Brachycythere smithsoniana Antonietto & Carmo & Viviers & Neto & Hunt 2016, sp. nov.

Brachycythere smithsoniana sp. nov.

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Fig. 9 F–K

“ Veenia ” GAD 34 – Grosdidier 1979: 8, fig. 28a–c.

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.

Type material

Holotype

BRAZIL: ♀, carapace, stored dry in a micropaleontological slide (CP-721).

Allotype

BRAZIL: ³, carapace, stored as the holotype (CP-722).

Paratypes

BRAZIL: ♀, carapace (CP-723), ♀, carapace (CP-724), ³, valve (CP-725) and ³, carapace (CP-726); stored as the holotype.

Other material examined

BRAZIL: Massapê outcrop, Riachuelo Municipality, Sergipe State (samples MP-1514, MP-1516 to MP-1520, MP-1522, MP-1542, MP-1543 and MP-1545 to MP-1547), approximate coordinates 10°06' S, 37°10' W.

Type locality and stratum

Sample MP-1515, Massapê outcrop, Riachuelo Municipality, Sergipe State, Brazil, approximate coordinates: 10°06' S, 37°10' W; Riachuelo Formation, Sergipe-Alagoas Basin, uppermost Albian, Aracajuia antiqua zone (MSA-1).

Measurements

Holotype (CP-721): ♀, carapace: length = 0.65 mm, height = 0.39 mm, width = 0.28 mm. Allotype (CP-722): ³, carapace: length = 0.66 mm, height = 0.35 mm, width = 0.31 mm. Paratype (CP-723): ♀, carapace: length = 0.64 mm, height = 0.39 mm, width = 0.31 mm. Paratype (CP-724): ♀, carapace: length = 0.63 mm, height = 0.37 mm, width = 0.30 mm.

Paratype (CP-725): possible ♀, left valve: length = 0.62 mm, height = 0.35 mm.

Paratype (CP-726): ³, carapace: length = 0.69 mm, height = 0.38 mm, width = 0.33 mm.

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.

Paleoecology and distribution

Brachycythere smithsoniana sp. nov. is a marine, shelf species occurring in the following localities and stages: Madiéla Formation, Gabon Basin, Gabon, upper Aptian–middle Albian (Grosdidier 1979); and in the present work, Angico, Maruim and Taquari Members, Riachuelo Formation, Sergipe-Alagoas Basin, Brazil, uppermost Albian, Aracajuia antiqua zone (MSA-1).