Aulacoseira giraffensis (Bacillariophyceae), a new diatom species forming massive populations in an Eocene lake

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.

History of the Giraffe Pipe locality inferred from microfossil remains: a thriving freshwater ecosystem near the Arctic Circle during the warm Eocene

“…A laser-ablation inductively coupled plasma mass spectrometry 206 Pb/ 238 U estimate of zircon crystals from two tephra layers found near the end of the lacustrine deposits yielded an age estimate only slightly younger than the emplacement age (Reyes et al, 2020). These results support the idea that the 72 m of lacustrine deposits were deposited shortly after kimberlite emplacement and over a period of thousands of years (Siver et al, 2019). Although previous studies do not suggest that any discontinuities exist within the lacustrine section of the core, including the transition points indicated in our study, we can't rule out this possibility.…”

“…Remains of sponge spicules (e.g., Fig 3.10) representing the family Spongillidae, and to a lesser extent family Potamolepidae, were found throughout the core, but in low numbers in most samples.

Figure 2.Scanning electron micrographs of untreated rock fragments from four representative stratigraphic intervals of the Giraffe Pipe core dominated by ( 1 ) chrysophyte cysts; ( 2 ) scales of Mallomonas insignis Penard, 1919; ( 3 ) the diatom Aulacoseira giraffensis Siver, Wolfe, and Edlund in Siver et al, 2019; and ( 4 ) Choanocystis heliozoan scales. Scale bars = ( 1, 2 ) 20 μm; ( 3, 4 ) 10 μm.

Figure 3.Scanning electron micrographs of microfossil specimens representing 19 of the most important organisms uncovered in the Giraffe Pipe core.

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“…Scanning electron micrographs of untreated rock fragments from four representative stratigraphic intervals of the Giraffe Pipe core dominated by ( 1 ) chrysophyte cysts; ( 2 ) scales of Mallomonas insignis Penard, 1919; ( 3 ) the diatom Aulacoseira giraffensis Siver, Wolfe, and Edlund in Siver et al, 2019; and ( 4 ) Choanocystis heliozoan scales. Scale bars = ( 1, 2 ) 20 μm; ( 3, 4 ) 10 μm.…”

Remarkably preserved cysts of the extinct synurophyte, Mallomonas ampla, uncovered from a 48 Ma freshwater Eocene lake

“…This region of the Arctic was warm and wet compared to today, with reconstructed mean annual temperature and mean annual precipitation values 17 °C higher and 4 times greater, respectively, than present, and the region supported a warm mixed forest 28 . The entire lacustrine phase of the core contains an extensive array of microfossils which reflect a lake teaming with organisms, including chrysophytes, synurophytes, diatoms, euglyphids, heliozoans and sponges 26,[29][30][31][32][33] . Shifts in the complement of organisms indicate changing lake conditions, including between alkaline and acidic periods, and shallow versus deep phases 32 .…”

“…The entire lacustrine phase of the core contains an extensive array of microfossils which reflect a lake teaming with organisms, including chrysophytes, synurophytes, diatoms, euglyphids, heliozoans and sponges 26,[29][30][31][32][33] . Shifts in the complement of organisms indicate changing lake conditions, including between alkaline and acidic periods, and shallow versus deep phases 32 . The presence of synurophytes, diatoms and sponge lineages found today largely in tropical regions reflect the warm climate of the region 26 .…”