The ichthyosaurs are a group of extinct reptiles with a dolphin-shaped body, adapted to a marine life-style that are known from many well preserved fossils but lack modern descendants. Their origin has been controversial, but cladistic methods have recovered them as diapsid reptiles, a clade that includes the Lepidosauromorpha (Beak-heads + Lizards and Snakes) and Archosauromorpha (many extinct groups + dinosaurs + crocodiles). And, it is possible that ichthyosaurs are the sister to the lizards (Sauria). In time, they are known from the Triassic to the Late Cretaceous (about 250 to 95 million years ago).
Mixosaurus fossils are cosmopolitan and have been recovered from Triassic rocks in continental Asia, Indonesia, Europe, Canada, Alaska, and the lower USA. The name, provided by George Baur in 1887, refers to its mixed morphology suggesting it was in transition between a terrestrial and marine lifestyle. Less than two meters long, Mixosaurus had: a long tail with a low fin; a dorsal fin; paddle-shaped limbs with five digits; and long, narrow jaws. Thus, it may have been a slow swimmer, which ate fish. Because of these characters Mixosaurus are considered basal ichthyosaurs – species close to the ancestral ichthyosaur. .
Christian Klob and colleagues (2011) have now done the first histological examination of ichthyosaur bone on a series of specimens that range from very young specimens (about 50 cm) to large adults (about 1.2 m). Previous research had shown that juvenile Cretaceous ichthyosaur (non-Mixosaurs) bones had woven-fiberous tissue accompanied highly vascularized spongy bone was replaced with more dense bone. But, Mixosaurus had not been studied and were of interest because of their basal characters and early appearance in the geological record.
The study used bones (femur, fibula, ischium, ulna, phalanges, scapula, rib, and gastral rib fossils) that were cut, ground, and section so that the fossilized cellular structure could be seen with a light microscope. Growth marks were present in most of the bone samples and ontogenetic changes could be traced, although resorption had deleted part of the growth record. The high growth rate of Mixosaurus implies relatively high metabolic rates, a precondition for being homoeothermic. Warm-blooded physiology had been previously proposed for some of the large deep-water, ocean-cruising ichthyosaurs and it now seems likely that it was also present in their ancestors.
Kolb, C., M. R. Sánchez-Villagra and T. M. Scheyer 2011. The palaeohistology of the basal ichthyosaur Mixosaurus (Ichthyopterygia, Mixosauridae) from the Middle Triassic: Palaeobiological implications. Comptes Rendus Palevol, doi:10.1016/j.crpv.2010.10.008