Morphometric analysis of Paleogene Loxoconcha species (Crustacea, Ostracoda) from Egypt and Tunisia

Abstract

Three Loxoconcha species from the Paleocene–Eocene successions of Egypt are closely related to each other; namely: Loxoconcha saharaensis (L. saharaensis) Bassiouni & Luger from the Late Paleocene of Egypt, Loxoconcha blanckenhorni (L. blanckenhorni) Bassiouni & Luger from the Early Eocene of Egypt, and Loxoconcha pseudopunctatella (L. pseudopunctatella) Cronin & Khalifa from the Middle to Late Eocene of Egypt. Other related forms have been identified from the Early Eocene of Egypt and the Late Paleocene of Tunisia. Relative warp analysis could differentiate among the three studied species on the basis of sexual dimorphism. Thin-plate spline analysis suggests that the related forms from Egypt and Tunisia are intermediate forms between L. saharaensis and L. blanckenhorni, with closer similarity to L. saharaensis. Therefore, the holotype of L. saharaensis is defined herein as morphotype A, the specimen from Tunisia as morphotype B, and the specimen from Egypt as morphotype C. Moreover, the results indicate that the Middle Eocene species (L. pseudopunctatella) is closer to the Early Eocene species (L. blanckenhorni) than to the Late Paleocene species (L. saharaensis). A probable evolutionary trend is detected within the studied L. saharaensis morphs from the Late Paleocene of Egypt and Tunisia to the Early Eocene of Egypt. This trend was more or less simultaneous with decreasing size above the PETM level. A similar lineage and size reduction was detected between the Early Eocene species L. blanckenhorni and the Middle Eocene species L. pseudopunctatella.