Albian bivalves from Madiela Formation in north Gabonese coastal basin

A study of Albian bivalves from Madiela Formation in north of Gabonese coastal basin has been carried out from N’Toum quarry geological section. Four genera including six species are identified: Liopistha (Psilomya) sp. 1, ? Liopistha (Psilomya) sp. 2, Megaporomya sp., Neithea (Neithea) dutrugei (Coquand, 1862), Pleuromya sp. 1 and Pleuromya sp. 2. One morphotype is identified as Bivalvia gen. et sp. indet. The genus Megaporomya Ayoub-Hannaa et al., 2013 and all identified species, except Neithea (Neithea) dutrugei (Coquand, 1862), are found for the first time in Gabonese coastal basin. Now, a total of twelve species of bivalves have been reported from the Madiela Formation. In Gabon, the presence for the first time of genera Liopistha and Pleuromya in N’Toum quarry geological section which is assigned to the Albian permits us to expand their geographical distribution until N’Toum region, and their age range to Albian.


Introduction
The Gabonese coastal basin is one of the West Africa coastal basins which extend from Cameroon to Angola-Namibia and contains sediments with different geological stories. The tectonic and sedimentary evolution (Mbina Mounguengui, 1998;Mbina Mounguengui et al., 2002;Mbina Mounguengui and Lang, 2003) allow to recognize three phases: a prerift phase (Precambrian-Jurassic), a rift phase (Neocomian-Barremian) and a drift phase (Aptian-present). Prerift and rift phases are characterized mainly by continental, lacustrine and fluvial sediments whereas the drift phase is characterized by marine sediments.
The aim of this study is to: provide, after half a century, systematic descriptions; present the new diversity; discuss the distribution and stratigraphy of the bivalve fauna from the Madiela Formation based on new data from N'Toum quarry section.

Material and methods
The material for this study comprises 16 specimens from N'Toum quarry section. In general, all specimens are preserved as internal moulds. Some specimens are laterally deformed due to compaction.  Lanau (1985).
Linear measurements are in millimeters. Abbreviations of measured parameters: L: shell length; H: shell height; W: width of articulated valves.
The systematic classification follows that of Carter et al. (2011). The morphological terminology follows the glossary of Cox (1969) in the Treatise on Invertebrate Paleontology.
The material is deposited at the collection of the Geology Department, Faculty of Sciences Masuku University of Franceville (Gabon), under collection number MDG/GBN/Lm.
Here we describe species which are mentioned for the first time in the Albian of the Gabonese coastal basin.
Remarks. In Gabon, this genus is represented only by one species recorded in Aptian (Dartevelle and Freneix, 1957 and this study).
Description. Shell large, elongate-ovate, strongly inequilateral, equivalved, little inflated. Maximum inflation slightly below umbonal area, decreasing gradually towards posterior end. Umbonal area wide and inflated. Beaks prominent, prosogyrate, situated one-fourth of the total valve length from the anterior end. Anterior and posterior margins rounded, joining ventral margin in rounded angle. No lunule. Anterodorsal margin truncated obliquely, joining anterior margin in acute angle. Postero-dorsal margin slightly concave. Posterior end wadely gaping. This gap extends until postero-ventral margin where becomes narrow. Ventral margin straight to sighlty convex. Ornament consisting of fine, commarginal folds separated by regular interspaces. Ribs erasing towards umbonal area.
Description. Shell large, elongate-ovate to oblong, strongly inequilateral, nearly equivalved, inflated. Maximum inflation slightly below umbonal area, decreasing gradually towards posterior end. Umbonal area wide and inflated. Beaks prominent, prosogyrate, situated one-fourth of the total valve length from the anterior end. Anterior and posterior margins rounded, joining ventral margin in nearly rounded angle. No lunule. Antero-dorsal margin short, straight, joining anterior margin in rounded angle. Postero-dorsal margin straight. Anterior and posterior margins slightly gaping. Ornament consisting of fine commarginal folds.
Occurrence. Albian of Gabon (this study). In Gabon this species is found for the first time. Description. Shell very large, elongate-ovate, globular, wider than high, slightly equivalved, strongly inequilateral, strongly inflated, slightly compressed posteriorly. Maximum inflation below the umbonal area. Umbonal area wide and strongly convex. Beaks large, prominent, slightly incurved anteriorly and situated approximately one-third of the total valve length from the anterior end. Anterior umbonal round ridge well-developed, extending from umbonal area to middle of anterior margin, forming boundary of shallow depression on anterior part. Anterior margin short, slightly convex, meeting the ventral margin in a blunt, nearly right angle. Posterior margin obliquely truncated, forming an acute angle with the ventral margin. Posterior end strongly gaping. Antero-dorsal margin straight, meeting the anterior margin in a rounded angle. Postero-dorsal margin straight to slightly convex, slightly sloping, forming an obtuse angle with the posterior margin. Ventral margin slightly convex. Ornamentation consisting of two strong commarginal ribs widely interspaced towards ventral margin.
Remarks. Megaporomya reymenti Ayoub-Hannaa et al. (2013) from Upper Turonian of Sergippe, Brazil resembles this species in outline and form and position of beaks but differs in lacking two strong commarginal folds widely interspaced towards ventral margin.
Occurrence. Albian of Gabon (this study). In Gabon, this species is found for the first time.
Occurrence. Early to Late Cretaceous.
Description. Shell medium to large-sized, subtrigonal to transversely oblong, inequilateral, slightly inequivalved, strongly compressed posteriorly, moderately inflated with maximum inflation slightly below umbonal area. Umbonal area broad and slightly convex. Beaks prominent, elevated above hinge line, sharply pointed, prosogyrate, situated approximately one-fourth of the total valve length from the anterior end. Anterior umbonal ridge well-developed, extending from umbonal area to anterior end, forming boundary of deep depression on anterior part. Posterior umbonal ridge welldeveloped. Anterior end short slightly rounded, joining ventral margin in rounded curve. Posterior end elongate. No posterior gap. Anterodorsal margin short, straight, joining anterior end in obtuse angle. Postero-dorsal margin straight to slightly concave, gradually sloping towards posterior margin. Ventral margin broadly rounded. Shell smooth.
Measurements ( Description. Shell large-sized, elongate-ovate, strongly inequilateral, slightly inequivalved, strongly compressed posteriorly, moderately inflated with maximum inflation slightly below umbonal area. Umbonal area broad and slightly convex. Beaks prominent, elevated above hinge line, prosogyrate, situated approximately one-third of the total valve length from the anterior end. Escutcheon narrow, elongate and shallow. Anterior end short slightly rounded, joining ventral margin in rounded curve. Posterior end elongate. No posterior gap. Anterodorsal margin short, straight, steeply sloping towards anterior margin. Postero-dorsal margin straight to slightly concave. Ventral margin rounded. Ornament consisting of weak irregular commarginal folds. Remark. By its strongly compressed posterior end and well-developed prosogyrate beaks, specimens recorded herein resembles to Liopistha (Psilomya) genus to which it is questionably assigned because of the absence of lessdeveloped radial ornament. It differs from Liopistha (Psilomya) concentrica Stanton, 1894 in having elongate ovate shell and weak irregular commarginal ribs. ? Liopistha (Psilomya) sp. shows similarities with Liopistha (Psilomya) elongata Stanton, 1894 but differs in lacking faint axial tubercles.
Occurrence. Albian of Gabon (this study). In Gabon, this species is found for the first time.
Remark This morphotype shows some similarities to genus Lucina Bruguière, 1797 in general outline and in having dorsal area and concentric lamellae but the poor preservation doesn't permit to distinguish ligament, hinge and muscle scars which are others capital characters form this genus.

Diversity and abundance
The bivalve fauna recorded from the N'Toum quarry geological section is composed of seven species including five genera and four families (Fig. 2). Bivalve faunas previously described and illustrated from the Madiela Formation (Choubert, 1935;Hourcq, 1943;Dartevelle and Brébion, 1957) differ with the one described from N'Toum quarry geological section herein. The particularity of the latter one is the presence of Liopistha (Psilomya) sp. 1, ? Liopistha (Psilomya) sp. 2, Megaporomya sp., Pleuromya sp. 1, Pleuromya sp. 2 and Bivalvia gen. et sp. indet. Now, a total of twelve species of bivalves have been reported from the Madiela Formation.
From the abundance point of view, the bivalve fauna from N'Toum quarry geological section is characterized by low abundance. Only 15 specimens have been recorded. This fauna is dominated by representatives of Pleuromyidae Zittel, 1895 and Poromyidae Dall, 1886 families with respectively 7 and 6 specimens. The other families are represented by one or two specimens. At a generic level, only Pleuromya has more five specimens.

Distribution and stratigraphy
From Gabon, the genera Liopistha and Pleuromya are only known respectively within the Senonian and Turonian (Dartevelle and Freneix, 1957). The N'Toum quarry geological section, which is assigned to the Albian, provides representatives of the genera Liopistha and Pleuromya. Their presence permits us herein to expand their geographical distribution until N'Toum regions, and their age range to Albian.
The genus Megaporomya, erected by Ayoub-Hannaa et al. (2013) in upper Turonian of Brazil, has been recorded in Albian of Morrocco (Benzaggagh, 2016). In Gabon, this is the first record of this genus.

Conclusion
The study of Albian bivalves from the Madiela Formation from N'Toum quarry geological section reveals the following conclusions: in comparison with previous studies of bivalve fauna from the Albian Madiela Formation, bivalves from N'Toum quarry section differ on its systematic content; the fauna of N'Toum quarry geological section is dominated by representatives of Pleuromyidae Zittel, 1895 and Poromyidae Dall, 1886 families; in Gabon, the stratigraphic and geographic range of Liopistha and Pleuromya genera are extended into Albian and N'Toum region respectively; -Megaporomya genus is recorded herein for the first time in the Gabonese coastal basin.