Coprolite evidence for marine vertebrate migration in the warm Cretaceous Arctic

Vertebrate migrations significantly impact the ecosystems they traverse, yet evidence for migration in the fossil record is difficult to identify. Here we infer the occurrence of seasonally driven marine migration in two exposures of the Upper Cretaceous Kanguk Formation on Devon Island, Nunavut, Canada. Four lines of evidence support the occurrence of migration at these sites: 1) the paleolatitude of Devon Island above the Arctic Circle indicates seasonal extremes in photoperiod, and abundant fossil diatoms in both sediments and coprolites reflect phytoplankton blooms that provided seasonally abundant food sources; 2) the greater abundance of coprolites relative to skeletal fossils suggests temporary vertebrate visitation; 3) coprolite contents and the internal tube-shaped structures we interpret as lumens suggest that at least 45 of the Devon Island coprolites can be attributed to sturgeons, a highly conserved clade with extant members that are predominantly migratory; and 4) paleobiogeographical occurrences of sturgeons and other Cretaceous Devon Island taxa in North America support migratory behaviour. Our analyses suggest sturgeons were migratory visitors that exploited rich food resources supported by seasonal planktonic blooms. This Cretaceous fossil assemblage thus offers rare coprolite evidence that supports the occurrence of migration in the Arctic that likely impacted resource cycling within the Western Interior Seaway.