Endoskeleton

Part of a series related to
Biomineralization
General Mineralized tissues Remineralisation Biocrystallization Biointerface Biofilm
Exoskeletons (shells) Arthropod exoskeleton cuticle Brachiopod shell Cephalopod shell cirrate shell cuttlebone gladius Lorica Choanoflagellate lorica Protist shell coccosphere coccolith diatom frustule foraminifera test testate amoebae Seashell echinoderm stereom mollusc shell nacre chiton shell gastropod shell small shelly fauna scaly-foot snail shell estuary shells Sponge spicule Test
Endoskeletons (bones) Vertebrate skeleton Bone mineral Ossification
Teeth, scales, tusks etc Limpet teeth Otolith otolithic membrane Scale microfossils Tusk
Calcification amorphous calcium carbonate marine biogenic calcification calcareous nannofossils Aragonite oolitic aragonite sand aragonite sea Calcite microbial calcite precipitation calcite sea Great Calcite Belt
Silicification biogenic silica siliceous ooze diatomaceous earth
Other forms Bone bed Kerogen alginite oil shale Phosphate phosphorite Pyrena
Related Mineral evolution In soil mineralization immobilization Ballast minerals Magnetofossil Magnetosome Magnetotactic bacteria Magnetoreception Microfossils engrailed gene Druse Cupriavidus metallidurans Biomineralising polychaetes Mineral nutrients Microbial mat Fossilization permineralization petrifaction Burgess Shale preservation
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An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is a structural frame (skeleton) on the inside of an animal, overlaid by soft tissues and usually composed of mineralized tissue.^[1][2] Endoskeletons serve as structural support against gravity and mechanical loads, and provide anchoring attachment sites for skeletal muscles to transmit force and allow movements and locomotion.

Vertebrates and the closely related cephalochordates are the predominant animal clade with endoskeletons (made of mostly bone and sometimes cartilage), although invertebrates such as sponges also have evolved a form of endoskeletons made of diffuse meshworks of calcite/silica structural elements called spicules, and echinoderms have a dermal calcite endoskeleton known as ossicles. Some coleoid cephalopods (squids and cuttlefish) have an internalized vestigial aragonite/calcite-chitin shell known as gladius or cuttlebone, which can serve as muscle attachments but the main function is often to maintain buoyancy rather than to give structural support, and their body shape is largely maintained by hydroskeleton.

Compared to the exoskeletons of many invertebrates, endoskeletons allow much larger overall body sizes for the same skeletal mass, as most soft tissues and organs are positioned outside the skeleton rather than within it. Being more centralized in structure also means more compact volume, making it easier for the circulatory system to perfuse and oxygenate, as well as higher tissue density against stress. The external nature of muscle attachments also allows thicker and more diverse muscle architectures, as well as more versatile range of motions.