rhynchocephalians

Rhynchocephalians are an ancient lineage of reptiles within Lepidosauria that includes the sole living member, the tuatara, and a broad fossil record of extinct relatives. The clade Rhynchocephalia first appears in the late Triassic and was once diverse and widespread across multiple continents. Today, only one lineage survives: the tuatara (Sphenodon punctatus), native to New Zealand’s offshore islands, while numerous extinct rhynchocephalians are known from fossil deposits around the world.

Anatomy and biology common to the group include a diapsid skull with two temporal openings and, in many species, acrodont dentition, where teeth are fused to the upper or lower jaws rather than set in sockets. A distinctive feature of rhynchocephalians is the well-developed parietal or pineal organ, visible as a third eye in juveniles and present in adults in various degrees. They are generally slow-growing, with long lifespans relative to body size, and most have a robust, lizard-like body plan adapted to various ecological niches.

Ecology and life history vary widely among extinct rhynchocephalians, from aquatic to terrestrial forms; the living tuatara is a burrow-dwelling predator or scavenger that feeds on invertebrates, small vertebrates, and eggs. Reproduction in tuatara is oviparous, with eggs laid infrequently and incubation lasting many months, and individuals reaching sexual maturity after many years.

Conservation status is of concern for the living lineage. The tuatara faces threats from introduced predators and habitat disturbance; it is protected and subject to island restoration and monitoring programs to preserve this once-abundant but now fragmentary lineage.