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, such as dense and heavy bones to counteract buoyancy, the internal nostrils migrating farther into the head to help with breathing while completely submerged, the snout becoming wider and more elongated to consume aquatic plants better, and the head angling 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). Thalassocnus probably walked across the seafloor and dug up food with its claws. They probably could not do high-powered swimming, relying on paddling if necessary. Early Thalassocnus were probably generalist grazers eating seaweed and seagrasses close to shore, whereas later species specialized on seagrasses farther off the coast. They were probably preyed upon by sharks and macroraptorial sperm whales such as Acrophyseter. Thalassocnus were found in formations with large marine mammal and shark assemblages.
Taxonomy
Type specimens
Thalassocnus were ground sloths that lived from the Late Miocene to the end of the Pliocene—Late Huayquerian to Early Uquian in the SALMA classification—and all five species were discovered in different horizons of the Pisco Formation in Peru. T antiquus was discovered in the Aguada de Lomas Horizon in 7 or 8 million year old strata; T. natans (the type species) from the Montemar Horizon lived around 6 million years ago (mya); T. littoralis from the Sud-Sacaco Horizon lived around 5 mya; T. carolomartini from the Sacaco Horizon lived between 3 and 4 mya; and T. yaucensis from the Yuaca Horizon lived 3 to 1.5 mya. Specimens were also found in the Bahía Inglesa Formation, the Coquimbo Formation, and the Horcón Formation in Chile. A total of three species has been identified with certainty in Chilean formations, T. carolomartini, T. natans, T. antiquus while the presence of T. yaucensis is judged likely. In 1995, the genus Thalassocnus was formally described with the species T. natans, with a partial skeleton, MUSM 433, by paleontologists Christian de Muizon and H. Gregory McDonald. T. littoralis was described from a nearly complete skull, MUSM SAS 1615, in 2002. T. carolomartini from a skull, SMNK PAL 3814, and hands, SMNK PAL 3814, was also described in 2002, and the two T. carolomartini specimens may represent one individual. T. antiquus was described by MUSM 228 in 2003 comprising a skull, jaw, and most of the rest of the body, though the latter is badly damaged. T. yuacensis was described in 2004 from a nearly-complete skeleton, MUSM 1034, and skull, MUSM 37.
Etymology The generic epithet Thalassocnus derives from the Greek word thalassa "sea" and Ocnus, an allegorical deity from Greek and Roman mythology that represents the wasting of time, or slothfulness. The species name carolomartini is named in honor of Carlos Martin, the late owner of the Sacaco hacienda and finder of several bones in the Pisco Formation, including the holotype specimen; and yaucensis after the village Yauca which is near where the species was found.
Phylogeny In 1968, taxonomist Robert Hoffstetter placed undescribed sloth remains into the family Megatheriidae, possibly belonging to the now-defunct subfamily Planopsinae, mainly based on similarities with the ankle bone and femur. Upon species description in 1995, they were moved into the subfamily Nothrotheriinae. In 2004, this was later elevated to family Nothrotheriidae, and the sloths were put into the new subfamily Thalassocninae. In 2017, the sloths were moved back to the family Megatheriidae. Thalassocninae may have diverged from Megatheriinae during the Friasian age of the Miocene around 16 mya. However, a 2018 analysis retains Thalassocninae within Nothrotheriidae. Given that the two families in question may be sisters, and that the position of Thalassocninae within either would likely be fairly basal, correct family placement may be difficult. The five species seem to form one direct lineage (chronospecies), however, it is possible T. antiquus is not the ancestor of T. natans.
Description
Size
Thalassocnus is the only aquatic xenarthran—a group that includes sloths, anteaters, and armadillos—though the ground sloth Eionaletherium from the Miocene of Venezuela may have adapted to nearshore life, as well as Ahytherium from the Pleistocene of Brazil. However, Thalassocnus may have also been adapted to a terrestrial lifestyle based on its record in Argentina. Thalassocnus, as time progressed, increased in size. T. natans has the most complete skeleton preserved and measures from snout to tail 2.55 meters (8.4 ft). Based on a femur-to-body-length ratio, the T. littoralis specimen—probably a female—measured 2.1 meters (6.9 ft) in life, and the T. yuacensis specimen 3.3 meters (11 ft).
Skull
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