Belongs within: Pilosa.
Contains: Mylodonta, Ortotheriinae.
The Folivora, sloths, are a group of herbivorous mammals known from Central and South America. Living members of the group are arboreal though many fossil members were terrestrial and could reach massive sizes.
The swimming sloth
Published 3 April 2008
Sometimes it seems that there is nothing that is so bizarre that some organism does not do it. Life at temperatures that approach boiling? No problem. Male-only species? We can do that. But sometimes something comes along that makes one think, “Diversity of life is all well and good, but that’s just stoopid“. Case in point—Thalassocnus, the marine sloth.
Like the recently covered Odobenocetops, Thalassocnus hailed from what is now the west coast of South America. Indeed, as the Miocene to Pliocene time range of Thalassocnus includes that of Odobenocetops, it is entirely possible, if not probable, that the two of them could have bumped into each other on a regular basis. Though it persisted for some four or five million years with five species described from successive time periods, Thalassocnus seems to have always had a quite restricted distribution. It has only been recorded from the Pisco Formation on the southern coast of Peru (de Muizon et al. 2003).
Though the idea of a marine sloth seems at first quite illogical, sloths are actually quite adept swimmers. The coastline along what is now the Pisco Formation was a barren desert at the time that Thalassocnus inhabited it (de Muizon et al. 2004a), which would have meant that the only food source available to the herbivorous sloths would have been seagrasses and seaweeds growing offshore. Earlier species of Thalassocnus show a high degree of wear on their teeth caused by chewing lots of sand with their food (de Muizon et al. 2003b), suggesting that Thalassocnus started out collecting plant matter washed onshore, or feeding in close enough that swimming would have brought sand into suspension. As time progressed, Thalassocnus would have ventured further and further out. Later species show almost no sign of sand-induced wear, indicating that by that time they were feeding entirely aquatically at some distance from the shore.
In fact, the most fascinating thing about Thalassocnus, other than its sheer existence, is the almost perfect evolutionary series it offers. Each of the five species is known from a different level of the formation, and while cladistic analysis was not entirely able to eliminate other options (de Muizon et al. 2003), the most likely explanation seems to be that only one species inhabited the relatively restricted distribution at a time, with each successive species giving rise to the next anagenetically (for those not in the know, ‘anagenesis’ refers to change within a single lineage, while ‘cladogenesis’ refers to species splitting off from each other). It has been suggested that anagenetic change is more likely to be purely adaptive than cladogenetic change, and Thalassocnus does not disappoint in this regard. Each successive species shows a higher degree of adaptation for an aquatic lifestyle and aquatic feeding, as shown for instance in the figure below from de Muizon et al. (2004a):
This figure shows the lower jaws of all five Thalassocnus species from the oldest (T. antiquus) on the left to the youngest (T. yaucensis) on the right. Note the progressive lengthening and change in shape of the end of the jaw, which de Muizon et al. (2004b) suggest was related to the development of thick muscular lips for collecting seagrasses. Later species also showed adaptations for a grazing rather than browsing feeding mode.
One aquatic adaptation that is notable by its absence is that even the later species of Thalassocnus show no sign of pachyostosis, thickened and heavy bones as found in other aquatic bottom-feeders such as sirenians. This is a curious deficiency in regards to Thalassocnus‘ probable lifestyle. If you have ever tried to sit or lie on the bottom of a pool, you would have noticed that staying submerged requires continuous and quite strenuous effort, as the body tends to float to the surface. Most bottom-feeding aquatic vertebrates evolve denser builds to remove this requirement. De Muizon et al. (2004b) suggest that instead of evolving greater density, Thalassocnus used the enlarged claws it retained from its sloth ancestry to anchor itself to the bottom. They compare it in this regard to the seaweed-grazing marine iguana Amblyrhynchus cristatus, which also has enlarged claws. Such claws would have also aided Thalassocnus (as they do the iguana) in clinging to rocks when entering and exiting the water, even in the presence of heavy surf.
Thalassocnus must have been a remarkable animal to see in life. However, this brings me to question what exactly it did look like in life. Sloths have a reputation for being decidedly shaggy animals, and most extinct ground sloths are reconstructed with similarly long, flowing hair. However, such a pelt seems poorly suited for an aquatic lifestyle, and so unlikely for Thalassocnus. Many aquatic mammals, such as cetaceans and sirenians, have almost entirely lost their hair in favour of an insulating fat layer. Others, such as sea lions and the sea otter Enhydra lutris, have evolved a dense pelt. Personally, I find it easier to imagine a sloth with a pelt, albeit a shorter one than its terrestrial relatives, than an entirely naked sloth, and point out in my support that the pelted marine mammals include a higher proportion of those that divide their lives between land and sea. Still, barring remarkable good fortune, either option must remain entirely hypothetical when dealing with an extinct animal.
Systematics of Folivora
Folivora [Gravigrada, Phyllophaga, Tardigrada]DSD03
| i. s.: Neomylodon lighaiP92
|--EntelopidaeR06
| |--Entelops Ameghino 1887SM93
| `--Delotherium Ameghino 1889SM93
|--Bradypus Linnaeus 1758C57 (see below for synonymy)
| | i. s.: B. boliviensis Gray 1871 [=B. tridactylus boliviensis]C57
| | B. infuscatus Wagler 1831C57
| | |--B. i. infuscatus (see below for synonymy)C57
| | |--B. i. brasiliensis Blainville 1840 (see below for synonymy)C57
| | |--B. i. ephippiger Philippi 1870 [incl. B. ignavus Goldman 1913, B. violeta Thomas 1917]C57
| | |--B. i. gorgon Thomas 1926C57
| | `--B. i. marmoratus (Gray 1849) (see below for synonymy)C57
| |--B. torquatus Illiger 1811FS15, C57 (see below for synonymy)
| `--+--*B. tridactylus Linnaeus 1758C57, FS15, C57 (see below for synonymy)
| `--+--B. pygmaeusFS15
| `--B. variegatus Schinz 1823FS15, C57 [=B. (Scaeopus) variegatusC57]
`--Megalonychidae [Megatheria]FS15
| i. s.: Eucholoeops ingensS32
| SchismotheriumS32
| HapaloidesS32
| Hapalops Ameghino 1887S32, BAA04
| |--H. cadensD07
| |--H. elongatusOB13
| |--H. indifferensD07
| `--H. longicersD07
| Proschismotherium Ameghino 1887SM93
| |--P. oppositumS32
| `--P. scarritti Simpson 1932S32
|--+--MylodontaFS15
| `--+--+--Eremotherium laurillardiFS15
| | `--MegatheriidaeSM93
| | |--Proprepotherium Ameghino 1904SM93
| | `--Megatherium Cuvier 1796FS15, H07
| | |--M. americanumH07
| | |--M. lundiiO05
| | `--M. tarijenseFS15
| `--NothrotheriinaeMM95
| |--PronothrotheriumMM95
| `--+--NothropusMM95
| `--+--+--Nothrotheriops shastenseMM95, FS15
| | `--Nothrotherium maquinenseFS15
| `--Thalassocnus Muizon & McDonald 1995MM95
| |--*T. natans Muizon & McDonald 1995MM95
| |--T. antiquusD07
| |--T. carolomartiniN10
| |--T. littoralisD07
| `--T. yaucensisN10
`--+--+--OrtotheriinaeFS15
| `--Choloepus Illiger 1811FS15, C57 [=Chaelopus Gray in Griffith 1827C57, Cholopus Sclater 1872C57]
| |--*C. didactylus (Linnaeus 1758) (see below for synonymy)C57
| `--C. hoffmanni Peters 1858C57
| |--C. h. hoffmanni [incl. C. didactylus var. columbianus Gray 1871]C57
| |--C. h. agustinus Allen 1913C57
| |--C. h. capitalis Allen 1913 [incl. C. andinus Allen 1913]C57
| |--C. h. florenciae Allen 1913 [incl. C. napensis Lönnberg 1922]C57
| |--C. h. juruanus Lönnberg 1942C57
| `--C. h. pallescens Lönnberg 1928 [incl. C. h. peruvianus Menegaux 1906 (n. d.)]C57
`--+--Megalonyx Jefferson 1799FS15, H07 [Megalonychidae]
| `--M. jeffersoniH07
`--+--+--Diabolotherium nordenskioldiFS15
| `--Acratocnus Anthony 1916FS15, AC98
| |--*A. odontrigonus Anthony 1916AC98
| `--A. yeFS15
`--MegalocninaePC56
|--Neomesocnus Arredondo 1961AC98
| `--*N. brevirrostris Arredondo 1961AC98
|--Megalocnus Leidy 1868 [=Megalochnus Ameghino 1881; incl. Myomorphus Pomel 1868]PC56
| |--*M. rodens Leidy 1868PC56 (see below for synonymy)
| `--M. zileFS15
|--Parocnus Miller 1929AC98
| |--*P. serus Miller 1929AC98
| `--P. browniFS15
`--Mesocnus Matthew 1931AC98
|--*M. browni Matthew 1931AC98
`--M. torrei Matthew 1931AC98Bradypus Linnaeus 1758C57 [incl. Acheus Cuvier 1825C57, Arctopithecus Gray 1849 non Virey 1819C57, Eubradypus Lönnberg 1942C57, Hemibradypus Anthony 1906C57, Ignavus Blumenbach 1779C57, Neobradypus Lönnberg 1942C57, Scaeopus Peters 1864C57; BradypodidaeDSD03]
Bradypus infuscatus brasiliensis Blainville 1840 [=B. braziliensis, B. tridactylus brasiliensis; incl. Arctopithecus blainvilli Gray 1849, B. dorsalis Fitzinger 1871, B. miritibae Lönnberg 1942, B. pallidus Wagner 1844]C57
Bradypus infuscatus infuscatus Wagler 1831 [incl. B. infuscatus var. brachydactylus Wagner 1844, B. marmoratus codajazensis Lönnberg 1942, B. macrodon Thomas 1917, B. speculiger Fitzinger 1871, B. trivittatus Cornalia in Osculati 1850]C57
Bradypus infuscatus marmoratus (Gray 1849) [=Arctopithecus marmoratus; incl. A. problematicus Gray 1849, A. flaccidus var. smithii Gray 1869, B. (Eubradypus) tocantinus Lönnberg 1942, B. unicolor Fitzinger 1871]C57
Bradypus torquatus Illiger 1811FS15, C57 [=B. (*Scaeopus) torquatusC57; incl. B. affinis Gray 1849C57, B. crinitus Gray 1849C57, Hemibradypus mareyi Anthony 1906C57]
*Bradypus tridactylus Linnaeus 1758C57, FS15, C57 [=Arctopithecus tridactylusC57; incl. Acheus ai Lesson 1827C57, B. aiC57, B. cristatus Temminck in Fitzinger 1871C57, B. cuculliger Wagler 1831C57, B. tridactylus guianensis Blainville 1940C57, B. gularis Rüppell 1845C57, Arctopithecus gularisC57]
*Choloepus didactylus (Linnaeus 1758) [=Bradypus didactylus, Cholopus didactylus; incl. Choloepus brasiliensis Fitzinger 1871, B. curi Link 1795, Choloepus guianensis Fitzinger 1871, B. unau Link 1795]C57
*Megalocnus rodens Leidy 1868PC56 [incl. M. casimbae Matthew 1959AC98, M. intermedius Mayo 1969AC98, M. junius Matthew 1959AC98, M. ursulus Matthew 1959AC98]
*Type species of generic name indicated
References
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