Squamates are members of the largest reptilian order, Squamata. Characterized by horny epidermal scales and periodic molting, it is also the second-largest order of living vertebrates consisting of lizards, snakes, and amphisbaenians.
They are the most diverse and the only extant group of reptiles having species that show all three modes of reproduction: oviparity (egg-laying), viviparity (birth of live young), and ovoviviparity (live young that completely develop in eggs).
Squamates are most closely related to tuatara, the only surviving member of the order Rhynchocephalia.
Size
As a wide group, squamates vary greatly in size, ranging from dwarf geckos measuring 16 mm (0.63 inches) in length to the reticulated python, which is 6.5 m (21 ft) long.
Taxonomy
Squamates are considered a sister group to tuatara, the only living member of the order Rhynchocephalia. Both orders form the subclass Lepidosauria, the sister group to Archosauria (a clade that contains crocodiles and birds).
The order Squamata was traditionally classified into three suborders: Lacertilia (lizards), Serpentes (snakes), and Amphisbaenia (worm lizards). While snakes and worm lizards were placed close together as monophyletic groups, lizards were treated separately as a paraphyletic group.
However, recent molecular studies place all venomous groups, namely Serpentes, Iguania (agamids, chameleons, iguanids, etc.), and Anguimorpha (monitor lizards, Gila monsters, glass lizards, etc.), under a single clade, Toxicofera.
The modern classification of the order Squamata is given below. Here, over 10,900 extant squamates are divided into 67 families.
Family: Xenodermidae (Odd-scaled snakes and relatives)
Family: Xenopeltidae (Sunbeam snakes)
Scolecophidia
Family: Anomalepidae (Dawn blind snakes)
Family: Gerrhopilidae (Indo-Malayan blindsnakes)
Family: Leptotyphlopidae (Slender blind snakes)
Family: Typhlopidae (Blind snakes)
Family: Xenotyphlopidae (Malagasy blind snakes)
Evolution
Since the fossils of rhynchocephalians date back to the Early Triassic Period, the early ancestors of squamates must also have existed around that time.
The oldest known squamate, or the stem squamate, belongs to the gecko-like genus Megachirella, which appeared in the Middle Triassic Period (around 240 million years ago), while anguimorphs, like Cryptovaranoides, emerged in the Late Triassic Period.
Scientists believe crown group squamates probably originated in the Early Jurassic Period, while the oldest unambiguous squamate fossils date to the Middle Jurassic Period (about 174 to 161 million years ago), around which modern groups, like snakes, originated.
The earlier forms diversified into iguanas, varanoids, and true snakes in the Cretaceous Period (145 to 66 million years ago). Around the same time, the groups Mosasauridae and Polyglyphanodontia also thrived; however, they were wiped off along with many squamate lineages in the Cretaceous-Palaeogene mass extinction event (around 66 million years ago).
Reproduction
Squamates primarily reproduce sexually, although some, like the Colombian rainbow boa (Epicrates maurus), can reproduce asexually through facultative parthenogenesis.
Sexual
Male squamates possess special copulatory organs called hemipenes, held inverted within their bodies. During mating, these organs are everted and erected by erectile tissues for inserting into the female’s body. In some species, the hemipenes are forked (two-tipped), while in others, they bear hooks or spines for better attachment to the mate.
In snakes and lizards, the males compete with each other to win over a female. For example, in Vipers, the males twist themselves around the bodies of their opponents, bite their necks, and pin them down on the ground. In some groups, such as sand lizards, the females mate with two or more males and preferentially select the sperm of the more distantly related male.
In oviparous squamates, the eggs laid are covered with rough, parchment-like shells, with the only exceptions being blind lizards and the three families of geckos: Gekkonidae, Phyllodactylidae, and Sphaerodactylidae, which lay calcified eggs.
Asexual
Some snakes, like the Colombian rainbow boa, the copperhead, and the cottonmouth species, switch from the sexual mode of reproduction to facultative parthenogenesis, in which the eggs of the females develop into embryos without being fertilized by a male gamete. In this type of parthenogenesis, haploid gametes derived from the same individual fuse to form the diploid embryo (automixis).
Conservation Status
Many squamates are currently endangered due to anthropogenic causes such as habitat loss, poaching, illegal trade, and the introduction of invasive species. Most extinct species of lizards and snakes belonged to Africa.
However, different wildlife breeding programs, zoos, and private hobbyists are raising awareness about the urgency of the situation and taking necessary steps to conserve these reptiles.
Interesting Facts
Venom is found in members of Iguania, Anguimorpha, and Caenophidia and is believed to have evolved only once before the three lineages diverged.
