Primary species in this group is Archaeopteryx, which was a crow-sized, bipedal reptile with a blunt snout and reptilian teeth. First discovered in 1877 in Bavaria, at this time there are still only 6 specimens in existence Modern birds include over 8700 species from 23 orders. Tremendous amount of information on birds exists, primarily due to their abundance and to common interest in them by birdwatcher-s and researchers 7. Class Mammalia Hair and mammary glands are the only unique characteristic of this class. Contains more nocturnal species than other classes.
Mammals also vary in size more than any other vertebrates.
Includes the duck-billed platypus and the spiny anteater. Placenta composed of parts of the uterine lining and fetal extraembryonic membranes through which exchanges between mother and embryo occur. Greatest adaptive radiation of marsupials occurred in Australia.