Phylogenetic relationships and evolutionary history of the Early Paleogene genus Igorina through parsimony analysis

The evolution of planktonic foraminifera in the Paleocene-Eocene time interval is characterized by a high rate of diversification after the major extinction event observed at the Cretaceous/Paleogene boundary. The accelerated speciation rate resulted in the appearance of several new genera (i.e., Igorina and Acarinina among others) each of them identified on the basis of distinctive wall texture of the shell. Phylogenetic relationships within many genera are still poor understood including the origin and phylogeny of the genus Igorina. This group, characterized by a thick, nonspinose and incrusted wall, appears in Subzone P3a (early late Paleocene) and disappears in Zone E9 (middle Eocene). To date, nine species have been assigned to the genus Igorina (I. pusilla, I. trichotrocha, I. tadjikistanensis, I. convexa, I. albeari, I. laevigata, I. lodoensis, I. broedermanni and I. anapetes) based on both wall texture and morphologic similarities. This study is aimed to reconstruct the phylogeny and evolution of the igorinid species with cladistic analysis by applying the method of parsimony. To perform the study, two hundred samples have been analysed from Ocean Drilling Program (ODP) Leg 198 Hole 1209B (Shatsky Rise, Central Pacific Ocean), Leg 143 Hole 865B (Allison Guyot, Central Pacific Ocean), Leg 15 Sites 151 and 152 (Caribbean Sea) and Leg 113 Hole 690B. The biostratigraphic analysis of sub-tropical localities revealed some problems in applying the previous biostratigraphic scheme (Berggren and Pearson, 2005) mainly because some marker species 1) have been misinterpreted and/or misidentified several times in previous studies, 2) show a delayed appearance with respect to what reported from other localities, and 3) are very rare or absent in the studied samples. Species identification was mainly performed through comparison with the original descriptions and illustrations and partially follows Subbotina (1953), Blow (1979), Olsson and others (1999), Pearson and others (2006). Phylogenetic relationships of the species assigned to Igorina are determined through stratocladistic analysis by using a data matrix of 23 taxa (including key species of Acarinina), 31 morphological characters (unordered), and a stratigraphic character (ordered) mapping the first occurrence of the taxa under investigation. The morphological characters included in this analysis are based on morphometric measurements such as the angle of chambers, maximum diameter of the inner whorls, angle of peripheral margin, among others. Some morphotypes of uncertain taxonomic identification but showing consistent morphological and wall texture features have been coded and analysed separately as morphotypes A, B, C, D, E and F to determine their ancestor-descendant relationships and evaluate their validity as discrete species. The software PAUP* (Swofford 2002) has been used to process the data and to obtain a numerical matrix with codified the selected characters, then the matrix has been processed using the heuristic search option to discover the most parsimonious trees. Results suggest that Igorina pusilla is the first representatives of the Igorina lineage and is subsequently followed by I. laevigata. I. albeari, and I. tadjikistanensis in agreement with their stratigraphic distribution. Morphotypes (C, E, F) have been included in Igorina paraspiralis (Soldan and others, 2011) while Igorina morphotype A represents a single species (Igorina praecarinata Soldan and others, 2011). Morphotypes B and D fall in the variability of well-know species. Moreover, the analysis provides evidence that I. trichotrocha, I. lodoensis, I. broedermanni and I. anapetes are more closely related to genus Acarinina than Igorina and clearly belong to a different lineage. A preliminary analysis of the wall texture architectures the broedermanni group has been performed to assess their ancestor-descendant relationships and to evaluate the possibility to place them in a discrete new genus.