Thalassocnus

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, 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. Upon species description in 1995, they were moved into the subfamily Nothrotheriinae. 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. In later species, to adapt to feeding underwater, the soft palate of the mouth separating the trachea from the esophagus was more developed, and the internal nostrils between the nasal cavity and the throat were farther inside the head. This also increased the size of the mouth. However, these adaptations also developed in some terrestrial mammals, and so could instead be related to chewing efficiency. The masseter muscle on the skull was probably the main muscle for biting down. The later species had a more powerful bite to better grasp seagrass. The pterygoid muscle in later species was larger to adapt to grinding rather than cutting while chewing. The latest species, T. carolomartini and T. yuacensis show some evidence of having a short trunk similar to tapirs and elephant seals. Thalassocnus had a hypsodont dentition pattern with high tooth crowns and the tooth enamel extending beyond the gums. Thalassocnus lacked canine teeth, and had four upper and three lower molars on either side of the mouth. Similar to other sloths, the teeth had an outer coating of durodentine, a bonelike version of dentine, and had softer vasodentine inside, a form of dentine that allows blood flow. The teeth were prism-shaped with a circular cross section, and the teeth interlocked tightly while chewing in the later species. In earlier species, the chewing pattern sharpened their teeth. The earlier species had more rectangular teeth that had a similar build to the giant ground sloth Megatherium americanum, whereas the later species had squarer and larger teeth. From earlier species to later species, the teeth show a change of function from cutting food to grinding food. Like in whales, the head could align directly with the spine. == Paleobiology ==

Bone density The thick and dense bones of later species (pachyosteosclerosis) allowed the animal to combat buoyancy and sink to the seafloor, similar to modern day manatees. T. antiquus had a bone density comparable to terrestrial ground sloths. In later species, the bone grew to be so compact that the medullary cavity, which holds the bone marrow, was nearly absent in the limbs. Likewise, the limbs made the heaviest contribution to overall skeletal weight. This condition has been seen in ancient archaeocete whales with reduced limbs. This developed over a relatively short period of time, around 4 million years. Modern pilosan sloths and anteaters already have denser bones than most other mammals, so the sloth may have had a predisposition to dense bones and developed them much faster (exaptation). Metabolism Based on δ18Op 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. Foraging like desmostylians (Paleoparadoxia above). Thalassocnus were nearshore herbivores which likely became aquatic due to the desertification of the land and a lack of terrestrial food. Later Thalassocnus probably mainly walked across the seafloor instead of swimming through the water. They possessed no adaptations for high-powered swimming, probably paddling like terrestrial mammals. The later species probably dug up seagrasses to the roots with their claws like the beaver and platypus, though, like sirenians, also used strong lips to rip out grasses. Their diet may have also included buried food. They may have been preyed upon by the macroraptorial sperm whale Acrophyseter, and injured individuals vulnerable to shark attacks. Sexual variation 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.). == Paleoecology ==

Thalassocnus fossils have been found in Peru and Chile, an area which has been a desert since the Middle Miocene. Other vertebrates include the seals Acrophoca and Hadrokirus, sea turtles such as Pacifichelys urbinai, the gavialid Piscogavialis, cormorants, petrels, and boobies (Sula spp.). The formation has a large cartilaginous fish assemblage, mainly belonging to the ground sharks, such as requiem sharks and hammerhead sharks; and, to a lesser extent, mackerel sharks, such as white sharks, sand sharks, and Otodontidae. Many shark teeth were associated with the extinct broad-tooth mako (Cosmopolitodus hastalis) and megalodon (Carcharocles megalodon). Other cartilaginous fish include eagle rays, sawfish, and angelsharks. Most of the bony fish findings represent tunas and croakers. Livyatan and megalodon were probably the apex predators. In Chile, the Bahía Inglesa Formation in the Caldera Basin is known for a large shark population, especially megalodon, the broad-toothed mako and the great white shark (Carcharodon carcharias). Others include the blue shark (Prionace glauca), the smalltooth sand tiger (Odontaspis ferox), Pristiophorus sawsharks, Squatina angelsharks, bullhead sharks (Heterodontus spp.), Myliobatis eagle rays, and elephantfish (Callorhinchus spp.). Pachyptila prion seabirds were found here. It also yielded several marine mammals: a rorqual, Acrophoca, the sperm whale Scaldicetus, and Odobenocetops. Several penguin species are known from all three of these formations, such as ancient banded penguins (Spheniscus spp.). Extinction Thalassocnus went extinct at the end of the Pliocene due to a cooling trend that followed the closing of the Central American Seaway which killed off much of the seagrass of the Pacific South American coast. As seagrass specialists, the later species of Thalassocnus had evolved negative buoyancy to allow them to feed at the sea floor, similar to modern dugongs. This feeding style probably involved bottom-walking, like hippopotamuses and the extinct desmostylians, and digging. Negative buoyancy requires dense bones and a limited layer of blubber, which would make thermoregulation difficult for them in cooler water temperatures, particularly in view of the low metabolic rate of xenarthrans. Thus, Thalassocnus would have been poorly adapted to the changing conditions even if enough seagrass had remained to subsist on. == See also ==