Evolution of the silicoflagellate naviculopsid skeletal morphology in the Cenozoic

Highlights

• Silicoflagellates with naviculopsid skeletal morphology include Naviculopsis, Pseudonaviculopsis and Bachmannocena.

• Naviculopsis diverged into two groups in late Paleocene, with plexid groups in each lineage.

• The last prominent occurrence of naviculopsid silicoflagellate morphologies is within Bachmannocena diodon subsp. nodosa in the late Miocene.

• The skeletal morphology may be an adaption to reduced portal size.

Abstract

The naviculopsid morphology, which consists of an arch across the minor axis of a basal ring with two corners and sides, is the simplest of the Cenozoic silicoflagellate skeletal designs that include an apical structure. This skeletal design is most often associated with the genus Naviculopsis (geologic range: middle Paleocene to early Miocene), but also occurs among other silicoflagellate genera. This paper interprets the evolutionary history for silicoflagellates of naviculopsid morphology, beginning with a group of large two-sided silicoflagellates from the late early Paleocene, recently placed in genus Pseudonaviculopsis, and ending with a late Miocene group of naviculopsid morphologies associated with Bachmnnocena diodon subsp. nodosa. Between these is Naviculopsis, which has more than twenty species recognized in deep sea sediments.

A significant portion of the Naviculopsis taxa discussed in this paper are associated with plexus episodes of abundant and unusual variability from the late Paleocene, late Eocene and early Miocene. A possible explanation of the development of this skeletal morphology, as well as silicoflagellates with more complicated apical structures, is a need to decrease the size of portals, which may better support the cell boundary during mitosis.

Introduction

Silicoflagellates are a group of marine single-celled algae with a geologic record that extends to at least the late Early Cretaceous, about 115 million years ago (Ma) (McCartney et al., 1990, McCartney et al., 2014a). Of the diverse Cenozoic silicoflagellate skeletal morphologies, the simplest is the naviculopsid design characterized by a 2-sided basal ring with major-axis spines and simple apical structure, best known from Naviculopsis (Fig. 1), which ranges from late middle Paleocene (~59.9 million years ago, McCartney et al., 2018) to early Miocene (~17.9 Ma, below). The apical structure of this morphology consists of an arch composed of either a single element or plate, or two elements that meet at a triple junction and apex spine (see McCartney and Witkowski, 2016). Other skeletal morphologies occur within Naviculopsis to include species characterized by three or four struts that meet at an apex, and 4-strutted morphologies with minor-axis bridge. Since a triple junction is established by two basal elements and an arch, there is no basal spine on the minor-axis. Skeletons of other genera can also, as part of species variability or as aberrant specimens, display the naviculopsid morphology.

General silicoflagellate phylogenies for the Cenozoic (Lipps, 1970; McCartney et al., 1990, McCartney et al., 2015a; Prema, 1996) begin with the 3-sided Corbisema that survived the K/Pg boundary event (Harwood, 1988; McCartney et al., 2010, McCartney et al., 2015a). McCartney et al. (2018) provided more constrained ages for the sequence of evolutionary events during the Paleocene. The first divergence of a new silicoflagellate genus – Pseudonaviculopsis – occurs in the early Paleocene before ~63.9 Ma. Dictyocha appears in the late early Paleocene (~61.4 Ma), prior to the first appearance of Naviculopsis in the late middle Paleocene (~59.9 Ma). Gleser, 1962, Gleser, 1966 is the only researcher to previously illustrate phylogenetic interpretations of various Paleogene silicoflagellates, including Naviculopsis, at the species level. The present paper provides more detailed phylogenetic interpretation and follows a study of silicoflagellate evolution in the Cretaceous (McCartney et al., 2010). A more general paper on silicoflagellate Cenozoic evolution is in preparation.