Thalassocnus

Thalassocnus Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓ [1]
T. natans skeleton in its hypothesized swimming pose, Muséum national d'histoire naturelle , Paris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Pilosa
Family:	†Nothrotheriidae
Subfamily:	†Thalassocninae
Genus:	†Thalassocnus de Muizon & McDonald, 1995
Type species
†Thalassocnus natans de Muizon & McDonald, 1995
Other species
†T. antiquus de Muizon et al., 2003 †T. carolomartini McDonald & Muizon, 2002 †T. littoralis McDonald & Muizon, 2002 †T. yuacensis de Muizon et al., 2004

Thalassocnus is an extinct

Thalassocnus were

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

In 1968,

The five species seem to form one direct lineage (chronospecies), however, it is possible T. antiquus is not the ancestor of T. natans.^[14][11][9]

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.^[15] However, Thalassocnus may have also been adapted to a terrestrial lifestyle based on its record in Argentina.^[2] Thalassocnus, as time progressed, increased in size.^[10]

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).^[16]

The later Thalassocnus species had enlarged premaxillae and thus had a more elongated snout. The lower jaw progressively elongated and became more spoon-shaped, possibly mimicking the function of the splayed

The spinous process of the first thoracic vertebra is nearly vertical, but, unlike other sloths, the other vertebrae incline towards the tail; inclination increases in later species, with T. littoralis and T. carolomartini having a 70° inclination as opposed to T. antiquus and T. natans with a 30° inclination. Inclination begins to decrease at the ninth thoracic vertebra. This inclination may have caused less-developed back muscles that would have been needed for high-powered swimming.[16]

The structure of the

T. natans skull and limb bones from Cerro Ballena

Partial T. cf. natans humerus

Indicated by the large

The thick and dense bones of later species (

Based on δ¹⁸O_p values obtained from Thalassocnus bones and teeth, the genus appeared to have had an elevated metabolic rate relative to other marine mammals, a feature likely retained from its terrestrial ancestors.^[21]

Thalassocnus were nearshore herbivores which likely became aquatic due to the desertification of the land and a lack of terrestrial food.[10] Earlier species were likely general grazers that foraged for seagrass and seaweed along the sandy coastline, indicated by scratch marks on the teeth caused by chewing sand, probably foraging in areas with a depth of less than 1 meter (3.3 ft). T. antiquus probably did not enter the water to feed, instead eating plants that washed ashore. Conversely, the later species, T. carolomartini and T. yaucensis, lacking these marks, probably fed in deeper waters like manatees. The earlier species chewed with the jaws going up and down to mash food, whereas later species chewed with the jaws going front to back to grind it.^[14][16]

The later species fed entirely on the seafloor, similar to

Thalassocnus may have used their claws for loosening dirt, cutting vegetation, grasping food, or anchoring themselves to the seafloor. They may have also used the claws to grab onto rocks during strong waves, and there are tibiae and fibulae remains that have been broken and healed, indicating the individual may have been thrown against the rocks of the shore during a storm. This individual may have used its claws to drag itself onto shore.^[14][16]

Thalassocnus may have competed with dugongine sirenians for seagrasses, although the latter were rare in the area.^[10] They, along with the other marine mammals of the Bahía Inglesa Formation (specifically at Cerro Ballena), could have been killed by harmful algal blooms.^[23] They may have been preyed upon by the macroraptorial sperm whale Acrophyseter,^[24] and injured individuals vulnerable to shark attacks.^[14]

Thalassocnus may have exhibited sexual dimorphism, indicated by the variation of individual fossils of T. littoralis and between two skulls of T. carolomartini. The skulls show disparity in general size, slenderness of teeth, and the length of the premaxillae—which make up the snout. The size difference in the premaxillae are reminiscent of the developed upper lips or proboscis in males of modern mammals like the elephant seal (Mirounga spp.).^[14]

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Pisco Formation
Bahía Inglesa Formation
Horcón Formation
Coquimbo Formation

Thalassocnus fossils have been found in Peru and Chile, an area which has been a desert since the Middle Miocene.^[6][10]

The Pisco Formation of Peru is known for its wide assemblage of marine vertebrates. Several whales are known, most commonly the mid-sized

Several penguin species are known from all three of these formations, such as ancient banded penguins (Spheniscus spp.).^[32]

Thalassocnus went extinct at the end of the Pliocene due to a cooling trend that followed the closing of the

• Eionaletherium
• Pezosiren
• Metamynodon
• Desmostylia