Sauropterygia: Nothosauria and Pachypleurosauria

Nothosauria and Pachypleurosauria inhabited shallow marine environments of the Tethys Ocean during the Early to Middle Triassic. They shared a similar elongated body shape including long necks and tails and dorsoventrally flattened skulls. Pachypleurosaurs are small marine reptiles whereas Nothosauria include small to large forms, all following an active predatory lifestyle. Pachypleurosaurs swam by tail-driven propulsion whereas nothosaurs used paraxial locomotion in which the forelimb was actively involved in propulsion. Generally, the main tissue deposited is parallel-fibered bone but both groups show a certain intraspecific variability and developmental plasticity. Pachypleurosaurs and nothosaurs grew in general with lamellar-zonal bone tissue that is low to moderately vascularized in most pachypleurosaurs and usually moderately vascularized in nothosaurs. The pachypleurosaur Anarosaurus heterodontus, however, shows well-vascularized incipient fibrolamellar bone tissue and some nothosaurs show local layers of fibrolamellar bone. Humeri of pachypleurosaurs are osteosclerotic with a medullary cavity reduced or filled in by endosteal bone. Nothosaurs show a wide spectrum of microanatomical patterns spanning from thick-walled osteosclerotic to thin-walled (almost bird-like) humeri, most likely reflecting differences in habitat and swimming style.