Thalassocnus is an extinct genus of apparently semiaquatic (for the geologically oldest species) or fully aquatic (for the geologically most recent species) marine sloth from the Miocene and Pliocene of South America. Fossils found to date have been from the coast of Peru (Pisco Formation) and Chile (Bahía Inglesa Formation). They were apparently grazers of sea grass and seaweed. The various species of this genus provides the best-documented case of gradual adaptation to a secondarily aquatic lifestyle. This is documented both at the morphological level, such as a progressive flattening of the radius, and at the microanatomical level, which shows a progressive increase in thickness (pachyostosis) and compactness (osteosclerosis) of the long limb bones and ribs, providing ballast. They may have used their powerful claws to anchor themselves to the sea floor to facilitate feeding, similar to the behavior of the marine iguana.
Video Thalassocnus
Taxonomy
Thalassocnus lived from the Late Miocene to the end of the Pliocene, with all five species being discovered in different horizons of the Pisco Formation. T antiquus was discovered in the Aguada de Lomas Horizon in 7 or 8 mya strata; T. natans (the type species) from the Montemar Horizon lived around 6 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 also from the Sacaco Horizon lived 3 to 1.5 mya. A near complete skull is known from only two species, T. natans and T. yaucensis. These species seem to form one direct lineage (chronospecies), however it is possible T. antiquus is not the ancestor of T. natans.
Presumed phylogeny of Thalassocnus, with relationship to several nothrotheriines and a megatheriid. However, a later phylogenetic study conducted by Amson, de Muizon & Gaudin (2017) indicated that Thalassocnus was not a nothrotheriid but rather a member of the family Megatheriidae. The rib density of T. antiquus does not greatly differ from its terrestrial ancestor Nothrotherium, with neither osteosclerotic or amedullar ribs. However, it was observed that in the four recent ancestral lineages, T. natans, T. littoralis, T. carolomartini, and T. yaucensis, there was a general increase in bone density compared to the ancestral relative. A 22% increase in rib bone density occurred between T. antiquus and T. carolomartini. .
Maps Thalassocnus
Description
Thalassocnus were semi-marine sloths that grew to be slightly larger than the average human. The first finding of a Thalassocnus fossil was in Peru, at the Pisco Formation site. It was also here where the different species of Thalassocnus were identified. The most basal species of Thalassocnus did not have the same increased bone density seen in the other species which came after it. The latest species of Thalassocnus had such dense bones especially in the limbs with a density so great it had only also been seen in sirenians. Because of this, it is likely this species spent most of its time completely submerged. As time passed, a gradual thickening of the cortical bone was found in the ribs and limbs of Thalassocnus; an increase in density that helped to counteract the natural buoyancy of an air-breathing mammal. Later species also displayed elongation of the premaxillae and mandible symphysis, creating a long and wide snout, better adapted to consuming water plants. The earlier species of Thalassocnus have numerous striae on their teeth, possibly indicating intake of sand when feeding in shallow water. One of the main characteristics of the forelimb of Thalassocnus is the shortness of the humerus and radius. The late species of the genus are characterized by the development of the pronator ridge of the radius, stoutness of the ulna, widening of the proximal carpal row, and shortening of the metacarpals. The hind limb is characterized by a small iliac wing, a gracile femur with well-formed femoral neck, teardrop shaped patella, long and slender tibia, and triangular tuber calcis. Later species such as T. carolomartini and T. yaucensis, were apparently specialized grazers that fed in deeper water; they display distinct evidence of transverse mandibular movement and lack dental striae.
Thalossocnus is the only aquatic xenarthran-a group that includes sloths, anteaters, and armadillos. The genus progressively became more adapted to aquatic foraging, likewise, T. carolomartini and T. yaucensis are the most suited than the rest of the genus for grazing. For example, the two latest species have internal nostrils positioned further back into the head to facilitate breathing while fully submerged.
Like other nothrotheres, Thalossocnus lacked canine teeth, and had four upper and three lower molars. 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. The teeth were prism- or cylinder-shaped, and the teeth interlocked tightly while chewing in the later species. The positioning of the teeth and the chewing pattern of earlier species sharpened their teeth. The genus progresses from rectangular teeth to more square teeth.
Thalassocnus may have exhibited sexual dimorphism, indicated in the variation of individuals of T. littoralis and based on observed differences between two skulls of T. carolomartini, which show disparity in general size, slenderness of teeth, and slightly shorter premaxillae. These differences are reminiscent of the developed upper lips or proboscis in males of more recent mammals like the saiga antelope and elephant seals.
Adaptation to aquatic life
Because the Peruvian coast was a barren desert during the Neogene, Thalassocnus was once thought to be a beach-dwelling sloth that was obligated to enter the water to feed. However, based on the additional specimens found from different stratigraphic levels, it is now believed that Thalassocnus adapted to ocean life due a marine transgression along Peru's southern coast.
Thalassocnus possessed adaptations to aquatic life, analogues of which can be found in many vertebrates that adapted to a return to living in the sea. One such adaptation is increased bone density. Heavier bones help lower buoyancy and thus conserve energy. The bones of Thalassocnus species are heavier than other mammals', supporting their return to the water. The younger species show increasingly adapted bone structure; the younger species show decreasing medullary lumen diameter, with T. carolomartini almost completely losing the medulla in its bones [3]. Thalassocnus also possessed longer shinbones than their terrestrial relatives. This modification, along with strong forelimbs, must have allowed them to propel themselves with a paddling movement. Their spines also saw significant modifications. Relatively small vertebrae are found behind the head, not allowing for strong muscles to hold the head. In the water the buoyancy combats gravity already, and therefore in an aquatic lifestyle strong neck muscles are unneeded for this purpose. The general osteosclerosis and pachyostosis of bones of Thalassocnus and the position of the axial postcranium suggest frequent dives and a significant amount of time spent at the benthic region, and the strong, stabilized vertebral column was likely related to digging in this area. The extreme hardening of bones indicate that Thalassocnus used bottom-walking as a form of swimming. Even though the structure of the tail vertebrae hint at strong musculature, this is not necessarily evidence that Thalassocnus used their tails to swim. This pattern is also seen in tree-dwelling animals with strong prehensile tails. [II]
Paleobiology
Diet
Thalassocnus were herbivores, as evidenced from both osteological findings and coprolite findings from the related extinct terrestrial sloth Nothrotheriops. The tooth structure of the three early Thalassocnus species, T. antiquus, T. natans, and T. littoralis, is very similar to the type seen in Megatheria, with the molars being of the bilophodont type. This indicates that Megatheria and early thalassocninae had similar diets. Nothrotheriops are closely related to Thalassocnus. Their diet is known to have been herbivorous, as found from their faeces, preserved in the form of coprolites [I]. The foraging behaviour of the older representatives are thought to have involved the gathering of sea grasses and seaweed from shallow coastal waters, whose depth was less than one meter. In addition, the atlas or topmost bone of the vertebra especially of the later species indicates a downturned rostrum similar to dugongs which better suits Thalassocnus for foraging on seagrass. Because of the plentiful striae on their teeth, the species possibly ingested sand along with their food. The younger species, T. carolomartini and T. yaucensis, do not possess these signs of sand-induced wear on the teeth. This corresponds with osteological findings, which indicate that these younger representatives were better adapted to aquatic life than their predecessors. They possibly foraged in deeper waters, in a similar manner to today's manatees. Further changes to the skull structure as the species developed also show a change in adaptation to different food sources in the younger species, from terrestrial to aquatic.
The growth lines of enamel visible in the molars (striae of Retzius) in earlier species indicates they ate sand with their food, indicating they fed in shallow waters or on seaweed and seagrass. T. antiquus, the oldest of the genus, has the most striae, indicating this species did not feed in the water, eating only plants that had washed up onshore. Further, earlier species chewed with the jaws going up and down, whereas later species chewed with the jaws going front to back.
Extinction
Thalassocnus went extinct in the late Pliocene. This period was characterised by a higher temperature than today, as well as a higher sea level (see also: Pliocene). However, when the Isthmus of Panama formed, the Central American Seaway closed and the coastal water of Peru cooled down. Thalassocnus might not have had the chance to adapt to this changing environment, since the change in climate also caused the extinction of sea-grasses and seaweeds in the area. In order to cope with the changing temperatures, Thalassocnus would have needed blubber. However, blubber would counteract the negative buoyancy from the dense bones, and ultimately Thalassocnus would not be able to feed on bottom dwelling seagrass. The low metabolic rate of Thalassocnus made it especially difficult for these species to adapt to cooler temperatures.
Distribution and habitat
All Thalassocnus fossils found to date were found on the Pacific coast of South-America, generally in the same layers as whales, dolphins and seabirds. Most Thalassocnus fossils have been found in Peru, especially in the Pisco Formation. Another site of interest is the Bahía Inglesa Formation in Chile, where the first non-Pisco Thalassocnus fossil was found. Finding this fossil showed that the distribution area of Thalassocnus was 1600 km larger than originally thought. The Chilean fossil more closely resembled the older Thalassocnus species, T. antiquus and T. natans, than the more recent species.
Thalassocnus is described as having aquatic- or semiaquatic habits. Since the middle of the Miocene, what is now the coast of Peru has been mostly desert. Thalassocnus would live on land, but because of the lack of terrestrial plants, find its food in the ocean. Its diet most likely consisted of seaweeds and seagrasses. Changes in bone structure suggest that Thalassocnus used to live in a terrestrial habit and then slowly adapted to an aquatic one. For example, osteosclerosis found in Thalassocnus fossils seems to implicate that Thalassocnus not only swam, but walked as well. This trait would originate in the ancestors of Thalassocnus, the Megatheriidae, who lived on land. Further support for Thalassocnus having an aquatic habit comes from its limb bone proportions. The difference between the limb bone proportions of Thalassocnus and terrestrial sloths is significantly larger than the difference between the limb bone proportions of terrestrial and aquatic rodents. This indicates that, just like rodents, there were both terrestrial sloths and aquatic ones (Thalassocnus).
References
Bibliography
- McDonald, H. G.; Muizon, C. de (July 2002). "The cranial anatomy of Thalassocnus (Xenarthra, Mammalia), a derived nothrothere from the Neogene of the Pisco Formation (Peru)". Journal of Vertebrate Paleontology. Society of Vertebrate Paleontology. 22 (2): 349-365. doi:10.1671/0272-4634(2002)022[0349:TCAOTX]2.0.CO;2.
- William F. Perrin, Bernd Wursig, and J. G.M. Thewissen. Encyclopedia of Marine Mammals, Pg.72.
- Muizon, C. de; McDonald, H. G. (1995-05-18). "An aquatic sloth from the Pliocene of Peru". Nature. Nature Publishing Group. 375 (6528): 224-227. Bibcode:1995Natur.375..224M. doi:10.1038/375224a0.
- Amson, E.; Argot, C.; McDonald, H. G.; Muizon, C. de (2015-02-14). "Osteology and Functional Morphology of the Axial Postcranium of the Marine Sloth Thalassocnus (Mammalia, Tardigrada) with Paleobiological Implications". Journal of Mammalian Evolution. New York: Springer. 22 (4): 473-518. doi:10.1007/s10914-014-9280-7. Retrieved 1 November 2016.
- Thompson, R. S.; van Devender, T. R.; Martin, P. S.; Foppe, T.; Long, A. (13 March 1980). "Shasta Ground Sloth (Nothrotheriops shastense Hoffstetter) at Shelter Cave, New Mexico: Environment, Diet, and Extinction". Quaternary Research. 14: 360-376. Bibcode:1980QuRes..14..360T. doi:10.1016/0033-5894(80)90017-4.
- World Encyclopedia of Dinosaurs & Prehistoric Creatures: The Ultimate Visual Reference To 1000 Dinosaurs And Prehistoric Creatures Of Land, Air And Sea ... And Cretaceous Eras (World Encyclopedia) by Dougal Dixon
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