 (to which the present species was first assigned) is restricted to the Paleozoic period; the former also mentions that Bairdoppilata is virtually identical in external view to Bairdia, but presents a series of denticles ahead of both extremities of its hinge in internal view, and is confined to the Cretaceous-Recent. The present authors rely on both statements for the generic change herein performed. Several occurrences (including possible ones) of Bairdoppilata pseudoseptentrionalis Mertens, 1956(Shahin 1991Andreu & Bilotte 2006) are transferred to Bairdoppilata comanchensis comb. nov. based on differences in size and overall external shape from these specimens to the ones assigned by Mertens (1956). One of the specimens identified as Bairdoppilata comanchensis in Swain & Brown (1964: pl. 1, fig. 5a, c) belongs in fact to B. rotunda (Alexander, 1929) -see the generic review in Howe & Laurencich (1958); the other one is from an unnamed taxon. The specimen in Neufville (1973), also figured in Reyment (1981), might belong to B. comanchensis comb. nov., despite differences in carapace size and dorsal margin morphology, possibly caused, respectively, by environmental and diagenetic factors. One of the specimens of Bairdia spp. figured in Rosenfeld & Raab (1974: pl. 1, fig. 14) also belongs to the same species, but these authors do not specify its range and geographical distribution, instead grouping it with all the others identified as Bairdia; this problem was partly solved by Honigstein et al. (1985), who restricted the Bairdoppilata comanchensis specimens to the late Cenomanian-Turonian.

Remarks
The diagnosis follows Mertens (1956). Historically, Bairdoppilata pseudoseptentrionalis Mertens, 1956 has been mistaken for B. subdeltoidea (Münster, 1830), as the former species has never been adequately illustrated since its description by Münster (1830). Several sub-species were added to Bairdoppilata subdeltoidea during the 19 th century, which increases the difficulties in establishing its current taxonomic status. The most recent review of the species is in Deroo (1956), who renamed it as Bairdoppilata roemeri, with a lower Cretaceous-Tertiary stratigraphic range. Assuming the species is still valid, it has to be separated from Bairdoppilata pseudoseptentrionalis, at least in terms of age; the former is considered later Cretaceous-Tertiary (Münster 1830), while the latter is from the late Albian-Cenomanian (Mertens 1956). The specimens in Kaye (1965), Gründel (1966) and Jarvis et al. (1988) present a straighter posterior portion of the dorsal margin in the left valve, when compared to the more rounded one in B. pseudoseptentrionalis. In Keen & Siddiqui (1971) and Babinot et al. (1985b), the specimens presents a generally wider overlapping and less rounded overall shape.

Remarks
Bairdoppilata sp. 1 is remarkable among Bairdoppilata species of the lower Cretaceous because of the combination of the following features: an elongate overall shape, elongate posterior end, comparable to B. southerhamensis Weaver, 1982 (although this species' carapace displays greater height) (Weaver 1982), and punctuate ornamentation. The current specimens probably represent a new species, but due to the small amount of material (composed solely of valves) recovered, we opt to leave it in open nomenclature.

Remarks
The specimens of Bairdoppilata sp. 2 resemble those of Bairdoppilata sp. 1, although they are less elongate and present a smooth lateral view. Compared to B. southerhamensis in Weaver (1982), it is less rounded in lateral view and narrower than the latter.

Type locality and stratum
Not designated.

Remarks
The diagnosis follows Dingle (1984). The genus Robsoniella was originally placed in the family Healdiidae Harlton, 1933 by Kuznetsova (1956). After review by Gramm & Kuznetsova (1970), it was separated into its own family, Robsoniellidae Gramm & Kuznetsova, 1970. Dingle (1984 first noted its affiliation to the superfamily Bairdioidea Sars, 1888, despite classifying it as a Bairdiidae Sars, 1888. However, it shares several morphological characters with the Bythocyprididae Maddocks, 1969, as already pointed out by Babinot et al. (2009), although this study gave no further explanation. Bythocyprididae was originally proposed by Maddocks (1969) as a subfamily of Bairdiidae Sars, 1888, namely Bythocypridinae Maddocks, 1969. Its status was reviewed by Kozur (1972), who elevated it to the family level. Specimens in Oertli (1974) and Damotte (1992) present a very similar general layout in lateral view, although not enough to consider them as being Robsoniella falklandensis.

Remarks
The generic diagnosis follows Martin (1958). Cetacella GA D 25 in Grosdidier (1979) also belongs to this species.

Paleoecology and distribution
Cetacella sp. 1 is a marine, shelf species occurring in the following localities and stages: Madiéla Formation, Gabon Basin, Gabon, upper Aptian-lower Albian (Grosdidier 1979

Remarks
The diagnosis follows Piovesan et al. (2013), although no striate forms were observed in the present samples.

Remarks
The diagnosis follows Do .

Remarks
The suprageneric taxonomy follows Hou et al. (2002) and the generic one follows Do . However, although the specimens observed are very similar to Harbinia sinuata (Krömmelbein & Weber, 1971), they differ in the shape of the dorsal anterior margin adjacent to the "hump" region, characteristic of the genus Harbinia Tsao, 1959. In the specimens of Krömmelbein & Weber (1971) and Do , this area of the dorsal margin is flatter, which overall widens the anterior end.
The present specimens seem to be similarly, although more discretely, ornamented as other Harbinia species (Krömmelbein & Weber 1971;Hou 1984;Antonietto et al. 2012;Do Carmo et al. 2013). For these reasons, the current specimens are identified dubiously as Harbinia sinuata.

Paleoecology and distribution
Harbinia sinuata? is a freshwater to transitional species occurring in the following localities and stages: Bongor, Doba and Doseo Basins, Chad, upper Aptian (Colin & Dépêche 1997)

Remarks
The generic diagnosis follows Gramann (1963). Liasina sp. 1, in contrast to most Liasina species, is not smooth in lateral view, but rather pitted. The very restricted number of specimens does not allow a complete description of this probably new species. Specimens in Viviers et al. (2000), namely Bythocypris? sp. Se1, and Piovesan et al. (2013) also belong to Liasina sp. 1.

Diagnosis
A species of Praebythoceratina distinguished by the combination of the following features: ornamentation consisting of irregularly shaped reticulations at the postero-central and ventro-posterior areas, a tubercle in the dorso-central area and an irregularly reticulate spine in the ventro-central area. In dorsal view, the spine presents a widened basis, forming a triangle-shaped ala running from the postero-ventral to the antero-ventral areas.

Etymology
From the Greek delta, meaning "d", and the Latin alata, meaning "with a wing". Praebythoceratina deltalata sp. nov. presents a spine in its ventro-posterior region that is shaped like a triangular ala (hence delta, as the letter "d" in the Greek alphabet resembles a triangle).

Type Material
Holotype CP-683 (right valve), stored dry in a micropaleontological slide.

Description
In lateral view, medium-sized subtriangular to sub-rectangular carapace, with greatest height at posterocentral and greatest length at centro-dorsal regions. Round anterior end, becoming slightly rectilinear close to dorsal margin, where an obtuse cardinal angle is formed. Sharp, supra-curvate posterior end, forming dorsal caudal process. Rectilinear dorsal margin, becoming concave only at centro-posterior region. Regularly straight ventral margin, with slight concavity at oral region. Primary ornamentation consisting of tubercle in dorso-central area, irregularly reticulate spine in ventro-central area and highly inconspicuous, large and low marginal rib running from dorso-posterior to antero-ventral margins. Very shallow S 1 and S 2 sulci. Secondary ornamentation includes irregularly shaped reticulations at posterocentral and ventro-posterior areas. Weakly developed rounded eye tubercle. In dorsal view, strongly sagitiform; spine presents a widened basis, forming wide triangle-shaped ala running from posteroventral to antero-ventral areas. Presence of dorsal ridge creates a flat surface adjacent to hinge line. In internal view, adont hinge, composed in right valve by sulcus running along most of dorsal margin.
Weakly developed inner lamella, observable along anterior end and ventral region of posterior end; not visible in oral region, where valve margin bends inwards expressively.

Remarks
Subfamiliar and tribal identifications follow Gründel & Kozur (1971), while the generic is based on Becker (1990). Occurrences herein described correspond to the first early Cretaceous records of Praebythoceratina Gründel & Kozur, 1972, hitherto restricted to the Triassic-Jurassic. Most specimens of Praebythoceratina deltalata sp. nov. were found broken, thereby limiting the designation of type specimens. This species is very similar in general layout to Bythoceratina umbonatoides (Kaye, 1964), B. umbonata (Williamson, 1847 and Monoceratina longispina (Bosquet, 1854) from the Albian-Cenomanian of England (Kaye 1964(Kaye , 1965Weaver 1982). However, they differ in the position of the dorsal tubercle (postero-dorsal in the former, antero-dorsal in the latter) and in the carina-like morphology of the spine basis in P. deltalata sp. nov. (absent in B. umbonatoides). Several species, including P. amsittenensis (Andreu-Boussut, 1991) comb. nov. from the Albian of Morocco (Andreu-Boussut 1991), B. tamarae Rosenfeld, 1974 andB. avnonensis Rosenfeld, 1974 from the upper Cenomanian of Israel (Rosenfeld & Raab 1974), are distinguished from P. deltalata sp. nov. by the same features. Donze (1964) illustrated Monoceratina sp. A from the Berriasian of France in dorsal view, which is strongly similar. However, there is not enough information to perform length/height/width measurements of these specimens, and therefore they cannot safely be assumed to belong to P. deltalata sp. nov.

Remarks
The generic identification is based on Becker (1990), while the specific follows Andreu-Boussut (1991). Some of the specimens from the Aptian of Gabon identified by Grosdidier (1979) as "Patellacythere" GA E 27 belong to this species, while others are related to Patellacythere shimonensis (Rosenfeld & Raab, 1983) comb. nov. Bythoceratina? sp. P1 in Viviers et al. (2000) probably also belongs to the present species, but it is not possible to fully confirm this, as the figured specimen is poorly preserved.

Remarks
The generic identification follows Becker (1990), while the specific follows Alexander (1934). Despite being poorly preserved, the holotype of Praebythoceratina trinodosa (Alexander, 1934) comb. nov. presents typical morphological traits that are not common in other Early Cretaceous bythocytherid species -a postero-dorsal tubercle over the lateral u-shaped inflation and the presence of reticulate ornamentation at the central area that slightly fades along adjacent areas -but observable in the present specimens. Compared to P. amsittenensis comb. nov., P. trinodosa comb. nov. is more slender in lateral view and presents more developed antero-dorsal and postero-dorsal tubercles.

Remarks
The only specimen of Patellacythere sp. 1 found in the present samples resembles those of P. parva Weaver, 1982, although not the ones in Witte (1992), which are incomplete, having lost most of their posterior ends. However, Patellacythere sp. 1 has a slightly less developed lateral u-shaped swelling, and therefore is dubiously placed in the current species.

Remarks
The present specimens share the typical morphology of Xestoleberis Sars, 1928: a small and smooth carapace, with greatest width at the postero-dorsal region. None of the specimens collected are disarticulated, and thus none of their internal features can be observed. According to Kempf (1986), more than 300 taxa have been described and assigned to this genus, despite its quite conservative shell morphology and the fact that it was originally described based on soft part morphology (Puckett et al. 2012). Many papers describe, identify or partially identify Xestoleberis species in the Cretaceous (Veen 1936;Bonnema 1941;Schmidt 1948;Benson & Tatro 1964;van den Bold 1964;Holden 1964;Crane 1965;Herrig 1966;Rosenfeld & Raab 1974;Dingle 1980;Weaver 1982

Remarks
For uncertainties in the generic diagnosis, see remarks on Xestoleberis? sp. 1. Xestoleberis? sp. 2 differs from the former by its larger size and sturdier general layout, both in lateral and dorsal views.

Remarks
The suprageneric diagnosis follows Neale (1982). No internal features were observable in the present specimens, although their general shape and size resemble those of Apatocythere species in Triebel (1940). Because of that, the present authors maintain the dubious identification of Apatocythere? sp. 1.

Diagnosis
A genus of Dicrorygmini with the following distinguishing features: sub-rounded to sub-rectangular carapace with rounded, convex dorsal margin, smooth or punctate ornamentation, weakly developed hinge line elements, narrow marginal area and lamellae and valves highly bent inwards in the oral region, sometimes partially obliterating the view of the inner margin and selvage.

Etymology
From the Latin Gabon, meaning "Gabon", a country where specimens of the genus are common in Early Cretaceous strata; and the Greek orygma, meaning "tunnel", a reference to this genus belonging to the tribe Dicrorygmini Gründel & Kozur, 1975.

Remarks
The tribal and subfamiliar diagnoses follow Gründel (1978b). The Dicrorygmini is a small tribe probably composed by the following genera: Archeocuneocythere Mandelstam, 1947, Dicrorygma Poag Jr, 1965, Orthorygma Christensen, 1965(= Oertliana Kilenyi, 1965, here raised to the generic level, and Gabonorygma gen. nov. Species of Gabonorygma gen. nov. so far include G. brotzeni (Christensen, 1965), from the Kimmeridgian-early Tithonian of Denmark, and the presently described G. sergipana gen. et sp. nov. Pattersoncypris? sp. 1 in Piovesan et al. (2013) is also a species to be included in Gabonorygma gen. nov., if properly reviewed. Cytherideinarum gen. et sp. nov. 2 in Oertli et al. (1961), from the Valanginian-Barremian of Denmark, also belongs to this new genus, although it is not officially described; Cytherideinarum itself cannot be accepted as a valid generic name, as it never received a proper diagnosis. Despite the small number of classified taxa, their number could be considerably larger, since Gabonorygma gen. nov. represents what is in general the most neglected carapace morphology among ostracods, namely, a small, rounded, smooth, mostly featureless general layout.

Diagnosis
A species of Gabonorygma gen. nov. distinguished by the following features: carapace with greatest height at the antero-central region and greatest width at the centro-posterior region, round, slightly infra-curvate posterior end and weakly developed inner margin, not visible in the oral region, where the selvage bends inwards expressively.

Etymology
From Sergipe State, Brazil, where the species was described.

Description
In lateral view, sub-rounded to sub-rectangular carapace, with greatest height at antero-central and greatest length at central regions. Left valve larger than the right, slightly overlapping it at ventral margin and posterior end. Round anterior end, becoming slightly rectilinear close to dorsal margin, forming an obtuse cardinal angle. Round, slightly infra-curvate posterior end; obtuse posterior cardinal angle. Slightly rectilinear to concave dorsal margin. Rectilinear ventral margin. Smooth overall ornamentation. In dorsal view, elliptical, with greatest width at centro-posterior region. In internal view, adont hinge, composed of smooth bar in left valve. Weakly developed inner lamella, not visible in oral region, where valve margin bends inwards expressively. Selvage visible through entire free margin. Central muscle scar group consisting of at least three elliptical central scars oriented in its greatest axis from anteroventral to postero-dorsal margins and an elliptical frontal scar. Weak sexual dimorphism: females more sub-rounded and wider in dorsal view, males more sub-rectangular and narrower.

Remarks
Gabonorygma sergipana gen. et sp. nov. can be separated from Gabonorygma brotzeni by the more rounded shape of its posterior end, the more anterior position of its greatest height and the more central position of its greatest width. Specimens in Grosdidier et al. (1979), Piovesan et al. (2013) and Viviers et al. (2000) also belong to the new species.

Remarks
The suprageneric diagnosis follows Gründel (1977) and Liebau (2005), while the specific diagnosis is based on Gründel (1966). Although the present species and Neocythere? pseudovanveeni Gründel, 1966 are similar in size and general layout, the style of ornamentation and lack of distinctive morphological traits, i.e., the dorsal marginal flange and the pair of ventral ribs, leave the attribution to the former uncertain. Specimens identified as Neocythere GA B 21 in Grosdidier (1979) and Perissocytheridea? sp. 1 in Piovesan et al. (2013) also belong to Neocythere? aff. pseudovanveeni.

Remarks
The diagnosis follows Antonietto et al. (2013) for the generic and Rosenfeld & Raab (1983) for the specific level.

Remarks
The diagnosis follows Piovesan et al. (2013). After a major review of the genera Amphicytherura and Aracajuia by Antonietto et al. (2013), the species has been transferred to the latter.

Remarks
The generic diagnosis follows Alexander (1933) and the suprageneric diagnosis follows Gründel (1976). Due to the paucity of well-preserved specimens, Eocytheropteron sp. 1 cannot be properly described.
The reclassification of Metacytheropteron minuta (Swain, 1976) has been suggested by several previous works, including Babinot & Colin (1988) and Piovesan et al. (2012), so the present authors have opted to perform it. The figured specimen possibly belongs to this species, although it differs significantly in the dorsal overlap (strong in Metacytheropteron minuta). Still, it is co-specific with the ones identified as Metacytheropteron GA B 14 by Grosdidier (1979) and Metacytheropteron aff. sp. GA C 26 by Viviers et al. (2000).

Remarks
The diagnosis follows Colin et al. (2005). Additionally, Microceratina sp. in Babinot et al. (2009) is included in the synonymic list of Microceratina azazoulensis Andreu & Colin, 2005. Chapmanicythereis? sp. 1 in Hart & Critenden (1985) is also tentatively herein considered co-specific, as the figured specimen presents the same general shape, but not the ornamentation, which could be due to taphonomic processes.

Remarks
Microceratina? sp. 1 vaguely resembles other species of Microceratina in Colin et al. (2005) in general shape (which is incomplete in the present specimen). Its ornamentation, however, bears no resemblance to any of the aforementioned, and for this reason it is only tentatively placed in this genus.

Diagnosis
A species of Brachycythere distinguished by the following features: in lateral view, subtriangular to subelliptical carapace, with greatest height at the antero-central and greatest length at the central regions.
Left valve is larger than the right, overlapping it markedly through the entire free margin, except at the postero-ventral margin, where the overlap is modest. Ornamentation differing from left to right valves: in the right one, it consists of three weakly distinguishable smooth ribs at dorsal, central and centroventral positions; the left one is entirely smooth.

Etymology
Named for the Smithsonian Institution, the organization to which the National Museum of Natural History of Washington, D.C. belongs, where an important part of this work was carried out.

Description
In lateral view, subtriangular carapace, with greatest height at antero-central and greatest length at central regions. Left valve larger than right valve, overlapping it pronouncedly through entire free margin, except at postero-ventral margin, where overlap is discrete. Round anterior end, becoming more rectilinear close to dorsal margin, where a slightly obtuse cardinal angle is formed. Lightly pointed posterior end, forming small caudal process, rectilinear below and above its rounded, more extreme point; with dorsal margin it forms an obtuse cardinal angle. Rectilinear to slightly concave dorsal margin. Rounded ventral margin, with slightly conspicuous concavity at oral region at left valve. Ornamentation differing from left to right valves: in right one three weakly distinguishable smooth ribs at dorsal, central and centroventral position: the generally curved dorsal one runs from centro-posterior to antero-dorsal region, slightly projecting behind dorsal margin at antero-dorsal region; central one runs from postero-central to central region; lightly curved ventral one runs from postero-ventral to antero-ventral region. Left valve entirely smooth. Flattened anterior and posterior regions. Weakly developed elliptical eye tubercle. In dorsal view, elliptical, with greatest width at centro-posterior region; pointed anterior and posterior ends. In internal view, holamphidont hinge, composed in left valve of frontal rounded socket and smaller, rounded bilobate tooth, a central smooth bar and a posterior elliptical, elongate socket. Well-developed inner lamella, not visible in oral region, where valve margin bends inwards expressively. Selvage not visible at postero-ventral and postero-central regions. Mild sexual dimorphism: females shorter in dorsal view and more rounded in lateral view than males, presenting more valve overlap through carapace margins than males.

Remarks
The suprageneric diagnosis follows Puckett (2002). Brachycythere smithsoniana sp. nov. presents the very distinctive feature among species of Brachycythere of having differences in ornamentation between its left and right valves as following: the right valve presents three weakly distinguishable ribs in dorsal, central and ventral positions, while the left is smooth. Brachycythere asymmetrica Puckett, 1994 also has dissimilar ornamentation between its left and right valves, but in a different pattern: in this species, only a ventral rib is present and this in both valves, although developed in varied levels from left to right; it is also distinct between male and females of the species (Puckett 1994). "Veenia" GAD 34 in Grosdidier (1979) is co-specific to Brachycythere smithsoniana sp. nov.

Remarks
The generic diagnosis partially follows Majoran (1989). The main difference between the present specimen and previously described nominal species of Algeriana is the presence in the former of an additional anterior sub-marginal ridge originating in the eye tubercle. Due to the restricted number of specimens, it is not possible to make any further comments on the taxonomic placement of Algeriana? sp. 1.

Remarks
The diagnosis follows Do .

Taxonomic summary
The present taxonomic study reviews the identification of the ostracod species found in the Angico, Taquari and Maruim Members of the Riachuelo Formation. Thirty-nine species of 25 genera in 16 families were found and the new taxa include a genus, Gabonorygma gen. nov., and three species:

Paleoecology
According to Campos Neto et al. (2007), the sediments of the Riachuelo Formation were deposited during the drift phase of the proto-South Atlantic Ocean. The sedimentary basin evolved from an initially transitional (local late Alagoas) to an initial shelf system (early Albian) and then gradually to more open sea habitats (late Albian-Cenomanian). This is evidenced by the temporal and spatial distributions of the Angico, Maruim and Taquari Members, which respectively represent inner shelf, reef and outer shelf shallow marine deposits (the Taquari Member also preserves lagoon and back reef environments).
Paleoecological data of the Albian species herein found (Fig. 10) endorse the lithological interpretations above, as 38 of the 39 identified species displayed shallow marine life habits, according to the literature. This is coincident with the paleoenvironments mainly observed in the Albian of the Riachuelo Formation. Seven species were also characteristic of moderately deep waters, and five were transitional (two of them even freshwater). However, our data are not perfectly congruent with data pertaining to the stratigraphic occurrence, because species living in the transitional zone do not necessarily occur in strata expected to be more transitional (MSA-0.1 and MSA-0.2 subzones). Moderately deep species are also equally found in both transitional and shallow marine to moderately deep marine strata (MSA-0.3, MSA-0.4 and MSA-0.5 subzones and MSA-1 zone). Therefore, further analysis of additional data is needed to elucidate the distribution of these species in late Aptian-Albian strata of the Riachuelo Formation.