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Revisiting the taxonomy of the intermediate stages in the Globigerinoides-Praeorbulina lineage

机译：再次探讨Globigerinoides-Praeorbulina世系中间阶段的分类

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摘要

Despite the biostratigraphic and chronostratigraphic importance of the Globigerinoides trilobus-Praeorbulina evolutionary lineage in the upper Burdigalian to Langhian interval, some uncertainties concern the taxonomic interpretation of the intermediate stages of this lineage, in particular the generic attribution of the species sicanus De Stefani 1952 and its relationship with Globigerinoides bisphericus Todd 1954 and Praeorbulina glomerosa curva (Blow 1956). In this study, we present: 1) a review of the intermediate stages of the Globigerinoides trilobus-Praeorbulina evolutionary lineage focussing on the different taxonomic concepts of the species bisphericus, sicanus and glomerosa curva which according to Blow (1956; 1969) and Jenkins et al. (1981) gave rise to two different lines of evolution, and 2) our concepts based on the study of assemblages from several Mediterranean successions which have been compared with those of mid- and low-latitude Atlantic Ocean. On the basis of several diagnostic characters, we identified three morphotypes (Morphotypes 1, 2 and 3) within the populations transitional from G. trilobus to Praeorbulina. In our assemblages, Morphotype 3 and Morphotype 2 are the most representative ones and are considered as Globigerinoides sicanus sensu Blow (1956; 1969) and Kennett and Srinivasan (1983) and as a variant of G. sicanus, respectively. In fact, the taxonomic concepts of Blow, which take both the population variability and the gradual stratigraphic evolution of the biocharacters into account, can be better applied to our assemblages than those of Jenkins et al. (1981), which are mainly based on the characteristics of the holotypes without considering the variability of the species. Accordingly, the evolution of Praeorbulina via G. sicanus (senior synonym of G. bisphericus) proposed by Blow (1956; 1969) can he followed in the studied assemblages and the Praeorbulina datum is represented by the first appearance of P. glomerosa curva. On the basis of our data the evolution of P. glomerosa curva from G. sicanus lasts more than one myr. The main evolutionary changes within G. sicanus populations leading to Praeorbulina are the appearance of specimens with three apertures and the gradual increase in the outline sphericity. Near-spherical individuals of G. sicanus with three apertures can be considered very close to P. glomerosa curva, which in turn is characterized by a spherical outline, at least four apertures with the primary aperture nearly undistinguishable from the supplementary ones and a close umbilicus. Ultimately, our re-examination of the intermediate stages between G. trilobus and Praeorbulina results in a re-evaluation of the evolution proposed by Blow (1956; 1969) with significant bio- and chronostratigraphic implications.
机译：尽管在上Burdigalian到Langhian间隔的Globigerinoides trilobus-Praeorbulina进化世系在生物地层和年代地层上都很重要，但仍存在一些不确定性涉及该世系的中间阶段的分类学解释，尤其是西卡努斯·德·斯特凡尼（Sicanus De Stefani）1952及其物种的一般归因与Globigerinoides bisphericus Todd 1954和Praeorbulina glomerosa curva的关系（Blow 1956）。在这项研究中，我们提出：1）综述Globigerinoides trilobus-Praeorbulina进化谱系的中间阶段，着眼于Blow（1956; 1969）和Jenkins等的双球菌，西卡努斯和肾小球弯曲菌的不同分类学概念。等（1981）提出了两种不同的发展思路，2）我们的概念基于对几个地中海演替组合的研究，并与中低纬度大西洋进行了比较。基于几个诊断特征，我们在从三叶lob过渡到to鳞的种群中鉴定了三种形态型（形态型1、2和3）。在我们的组合中，形态3和形态2是最具代表性的形态，分别被视为Globigerinoides sicanus sensu Blow（1956; 1969）和Kennett和Srinivasan（1983）以及G. sicanus的变体。实际上，考虑到种群变异性和生物特征的逐步地层演化，Blow的分类学概念比Jenkins等人的分类学概念可以更好地应用于我们的组合。 （1981），这主要是基于整体型的特征，而没有考虑物种的变异性。因此，Blow（1956; 1969）提出的通过G. sicanus（G. bisphericus的高级同义词）的Praeorbulina的进化可以跟随研究的组合，而Praeorbulina基准面则以P. glomerosa curva的首次出现为代表。根据我们的数据，从G. sicanus进化出的P. glomerosa curva持续超过一个myr。导致Praororbulina的G. sicanus种群内部的主要进化变化是具有三个孔的标本的出现以及轮廓球度的逐渐增加。可以认为具有三个孔的西卡氏假单胞菌的近球形个体非常接近球状假单胞菌，其特征是球形轮廓，至少四个孔，主要孔与补充孔几乎没有区别，并且脐部紧密。最终，我们对三叶松线虫和Praeorbulina之间的中间阶段的重新检查导致对Blow（1956; 1969）提出的演化进行了重新评估，具有重要的生物和年代地层意义。

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