Editing Endoskeleton (section)

=== Chordates ===
{{Skeleton}}
All [[chordate]]s have a [[notochord]], a flexible [[glycoprotein]] rod cross-wrapped by two [[collagen]]-[[elastin]] helices, which their [[body plan]]s develop around as [[embryo]]s. With the exception of the [[subphylum]] [[Tunicata]] (whose members only retain the notochord during [[larva]]l [[biological life cycle|stage]]s and as [[adult]]s are either [[soft-bodied organism|soft-bodied]] or, in the case of [[sea squirt]]s, supported by a [[cellulose]] exoskeleton known as a [[test (biology)|test]]), chordate bodies are developed along an [[axial skeleton|axial]] endoskeleton derived from the notochord. Like many macroscopically [[motile]] [[bilaterian]] animals that need to be capable of sufficient [[animal locomotion|locomotive]] [[propulsion]], chordates evolved specialized [[striated muscle]]s over their endoskeletons, which have serialized [[sarcomere]]s and parallel [[myofibril]]s bundled in [[muscle fascicle|fascicle]]s to both generate greater [[force]] and optimize [[muscle contraction|contractile]] speed.

==== Cephalochordates ====
In the more [[basal (phylogenetics)|basal]] subphylum [[Cephalochordata]] ([[lancelet]]s), the endoskeleton solely consists of a single notochord. Alternating muscle contractions bend the notochord from side to side, which stores and releases [[elastic energy]] like a [[spring (device)|spring]], resulting in a [[body-caudal fin locomotion]] with better energy efficiency, although [[extant taxon|extant]] cephalochordates (only three [[genera]] with 32 [[species]] from the family [[Branchiostomatidae]]) are [[burrowing]] [[filter feeder]]s who mostly remain immobile in the [[substrate (aquatic environment)|substrate]].

==== Vertebrates ====
Chordates in the [[crown group]] subphylum [[Vertebrata]] (i.e. ''vertebrates'', such as [[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s and [[mammal]]s), the endoskeleton is greatly expanded. During [[embryonic development]], the notochord becomes [[body segment|segment]]ally replaced by a much tougher [[vertebral column]] (i.e. the ''spine'') composed of stiffer [[structural element]]s called [[vertebra]]e. Notochord [[vestigiality|remnant]]s are transformed into [[intervertebral disc]]s, which give some [[range of motion]] between the adjacent vertebrae, allowing the overall spinal column to flex and rotate. The vertebrate endoskeleton is made up of two types of [[mineralized tissues]], i.e. [[bone]] and [[cartilage]], with the [[joint]]s reinforced by [[ligament]]s made of [[Type I collagen]]. Unlike the singular axial skeleton of cephalochordates, the vertebrate skeletal elements expand axially, ventrally and laterally to form the [[cranium]], [[rib cage]] and [[appendicular skeleton]], giving vertebrates a much more widened endoskeleton.

Vertebrates also have bulkier, more complexly organized striated muscles called [[skeletal muscle]]s inserted over both the axial and appendicular skeletons, which can transmit significant forces via [[dense connective tissue]] cords/bands called [[tendon]]s and [[aponeuroses]]. In [[terrestrial animal|terrestrial]] vertebrates ([[tetrapod]]s), both the axial and ''especially'' the appendicular endoskeleton (the latter of which [[evolution|evolve]]d into [[limb (anatomy)|limb]]s) have become significantly strengthened to adapt for the added burden of [[gravity]] and [[terrestrial locomotion|locomotion on dry land]], as their bodies' weight is not offset by [[buoyancy]] as in aquatic environments. In some vertebrate species, parts of the endoskeleton become specialized for [[animal flight|flight]] (as [[wing]]s), [[balance (ability)|balance]] (in [[arboreal]] species), [[animal communication|communication]] (as [[animal language|vocalization]]s or [[fish fin|fin]]/[[neural spinal sail|sail]]/[[crest (anatomy)|crest]] [[display (zoology)|display]]), [[hearing]] ([[mammalian]] [[ossicle]]s), [[digestion]] (particularly [[mastication]]) and [[prehensility]] ([[grasping]], [[object manipulation]] and [[fine motor skill|fine motor activities]]).

The combination of a more [[robust]] endoskeleton and a stronger, more versatile [[muscular system]], supported by a [[heart]]-pumped [[closed circulatory system]], a [[myelin]]ated [[nervous system]] with faster [[saltatory conduction]]s (in all [[jawed vertebrate]]s) and [[centralized]] neural control by an highly functional [[brain]], have allowed the vertebrates to achieve much larger body sizes than [[invertebrate]]s while still maintaining responsive [[sensory perception]] and [[motor control]]. As a result, vertebrates have gradually dominated all [[trophic level|high-level]] [[ecological niche|niche]]s in both [[aquatic ecosystem|aquatic]] and [[terrestrial ecosystem]]s since the [[Devonian]] (circa. 420-359&nbsp;[[million years ago|Mya]]).