Editing Archelon (section)

==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/>