Thalassocnus fossil map
Where fossils of Thalassocnus have been found, shown on the Earth as it looked about 5 million years ago and on today’s map.
Dots are fossil collections recorded in the Paleobiology Database. The “then” view moves today’s fossil sites back to their positions 5 million years ago (the time slice nearest the median age of the fossils) using the Merdith et al. (2021) plate model; positions are approximate. Open on the timeline map →
| Group | Early mammals |
|---|---|
| Lived | 11.63–2.58 million years ago (Tortonian to Piacenzian) |
| Periods | Neogene |
| Family | Nothrotheriidae |
| Named by | Muizon and McDonald 1995 |
| Fossil records | 30 occurrences at 15 locations (PBDB) |
| Found in | Peru, Chile, Argentina |
| Rock formations | Pisco, Bahía Inglesa, Coquimbo, txn:134630, Tafna |
About Thalassocnus
Thalassocnus is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast. It is monotypic within the subfamily Thalassocninae. The five species—T. antiquus, T. natans, T. littoralis, T. carolomartini, and T. yuacensis—represent a chronospecies, a population gradually adapting to marine life in one direct lineage. They are the only known aquatic sloths, but they may have also been adapted to a terrestrial lifestyle. They have been found in the Pisco Formation of Peru, the Tafna Formation of Argentina, and the Bahía Inglesa, Coquimbo, and Horcón formations of Chile. Thalassocninae has been placed in both the families Megatheriidae and Nothrotheriidae.
Thalassocnus evolved several marine adaptations over 4 million years, developing dense and heavy bones to counteract buoyancy, internal nostrils that migrated farther into the head to facilitate breathing while completely submerged, a wider and more elongated snout to consume aquatic plants, and a head that angled progressively farther and farther downwards to aid in bottom feeding. The long tail was probably used for diving and balance similar to the modern day beaver (Castor spp.) and platypus (Ornithorhynchus anatinus).
Text in “About” is quoted from the Wikipedia article Thalassocnus (revision 1374910372), licensed under CC BY-SA 4.0; introduction excerpted verbatim, formatting removed. Ages, family, authority and fossil records: Paleobiology Database, accessed 23 October 2026. Ages are those of the rock layers the fossils were found in. Wikidata: Q137429.
