Editing Turtle (section)

===Shell===
{{main article|Turtle shell}}
[[File:Turtle skeleton cross-section, labelled as infographic.svg|thumb|left|upright=1.3|[[Sagittal section]] of a tortoise skeleton|alt=Photograph of one half of a tortoise skeleton, cut in half vertically showing the vertebrae following curving along the carapace]]

The shell of a turtle is unique among [[vertebrate]]s and serves to protect the animal and provide shelter from the elements.<ref name=Firefly>{{cite book |last1=Iverson|first1=John |last2=Moll|first2=Edward O|year=2002 |contribution=Turtles and tortoises |title=The Firefly Encyclopedia of Reptiles and Amphibians |editor=Halliday, Tim |editor2=Adler, Kraig |publisher=Firefly Books |pages=118–129 |isbn=978-1-55297-613-5 }}</ref>{{sfn|Orenstein|2012|p=22}}<ref>{{Cite web |last=Hutchinson |first=J. Howard |year=1996 |title=Introduction to Testudines: The Turtles |url=http://www.ucmp.berkeley.edu/anapsids/testudines/testudines.html |publisher=University of California Museum of Paleontology |access-date=June 4, 2003 |archive-date=June 29, 2016 |archive-url=https://web.archive.org/web/20160629134148/http://www.ucmp.berkeley.edu/anapsids/testudines/testudines.html |url-status=live }}</ref> It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) [[Turtle shell#Carapace|carapace]] and the flatter, ventral (belly) [[Turtle shell#Plastron|plastron]]. They are connected by lateral (side) extensions of the plastron.<ref name=Firefly/>{{sfn|Orenstein|2012|p=16}}

The [[carapace]] is fused with the vertebrae and ribs while the plastron is formed from bones of the [[shoulder girdle]], [[sternum]], and [[gastralia]] (abdominal ribs).<ref name=Firefly/> During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the [[dermis]] (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as [[fibroblast growth factor]]s that include [[FGF10]].<ref name="Cebra-Thomas Tan 2005">{{cite journal |last1=Cebra-Thomas |first1=Judith |last2=Tan |first2=Fraser |last3=Sistla |first3=Seeta |last4=Estes |first4=Eileen |last5=Bender |first5=Gunes |last6=Kim |first6=Christine |last7=Riccio |first7=Paul |last8=Gilbert |first8=Scott F. |title=How the Turtle Forms its Shell: a Paracrine Hypothesis of Carapace Formation |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=304B |issue=6 |year=2005 |pages=558–569 |issn=1552-5007 |doi=10.1002/jez.b.21059 |pmid=15968684 |bibcode=2005JEZB..304..558C |s2cid=2484583 |url=http://doc.rero.ch/record/15491/files/PAL_E2884.pdf }}</ref> The shoulder girdle in turtles is made up of two bones, the scapula and the [[coracoid]].<ref name=gaffney>{{cite journal |last=Gaffney |first=Eugene S. |year=1990 |title=The Comparative Osteology of the Triassic Turtle ''Proganochelys'' |journal=Bulletin of the American Museum of Natural History |issue=194 |pages=1–263 |hdl=2246/884 |url=http://digitallibrary.amnh.org/handle/2246/884 |access-date=May 16, 2021 |archive-date=May 16, 2021 |archive-url=https://web.archive.org/web/20210516221736/http://digitallibrary.amnh.org/handle/2246/884 |url-status=live |oclc=263164288 }}</ref> Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development.<ref name="Schoch Sues 2019"/>
[[File:How the Turtle Gets its Shell.svg|thumb|upright=1.6|right|[[Embryogenesis|Development]] of the shell. The ribs are growing sideways into the carapacial ridge, seen here as a bud, to support the carapace.<ref name="Cebra-Thomas Tan 2005"/>|alt=Drawing of a section through a turtle embryo showing formation of the shell, with the ribs growing sideways]]
The shell is covered in [[epidermal]] (outer skin) scales known as [[scute]]s that are made of [[keratin]], the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total. Carapace scutes are divided into "marginals" around the margin and "vertebrals" over the vertebral column, though the scute that overlays the neck is called the "cervical". "Pleurals" are present between the marginals and vertebrals.{{sfn|Orenstein|2012|pp=16–17}} Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals. [[Pleurodira|Side-necked turtles]] additionally have "intergular" scutes between the gulars.{{sfn|Orenstein|2012|p=16}}<ref>{{cite book |last=Pritchard |first=Peter C. H. |chapter=Evolution and Structure of the Turtle Shell |editor-last=Wyneken |editor-first=Jeanette |editor-last2=Bels |editor-first2=V. L. |editor-last3=Godfrey |editor-first3=Matthew H. |title=Biology of Turtles |year=2008 |publisher=CRC Press |isbn=978-0-8493-3339-2 |oclc=144570900 |page=56}}</ref> Turtle scutes are usually structured like [[mosaic]] tiles, but some species, like the [[hawksbill sea turtle]], have overlapping scutes on the carapace.{{sfn|Orenstein|2012|p=16}}

The shapes of turtle shells vary with the adaptations of the individual species, and [[sexual dimorphism|sometimes with sex]]. Land-dwelling turtles are more dome-shaped, which appears to make them more resistant to being crushed by large animals. Aquatic turtles have flatter, smoother shells that allow them to cut through the water. Sea turtles in particular have streamlined shells that reduce [[drag (physics)|drag]] and increase stability in the open ocean. Some turtle species have pointy or spiked shells that provide extra [[Anti-predator adaptation|protection from predators]] and [[camouflage]] against the leafy ground. The lumps of a tortoise shell can tilt its body when it gets flipped over, allowing it to flip back. In male tortoises, the tip of the plastron is thickened and used for butting and ramming during combat.{{sfn|Orenstein|2012|pp=22–26}}

Shells vary in flexibility. Some species, such as [[box turtle]]s, lack the lateral extensions and instead have the carapace bones fully fused or [[ankylosis|ankylosed]] together. Several species have hinges on their shells, usually on the plastron, which allow them to expand and contract. [[Softshell turtle]]s have rubbery edges, due to the loss of bones. The leatherback turtle has hardly any bones in its shell, but has thick [[connective tissue]] and an outer layer of leathery skin.{{sfn|Orenstein|2012|pp=22–23, 26–27}}