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.
Citation
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 Labels: bone structure, homeothermy, mixosaurus