Skeleton Ichthyosaur

1 skeleton

1.1 skull
1.2 lower jaws
1.3 teeth
1.4 postcrania

1.4.1 vertebral column
1.4.2 appendicular skeleton

skeleton
skull

the skull of temnodontosaurus platyodon has typical ichthyosaurian shape elongated snout , large eye sockets.

basal ichthyopterygia had elongated triangular skull. ichthyosaurs in narrow sense, snout became pointy. snout formed premaxilla. maxilla behind shorter , excluded external nostril rear branch of premaxilla. accordingly, number of premaxillary teeth high, while maxillary teeth fewer in number or absent. rear top of snout formed nasal bones. derived species have foramen internasale, midline opening separating rear of nasal bones. nasal bone forms top , front rim of bony nostril, placed in front of eye socket. however, triassic species, premaxilla extended @ excludes nasal nostril.

the rear of skull dominated large eye socket, covering major part of rear side surface. in socket, large scleral ring present; circular structure of small, overlapping bone segments protecting eye against water pressure. both in relative , absolute senses, ichthyosaurs have largest eye sockets of known vertebrates. other rear skull elements typically compressed , fused difficult identify. top rear element of skull assumed supratemporal bone, while squamosal , quadratojugal fused. however, in 1968, alfred sherwood romer stated presumed supratemporal in fact squamosal, in 1973 confirmed mcgowan. in 1990, though, john steve massare convinced researchers original identification had been correct 1 after all. supratemporal forms rear rim of supratemporal opening; lower temporal opening @ side lacking. front rim of supratemporal opening typically formed postfrontal; basal utatsusaurus postorbital , squamosal still reach edge. between paired supratemporal openings, skull roof narrow; species have longitudinal crest on attachment jaw muscles. basal ichthyopterygia have parietal eye opening between paired parietal bones. ichthyosaurs proper, opening moves front, first border between parietals , frontals , between frontals, condition shown derived species. postparietal , tabular bones lacking. often, bones of of skull , palate incompletely ossified, apparently having partly remained cartilage. occipital condyle typically convex. stapes, bone transmitting sound waves eardrum middle ear, elongated , not pierced foramen. pterygoid teeth typically lacking.

lower jaws

like snout, lower jaws elongated. however, in species, eurhinosaurus , excalibosaurus, front of snout far protrudes beyond lower jaws. while front of lower jaw typically low, rear depth variable. greater part of lower jaw formed front dentary, tooth-bearing bone. @ inner side dentary covered splenial extends forwards until symphysis, common contact surface both lower jaws grown together. jaw joints not allow horizontal chewing movement: function simple hinges vertically open or close jaws.

teeth

ichthyosaur teeth typically conical. fish-eating species have long , slender tooth crowns recurved. forms specialised in catching larger prey have shorter, broader, , straighter teeth; sometimes, cutting edges present. thalattoarchon, apex predator, had larger teeth formed flattened blades. durophagous species, ate shellfish, have low, convex teeth closely packed. many ichthyosaur dentitions heterodont, combining several tooth shapes, e.g. small teeth in front , larger teeth @ rear. teeth placed in tooth sockets; derived species possess common tooth groove. in latter case, adult individuals become toothless. teeth in tooth sockets fuse jawbone. ichthyosaur teeth, dentine shows prominent vertical wrinkles. durophagous forms have teeth deep vertical grooves , wrinkles in enamel.

postcrania
vertebral column

ichthyosaur vertebra sundance formation (jurassic) of natrona county, wyoming. note characteristic hourglass cross-section. scale in millimetres

basal ichthyopterygia, land-dwelling ancestors, still had vertebrae possessed full set of processes allowed them interlock , articulate, forming vertebral column supporting weight of body. ichthyosaurs aquatic, bodies supported archimedes force exerted water; in other words, buoyant. therefore, vertebral processes had lost of function. ichthyosaurs proper had rear dorsal vertebrae had become disc-shaped, of typical fishes. more derived species, front dorsals became discs. gradually, processes lost, including rib attachment. vertebral bodies became shorter. front , rear sides of discs hollowed out, resulting in so-called amphicoelous condition. transverse cross-section of such vertebra has hourglass shape. morphology unique within amniota , makes discerning ichthyosaur vertebrae of other marine reptiles easy. process kept function spine @ top, serving attachment dorsal muscles. however, spine became simple structure. neural arch, of outgrowth, typically no longer fused vertebral centre.

the neck short, , derived species show reduction in number of cervical vertebrae. short neck positions skull close trunk, in slight oblique elevation it. derived species have reduced number of dorsals, total of presacral vertebrae totalling forty fifty. vertebral column little differentiated. basal ichthyopterygia still have 2 sacral vertebrae, these not fused. triassic forms have transversely flattened tail base high spines undulating tail movement. derived forms have shorter tail characteristic kink @ end; section of wedge-shaped vertebrae, supporting fleshy upper tail fin lobe, forced tail end lower fin lobe.

as derived species no longer have transversal processes on vertebrae—again condition unique in amniota—the parapophyseal , diapophysael rib joints have been reduced flat facets, @ least 1 of located on vertebral body. number of facets can 1 or two; profile can circular or oval. shape differs according position of vertebra within column. presence of 2 facets per side not imply rib double-headed: often, in case, has single head. ribs typically thin , possess longitudinal groove on both inner , outer sides. lower side of chest formed gastralia. these belly ribs have single centre segment , 1 or 2 outer segments per side. not fused real plastron. 2 gastralia present per dorsal rib.

appendicular skeleton

in specimen seen below, looks breastbone in fact fused coracoids

the shoulder girdle of ichthyosaurs not modified original condition. basal forms show hatchet- or crescent-shaped shoulder blade or scapula; derived forms have elongated blade positioned on broader base. scapula not fused coracoid scapulocoracoid, indicating forces exerted on shoulder girdle moderate. shoulder joint positioned on border between scapula , coracoid. both coracoids fused on common midline. coracoid shape variable, rather low. upper part of shoulder girdle formed 2 long , slender clavicles, crowned central interclavicular bone large , triangular basal forms, small , t-shaped in jurassic species. breast bones or sterna absent.

ichthyosaur paddle (charmouth heritage coast centre)

basal forms have forelimb still functionally differentiated, in details resembling arm of land-dwelling forebears; ulna , radius elongated , separated; carpals rounded, allowing wrist rotate; number of phalanges within range shown land animals. ichthyosaurs proper, contrary, have forelimb adapted function flipper. however, adaptations variable. triassic species typically have derived humerus, changed disc. jurassic species tend have more elongated humeral form rounded head, narrow shaft, , expanded lower end. radius , ulna flattened, can circular, or without notch, or have waist. notches can homologous original shafts, newly formed. jurassic forms no longer have space, spatium interosseum, between radius , ulna. often, latter bones gradually merge lower, disc-shaped elements - 4 carpals again differ little in form 5 metacarpals.

in arm of ophthalmosaurus icenius, additional upper row of elements has developed, ending above in lower arm bone.

a derived condition show phalanges, small disc-shaped elements positioned in long rows. sometimes, number of fingers reduced, low two. rather common phenomenon within tetrapoda. unique, however, derived tetrapods, fact species show nonpathological polydactyly, number of fingers being higher five. species have ten fingers per hand. these fingers again, can have increased number of phalanges, thirty, phenomenon called hyperphalangy, known plesiosauria, mosasaurs, , cetacea. high number of elements allows flipper shaped hydrofoil. when high number of fingers present, identity difficult determine. assumed fingers added @ both front , @ rear, perhaps core of 4 original fingers. if fingers added, number of metacarpals , carpals increased; lower arm element present. earlier, common divide ichthyosaurs longipinnate , latipinnate forms, according long or wide shape of front flippers, recent research has shown these not natural groups, ichthyosaur clades containing both species , without elongated forelimbs.

the ichthyosaur pelvis typically rather reduced. 3 pelvic bones: ilium, ischium, , pubic bone, not fused , not touch each other. also, left , right pelvic sides not longer touch; basal forms still have sacral ribs connecting ilia vertebral column. hip joint not closed on inside. pubic bone typically not connect ischium behind it; space in between workers identified fenestra thyreoidea; other researchers deny term applicable given general loose structure of pelvis. later species have connected pubic bone , ischium, in case, femoral head no longer articulates hip joint. triassic species have plate-like pubic bones , ischia; in later species these elements become elongated narrow shaft , can form single rod.

typically, hindlimbs shorter forelimbs, possessing lesser number of elements. often, rear flipper half length of front flipper. thighbone short , broad, narrow waist , expanded lower end. tibia, fibula , metatarsals merged mosaic of bone discs supporting hydrofoil. there 3 6 toes present. toe phalanges show hyperphalangy; exceptionally, ophthalmosaurus shows reduced number of phalanges.