Walking sea cow — Pezosiren portelli

Geologic period: Late Early to early Middle Eocene

Age: Late Early or early Middle Eocene.

Locomotion: Four-legged and amphibious; capable of terrestrial weight bearing.

Evidence: Pelvis, limbs, vertebral column and dense ribs directly preserve its anatomy. Amphibious locomotion is inferred from the combined terrestrial and aquatic features.

Why it matters: Pezosiren preserves one of the clearest transitional body plans in sirenian evolution: fully developed weight-bearing limbs combined with clear aquatic adaptations.

Plain Language Identity: A four-legged Eocene stem sirenian showing an early stage in the transition from land-capable mammals to fully aquatic sea cows.

What You Are Looking At: A cast reconstruction based on bones of Pezosiren portelli.

Ecosystem: Lagoonal, estuarine or deltaic Eocene environment of Jamaica.

Source Urls: https://naturalhistory.si.edu/visit/virtual-tour/current-exhibits/deep-time-virtual-tour-fossil-list; https://link.springer.com/article/10.1007/s10914-026-09804-8

Formation: Chapelton Formation, Guys Hill Member

Find Location: Seven Rivers, western Jamaica

Enrichment Method: SAGE curated research pass; title + scientific name + source scene matched; chronology and taxonomy conflicts retained rather than silently corrected; evidence separated from inference

Scientific name: Pezosiren portelli

Evidence / representation: Representative evidence-type visual; not the exact specimen.

Representative evidence-type visual; not the exact specimen.

After the Dinosaurs: Mammals, Ice Ages and the World We Inherited.