Lower Cretaceous Orbitolinas from Venezuela
1996; Micropaleontology Press; Volume: 42; Issue: 1 Linguagem: Inglês
10.2307/1485984
ISSN1937-2795
AutoresÁgnes Görög, Annie Arnaud‐Vanneau, Ágnes Görög,
Tópico(s)Evolution and Paleontology Studies
ResumoTwo Orbitolina species were found in the Late Aptian-Early Albian of Venezuela. The first one, Orbitolina (Mesorbitolina) pervia Douglass 1960 was collected in the Eastern allochthonous area, at Bergantin, La Maravilla, in the Chimana Formation. This species is well known in different Tethyan regions. The second one, Orbitolina (Mesorbitolina) ovalis n. sp., is a new oval species coming from the Western autochthonous area, at Jabon, in the Upper part of the Penas-Altas Formation. This species, also identified in Peru, could be characteristic of a Western Province. INTRODUCTION Orbitolinas have been collected from different regions of Venezuela for more than a century and several species were first described from this region. The first mention of Orbitolina from Venezuela was by Karsten (1858) who indicated their presence in the bergantin (corresponding to Chimana Formation). He described the species as Orbulites lenticularis. In 1886, he renamed these orbitolinas and identified them as a new species calledOrbitulina venezuelana coming from 'Trujillo, Bottijoque, Escuque, etc... (Western Venezuela). The figured orbitolina is a small, conical, isolated specimen, about 3 mm in diameter (P1. VI, fig. 6). No thin sections were figured at that time. In 1924, W.O. Dietrich studied the specimens collected by Karsten and assigned them to varieties of Orbitolina lenticularis (Blumenbach) (p. 183). H.K. Hodson (1926) described and figured without thin sections three new species from Rio Asagua, state of Monagas, probably from the Cantil Formation (corresponding to Upper Borracha and Chimana Formations). These are Orbitolina texana asaguana, 0. texana monagasana and 0. thompsoni (p. 5-6, pl. 1, fig. 1-9). T.W.Vaughan (1932) investigated orbitolinas from Venezuela and Guatemala, probably from the Bergantin Beds(= Chimana Formation). He suggested that Orbitolina venezuelana (Karsten) might be a synonym of Orbitolina texana (Roemer). H.D. Hedberg (1937) figured O. concava texana from the base of the Rio Querecual section and from the lower part of the Bergantin Beds. The specimen is in a thin section and the embryonic apparatus is missing. He compared the specimens from the Bergantin Beds with those from the Glen Rose Formation, Texas, suggesting that they belong to the same species. E. Rod and W. Maync (1954) mentioned orbitolinas from several localities in Venezuela, without description or illustrations. Two specimens of Maync's collection, found in the Upper Borracha Formation, Rio Punceres, Eastern Venezuela, were figured by J. Hofker in 1963 (pl. XVI, fig. 9-10). For the first time, these specimens show clearly the embryonic apparatus and permit them to be identified as the subgenus Mesorbitolina. Rod and Maync (1954) also mentioned Orbitolina concava from the lowermost beds of the Cogollo Group (it is underlain by the Aguardiente Formation). On the other hand, J. Hofker (1963) mentioned orbitolinas (form group III) from the Albian of the Trujillo State, collected from the middle part of the Aguardiente Formation. To summarize, except for Hofker's paper, the previous papers record a few isolated specimens, and/or poor thin sections without illustration of embryonic apparatuses. Today, it is impossible to identify these species because external characters are not sufficient to enable generic or specific identifications. The studied specimens in this paper come from two localities of late Aptian (Clansayesian)-Albian age (text-fig. 1). They were collected in marly levels interbedded in limestones. The first locality, 4703, from the Cantil Formation, is situated in La Maravilla section, Bergantin, allochtonous area, Eastern Venezuela (text-fig. 2). Ammonites coming from this level indicate late Aptian-Clansayesian ( MacSotay 1990, oral communication). The second locality, 4420, 4421, belongs to the upper part of the Penas Altas Formation, along the La Pastora-Jabon road, autochtonous area, Western Venezuela (text-fig. 2). The age is probably Early Albian. Preparation of samples Samples of Orbitolina-bearing layers from these localities were collected by Hubert Arnaud in 1990. After washing, more than one hundred well-preserved specimens per level were choosen. Each test was identified by a number. Diameter and height of tests were measured and then plotted on graphs. The plots helped to choose specimens for preparation of oriented thin sections. Axial, horizontal and tangential sections were made. The dimensions of embryonic apparatus and protoconch were then measured and plotted. The statistical methods allow recognition of the number of species that exist at a level and are of great help in comparing different populations. If statistical study is necessary, it not sufficient to recognize species, it is necessary to identify inner structures of the test and the aspect of the embryonic apparatus. micropaleontology, vol. 42, no. 1, pp. 65-78, text-figures 1-5, plates 1-4, 1996 65 This content downloaded from 207.46.13.172 on Tue, 23 Aug 2016 06:11:32 UTC All use subject to http://about.jstor.org/terms Agnes Gorog and Annie Arnaud-Vanneau: Lower Cretaceous Orbitolinas from Venezuela TEXT-FIGURE 1 Schematic map of Venezuela showing the location of the two studied sections containing Orbitolinas (according to H. Amaud and A. Amaud Vanneau, 1991). La Pastora-Jabon road section is located in the autochthon i.e. on the southern Margin of the Tethys. La Maravilla section is located in the allochthon coming from the southern Margin of the Tethys. SYSTEMATIC DESCRIPTION Order Foraminiferida EICHWALD 1930 Suborder Textulariina DELAGE and HEROUARD 1896 Superfamily Ataxophragmiacea SCHWAGER 1877 Family Orbitolinidae MARTIN 1890 Genus Orbitolina D'ORBIGNY 1850 Subgenus Mesorbitolina SCHROEDER 1962 Orbitolina (Mesorbitolina) pervia Douglass 1960 Plate 1, figures 1-8; text-figures 3-4 Orbitolina pervia DOUGLASS 1960. -p. 41-42, pl. 11, fig. 1-18. Orbitolina texana Douglass DOUGLASS 1960, p. 34-36, pl. 4, fig. 1-18, pl. 5, figs. 1-30; pl. 6, figs. 1-14. Orbitolina morelensis AYALA CASTANARES 1960, pl. 3, fig. 3, 9 (non holotype pl. 3, figs. 10, 14; pl. 3, figs. 4, 6 = Orbitolina (Mesorbitolina) texana (Roemer)). Orbitolina lenticularis (Blumenbach). HOFKER 1963, p. 198, P1. XVI, fig. 6, 7?, 8-13, 14?, 15, 16?, 17?, 18?. Orbitolina(Mesorbitolina) texana (Roemer). MOULLADE & SAINTMARC 1975, pl. XIII, fig. 8, 10; pl. XIV, fig. 1? Orbitolina (Mesorbitolina) texana texana (Roemer) MEZA ?1980, pl. III, fig. 1-7; pl. IV, fig. 1-4; pl.V, fig. 1-6. Orbitolina (Orbitolina) birmanica Sahni. ZHANG 1982, pl. 10; fig. 1 4. Holotype and paratypes: originally described by Douglass in 1960. The holotype corresponds to the numbered specimen 20124, slide 15 USNM P 5525 deposited in the US National Museum. It is figured on plate 11, fig. 6 of Douglass's paper (1960). Diagnosis: Small conical test, flattened and with a rounded base. The embryonic apparatus shows a characteristic plano-convex protoconch with a flat base. The marginal zone is narrow, rafters are only present in the latest chambers. The main septula are narrow and not thickened to a triangular shape. Type locality: the holotype was found in the lower part of Glen Rose limestone in the Hood Spring quadrangle, Brewster County, Texas, USA. Age of the type: probably Early Albian Description External characters: The test is small (350gm to 3775gm in diameter, text-fig. 4C). It is low, conical or conico-concave, 66 This content downloaded from 207.46.13.172 on Tue, 23 Aug 2016 06:11:32 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 42, no. 1, 1996 La Pastora Jabon Road
Referência(s)