an order of extinct mesozoic reptiles, mostly of large size (whence the name). notwithstanding their size, they present birdlike characters in the skeleton, esp. in the pelvis and hind limbs. some walked on their three-toed hind feet, thus producing the large "bird tracks," so-called, of mesozoic sandstones; others were five-toed and quadrupedal. see illust. of compsognathus, also illustration of dinosaur in appendix.