The frozen expanse of Alaska today belies a vastly different past, where the Arctic’s freshwater ecosystems during the Cretaceous period harbored a surprising diversity of fish species closely related to those inhabiting northern waters today. Recent groundbreaking research has uncovered multiple previously unknown species of ancient fish dating back roughly 73 million years, revealing that the ecosystems of prehistory were teeming not only with dinosaurs but also with ancestors of modern salmon and pike. Published in the journal Papers in Palaeontology, this study reshapes our understanding of fish evolution and the ecological dynamics of ancient northern latitudes.
At the heart of this discovery lies the Prince Creek Formation, a fossil-rich geological deposit along Alaska’s Colville River on the North Slope. Although famously associated with dinosaur remains, this site has yielded a wealth of smaller vertebrate fossils such as fish bones and teeth, which until recently had been difficult to analyze due to their minuscule size. A team of paleontologists meticulously collected and examined sediment samples from this formation, employing advanced micro-computed tomography (micro-CT) to digitally reconstruct detailed 3D models of these fossilized jaws and other skeletal fragments. These virtual dissections have unveiled an extraordinary glimpse into freshwater ecosystems coexisting with polar dinosaurs.
Among the most significant findings is a newly identified species of salmonid fish named Sivulliusalmo alaskensis. The genus name draws from the Inupiaq language meaning “to be first” and Latin for “salmon,” underscoring the species’ pivotal place in evolutionary history. This species now holds the distinction of being the oldest known salmonid in the fossil record, extending the documented existence of this family by approximately 20 million years. Prior to this, the earliest salmon fossils had been found in southern regions like British Columbia and Washington.
The presence of Sivulliusalmo alaskensis, alongside two newly identified species of pike and the earliest representatives of the carp and minnow group, illustrates that many modern freshwater fish lineages were already adapting to high-latitude environments during the Late Cretaceous. This challenges the traditional perception that such fish families arose and primarily diversified in southern or temperate climates. Instead, it appears that northern polar regions were crucial evolutionary “crucibles,” where these taxa first developed and flourished.
Cretaceous Alaska was positioned much closer to the North Pole than it is currently and experienced markedly different climatic conditions, including elevated average temperatures compared to today’s Arctic. Despite this overall warmth, the region still exhibited extreme seasonal variations in daylight and temperature, factors that would have challenged aquatic life. The discovery that salmonids—a group known for their cold-water affinity—even thrived under these fluctuating polar conditions offers new insights into their ecological resilience and the adaptations that have allowed their descendants to dominate northern freshwater habitats for millions of years.
The detailed fossil examination stems from the painstaking work of collecting and processing sediment samples that often contained bones so minute they could fit atop a pencil eraser. Using microscopy complemented by micro-CT scanning, researchers from institutions including the University of Alaska Fairbanks, Western University in Ontario, and the University of Colorado Boulder digitally reconstructed these diminutive fossils. This non-destructive imaging technique allowed the team to explore internal structures and fine anatomical details that would be impossible to discern in traditional fossil preparations, dramatically enhancing the taxonomic resolution of the finds.
Furthermore, these freshwater fish fossils reveal an ecological tapestry that is intimately connected to the contemporaneous dinosaur fauna inhabiting Cretaceous Alaska. They extend the ecological narrative beyond the charismatic giant reptiles and include foundational elements of the aquatic food web, shedding light on how ancient polar ecosystems functioned. The coexistence of these fish with ancient mammals, birds, and dinosaurs underscores the complexity and diversity of high-latitude biomes during the Mesozoic Era.
Notably, the discovery suggests that the evolutionary trajectory of important fish groups, such as salmonids, was influenced significantly by polar environments. The absence of these fish types in fossil assemblages from contemporaneous lower latitude regions lends weight to the hypothesis that northern freshwater ecosystems were a central geographic origin for these lineages. This has profound implications for biogeographical models tracing the historical dispersal and diversification pathways of aquatic fauna.
The research team, led by Donald Brinkman of the Royal Tyrrell Museum of Palaeontology and senior authors including Patrick Druckenmiller of the University of Alaska Museum of the North, emphasize that their approach involved comprehensive sampling strategies targeting even the smallest fossil remains. This holistic methodology ensures that the full spectrum of vertebrate biodiversity from the Prince Creek Formation contributes to the paleoecological reconstruction, moving beyond the traditional focus on large, conspicuous fossils.
The implications of this study reach beyond paleontology, offering valuable perspectives for evolutionary biology, climatology, and conservation science. Understanding how ancient fish species endured and adapted through diverse and shifting climatic regimes provides potential analogues for predicting how modern freshwater ecosystems might respond to current and future environmental changes, particularly in Arctic and sub-Arctic regions.
In summation, the revelation of Sivulliusalmo alaskensis and its kin enriches our comprehension of ancient polar freshwater ecosystems and the evolutionary history of a globally important fish lineage. These findings underscore the delicate interplay between environmental factors and biodiversity that has shaped life on Earth across geological epochs. As scientific methods continue to advance, further discoveries in Alaska’s fossil record promise to deepen our knowledge, bridging the past and present of Arctic life.
Subject of Research: Animals
Article Title: Fishes from the Upper Cretaceous Prince Creek Formation, North Slope of Alaska, and their palaeobiogeographical significance
News Publication Date: 7-May-2025
Web References:
https://onlinelibrary.wiley.com/doi/full/10.1002/spp2.70014
References:
Brinkman, D. et al. (2025). Fishes from the Upper Cretaceous Prince Creek Formation, North Slope of Alaska, and their palaeobiogeographical significance. Papers in Palaeontology. DOI: 10.1002/spp2.70014
Image Credits:
Credit: UAF photo by Kevin May
Keywords:
Cretaceous fish fossils, Arctic paleontology, salmonid evolution, Prince Creek Formation, micro-computed tomography, high-latitude freshwater ecosystems, paleobiogeography, ancient salmon, fossil jaw reconstruction, North Slope Alaska, Late Cretaceous aquatic fauna
