Squamates

Evolutionary history: Squamates (Squamata) form by far the most species-rich evolutionary radiation of living reptiles. They encompass the many lineages traditionally called lizards, as well as snakes and worm lizards (amphisbaenians). This familiar terminology hides an important evolutionary fact: snakes did not evolve as a separate group alongside lizards – they arose from within the broader radiation of lizard-like squamates. As a result, “lizards” excluding snakes do not constitute a complete natural clade. The squamate stem lineage reaches at least into the Middle Triassic. The approximately 240-million-year-old Megachirella wachtleri already possessed numerous squamate characteristics, revealing that the ancestry of modern lizards and snakes extends back well before dinosaurs came to dominate terrestrial ecosystems.

Appearance and lifestyle: One reason for the extraordinary success of Squamata is the evolutionary flexibility of their body plan. Their keratinised scales are periodically renewed, while modifications to the bony connections of the skull have allowed considerable cranial mobility in many lineages, reaching spectacular extremes in snakes. Male squamates characteristically possess paired copulatory organs called hemipenes, one of the group’s distinctive anatomical traits. Their body forms range from robust four-legged monitors and iguanas to delicate geckos, burrowing specialists and completely limbless species. Limb reduction and loss evolved independently again and again across squamate evolution rather than occurring only once in the ancestors of snakes. Reproduction is equally diverse: alongside egg laying, live birth has evolved independently more than one hundred times, while several lineages have also developed parthenogenetic reproduction.

Ecological role: Squamates inhabit every continent except Antarctica and occupy environments ranging from deserts, rainforests and mountains to subterranean habitats, freshwater ecosystems and the open ocean. They include tiny arthropod predators, herbivores, fruit and nectar feeders, scavengers and large apex predators. Many regulate populations of insects and vertebrate prey, while squamates themselves form an essential food source for countless birds, mammals and other predators. Some lizards also contribute to pollination and seed dispersal, particularly in island ecosystems. More than twelve thousand living squamate species are currently recognised, making Squamata one of the great evolutionary radiations of vertebrates. Habitat destruction, invasive species, climate change and unsustainable wildlife trade nevertheless threaten many specialised and range-restricted species.