Editing Steller's sea cow (section)

==Description==
[[File:Steller's sea cow skull.jpg|thumb|left|The skull of a Steller's sea cow, [[Natural History Museum, London|Natural History Museum of London]]|alt=The skull has a hole on the snout and large eye sockets on either side and flattens out on the top; no teeth are visible.]]
Steller's sea cows are reported to have grown to {{convert|8|to|9|m|ft|abbr=on|sp=us}} long as adults, much larger than [[extant species|extant]] [[sirenia]]ns.<ref name=marsh2011>{{cite book |last1=Marsh |first1=Helene |author-link1=Helene Marsh |last2=O'Shea |first2=Thomas J. |last3=Reynolds III |first3=John E. |year=2011 |title=Ecology and Conservation of the Sirenia: Dugongs and Manatees |chapter=Steller's sea cow: discovery, biology and exploitation of a relict giant sirenian |publisher=[[Cambridge University Press]] |location=New York, New York |isbn=978-0-521-88828-8 |oclc=778803577 |pages=18–35 |chapter-url={{Google books|plainurl=yes|id=7pQgAwAAQBAJ|page=front}}}}</ref> In 1987, a rather complete skeleton was found on Bering Island measuring {{cvt|3|m}}.<ref>{{cite web|url=https://www.atlasobscura.com/articles/found-skeleton-extinct-stellers-sea-cow-commander-islands-bering|title=Found: The Massive Skeleton of a Steller's Sea Cow|date=17 November 2017|access-date=6 December 2017}}</ref><ref>{{cite web|url= http://www.amiq.org/galleries/seacow/seacow.html|title= Steller's sea cow – Sunken flagship of the Bering Sea... – The AMIQ Institute|access-date=6 December 2017}}</ref> In 2017, another such skeleton was found on Bering Island measuring {{cvt|5.2|m}}, and in life probably about {{cvt|6|m}}.<ref name=":0">{{cite web|url=http://komandorsky.ru/en/might-be-miracle.html|title=Skeleton of Ancient Sea Cow Found on Bering Island|publisher=The Commander Islands Nature and Biosphere Reserve Named Marakov S.V.|access-date=3 December 2017}}</ref> [[Georg Wilhelm Steller|Georg Steller]]'s writings contain two contradictory estimates of weight: {{convert|4|and|24.3|MT|abbr=on|ST}}. The true value is estimated to fall between these figures, at about {{convert|8|-|10|MT|abbr=on|ST}}.<ref>{{cite journal |last=Scheffer |first=Victor B. |author-link=Victor Blanchard Scheffer |date=November 1972 |title=The Weight of the Steller Sea Cow |journal=[[Journal of Mammalogy]] |doi=10.2307/1379236 |jstor=1379236 |volume=53 |number=4 |pages=912–914 }}</ref> This size made the sea cow one of the largest mammals of the [[Holocene]] [[epoch (geology)|epoch]], along with baleen whales and some few toothed whales,<ref name=turvey2006/> and was likely an adaptation to reduce its [[Bergmann's rule|surface-area to volume ratio]] and conserve heat.<ref name=whitmore/>

Unlike other sirenians, Steller's sea cow was positively [[buoyancy|buoyant]], meaning that it was unable to submerge completely. It had a very thick [[epidermis|outer skin]], {{convert|2.5|cm|in|0|abbr=on}}, to prevent injury from sharp rocks and ice and possibly to prevent unsubmerged skin from drying out.<ref name=marsh2011/><ref name=mattioli2006/> The sea cow's [[blubber]] was {{convert|8|-|10|cm|0|abbr=on}} thick, another adaptation to the frigid climate of the Bering Sea.<ref>{{cite book |last=Berta |first=Annalisa |year=2012 |title=Return to the Sea: The Life and Evolutionary Times of Marine Mammals |publisher=[[University of California Press]] |location=Berkeley, California |isbn=978-0-520-27057-2 |oclc=757476446 |page=131 |url={{Google books|plainurl=yes|id=c1YyWgK613oC|page=131}}|quote=Steller described the sea cow's blubber, {{convert|8|–|10|cm|in|sp=us}} thick, as...}}</ref> Its skin was brownish-black, with white patches on some individuals. It was smooth along its back and rough on its sides, with crater-like depressions most likely caused by [[ectoparasite|parasite]]s. This rough texture led to the animal being nicknamed the "bark animal". Hair on its body was sparse, but the insides of the sea cow's [[flipper (anatomy)|flippers]] were covered in bristles.<ref name=forsten/> The fore limbs were roughly {{convert|67|cm|in|abbr=on}} long, and the tail fluke was forked.<ref name=forsten/>

The sea cow's head was small and short in comparison to its huge body. The animal's upper lip was large and broad, extending so far beyond the [[mandible|lower jaw]] that the mouth appeared to be located underneath the skull. Unlike other sirenians, Steller's sea cow was toothless and instead had a dense array of interlacing white bristles on its upper lip. The bristles were about {{convert|1.5|in|cm|order=flip|abbr=on}} in length and were used to tear [[seaweed]] stalks and hold food.<ref name=forsten/> The sea cow also had two [[keratin]]ous plates, called [[ceratodontes]], located on its [[palate]] and [[mandible]], used for chewing.<ref name=andersonanddomning/><ref name="Haeckel1895">{{cite book|last=Haeckel|url=https://books.google.com/books?id=AOi2AAAAIAAJ&pg=PA142|title=Systematische Phylogenie der Wirbelthiere (Vertebrata)|publisher=Georg Reimer|year=1895|edition=1|series=Entwurf einer systematischen Stammesgeschichte|volume=3|location=Berlin|pages=142–143|language=de|access-date=16 July 2021}}</ref> According to Steller, these plates (or "masticatory pads") were held together by [[interdental papilla]]e, a part of the [[gums]], and had many small holes containing [[nerve]]s and [[artery|arteries]].<ref name=forsten/>
[[File:Em - Hydrodamalis gigas model.jpg|thumb|upright=1.3|Model in the [[Natural History Museum, London]]|alt=Side view of a brown-green dugong: It is similar to a manatee in that the head is pointed downwards, the eyes are small, and the body is stocky. The arms are perpendicular to the body and bend backwards toward the tail. There are no fingernails. The tail is knotched, much like a dolphin tail.]]
As with all sirenians, the sea cow's [[rostrum (anatomy)|snout]] pointed downwards, which allowed it to better grasp [[kelp]]. The sea cow's nostrils were roughly {{convert|2|in|cm|order=flip|sigfig=1|abbr=on}} long and wide. In addition to those within its mouth, the sea cow also had stiff bristles {{convert|10|-|12.7|cm|abbr=on}} long protruding from its muzzle.<ref name=whitmore>{{cite journal |last1=Whitmore Jr. |first1=Frank C. |last2=Gard Jr. |first2=L. M. |year=1977 |title=Steller's Sea Cow (''Hydrodamalis gigas'') of Late Pleistocene Age from Amchitka, Aleutian Islands, Alaska |journal=Geological Survey Professional Paper |volume=1036<!--|pages=1–18--> |url=http://pubs.usgs.gov/pp/1036/report.pdf |doi=10.3133/pp1036 |series=Professional Paper |page=16 |bibcode=1977usgs.rept...16W }}</ref><ref name=forsten/> Steller's sea cow had small eyes located halfway between its nostrils and ears with black [[iris (anatomy)|irises]], [[wikt:livid|livid]] eyeballs, and [[canthus|canthi]] which were not externally visible. The animal had no eyelashes, but like other diving creatures such as [[sea otter]]s, Steller's sea cow had a [[nictitating membrane]], which covered its eyes to prevent injury while feeding. The tongue was small and remained in the back of the mouth, unable to reach the masticatory (chewing) pads.<ref name=whitmore/><ref name=forsten>{{cite journal |last1=Forsten |first1=Ann |last2=Youngman |first2=Phillip M. |year=1982 |title=''Hydrodamalis gigas'' |journal=[[Mammalian Species]] |doi=10.2307/3503855 |jstor=3503855 |number=165 |pages=1–3 |url=http://www.science.smith.edu/msi/pdf/i0076-3519-165-01-0001.pdf  |archive-url=https://web.archive.org/web/20161020122625/http://www.science.smith.edu/msi/pdf/i0076-3519-165-01-0001.pdf |archive-date=2016-10-20 }}</ref>

The sea cow's spine is believed to have had seven [[cervical vertebrae|cervical]] (neck), 17 [[thoracic vertebrae|thoracic]], three [[lumbar vertebrae|lumbar]], and 34 caudal (tail) vertebrae. Its ribs were large, with five of 17 pairs making contact with the [[sternum]]; it had no [[clavicle]]s.<ref name=forsten/> As in all sirenians, the [[scapula]] of Steller's sea cow was fan-shaped, being larger on the [[Anatomical terms of location#Anterior and posterior|posterior]] side and narrower towards the neck. The anterior border of the scapula was nearly straight, whereas those of modern sirenians are curved. Like other sirenians, the bones of Steller's sea cow were [[pachyosteosclerosis|pachyosteosclerotic]], meaning they were both bulky ([[pachyostotic]]) and dense ([[osteosclerotic]]).<ref name=whitmore/><ref name=berta2015>{{cite book |last1=Berta |first1=A. |last2=Sumich |first2=J. L. |last3=Kovacs |first3=K. M. |year=2015 |title=Marine Mammals: Evolutionary Biology |edition=3rd |chapter=Sirenians |publisher=[[Academic Press]] |location=Boston, Massachusetts |isbn=978-0-12-397002-2 |oclc=953575838 |page=105 |chapter-url={{Google books|plainurl=yes|id=zcycBAAAQBAJ|page=105}}|quote=The skeleton of sirenians displays both pachyostosis and osteosclerosis...}}</ref> In all collected skeletons of the sea cow, the [[manus (anatomy)|manus]] is missing; since ''[[Dusisiren]]''—the [[sister taxon]] of ''[[Hydrodamalis]]''—had reduced [[phalanges]] (finger bones), Steller's sea cow possibly did not have a manus at all.<ref>{{cite journal |last1=Takahashi |first1=S. |last2=Domning |first2=D. P. |last3=Saito |first3=T. |year=1986 |title=''Dusisiren dewana'', n. sp. (Mammalia: Sirenia), a new ancestor of Steller's sea cow from the upper Miocene of Yamagata Prefecture, northeastern Japan |journal=Transactions and Proceedings of the Paleontological Society of Japan |series=New Series |issue=141 |pages=296–321 |quote=...the phalanges were even more reduced, and possibly even completely lost, in Steller's sea cow. |url=https://www.jstage.jst.go.jp/article/prpsj1951/1986/141/1986_141_296/_pdf |format=PDF}}</ref>

The sea cow's heart was {{convert|35|lb|kg|order=flip|abbr=on}} in weight; its stomach measured {{convert|6|ft|m|order=flip|abbr=on}} long and {{convert|5|ft|m|order=flip|abbr=on}} wide. The full length of its [[intestinal tract]] was about {{convert|151|m|ft|sigfig=2|abbr=on}}, equaling more than 20 times the animal's length. The sea cow had no [[gallbladder]], but did have a wide [[common bile duct]]. Its anus was {{convert|10|cm|ft|sigfig=2|abbr=on}} in width, with its [[feces]] resembling those of horses. The male's penis was {{convert|80|cm|ft|sigfig=2|abbr=on}} long.<ref name=forsten/> Genetic evidence indicates [[convergent evolution]] with other marine mammals of genes related to metabolic and immune function, including [[leptin]] associated with energy [[homeostasis]] and reproductive regulation.<ref name="Sharko2021">{{cite journal |last1=Sharko |first1=F. S. |last2=Boulygina |first2=E. S. |display-authors=et al. |year=2021 |title=Steller's sea cow genome suggests this species began going extinct before the arrival of Paleolithic humans |journal=Nature Communications |volume=12 |issue=2215 |page=2215 |bibcode=2021NatCo..12.2215S |doi=10.1038/s41467-021-22567-5 |pmc=8044168 |pmid=33850161}}</ref>