Abstract
Background and aims – Diatoms began to inhabit freshwater by at least the Late Cretaceous, becoming well established by the early to middle Eocene. Aulacoseira, an important diatom in numerous ponds, lakes and rivers today, was one of the earliest known genera to colonize freshwater ecosystems. Members of this genus with characteristics familiar to those found on modern species became increasingly more abundant by the Eocene, and continued to thrive throughout the Miocene to the present. We describe a new species of Aulacoseira from an early to middle Eocene site near the Arctic Circle in northern Canada.
 Methods – Twelve samples taken from the Giraffe Pipe core were analysed in this study. Light and scanning electron microscopy were used to document morphological characters. Morphometric measurements were made from 200 specimens per sample (n = 1200), and used to investigate changes in valve size over time.
 Key results – The new species, Aulacoseira giraffensis, has valves with a length:width ratio close to 1, a hyaline valve face, straight mantle striae, a shallow ringleiste, branched linking spines, concave-convex complementarity on adjacent valve faces, and rimoportulae with simple papillae-like structure. The suite of characters, especially the highly branched spines, concave-convex valves and simple rimoportulae, is unique for this species. Large numbers of A. giraffensis specimens were found over a ten-metre section of the core, representing thousands of years. These high concentrations are indicative of abundant, bloom-like, growth.
 Conclusions – The locality represents one of the earliest known records of Aulacoseira dominating a freshwater community. Findings confirm that the morphological body plan for the genus was well established by the Eocene. Although findings indicate evolutionary stasis in morphological structure for A. giraffensis over a time scale of thousands of years, oscillations in valve morphometrics could potentially be used to trace changes in the environment of this ancient Arctic waterbody.
Highlights
Estimates based on molecular clock studies yield a Mesozoic origin for the diatoms, one of the most successful protistan lineages in recent geologic history (Kooistra & Medlin 1996, Sims et al 2006, Medlin 2016)
The Battle Formation stretches across southern Alberta into Saskatch-Figure 3 – SEM images of Aulacoseira giraffensis specimens from the Giraffe Pipe locality depicting intact short filaments or frustules (A–C), valves (D–E), and close-ups of typical linking spines
Spine morphology on the Battle specimens differs from A. giraffensis, both species had valves of similar size and length to width ratios, straight mantle striae, a solid valve face, and a shallow ringleiste
Summary
Estimates based on molecular clock studies yield a Mesozoic origin for the diatoms, one of the most successful protistan lineages in recent geologic history (Kooistra & Medlin 1996, Sims et al 2006, Medlin 2016). Their radiation in the world’s oceans commenced in the early Cretaceous In western North America, freshwater diatomite deposits, many of which represent the remains of large ancient waterbodies, are known from the middle Eocene, and become progressively more abundant by the Miocene (Krebs 1994). Details of the Aulacoseira taxa present in the earlier (e.g. early to middle Eocene) deposits are few
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