Editing Archelon

{{Short description|Cretaceous marine turtle and the largest turtle ever discovered}}
{{Distinguish|ARCHELON, the Sea Turtle Protection Society of Greece}}
{{Italic title}}
{{Good article}}
{{Use American English|date=September 2020}}
{{Use mdy dates|date=September 2020}}
{{Speciesbox
| fossil_range = [[Campanian]], {{fossil range|80.21|74.21}}<ref name=GeologicTime>{{citation|last1=Ogg|first1=J. G.|last2=Hinnov|first2=L. A.|chapter=Cretaceous|year=2012|pages=793–853|editor-last1=Gradstein|editor-first1=F. M.|editor-last2=Ogg|editor-first2=J. G.|editor-last3=Schmitz|editor-first3=M. D.|editor-last4=Ogg|editor-first4=G. M.|title=The Geologic Time Scale|publisher=Elsevier|location=Oxford|doi=10.1016/B978-0-444-59425-9.00027-5|isbn=978-0-444-59425-9|s2cid=127523816}}</ref><ref name=MartinandStewart>{{cite journal|author1=Martin, L. D.|author2= Stewart, J. D.|title=An ichthyornithiform bird from the Campanian of Canada|year=1981|journal=Canadian Journal of Earth Sciences|volume=19|issue=2|pages=324–327|doi=10.1139/e82-024}}</ref><ref name=hoganson>{{cite journal|first1=J. W.|last1=Hoganson|first2=B.|last2=Woodward|year=2004|title=Skeleton of the Rare Giant Sea Turtle, ''Archelon'', Recovered from the Cretaceous DeGrey Member of the Pierre Shale near Cooperstown, Griggs County, North Dakota|journal=North Dakota Geological Society Newsletter|volume=32|issue=1|pages=1–4|url=https://www.dmr.nd.gov/ndfossil/education/pdf/Archelon.pdf|access-date=December 23, 2018|archive-date=October 18, 2020|archive-url=https://web.archive.org/web/20201018221547/https://www.dmr.nd.gov/ndfossil/education/pdf/Archelon.pdf|url-status=dead}}</ref><ref name=DeGrey>{{cite journal|first1=J. E.|last1=Fox|title=Mollusks from the late Campanian upper DeGrey Formation of the Pierre Shale Group, Missouri River Valley, central South Dakota|journal=Geological Society of America Special Papers|year=2007|volume=427|pages=85–98|doi=10.1130/2007.2427(06)|isbn=978-0-8137-2427-0 |s2cid=133174376|url={{google books|plainurl=yes|id=RVqA3-Ay1wkC|page=85}}}}</ref>
| image = Big fossil turtle.jpg
| image_caption = Mounted cast at the [[University of Manitoba]]
| image_alt = A suspended display
| parent_authority = [[George Reber Wieland|Wieland]], 1896
| genus = Archelon
| species = ischyros
| authority = [[George Reber Wieland|Wieland]], 1896
| display_parents = 2
}}

'''''Archelon''''' is an extinct marine turtle from the [[Late Cretaceous]], and is  the largest turtle ever to have been documented, with the biggest specimen measuring {{cvt|4.6|m|ft}} from head to tail and {{cvt|2.2|-|3.2|MT|ST}} in body mass. It is known only from the [[Pierre Shale]] and has one species, '''''A. ischyros'''''. In the past, the genus also contained ''A. marshii'' and ''A. copei'', though these have been reassigned to ''[[Protostega]]'' and ''[[Kansastega]]'', respectively. The genus was named in 1896 by American paleontologist George Reber Wieland based on a skeleton from South Dakota, who placed it into the extinct [[family (biology)|family]] [[Protostegidae]]. The [[leatherback sea turtle]] (''Dermochelys coriacea'') was once thought to be its closest living relative, but now, Protostegidae is thought to be a completely separate lineage from any living [[sea turtle]].

''Archelon'' had a leathery [[carapace]] instead of the hard shell seen in most sea turtles. The carapace may have featured a row of small ridges, each peaking at {{cvt|2.5|or|5|cm|in|sigfig=1}} in height. It had an especially hooked beak and its jaws were adept at crushing, so it probably ate hard-shelled [[crustaceans]], [[mollusks]], and possibly even [[sponges]], while slowly moving over the seafloor. It also potentially consumed other animals, whilst swimming closer to the surface, like [[jellyfish]], [[squid]], or [[nautiloids]]. However, its beak may have been better-adapted for [[shearing (physics)|shearing]] flesh, with fish being another possible prey choice. With its large and strong foreflippers, ''Archelon'' was likely able to produce powerful strokes necessary for open-ocean travel and, if need be, escape from fellow marine predators. It inhabited the northern [[Western Interior Seaway]], a mild to cool temperate area, dominated by [[plesiosaur]]s, [[hesperornithiform]] seabirds, and [[mosasaur]]s. It may have gone extinct due to the shrinking of the seaway, increased infant mortality rates (in the sea), higher instances of egg and hatchling predation (on land), and a rapidly cooling climate.

==Research history==
[[File:Archelon skeleton.jpg|upright|thumb|left|Holotype (YPM 3000) at the [[Yale Peabody Museum]]|alt=A black and white photo of the turtle skeleton and a man standing next to it for scale. The man appears to be about half the size of the display]]
The [[holotype specimen]], [[Peabody Museum of Natural History|YPM]] 3000, was collected from the [[Late Campanian]]-age [[Pierre Shale]] of [[South Dakota]] along the [[Cheyenne River]] in [[Custer County, South Dakota|Custer County]] by American paleontologist [[George Reber Wieland]] in 1895, and described by him the following year based on a mostly complete skeleton excluding the skull. He named it ''Archelon ischyros'',<ref name=wieland1896>{{cite journal|last=Wieland|first=G. R.|year=1896|title=''Archelon ischyros'': a new gigantic cryptodire testudinate from the Fort Pierre Cretaceous of South Dakota|journal=American Journal of Science|series=4th series|volume=2|issue=12|pages=399–412|url=https://archive.org/details/americanjournal56unkngoog/page/n414|doi=10.2475/ajs.s4-2.12.399|bibcode=1896AmJS....2..399W}}</ref> genus name from the [[Ancient Greek]] {{lang|grc|ἀρχη}}- ({{transliteration|grc|arkhe}}-) 'first/early',{{sfn|Liddell|Scott|1980|pp=106–107}} {{lang|grc|χελώνη}} ({{transliteration|grc|chelone}}) 'turtle',{{sfn|Liddell|Scott|1980|p=781}} and species name from {{lang|grc|ἰσχυρός}} ({{transliteration|grc|ischyros}}) 'mighty' or 'powerful'.{{sfn|Liddell|Scott|1980|p=336}} A second specimen, a skull, was discovered in 1897 in the same region.<ref name=wieland1900/>
[[File:Archelon.png|thumb|1914 restoration by American paleontologist [[Samuel Wendell Williston]]|alt=Pencil drawing of the left-side view on the left and the top-side view on the right, with some fish in the background]]
In 1900, Wieland described a second species, ''A. marshii'', from remains collected in 1898 by American paleontologist [[Othniel Charles Marsh]], to whom the [[species name]] refers, on the basis that the shell underside ([[plastron]]) was thicker and the [[Humerus|humeri]] were straighter.<ref name=wieland1900/> However, in 1909, Wieland reclassified it as ''Protostega marshii''. In 1902, a third, mostly complete specimen was collected also along the Cheyenne River. In the same study, ''Protostega copei'' from [[Kansas]], which was first described by Wieland in 1909 and named in honor of [[Edward Drinker Cope]] who first erected the family Protostegidae,<ref name=wieland1909>{{cite journal|last=Wieland|first=G. R.|year=1909|title=Revision of the Protostegidae|journal=American Journal of Science|series=4th series|volume=27|issue=158|pages=101–130|url={{google books|plainurl=yes|id=8BBHAQAAMAAJ|page=101}}|doi=10.2475/ajs.s4-27.158.101|bibcode=1909AmJS...27..101W}}</ref> was moved to the genus ''Archelon'' as ''A. copei''.<ref name=zangerl>{{cite book|url=https://www.biodiversitylibrary.org/item/25267#page/34|first=R.|last=Zangerl|year=1953|title=The Vertebrate Fauna of the Selma Formation of Alabama. Part III: The Turtles of the Family Protostegidae|series=Fieldiana: Geology Memoirs|volume=3|number=3|publisher=Chicago Natural History Museum|pages=78–79, 128–130}}</ref> In 1998, ''A. copei'' was moved to the new genus (originally named ''Microstega'', but subsequently renamed ''Kansastega'') as ''K. copei''.<ref name=hooks>{{cite journal|first=G. E.|last=Hooks III|year=1998|title=Systematic Revision of the Protostegidae, with a Redescription of ''Calcarichelys gemma'' Zangerl, 1953|journal=Journal of Vertebrate Paleontology|volume=18|issue=1|pages=85–98|doi=10.1080/02724634.1998.10011036|bibcode=1998JVPal..18...85H }}</ref><ref>{{Cite journal |last1=McDavid |first1=S. N. |last2=Hooks |first2=G. E. |year=2023 |title=''Kansastega'', nom. nov., a replacement name for ''Microstega'' Hooks, preoccupied by Meyrick |journal=Journal of Vertebrate Paleontology |volume=43 |issue=1 |at=e2252648 |doi=10.1080/02724634.2023.2252648 |s2cid=261927100 |doi-access=free }}</ref> In 1992, a fourth and the largest specimen to date, nicknamed "Brigitta", was discovered in [[Oglala Lakota County, South Dakota]] and resides in the [[Natural History Museum Vienna]].<ref name=derstler/> In 2002, a fifth specimen, a partial skeleton, was discovered from the Pierre Shale of [[North Dakota]] along the [[Sheyenne River]] near [[Cooperstown, North Dakota|Cooperstown]].<ref name=hoganson/>

==Description==
The holotype measures {{cvt|352|cm|ft|sigfig=3}} from head to tail, with the head measuring {{cvt|60|cm|ft|sigfig=1}}, the [[cervical vertebra|neck]] {{cvt|72|cm|ft|sigfig=2}}, the [[thoracic vertebra]]e {{cvt|135|cm|ft|sigfig=2}}, the [[sacrum]] {{cvt|15|cm|ft|sigfig=1}}, and the [[caudal vertebra|tail]] {{cvt|70|cm|ft}}.<ref name=wieland1896/> The largest specimen, Brigitta, measures around {{cvt|4.6|m|ft|sigfig=2}} from head to tail and {{cvt|13|ft|m|order=flip}} from flipper to flipper,<ref name=hoganson/><ref name=derstler>{{cite journal|last1=Derstler|first1=K.|first2=A. D.|last2=Leitch|first3=P. L.|last3=Larson|first4=C.|last4=Finsley|first5=L.|last5=Hill|year=1993|title=The World's Largest Turtles - The Vienna ''Archelon'' (4.6 m) and the Dallas ''Protostega'' (4.2 m), Upper Cretaceous of South Dakota and Texas|journal=Journal of Vertebrate Paleontology|volume=13|number=suppl. to 3|page=33A|doi=10.1080/02724634.1993.10011533|url=https://www.academia.edu/2394297}}</ref> and, in life, weighed around {{cvt|2.2|-|3.2|MT|ST}}.<ref name=autogenerated2/> Skulls of ''Archelon'' measured at up to {{cvt|100|cm|ft}}.<ref>{{cite book| last1 =Lutz| first1 =Peter L. | author-link =Peter Lutz |first2=John A. |last2=Musick | title =The Biology of Sea Turtles | publisher =CRC Press | year =1996 | url =https://books.google.com/books?id=fhm2yGTBiN8C&q=Archelon&pg=PA10 | isbn =978-0-8493-8422-6| page =10}}</ref>
[[File:The American journal of science (1900) (18155231621).jpg|thumb|left|upright|''Archelon'' had a pronounced beak.|alt=A view of the right side of the skull, the beak is noticeably hooked]]

''Archelon'' had a distinctly elongated and narrow head. It had a defined hooked beak which was probably covered in a sheath in life, reminiscent of the beaks of [[birds of prey]]. However, in the back, the cutting edge of the beak is dull compared to such animals. Much of the length of the head derives from the elongated [[premaxilla]]e–which is the front part of the beak in this animal–and [[maxilla]]e. The [[jugal bone]]s, the cheek bones, due to the elongate head, do not project as far as they do in other turtles. The nostrils are elongated and rest on the top of the skull, slightly posited forward, and are unusually horizontal compared to sea turtles. The [[jugal bone]]s (cheekbones) are rounded as opposed to triangular in sea turtles. The [[articular bone]], which formed the jaw joint, was probably heavily encased in [[cartilage]]. The jaw probably moved in a hammering motion.<ref name=wieland1900>{{cite journal|first=G. R.|last=Wieland|year=1900|title=The Skull, Pelvis, and Probable Relationships of the Huge Turtles of the Genus ''Archelon'' from the Fort Pierre Cretaceous of South Dakota|journal=American Journal of Science|series=4th series|volume=9|issue=52|pages=237–251|doi=10.2475/ajs.s4-9.52.237|bibcode=1900AmJS....9..237W|url=https://archive.org/stream/americanjournal491900newh/#page/n264}}</ref>
[[File:ArchelonCMNHand.jpg|thumb|upright=0.7|Foreflipper of ''Archelon'']]

Five neck vertebrae were recovered from the holotype, and it probably had eight in total in life; they are X-shaped, procoelous–concave on the side towards the head and convex on the other–and their thick frame indicates strong neck muscles. Ten thoracic vertebrae were found, increasing in size until the sixth then rapidly decreasing, and they have little connection with the carapace. The three vertebrae of the sacrum are short and flat. It probably had eighteen tail vertebrae; the first eight to ten (probably in the same area as the carapace) had neural arches, whereas the remaining did not.<ref name=wieland1896/> Its tail likely had a wide range of mobility, and the tail is thought to have been able to bend at nearly a 90° angle horizontally.<ref name=wendell/>

The humeri in the upper arms are proportionally massive, and the [[radius (bone)|radii]] and [[ulna]]e of the forearms are short and compact, indicating the animal had strong flippers in life. The flippers would have had a spread of between {{cvt|16|and|20|ft|cm|order=flip}}, though most likely the more conservative estimate.<ref name=wieland1903/> Stretch marks on the limb bones indicate fast growth,<ref name=scheyer2007/> with similarities to the leatherback sea turtle, the fastest growing turtle known,<ref>{{cite journal|first=A. G. J.|last=Rhonin|year=1985|title=Comparative Chondro-Osseous Development and Growth of Marine Turtles|journal=Copeia|volume=1985|issue=3|page=763|doi=10.2307/1444768|jstor=1444768}}</ref> whose juveniles have an average growth rate of {{cvt|8.5|cm|in}} per year.<ref name=scheyer2007/>

===Carapace===
[[File:Archelon1DB.jpg|thumb|left|Restoration|alt=A bluish-gray turtle with some yellow-green spots on the neck and striations along the back edge of the flippers]]
The carapace comprises on either side eight neuralia–the plates closest to the midline–and nine pleuralia–the plates that connect the midline to the ribs. The plates of the carapace are mostly uniform in dimensions, with the exception of the two pairs of plates corresponding to the eighth thoracic vertebra which are smaller than the others, and the pygal plate closest to the tail which is larger. ''Archelon'' has ten pairs of ribs, and, like the leatherback sea turtle but unlike other sea turtles, the first rib does not meet the first pleural. As in sea turtles, the first rib is noticeably shorter than the second, in this case, three quarters of the length. The second to fifth ribs project at a right angle from the midline, and, in the holotype, each measure {{cvt|100|cm|ft}} in length. A rib increases in thickness in the vertical direction [[distal]]ly, as it gets farther from the midline, and the ribs are relatively larger and more well-developed than those of sea turtles. The second to fifth ribs, in the holotype, originate with a thickness of {{cvt|2.5|cm|in}} and terminate with around {{cvt|4|to|5|cm|in}} in thickness.<ref name=wieland1903/><ref name=wieland1909/>

The neuralia and pleuralia form highly irregular and finger-like [[suture (anatomy)|sutures]] where they meet, and one plate may have lain over the other plate while the bone was still developing and malleable. The neuralia and pleuralia–the bony portions of the carapace–are particularly thin, and the ribs, especially the first rib, and the [[shoulder girdle]] are unusually heavy and may have had to carry extra [[stress (mechanics)|stress]] to compensate, a condition seen in ancient ancestral turtles.<ref name=wieland1903/><ref name=wieland1909/> ''Archelon'' has [[Osteosclerosis#Animals|osteosclerotic]] structures, where the bone is dense and heavy, which probably served as [[ballast]]s in life similar to the limb bones of whales and other open-ocean animals.<ref>{{cite journal|first1=T. M.|last1=Scheyer|first2=I. G.|last2=Danilov|first3=V. B.|last3=Sukhanov|first4=E. V.|last4=Syromyatnikova|year=2014|title=The Shell Bone Histology of Fossil and Extant Marine Turtles Revisited|journal=Biological Journal of the Linnean Society|volume=112|issue=4|pages=701–718|doi=10.1111/bij.12265|doi-access=free}}</ref>

The carapace, in life, probably featured a row of ridges along the midline over the [[chest]] region, perhaps totaling in seven ridges, with each ridge peaking at either {{cvt|2.5|or|5|cm|in|sigfig=1}}.<ref name=wendell/> In the absence of firmly jointed neck and pleural plates, the skin over the carapace was probably thick, strong, and leathery in order to compensate and properly support the shoulder girdle.<ref name=wieland1903>{{cite journal|first=G. R.|last=Wieland|year=1903|title=Notes on the Marine Turtle ''Archelon''|journal=American Journal of Science|series=4th series|volume=15|issue=87|pages=211–216|doi=10.2475/ajs.s4-15.87.211|bibcode=1903AmJS...15..211W|url=https://zenodo.org/record/2504494}}</ref> This leathery carapace is also seen in the leatherback sea turtle. The spongy makeup is similar to the bones seen in open-ocean going vertebrates such as dolphins or [[ichthyosaur]]s, and was probably also an adaptation to reduce overall weight.<ref name=scheyer2007>{{cite journal|url=https://www.researchgate.net/publication/40663090|first1=T. M.|last1=Scheyer|first2=M. R.|last2=Sánchez-Villagra|year=2007|title=Carapace Bone Histology in the Giant Pleurodiran Turtle ''Stupendemys geographicus'': Phylogeny and Function|journal=Acta Palaeontologica Polonica|volume=52|issue=2|pages=137–154}}</ref>

===Plastron===
[[File:Archelon ischyros (ventral view).JPG|thumb|upright|''Archelon'' [[turtle shell#Plastron|Plastron]] at the [[North American Museum of Ancient Life]]|alt=The underside of a suspended skeleton]]
A turtle plastron, its underside, comprises (from head-most to tail-most) the epiplastron, the entoplastron, which is small and wedged in between the former and the hyoplastron; then is the hypoplastron and finally, the xiphiplastron. The plastron, as a whole, is thick,<ref name=wendell/> and measures (in a specimen described in 1898)  {{cvt|210|cm|ft|sigfig=1}} in length.<ref name=wieland1909/> Unlike the carapace, it features striations throughout.<ref name=wieland1898>{{cite journal|last=Wieland|first=G. R.|year=1898|title=The Protostegan Plastron|journal=American Journal of Science|volume=4–5|issue=25|pages=15–20|doi=10.2475/ajs.s4-5.25.15|url=https://archive.org/details/americanjournal27unkngoog/page/n28|bibcode=1898AmJS....5...15W}}</ref>

In protostegids, the epiplastron and entoplastron are fused together, forming a single unit called an "entepiplastron" or a "paraplastron." This entepiplastron is T-shaped, as opposed to the Y-shaped entoplastrons in other turtles. The top edge of the T rounds off, except at the center which features a small projection. The outward side is slightly convex and bends somewhat, away from the body. The two ends of the T flatten out, getting broader and thinner, as they get farther from the center.<ref name=wieland1898/>

A thick, continuous ridge connects the hyoplastron, hypoplastron, and xiphiplastron. The hyoplastron features a large number of spines projecting around the circumference. The hyoplastron is slightly elliptical, and grows thinner as it gets farther from the center, before the spines erupt. The spines grow thick and narrow towards their middle portion. The seven to nine spines projecting towards the head are short and triangular. The six middle spines are long and thin. The last 19 spines are flat. There are no marks indicating contact with the entepiplastron. The hypoplastron is similar to the hyoplastron, except it has more spines, a total of 54.<ref name=wieland1898/> The xiphiplastron is boomerang-shaped, a primitive characteristic in contrast to the straight ones seen in more modern turtles.<ref name=wendell/>

==Classification and evolution==
[[File:Leatherback Sea Turtle (17665415746).jpg|thumb|The [[leatherback sea turtle]] was previously thought to be the closest living relative.|alt=A leatherback sea turtle leaving a beach, possibly after laying eggs.]]
In its original 1896 description, Wieland placed ''Archelon'' into the [[family (biology)|family]] [[Protostegidae]], which included at the time the smaller ''[[Protostega]]'' and ''[[Protosphargis]]'',<ref name=wieland1896/> the latter being nowadays classified in a position closer to the family [[Cheloniidae]].<ref>{{cite journal|first=R.|last=Hirayama|title=Systematic position of ''Protosphargis veronensis'' Capellini, an enigmatic sea turtle from the Late Cretaceous of Italy|year=2005|journal=Journal of Vertebrate Paleontology|volume=25|issue=3|page=70A|url=https://www.researchgate.net/publication/297915862}}</ref> In 1953, Swiss paleontologist [[wikispecies:Rainer Zangerl|Rainer Zangerl]] split Protostegidae into two subfamilies: [[Chelospharginae]] and [[Protosteginae]]; to the former was assigned ''[[Chelosphargis]]'' and ''[[Calcarichelys]]'', and the latter ''Archelon'' and ''Protostega''.<ref name=zangerl/> The [[sister group]] of ''Protostegidae'' had, in the past, been considered to be [[Dermochelyidae]], and thus their closest living relative would have been the dermochelyid [[leatherback sea turtle]] (''Dermochelys coriacea'').<ref name=hooks/> However, [[phylogenetic]] studies conclude that protostegids represent a completely separate, [[basal (phylogenetics)|basal]] (ancient) lineage that originated in the [[Late Jurassic]], removing the family from the [[Superfamily (taxonomy)|superfamily]] ''[[Chelonioidea]]'' (which includes all sea turtles). In this model, ''Archelon'' does not share a marine ancestor with any sea turtle.<ref>{{cite journal|last=Joyce|first=W. G.|year=2007|title=Phylogenetic Relationships of Mesozoic Turtles|journal=Bulletin of the Peabody Museum of Natural History|volume=48|issue=1|page=66|doi=10.3374/0079-032x(2007)48[3:promt]2.0.co;2|s2cid=85998318 |url=http://doc.rero.ch/record/16104/files/PAL_E960.pdf }}</ref><ref>{{cite journal|first=J.|last=Anquetin|year=2011|title=Reassessment of the Phylogenetic Interrelationships of Basal Turtles (Testudinata)|journal=Journal of Systematic Palaeontology|volume=10|issue=1|page=35|doi=10.1080/14772019.2011.558928|s2cid=85295987|url=http://osf.io/dt6yj/}}</ref><ref>{{cite journal|first1=A. D.|last1=Gentry|first2=J. A.|last2=Ebersole|year=2018|title=The First Report of ''Toxochelys latiremis'' Cope, 1873 (Testudines: Panchelonioidea) from the Early Campanian of Alabama, USA|journal=PaleoBios|volume=35|page=1|doi=10.5070/P9351037558|url=http://escholarship.org/uc/item/23r690jk|doi-access=free}}</ref>

{{Cladogram|align=left|title=Protostegidae|caption=''Archelon'' with some protostegids<ref>{{cite journal|first1=I.|last1=Scavezzoni|first2=V.|last2=Fischer|year=2018|title=''Rhinochelys amaberti'' Moret (1935), a Protostegid Turtle from the Early Cretaceous of France|journal=PeerJ|volume=10|issue=6|page=e4594|doi=10.7717/peerj.4594|pmc=5898427|pmid=29666758 |doi-access=free }}</ref>
|cladogram={{clade|style=font-size:100%; line-height:100%;width:500px;
|label1=[[Protostegidae]]
|1={{clade
 |1=''[[Calcarichelys]]''
 |2=''[[Syllomus]]''
 |3={{clade
  |1={{clade
   |1=''[[Ocepechelon]]''
   |2=''[[Alienochelys]]''
   }}
  |2={{clade
   |1=''[[Protostega]]''
   |2='''''Archelon'''''
   }}
  }}
 |4={{clade
  |1=''[[Chelosphargis]]''
  |2={{clade
   |1=''[[Desmatochelys|Desmatochelys lowi]]''
   |2={{clade
    |1=''[[Bouliachelys]]''
    |2={{clade
     |1=''[[Santanachelys]]''
     |2=''[[Desmatochelys padillai]]''
     }}}}}}}}}}}}}}
[[File:Protostega_Gigas.jpg|thumb|''[[Protostega]]'', the [[type genus]] of the Protostegidae]]
{{clear}}

==Paleobiology==
[[File:Eutrephoceras dekayi var. montanensis - Naturhistorisches Museum Nürnberg - Nuremberg, Germany - DSC04178.jpg|thumb|left|''[[Eutrephoceras|Eutrephoceras dekayi]]'' may have been a food source.<ref name=wendell/> Above from the [[Natural History Museum Nuremberg]]|alt=A shiny ammonite shell is a piece of shiny rock]]
''Archelon'' was an [[obligate carnivore]]. The thick plastron indicates the animal probably spent a lot of time on the soft, muddy seafloor, likely a slow-moving [[bottom feeder]]. According to American paleontologist [[Samuel Wendell Williston]], the jaws were adapted for crushing, implying the turtle ate large [[mollusk]]s and [[crustacea]]ns. In 1914, he suggested that the abundant, thin-shelled, bottom-dwelling Cretaceous [[bivalve]]s–some exceeding {{cvt|4|ft|cm|order=flip}} in diameter–would have easily been able to sustain ''Archelon''.<ref name=wendell/> However, these were probably absent in the central Western Interior Seaway by the Early Campanian. Conversely, the beak may have been adapted for [[shearing (physics)|shearing]] flesh.<ref name=everhart>{{cite book|url={{google books|plainurl=yes|id=Ej4rDwAAQBAJ|page=150}}|first=M. J.|last=Everhart|year=2017|title=Oceans of Kansas: a Natural History of the Western Interior Sea|edition=2nd|publisher=Indiana University Press|pages=150–160|isbn=978-0-253-02715-3}}</ref> It might have been able to target larger fish and reptiles,<ref name=wieland1900/> as well as, similar to the leatherback sea turtle, soft-bodied creatures such as [[squid]] and [[jellyfish]].<ref name=hoganson/><ref name=autogenerated2>{{cite web|url=http://www.bhigr.com/pages/info/info_arch.htm|title=The ''Archelon''|publisher=Black Hills Institute of Geological Research, Inc.|access-date=23 December 2018|archive-date=March 12, 2016|archive-url=https://web.archive.org/web/20160312042116/http://www.bhigr.com/pages/info/info_arch.htm|url-status=dead}}</ref> However, it is possible the sharp beak was used only in combat against other ''Archelon''. The nautilus ''[[Eutrephoceras|Eutrephoceras dekayi]]'' was found in great number near an ''Archelon'' specimen, and may have been a potential food source.<ref name=wendell>{{cite book|chapter-url=https://archive.org/stream/waterreptilesofp1914will/waterreptilesofp1914will#page/231|first=S. W.|last=Williston|author-link=Samuel Wendell Williston|year=1914|chapter=Chelonia|title=Water Reptiles of the Past and Present|publisher=University of Chicago Press|pages=231–241|isbn=978-1-104-52662-7}}</ref> ''Archelon'' may have also occasionally scavenged off the surface water.<ref name=everhart/>
[[File:Archelon_ischyros_NHM_Wien.jpg|thumb|"Brigitta," the largest specimen, at the [[Natural History Museum Vienna]]<ref name=derstler/>|alt=Front view of the skeleton]]
''Archelon'' probably had weaker arms, and thus less swimming power, than the leatherback sea turtle, and so did not frequent the open ocean as much, preferring shallower, calmer waters. This is indicated by the similarity of the humerus/arm and hand/arm ratios of it and [[Cheloniidae|cheloniids]], which are known to have poor development of the limbs into flippers and a preference for shallow water.<ref>{{cite journal|first1=I.|last1=Gerheim|first2=P. S. R.|last2=Romano|year=2015|title=The Evolution of the Flippers and Paleoecology of Panchelonioidea (Testudines, Cryptodira)|journal=PeerJ PrePrints|doi=10.7287/peerj.preprints.1039v1|doi-access=free}}</ref> Conversely, the large flipper-to-carapace ratio of protostegids and the similarly large flipper spread, like that of the predatory cheloniid [[loggerhead sea turtle]] (''Caretta caretta''), combined with a broad body, indicate they could have pursued active prey, though they probably could not have sustained high speeds.<ref>{{cite journal|url=https://archive.org/details/osteologyofproto27wiel/page/294|last=Wieland|first=G. R.|year=1906|title=The Osteology of ''Protostega''|journal=Memoirs of the Carnegie Museum|volume=2|issue=7|page=294|doi=10.5962/p.234828 |s2cid=134854045 |doi-access=free}}</ref> Overall, it may have been a moderately-good swimmer, capable of open-ocean travel.<ref name=wendell/>

''Archelon'', like other marine turtles, probably would have had to have come onshore to nest; like other turtles, it probably dug out a hollow in the sand, laid several dozens of eggs, and took no part in child rearing. The right lower flipper of the holotype is missing, and stunted growth of remaining flipper bone indicates this occurred early in life. It may have been the result of attempted predation by a bird while a hatchling and trying to escape to the sea, bitten off by some large predator such as a [[mosasaur]] or a ''[[Xiphactinus]]'', or was crushed off by larger adults while herding on the shore. However, the latter is unlikely as juveniles probably did not frequent the coasts even during breeding season.<ref name=wendell/> Brigitta is estimated to have lived to 100 years, and may have died while partially covered in mud [[Dormancy#Brumation|brumating]]–a state of dormancy–on the ocean floor.<ref name=derstler/><ref name=autogenerated2 /> However, the longstanding belief that marine turtles brumate underwater like freshwater turtles may be incorrect given the high surfacing-frequency needed to prevent drowning.<ref>{{cite journal|first1=S.|last1=Hochscheid|first2=F.|author3-link=Graeme Hays|last2=Bentivegna|first3=G. C.|last3=Hays|year=2005|title=First Records of Dive Durations for a Hibernating Sea Turtle|journal=Biology Letters|volume=1|issue=1|pages=82–86|doi=10.1098/rsbl.2004.0250|pmc=1629053|pmid=17148134}}</ref>

==Paleoecology==
[[File:Archelon BW.jpg|thumb|left|Restoration in environment|alt=A green turtle with white spots, underwater, swimming downwards]]
''Archelon'' inhabited the shallow [[Western Interior Seaway]];<ref name=hoganson/> the muddy, oxygen-depleted seafloor was probably, on average, no more than {{cvt|600|ft|m|order=flip}} below the surface,<ref name=fossilmuseum/> and average water temperature may have been {{cvt|17|C|F}} in the Campanian.<ref name=Petersen2016>{{cite journal|first1=S. V.|last1=Petersen|first2=C.|last2=Tabor|first3=K. C.|last3=Lohmann|first4=C. J.|last4=Poulsen|last5=Meyer|first5=K. W.|last6=Carpenter|first6=S. J.|last7=Erickson|first7=J. M.|last8=Matsunaga|first8=K. K. S.|last9=Smith|first9=S. Y.|last10=Sheldon|first10=N. D.|year=2016|title=Temperature and salinity of the Late Cretaceous Western Interior Seaway|journal=Geology|volume=44|issue=11|pages=903–906|doi=10.1130/G38311.1|bibcode=2016Geo....44..903P|url=https://www.researchgate.net/publication/308036435}}</ref> The Late Cretaceous Dakotas were submerged in the Northern Inland Subprovince, an area characterized by moderate to cool temperatures, with an abundance of [[plesiosaur]]s, [[hesperornithiform]] seabirds, and mosasaurs, particularly ''[[Platecarpus]]''. There is no fossil evidence for vertebrate migration between the northern and southern provinces. Though sharks were generally more common in the southern province,<ref>{{cite journal|first1=E. L.|last1=Nicholls|first2=A. P.|last2=Russell|year=1990|title=Paleobiogeography of the Cretaceous Western Interior Seaway of North America: the Vertebrate Evidence|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=79|issue=1–2|pages=162–166|doi=10.1016/0031-0182(90)90110-S|bibcode=1990PPP....79..149N}}</ref> several sharks are known from the Pierre Shale, including ''[[Squalus]]'', ''[[Squalicorax]]'', ''[[Pseudocorax]]'', and ''[[Cretolamna]]''.<ref name=hoganson/> Other large predatory fish include [[Ichthyodectidae|ichthyodectid]]s such as ''Xiphactinus''.<ref name=fossilmuseum/> The Pierre Shale's invertebrate assemblage includes a variety of mollusks, namely [[ammonite]]s–from the Pierre Shale ''[[Placenticeras|Placenticeras placenta]]'', ''[[Scaphites|Scaphites nodosus]]'', ''[[Didymoceras]]'', and ''[[Baculites|Baculites ovatus]]''–bivalves–such as the giant ''[[Inoceramus]]''–<ref>{{cite thesis|url=https://commons.und.edu/cgi/viewcontent.cgi?article=1037&context=theses|first=K. F.|last=Brinster|year=1970|title=Molluscan Paleontology of the Pierre Shale (Upper Cretaceous), Bowman County, North Dakota|publisher=University of North Dakota|type=MS}}</ref> the squid-like [[Belemnitida|belemnites]],<ref name=fossilmuseum>{{cite web|url=http://www.fossilmuseum.net/fossil-art/maps/seaway/seaway100mya.htm|title=Western Interior Seaway|website=The Virtual Fossil Museum|access-date=25 December 2018}}</ref> and [[nautilus]].<ref name=wendell/>

As the seaway progressively migrated southward, it is possible ''Archelon'' was unable to migrate with it. The increasing threat of egg or hatchling predation by new marine or mammalian species may have led to the extinction of ''Archelon'', and the disappearance of gigantic protostegids seems to have coincided with the increasing size of dermochelyids.<ref name=wieland1909/> Protostegidae is more-or-less absent in [[Maastrichtian]] deposits, the latest Cretaceous, and probably died off due to a cooling trend<ref name=weems>{{cite journal|first=R. E.|last=Weems|year=1988|title=Paleocene turtles from the Aquia and Brightseat Formations, with a Discussion of their Bearing on Sea Turtle Evolution and Phylogeny|journal=Proceedings of the Biological Society of Washington|volume=101|issue=1|page=144|url=https://www.researchgate.net/publication/236103438}}</ref> which other turtles were able to survive due to some [[thermoregulatory]] capabilities.<ref>{{cite journal|first1=A. D.|last1=Gentry|first2=J. F.|last2=Parham|first3=D. J.|last3=Ehret|first4=J. A.|last4=Ebersole|year=2018|title=A New Species of ''Peritresius'' Leidy, 1856 (Testudines: Pan-Cheloniidae) from the Late Cretaceous (Campanian) of Alabama, USA, and the Occurrence of the Genus within the Mississippi Embayment of North America|journal=PLOS ONE|volume=13|issue=4|page=e0195651|pmc=5906092|pmid=29668704|doi=10.1371/journal.pone.0195651|bibcode=2018PLoSO..1395651G|doi-access=free}}</ref> Average water temperature may have decreased to {{cvt|7|or|12|C|F}} depending on estimated CO<sub>2</sub> levels.<ref name=Petersen2016/> However, some Maastrichtian-age Kansas Pierre Shale fossils may have been eroded millions of years ago, and it is possible ''Archelon'' survived well into the Maastrichtian.<ref name=everhart/>

==See also==
*''[[Stupendemys]]''
*''[[Psephophorus]]''
*''[[Atlantochelys]]''
*[[Largest prehistoric animals]]

==References==
{{Reflist|30em}}

===Cited text===
* {{cite book  | last1 = Liddell  | first1 = H. G.  | author-link = Henry Liddell  | last2 = Scott  | first2 = R.  | author-link2 = Robert Scott (philologist)  | year = 1980  | orig-year = 1871  | edition = abridged  | title = A Greek-English Lexicon  | publisher = [[Oxford University Press]]  | location = Oxford, United Kingdom  | title-link = A Greek-English Lexicon  }}

==Further reading==
{{Wikispecies}}
{{Commons category|Archelon}}
* {{cite book|author-link=Oliver Perry Hay|first=O. P.|last=Ray|title=The fossil turtles of North America|publisher=Carnegie Institution of Washington|location=Washington|volume=75|year=1908|url=https://www.biodiversitylibrary.org/item/45900#page/11/mode/1up}}
* {{cite journal|first=G. R.|last=Wieland|title=Notes on the Cretaceous turtles, ''Toxochelys'' and ''Archelon'', with a classification of the marine Testudinata|journal=American Journal of Science|year=1902|series=4|volume=14|issue=80|pages=95–108|doi=10.2475/AJS.S4-14.80.95|s2cid=127385958|bibcode=1902AmJS...14...95W|url=https://www.biodiversitylibrary.org/item/126199#page/115/mode/1up}}

==External links==
* [http://www.oceansofkansas.com/Turtles.html Oceans of Kansas Paleontology]

{{Testudines}}
{{Taxonbar|from=Q632618|from2=Q20717056}}

[[Category:Protostegidae]]
[[Category:Late Cretaceous turtles of North America]]
[[Category:Fossil taxa described in 1896]]
[[Category:Prehistoric turtle genera]]
[[Category:Monotypic prehistoric reptile genera]]