The deep sea, which is typically defined as depths of at least 200 feet, is home to 90% of marine fish life, but the fishes that inhabit it are poorly represented in the fossil record. Whole body fish fossils are rarely preserved in the briny depths, complicating efforts to better understand how life evolved in the Earth’s largest habitat.
In a recent study, Yale researchers introduced a new way to explore the origins of deep-sea fish species that seizes on a plentiful source of evolutionary evidence: tiny, fossilized fish teeth in sediments on the ocean floor.
For the study, the researchers matched distinctive spiral striations found on the teeth of extant species of Cyclothone — the world’s most numerically abundant fish genus — to striated patterns on fossilized teeth, including specimens from the Yale Peabody Museum, dating back more than 55 million years. The striations are unique to Cyclothone, appearing nowhere else in the fish tree of life.
The discovery pushes back the earliest known fossil occurrence of Cyclothone by more than 40 million years from the relatively cool middle Miocene (around 11 to 16 million years ago) to the extreme warmth of the early Eocene.
“Analyzing the surface texture of teeth microfossils provided us a clearer picture of the evolutionary history of Cyclothone,” said lead author Karinne Tennenbaum, a 2026 Yale graduate, who worked on the study as an undergraduate researcher in the lab of Pincelli Hull, associate professor of Earth and Planetary Sciences in Yale’s Faculty of Arts and Sciences and YIBS faculty affiliate. “The discovery that Cyclothone’s ancestors lived during an epoch when the oceans were very warm suggests that the genus may be more climate resilient than previously thought, which could be good news given that global temperatures continue climbing.”
The study is published in the journal Proceedings of the Royal Society B.
For the study, the researchers used high-resolution imaging to analyze the teeth of 236 fish specimen belonging to 63 of 68 existing orders of ray-finned fishes. Among them were 46 specimens representing 36 species of Stomiiformes, the order that includes Cyclothone, small fishes that inhabit deep-sea environments from tropical to temperate regions.
Most of the fishes had smooth teeth, but the teeth of many of the Stomiiformes species were textured. Some were furrowed while others had polygonal striations. The teeth of some species were striated on one side, like a fingerprint. The curved, conical teeth of Cyclothone species bore distinctive spiraling striations that are not found in other Stomiiformes — or elsewhere in the fish tree of life, according to the study.
Most deep-sea fish teeth are similar in shape, making it difficult to connect teeth fossils to specific fish lineages, the researchers explained.
“Microfossil fish teeth are perhaps the most abundant vertebrate fossils on the planet but can be really challenging to identify to specific taxonomic lineage — we can tell you a lot about the tooth but saying ‘this is the fish that made that tooth’ is not so simple,” said Elizabeth Sibert, a former Hutchinson Postdoctoral Fellow at the Yale Institute for Biospheric Studies and senior author of the paper. “This study capitalized on a decade-long database project I’ve been developing which allowed us to positively identify some unique tooth features and make a confident taxonomic identification.”
The mineral composition of fish teeth prevents them from dissolving in the ocean’s corrosive bottom waters. A gram of ocean sediment can contain hundreds of fossilized teeth, which can be precisely dated using techniques for dating marine sediment cores, researchers said.
As with the teeth of extant species, most of the fossilized teeth the researchers studied were smooth. They did, however, find several examples of conical, curved teeth fossils from the Southern Ocean that bore spiraling striations indistinguishable from those on the teeth of living Cyclothone species. They determined that these fossils belonged to ancient Cyclothone or a Cyclothone-like stem lineage that lived at least 55 million years ago.
The discovery aligns the fossil record of Cyclothone with time-calibrated molecular phylogenies — evolutionary trees derived from genetic data — that estimate the genus’ origins to between 35 and 56 million years ago, the researchers explained.
For more information, click here for an article published by Yale News or here for the full study in Proceedings of the Royal Society B.
