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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.

Heteropolar eunotioid diatoms (Bacillariophyceae) were common in the North American Arctic during the middle Eocene