Microfossil spiral teeth reveal mesopelagic Cyclothone (bristlemouths) fish evolved during the early Palaeogene greenhouse

Cyclothone (bristlemouths) is considered the most numerically abundant genus of fish in the world and primarily inhabits the mesopelagic and bathypelagic ocean zones. Cyclothone body fossils date to the middle Miocene (approximately 11-16 million years ago (Ma)), indicating a relatively recent evolution of the group, likely linked to global cooling and the expansion of modern, oxygen-poor mesopelagic habitats. However, deep-sea fishes are rarely preserved in the fossil record, leading to imprecise timing of their evolutionary origins. Ichthyoliths, microfossil fish teeth preserved in oceanic sediments, provide a detailed fossil record of deep-sea fishes and allow for precise dating of fossil occurrences, but identifying these isolated teeth to specific taxonomic lineages is challenging. Here, we demonstrate that spiral striations are found on the teeth of extant Cyclothone species but are absent on other taxa in the Order Stomiiformes or within the rest of the fish tree of life, linking this distinct micromorphology to Cyclothone. We found numerous microfossil fish teeth with these distinctive spiral striations from the Southern Ocean, dating back to 55.6 Ma, at International Ocean Discovery Program Site U1553, pushing the earliest known fossil occurrence of Cyclothone to the extreme greenhouse conditions of the Earliest Eocene.