Embryologi och tidig ontogeni hos mesozoiska fisködlor (Ichthyopterygia)
Sammanfattning: Ichthyosaurs (‘fish lizards’) are a group of marine reptiles that lived during the Mesozoic Era. They are all included in the superorder Ichthyopterygia, which consists of at least 36 genera and 80 species. The earliest ichthyosaurs occur in sediments deposited during the Olenekian in the Early Triassic (about 250 million years ago), and the last forms became extinct during the Cenoman in the Late Cretaceous (about 90 million years ago). Ichthyosaurs are characterized by large eyeballs (up to 25 cm in diameter) and a distinct, elongated snout. Some forms could reach well above 20 meters in length; however, the average length of most ichthyosaurs was around three to four meters. The earliest ichthyosaurs were lizard-like in appearance but they gradually developed a more fish-shaped body form indicating adjustments to a pelagic life. Ichthyosaurs had two pairs of fins formed by fingerlike bones; these indicate a terrestrial origin. Ichthyosaurs also had lungs instead of gills and thus were forced to swim to the surface to breathe air. Ichthyosaurs gave birth to live young, and there are a number of fossil finds of ichthyosaurs with associated embryos or juvenile individuals. This work describes a putative fossil embryo based on research done on the early ontogenetic development of ichthyosaurs. The potential embryo was found inside the body cavity of an adult individual belonging to the species Stenopterygius quadriscissus from the Early Jurassic of Holzmaden, south-western Germany. The embryo consists of millimeter-sized skeletal remains. The skeletal elements have been examined and photographed and most vertebral centra, a scapula, a clavicle and the teeth compare favorably in both size and appearance with other embryonic Stenopterygius, to suggest that skeletal remains belong to an embryo in an early stage of development. Two bones; i.e., a vertebral centrum and a putative tooth, differ in appearance from the rest of the assemblage, and hence it cannot be excluded that these elements represent stomach contents of the adult specimen. Notably though, all skeletal remains lack any trace of acid etching originating from digestive juices – an indication that the fossil elements indeed represent remains of an embryo rather than stomach contents. Nonetheless, there is a possibility that the ichthyosaur ate just before it died, which potentially could explain the absence of apparent acid etching.
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