''Thalassocnus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of semiaquatic

ground sloth Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. The term is used to refer to all extinct sloths because of the large size of the earliest forms discovered, compared to existing tree sloths. The Caribbe ...

s from the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

and

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

within the

subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end subfamily botanical names with "-oideae", and zoologi ...

Thalassocninae. The five species—''T. antiquus'', ''T. natans'', ''T. littoralis'', ''T. carolomartini'', and ''T. yuacensis''—represent a

chronospecies A chronospecies is a species derived from a anagenesis, sequential development pattern that involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. The sequence of alterations eventually produces a populatio ...

, 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 The Pisco Formation is a geologic formation located in Peru, on the southern coastal desert of Ica, Peru, Ica and Arequipa. The approximately thick formation was deposited in the Pisco Basin, spanning an age from the Miocene, Middle Miocene up to ...

of Peru, the Tafna Formation of Argentina, and the

Bahía Inglesa Bahia Inglesa ("English Bay") is a sand beach located near the port of Caldera in Atacama Region, Chile. With a population of 135 (census 2002), it owes its name to the visit of the English privateer A privateer is a private person or ship ...

Coquimbo Coquimbo is a port city, commune and capital of the Elqui Province, located on the Pan-American Highway, in the Coquimbo Region of Chile. Coquimbo is situated in a valley south of La Serena, with which it forms Greater La Serena with more than ...

, and Horcón formations of Chile. Thalassocninae has been placed in both the

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideall ...

Megatheriidae Megatheriidae is a family of extinct ground sloths that lived from approximately 23 mya—11,000 years ago. Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South America ...

and

Nothrotheriidae Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, they ...

. ''Thalassocnus'' evolved several marine adaptations over the course of 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 similarly to the modern day

beaver Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers ar ...

(''Castor'' spp.) and

platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Endemic (ecology), endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypu ...

(''Ornithorhynchus anatinus''). ''Thalassocnus'' probably walked across the

seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...

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 A generalist is a person with a wide array of knowledge on a variety of subjects, useful or not. It may also refer to: Occupations * a physician who provides general health care, as opposed to a medical specialist; see also: ** General pract ...

grazers eating

seaweed Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as ...

and

seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the orde ...

es close to shore, whereas later species specialized on seagrasses farther off the coast. They were probably preyed upon by sharks and

macroraptorial sperm whale Macroraptorial sperm whales were highly predatory whales of the sperm whale superfamily (Physeteroidea) of the Miocene epoch that hunted large marine mammals, including other whales, using their large teeth. They consist of five genera: ''Acrophy ...

s such as ''

Acrophyseter ''Acrophyseter'' is a genus of extinct sperm whales that lived in the Late Miocene off the coast of Peru comprising two species: ''A. deinodon'' and ''A. robustus''. It is part of a group of macroraptorial sperm whales which all shared several ...

''. ''Thalassocnus'' were found in formations with large

marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their reli ...

and shark assemblages.

Taxonomy

Type specimens

''Thalassocnus'' were

s that lived from the

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

to the end of the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Late Huayquerian to Early Uquian in the SALMA classification—and all five species were discovered in different horizons of the

in Peru. ''T antiquus'' was discovered in the Aguada de Lomas Horizon in 7 or 8 million year old

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

; ''T. natans'' (the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

) from the Montemar Horizon lived around 6

million years ago The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds. Usage Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (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 Caldera Basin ( es, Cuenca de Caldera) is a sedimentary basin located in the coast of northern Chile west of Copiapó. The basin has a fill of marine sediments of Late Cenozoic age. With a north-south extension of and an east-west width of the ...

, the

Coquimbo Formation Coquimbo Formation ( es, Formación Coquimbo) is a Miocene to Pleistocene, Middle Pleistocene sedimentary geologic formation, formation located in Coquimbo Region in Norte Chico, Chile, Norte Chico, Chile. The lowermost unit belongs to the lower Mi ...

, and the Horcón Formation in Chile. At 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

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

s 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 from 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 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 A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

; and ''yaucensis'' after the village Yauca which is near where the species was found.

Phylogeny

In 1968,

taxonomist In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...

Robert Hoffstetter Robert Julien Hoffstetter (11 June 1908 in Fargniers – 29 December 1999 in Gennevilliers) was a French taxonomist and herpetologist who was influential in categorizing reptiles. He described the snake families Bolyeriidae and Madtsoiidae Mad ...

placed undescribed sloth remains into the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

, possibly belonging to the now-defunct

Planopsinae Megatheriidae is a family of extinct ground sloths that lived from approximately 23 mya—11,000 years ago. Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South America. ...

, mainly based on similarities with the

ankle bone The talus (; Latin for ankle or ankle bone), talus bone, astragalus (), or ankle bone is one of the group of foot bones known as the tarsus. The tarsus forms the lower part of the ankle joint. It transmits the entire weight of the body from the ...

and femur. Upon species description in 1995, they were moved into the subfamily

Nothrotheriinae Nothrotheriidae is a family of extinct ground sloths that lived from approximately 17.5 mya—10,000 years ago, existing for approximately . Previously placed within the tribe Nothrotheriini or subfamily Nothrotheriinae within Megatheriidae, th ...

. In 2004, this was later elevated to family

, 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 Megatheriinae is a subfamily of the Megatheriidae, an extinct family of ground sloths that lived from the Middle Miocene to the Early Holocene. Classification Within the Megatheriidae there are two (possibly three) subfamilies; the Megatheriinae ...

during the

Friasian The Friasian age is a period of geologic time (16.3–15.5 Ma) within the Early Miocene epoch of the Neogene, used more specifically within the SALMA classification of South America. It follows the Santacrucian and precedes the Colloncuran age. ...

age of the

around 16 mya. However, a 2018 analysis retains Thalassocninae within Nothrotheriidae. Given that the two families in question may be

sisters A sister is a woman or a girl who shares one or more parents with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to refer to ...

, 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 (

), however it is possible ''T. antiquus'' is not the ancestor of ''T. natans''.

Description

''Thalassocnus'' is the only aquatic

xenarthra Xenarthra (; from Ancient Greek ξένος, xénos, "foreign, alien" + ἄρθρον, árthron, "joint") is a major clade of placental mammals native to the Americas. There are 31 living species: the anteaters, tree sloths, and armadillos. Ex ...

n—a group that includes

sloth Sloths are a group of Neotropical xenarthran mammals constituting the suborder Folivora, including the extant arboreal tree sloths and extinct terrestrial ground sloths. Noted for their slowness of movement, tree sloths spend most of their li ...

anteater Anteater is a common name for the four extant mammal species of the suborder Vermilingua (meaning "worm tongue") commonly known for eating ants and termites. The individual species have other names in English and other languages. Together with ...

s, and

armadillo Armadillos (meaning "little armored ones" in Spanish) are New World placental mammals in the order Cingulata. The Chlamyphoridae and Dasypodidae are the only surviving families in the order, which is part of the superorder Xenarthra, along wi ...

s—though the ground sloth '' Eionaletherium'' from the Miocene of Venezuela may have adapted to nearshore life, as well as ''

Ahytherium ''Ahytherium'' is an extinct genus of megalonychid sloth that lived during the Pleistocene of what is now Brazil. It contains a single species, ''A. aureum''. Discovery and taxonomy The almost-complete skeleton of ''Ahytherium'' alongside rema ...

'' from the Pleistocene of

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

. 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 . Based on a femur-to-body-length ratio, the ''T. littoralis'' specimen—probably a female—measured in life, and the ''T. yuacensis'' specimen . The thick and dense bones of younger species (

pachyosteosclerosis Pachyosteosclerosis is a combination of thickening (pachyostosis) and densification (osteosclerosis) of bones. It makes bones more heavy, but also more fragile. The condition often occurs in aquatic vertebrates, especially those living in shallow w ...

) allowed the animal to combat

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...

and sink to the

, similar to modern day

manatee Manatees (family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species ...

s. ''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 The medullary cavity (''medulla'', innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity. Located in the m ...

, which holds the

bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...

, 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 Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean Evolutionary radiation, radiation, t ...

whales with reduced limbs. This developed over a relatively short period of time, around 4 million years. Modern

pilosa The order Pilosa is a clade of xenarthran placental mammals, native to the Americas. It includes the anteaters and sloths (which includes the extinct ground sloths). The name comes from the Latin word for "hairy". Origins and taxonomy The b ...

n 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 Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common ...

). ''Thalassocnus'' may have exhibited

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

, 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

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has b ...

e—which make up the snout. The size difference in the premaxillae are reminiscent of the developed upper lips or

proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...

in males of modern mammals like the

elephant seal Elephant seals are very large, oceangoing earless seals in the genus ''Mirounga''. Both species, the northern elephant seal (''M. angustirostris'') and the southern elephant seal (''M. leonina''), were hunted to the brink of extinction for oil ...

(''Mirounga'' spp.).

Skull

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

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, whe ...

teeth in

ruminant Ruminants (suborder Ruminantia) are ungulate, hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by Enteric fermentation, fermenting it in a specialized stomach prior to digestion, principally ...

s. The later species had stronger lips, indicated by the large size of the

infraorbital foramen In human anatomy, the infraorbital foramen is one of two small holes in the skull's upper jawbone (maxillary bone), located below the eye socket and to the left and right of the nose. Both holes are used for blood vessels and nerves. In anatomica ...

which supplies blood vessels, and, like modern day

grazer Grazer may refer to: *grazer, an animal that grazes *Grazer, a native or inhabitant of Graz *GRAZER, the shoegaze band GRAZER Astronomy *Earth Grazer, Earth-grazing fireball that enters the Earth's atmosphere and leaves again *Mercury grazer, ast ...

s, probably had horny pads on the lips. Like in other grazers, the snout had a square shape as opposed to the triangular shape in

browsers Browse, browser or browsing may refer to: Programs * Web browser, a program used to access the World Wide Web *Code browser, a program for navigating source code * File browser or file manager, a program used to manage files and related objects * ...

. The nostrils moved from the front of the snout to the top of the snout, similar to seals. In later species, to adapt to feeding underwater, the

soft palate The soft palate (also known as the velum, palatal velum, or muscular palate) is, in mammals, the soft tissue constituting the back of the roof of the mouth. The soft palate is part of the palate of the mouth; the other part is the hard palate. ...

of the mouth separating the

trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...

from the

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

was more developed, and the internal nostrils between the

nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal c ...

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 In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

on the skull was probably the main muscle used for biting down. The later species had a more powerful bite to better grasp

. The pterygoid muscle in later species was larger to adapt to a grinding motion rather than a cutting motion while chewing. The latest species, T. carolomartini and T. yaucensis show some evidence of having a short trunk similar to tapirs and elephant seals. ''Thalassocnus'' had a

hypsodont Hypsodont is a pattern of dentition with high-crowned teeth and enamel extending past the gum line, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritt ...

dentition pattern with high

tooth crown In dentistry, crown refers to the anatomical area of teeth, usually covered by enamel. The crown is usually visible in the mouth after developing below the gingiva The gums or gingiva (plural: ''gingivae'') consist of the mucosal tiss ...

s and the

tooth enamel Tooth enamel is one of the four major Tissue (biology), tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the Crown (tooth), crown. The ...

extending beyond the

gums The gums or gingiva (plural: ''gingivae'') consist of the mucosal tissue that lies over the mandible and maxilla inside the mouth. Gum health and disease can have an effect on general health. Structure The gums are part of the soft tissue linin ...

. ''Thalassocnus'' lacked

canine teeth In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened howeve ...

, and had four upper and three lower

molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

on either side of the mouth. Similar to other sloths, the teeth had an outer coating of durodentine, a bonelike version of

dentine Dentin () (American English) or dentine ( or ) (British English) ( la, substantia eburnea) is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by ena ...

, and had softer vasodentine inside, a form of dentine that allows blood flow. The teeth were prism-shaped with a circular

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

, 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'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Pleistocene. It is best known for the elephant-sized type species ' ...

''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.

Vertebrae

''Thalassocnus'' had 7 neck vertebrae; 17

thoracic vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae and they are intermediate in size between the cervical ...

e, compared to the 18 in other megatheriid sloths; 3

lumbar vertebra The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse p ...

e, like other ground sloths; and 24 tail vertebrae, as opposed to other ground sloths which have less than 20. The

vertebral centra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

segments progressively became shorter in length, making the spine more stable, probably an adaptation for digging efficiency. In later species, the spinous processes which jut upwards from the vertebrae are markedly taller in the thoracic vertebrae than the neck vertebrae, as opposed to other sloths where they are around the same height. The small

neck vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In ...

e show they had weak neck muscles, as an aquatic creature does not need to hold its head up, and the neck probably faced downwards while at rest. However, the

atlanto-occipital joint The atlanto-occipital joint (''Capsula articularis atlantooccipitalis'') is an articulation between the atlas bone and the occipital bone. It consists of a pair of condyloid joints. It is a synovial joint. Structure The atlanto-occipital joint ...

, which controls neck movement, was stronger than it is in other sloths, which was probably an adaptation for bottom feeding to keep the head in a fixed position. Like in whales, the head could align directly with the spine. 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. The structure of the

tail vertebrae The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

indicates strong musculature and is similar to the

(''Castor'' spp.) and

(''Ornithorhynchus anatinus''), which use their tails for balance and diving rather than propulsion while swimming. The length of the tail, proportionally longer than other ground sloths, was probably a diving adaptation similar to modern day

cormorants Phalacrocoracidae is a family of approximately 40 species of aquatic birds commonly known as cormorants and shags. Several different classifications of the family have been proposed, but in 2021 the IOC adopted a consensus taxonomy of seven gen ...

(''Phalacrocorax'' spp.) which use their long tails to provide downward

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...

to resist buoyancy.

Limbs

Indicated by the large fossae, or depressions, on the

shoulder blade The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either ...

elbow The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the media ...

, and wrist, the later ''Thalassocnus'' species had strong arm muscles. These, and the relatively shorter arms, were probably adaptations for digging. ''Thalassocnus'' had five claws. The decreasing width of the legs in later species and the reduction of the

iliac crest The crest of the ilium (or iliac crest) is the superior border of the wing of ilium and the superiolateral margin of the greater pelvis. Structure The iliac crest stretches posteriorly from the anterior superior iliac spine (ASIS) to the posterior ...

s of the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

indicate less reliance on them for support—relying more on buoyancy—and the animal probably preferred to sit semi-submerged in the water while resting. The legs also became more flexible, with later species able to, at maximum extension, have the femora reach a horizontal position parallel to the torso. The femur and

kneecap The patella, also known as the kneecap, is a flat, rounded triangular bone which articulates with the femur (thigh bone) and covers and protects the anterior articular surface of the knee joint. The patella is found in many tetrapods, such as m ...

are smaller in ''T. yuacensis'' than in ''T. natans''. Unlike other ground sloths which put a lot of

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

on their hind limbs for locomotion—specifically from standing on two legs (

bipedalism Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' a ...

)—the leg bones of ''Thalassocnus'' are slender. Bipedalism also led to shorter

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

e in ground sloths; the opposite is seen in ''Thalassocnus'' where the tibiae and femora are about the same length. The earlier species, like other sloths, bore their weight on the sides of their feet (pedolateral), whereas the later species planted their feet flat (

plantigrade 151px, Portion of a human skeleton, showing plantigrade habit In terrestrial animals, plantigrade locomotion means walking with the toes and metatarsals flat on the ground. It is one of three forms of locomotion adopted by terrestrial mammals. T ...

) to better paddle and walk along the seafloor. The digits decreased in size in later species. The third digit was constantly flexed, perhaps acting like a

crampon A crampon is a traction device that is attached to footwear to improve mobility on snow and ice during ice climbing. Besides ice climbing, crampons are also used for secure travel on snow and ice, such as crossing glaciers, snowfields and ice ...

to anchor itself to the seafloor while digging. Like other ground sloths, the fifth digit was

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

and non-functioning.

Paleobiology

''Thalassocnus'' were nearshore herbivores which likely became aquatic due to the desertification of the land and a lack of terrestrial food. Earlier species were likely

general A general officer is an Officer (armed forces), officer of highest military ranks, high rank in the army, armies, and in some nations' air forces, space forces, and marines or naval infantry. In some usages the term "general officer" refers t ...

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 . ''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 in a manner similar to 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. The later species fed entirely on the seafloor, similar to

sirenia The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...

ns (manatees and

dugongs The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest m ...

) and the extinct

desmostylia The Desmostylia (from Greek δεσμά ''desma'', "bundle", and στῦλος ''stylos'', "pillar") are an extinct order of aquatic mammals that existed from the early Oligocene (Rupelian) to the late Miocene (Tortonian) (). Desmostylians are t ...

ns, becoming specialist bottom feeders of seagrasses. They probably fed on

Zosteraceae Zosteraceae (one of the four seagrasses families, Kubitzki ed. 1998) is a family of marine perennial flowering plants found in temperate and subtropical coastal waters, with the highest diversity located around Korea and Japan. Most seagrasses ...

seagrasses, which are known from the Pisco Formation, specifically the marine eelgrass '' Zostera tasmanica'', which is now only known from Australia. Later ''Thalassocnus'' probably mainly walked across the seafloor instead of swimming through the water. They possessed no adaptations for high-powered swimming, probably

paddling Paddling with regard to watercraft is the act of manually propelling a boat using a paddle. The paddle, which consists of one or two blades joined to a shaft, is also used to steer the vessel. The paddle is not connected to the boat (unlike in r ...

like terrestrial mammals. The later species probably dug up seagrasses to the

roots A root is the part of a plant, generally underground, that anchors the plant body, and absorbs and stores water and nutrients. Root or roots may also refer to: Art, entertainment, and media * ''The Root'' (magazine), an online magazine focusing ...

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. Acrophyseter deinodon skull flipped

''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

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity is ...

e 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. ''Thalassocnus'' may have competed with

dugong The dugong (; ''Dugong dugon'') is a marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest m ...

ine sirenians for seagrasses, although the latter were apparently rare in the area. They, along with the other marine mammals of the Bahía Inglesa Formation, could have been killed by

harmful algal bloom A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are sometimes ...

s. They may have been preyed upon by the

'', and injured individuals vulnerable to shark attacks.

Paleoecology

''Thalassocnus'' fossils have been found in Peru and Chile, an area which has been a desert since the

Middle Miocene The Middle Miocene is a sub-epoch of the Miocene Epoch made up of two stages: the Langhian and Serravallian stages. The Middle Miocene is preceded by the Early Miocene. The sub-epoch lasted from 15.97 ± 0.05 Ma to 11.608 ± 0.005 Ma (million y ...

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

cetotheriid Cetotheriidae is a family of baleen whales (parvorder Mysticeti). The family is known to have existed from the Late Oligocene to the Early Pleistocene before going extinct. Although some phylogenetic studies conducted by recovered the living ...

baleen whales Baleen whales (systematic name Mysticeti), also known as whalebone whales, are a parvorder of carnivorous marine mammals of the infraorder Cetacea (whales, dolphins and porpoises) which use keratinaceous baleen plates (or "whalebone") in their ...

; other whales found are the

rorqual Rorquals () are the largest group of baleen whales, which comprise the family Balaenopteridae, containing ten extant species in three genera. They include the largest animal that has ever lived, the blue whale, which can reach , and the fin wha ...

baleen whale ''

Balaenoptera siberi Balaenoptera siberi is an extinct species of baleen whale from the Late Miocene, described by Pilleri and Pilleri in 1989, based on fossils found in the Pisco Formation of the Pisco Basin in southwestern Peru.Demeré et al., 2005, p.115 Descri ...

'', the

beaked whale Beaked whales (systematic name Ziphiidae) are a family of cetaceans noted as being one of the least known groups of mammals because of their deep-sea habitat and apparent low abundance. Only three or four of the 24 species are reasonably well-k ...

'' Messapicetus gregarius'', the pontoporiid dolphin ''Brachydelphis mazeasi'', some oceanic dolphins, the tusked dolphin ''Odobenocetops'', and the macroraptorial sperm whales ''Acrophyseter'' and ''Livyatan''. Other vertebrates include the Phocidae, seals ''Acrophoca'' and ''Hadrokirus'', sea turtles such as ''Pacifichelys, Pacifichelys urbinai'', the Gavialidae, gavialid ''Piscogavialis'', cormorants, Procellariidae, petrels, and Sula (genus), 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 lamnidae, white sharks, sand sharks, and Otodontidae. Many shark teeth were associated with the extinct Cosmopolitodus hastalis, 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 Sciaenidae, 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 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 (bird), prion seabirds were found here. It also yielded several marine mammals: a rorqual, ''Acrophoca'', the sperm whale family, sperm whale ''Scaldicetus'', and ''Odobenocetops''. The Coquimbo Formation in Tongoy, Tongoy Bay and the Horcón Formation of the Valparaíso Basin also display diverse shark assemblages, including megalodon, the great white shark, ''Carcharodon plicatilis'', the shortfin mako (''Isurus oxyrinchus''), ''Pristiophorus'' sawsharks, the smalltooth sand tiger, ''Carcharhinus'' sharks, the school shark (''Galeorhinus galeus''), the copper shark (''Carcharhinus brachyurus''), and the bluntnose sixgill shark (''Hexanchus griseus''). 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

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

s. This feeding style probably involved bottom-walking, like hippopotamuses and the extinct desmostylians, and digging. Negative buoyancy requires Pachyosteosclerosis, 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.

References

External links

{{Taxonbar, from=Q137429 Prehistoric sloths Prehistoric marine mammals Prehistoric placental genera Miocene xenarthrans Pliocene xenarthrans Piacenzian extinctions Miocene mammals of South America Pliocene mammals of South America Uquian Chapadmalalan Montehermosan Huayquerian Neogene Argentina Neogene Chile Neogene Peru Fossils of Argentina Fossils of Chile Fossils of Peru Pisco Formation Fossil taxa described in 1995