Reproduction Ichthyosaur

chaohusaurus 3 juveniles

ichthyosaurs viviparous, i.e. bore live young instead of laying eggs. although reptiles , descended egg-laying, oviparous, ancestors, viviparity not unexpected first appears. air-breathing marine creatures must either come ashore lay eggs, turtles , sea snakes, or else give birth live young in surface waters, whales , dolphins. given streamlined , transversely flattened bodies, heavily adapted fast swimming, have been difficult if not impossible ichthyosaurs move far enough on land lay eggs. confirmed 9 december 1845 when naturalist joseph chaning pearce reported small embryo in fossil of ichthyosaurus communis. embryo, length of eleven centimetres, positioned in birth canal of two-and-a-half metre long mother, head pointed rear. pearce concluded fossil ichthyosaurs had have been viviparous.

detail of female stenopterygius great number of fetuses in belly, 1 of has been expelled

later, holzmaden deposits numerous adult fossils found containing fetuses. in 1880, harry govier seeley, heading special british paleontological committee studying problem of ichthyosaur reproduction, concluded birth given in water , fossils containing fetuses in birth canal represented cases of premature death of juvenile, causing demise of mother animal well. comparison has been made dolphins , whales, young need born tail-first prevent drowning; if juvenile born head-first, dies , mother if corpse gets stuck in birth canal. however, alternative explanation such fossils represent females had died other reasons while pregnant, after decomposition gasses drove out fetuses head-first. in 2014, study reported find of fossilized chaohusaurus female had died while giving birth 3 neonates. 2 had been expelled while third present in birth canal. fossil documented ichthyosaurs born head first, perhaps opposed later genera. chaohusaurus basal ichthypterygian—previously, basal genus of fetuses known, had been mixosaurus—this discovery suggests earliest land-dwelling ancestors of ichthyosaurs had been viviparous.

compared placental mammals or plesiosaurs, ichthyosaur fetuses tend small , number per litter high. in 1 female of stenopterygius 7 have been identified, in eleven. fetuses have @ quarter of length of mother animal. juveniles have same body proportions adult individuals. main ontogenetical changes during growth consist in fusion , greater robustness of skeletal elements.

crocodiles, sea turtles , lizards control offspring gender manipulating temperature of developing eggs environment; i.e. not have distinct sex chromosomes. live-bearing reptiles not regulate sex through incubation temperature. study in 2009, examined 94 living species of reptiles, birds , mammals, found genetic control of sex appears crucial live birth. concluded marine reptiles such control predated viviparity , adaptation stable sea-climate in coastal regions. genetics controlled sex in ichthyosaurs, mosasaurs , plesiosaurs.