Figure 1. Skeleton of the short-beaked echidna, T. aculeatus. The posttemporal canal (ptc); cervical ribs (cr); epipubic bones (eb) (a feature shared with marsupials); and archaic shoulder girdle are considered to be primitive characteristics retained by monotremes, while ossified sternal ribs (sr) occur in birds but not most mammals. The solid build of the skeleton is attributed to the echidna's fossorial lifestyle. Courtesy of Anne Musser.

Figure 2. The platypus, Ornithorhynchus anatinus. The platypus (Ornithorhynchus anatinus) is beautifully adapted to its aquatic lifestyle, with a thick, waterproof pelt; dorsally placed eyes; streamlined body and extensively webbed forefeet. Its extraordinarily sensitive, wide bill is its primary navigational tool when feeding underwater. Courtesy of Bernard Fanning.

Figure 3. The short-beaked echidna, T. aculeatus. The short-beaked echidna (T. aculeatus) is characterized by a short, straight beak and by a heavy coat of stout, sharp spines. T. aculeatus uses its beak, massive forearms and spatulate claws to break into ant and termite mounds or rotting logs in search of insect prey. Courtesy of Anne Musser.

Figure 4. The long-beaked echidna, Zaglossus bruijnii. The long-beaked echidna (Z. bruijnii) is the largest living monotreme, up to 17 kg in weight and up to 1 m in length. Its long beak has a marked downward curve, and its spines are smaller and less conspicuous than in T. aculeatus. Courtesy of Timothy Flannery.