Abstract
Eunotioid diatoms that express asymmetry in both the apical and transapical axes, forming heteropolar valves, are generally placed in the genus Actinella. The degree of heteropolarity varies between species, ranging from subtle differences between poles to highly differentiated head poles bearing an apical protuberance. Actinella species with less difference between the poles and lacking an apical protuberance are gradational with Eunotia. With over 100 known species reported globally, primarily in the tropics, only Actinella punctata Lewis, 1864 is currently known from North America. As part of a biotic survey and inventory project focused on the middle Eocene Giraffe crater locality near the Arctic Circle in northern Canada, we have uncovered a wealth of eunotioid diatoms including at least five heteropolar species attributed to Actinella, three of which are described formally herein as A. hickeyi sp. nov., A. goodwinii sp. nov. and A. kimberlitica sp. nov.. These diatoms all lack apical protuberances and bear resemblance to modern heteropolar counterparts within Eunotia. The objectives of this contribution are to report the findings from the Giraffe locality relative to modern and fossil eunotiophycid taxa, discuss the use of heteropolarity as a distinguishing character for the genus Actinella, and consider the palaeoclimatic and biogeographical implications of these observations.
Highlights
The subclass Eunotiophycidae of the class Bacillariophyceae represents a relatively small assemblage of taxa that are unique in possessing short raphes but lacking central nodules, yet are the only raphe-bearing diatoms having rimoportulae (Patrick & Reimer, 1966; Round et al, 1990)
Remains of two more species have been uncovered, but since we have examined a limited number of specimens to date, only preliminary observations are given
The position and reduced nature of the raphe, presence of rimoportulae, and features of the striae, areolae and helicoglossae are similar for Actinella and Eunotia, and both genera are, without doubt, evolutionarily closely related (Round et al, 1990; Kociolek, 2000)
Summary
The subclass Eunotiophycidae of the class Bacillariophyceae (diatoms) represents a relatively small assemblage of taxa that are unique in possessing short raphes (often referred to as rudimentary) but lacking central nodules, yet are the only raphe-bearing diatoms having rimoportulae (Patrick & Reimer, 1966; Round et al, 1990). There are eight genera recognized within the Eunotiophycidae: Actinella Lewis, Eunotia Ehrenberg, Desmogonium Ehrenberg (not recognized by some authors), Eunophora Vyverman, Sabbe & Mann, Peronia de Brébisson & Arnott ex Kitton, Semiorbis Patrick, Amphorotia Williams & Reid and Perinotia Metzeltin & LangeBertalot (Sabbe et al, 2001; Williams & Reid, 2006; Wetzel et al, 2012) These genera are distinguished on the basis of symmetry about the transapical axis, structure of the striae and position of the raphe either on the face of the valve (Peronia) or primarily on the valve mantle (all other genera). The heteropolar nature of the valve is the key feature separating Actinella from the other genera that have the raphe situated largely on or close to the mantle (Sabbe et al, 2001; Ripple & Kociolek, 2013)
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