In general appearance, tuatara resemble some agamid and iguanid lizards, but many morphologic features distinguish tuatara from most or all lizards. For example, tuatara have no ear holes, and the males lack a copulatory organ. Furthermore, tuatara have uncinate processes (rear-pointing extensions) on the ribs and their teeth are acrodont (tightly fused to the surface of the jawbone), and the young have a horny caruncle, or egg-breaker, to assist with hatching. The most
unusual feature, however, is a double row of teeth on the upper jaw into which fit those of the lower jaw.
Tuatara males are distinguishable from females. The males are larger than females and are often twice as heavy. Both sexes have a crest on the midline of the head and back, merging into toothlike projections down the tail. The crest of the males is larger than in females and can be inflated during aggressive and mating behavior. The Maori name, tuatara, refers to these crests on the back.
The largest tuatara are some populations of the northern form, males of which can reach more than 24 in (60 cm) in length and 2 lb (1 kg) in weight. Females are usually less than 16 in (40 cm) in length and rarely exceed 1 lb (454 g) in weight. In the northern form of S. punctatus, size varies broadly with latitude; the largest animals are also the most northern. Nonetheless, differences in mean body size can be found within the same island groups. The cause of this variation is unclear.
Tuatara vary in color from olive green to reddish to gray to almost black. The base color usually is overlaid with irregular darker markings and speckled with paler spots. Compared with S. punctatus, S. guntheri is more often olive green and tends to be more heavily speckled with paler spots. The crest is typically white in both species. Juveniles are often pale gray-brown, with paler V-shaped markings on the upper surface and distinctive darker markings radiating from the eyes. When newly hatched, they sometimes have a large triangular pale patch on top of the head from the snout to the eyes.
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