Fossil specimens of the autotrophic protist Mallomonas asmundiae bearing cysts and attached scales from an Eocene locality

Abstract

Abstract Mallomonas is a species-rich genus within the chrysophycean order Synurales that originated in the lower Cretaceous, and today is a common constituent of planktic communities in a wide array of aquatic environments. Mallomonas cells are motile unicells with a cell covering consisting of siliceous scales and bristles, and the organisms also produce siliceous cysts as part of their life cycle. Because scales and cysts have species-specific designs, their remains in sediments and fossil localities are commonly used to infer past habitat conditions. Since mature cysts are rarely found with the diagnostic scales, the vast majority of cyst morphotypes have not been linked to specific species. Numerous scales with a morphology matching the modern species M. asmundiae were uncovered in an Eocene fossil locality near the Arctic Circle in northern Canada. Many of the scales were still attached to the cysts in their original arrangements. These unique specimens effectively linked the scales to a specific cyst type, and were further used to examine scale variability and the cell covering of this 48 Ma old synurophyte. Although scale structure was found to be variable compared to its modern congener, the arrangement of scales within the cell covering was highly conserved.

Keywords

• Cyst
• Eocene
• Giraffe pipe
• Mallomonas
• Silica scales
• Synurales