Lizards possess a backbone, classifying them as vertebrates. As members of the Class Reptilia, they have a vertebral column—an internal skeleton, or endoskeleton—that provides the necessary structure for their characteristic movements and body shape. This long, flexible series of bones extends from the base of the skull down the length of the body.
What Defines a Vertebrate
The term vertebrate applies to any animal that possesses a vertebral column, commonly called a backbone or spine. This structure is composed of individual, serially arranged bony segments known as vertebrae. The primary function of the column is to enclose and protect the delicate central nervous system, specifically the spinal cord.
The vertebral column also serves as the primary axis for structural support, allowing the animal to maintain posture. Vertebrae link together through specialized joints, granting flexibility and enabling complex movements like walking, running, and burrowing. This internal framework separates vertebrates from invertebrates, such as insects or jellyfish, which lack bony spinal support.
The Structure of the Lizard Spine
The lizard’s vertebral column is highly specialized and divided into four distinct regions, providing varying degrees of mobility and support. The cervical region comprises the neck vertebrae, typically numbering about eight, allowing the lizard to move its head independently. The trunk region includes the thoracic vertebrae, where ribs attach to protect internal organs and aid in respiration.
The sacral region is the short segment where the spine connects to the pelvic girdle. It usually consists of two strong vertebrae that facilitate the transfer of locomotor forces from the hind limbs to the body. The most extensive part of the spine is the caudal region, which extends into the tail. These caudal vertebrae are numerous, sometimes exceeding one hundred segments in long-tailed species, and decrease in size toward the tail tip.
A unique feature in the caudal vertebrae of many lizard species is the presence of autotomic septa, or fracture planes. These specialized planes run transversely through the middle of the vertebral body. This is an adaptation that facilitates caudal autonomy, the ability to shed the tail as a defense mechanism against predators. If the tail is lost at one of these fracture points, the lizard can regrow a replacement tail, but the new structure is supported by a simple cartilaginous rod instead of true bony vertebrae.
Lizards in the Chordate Phylum
Lizards belong to the Kingdom Animalia and are classified under the Phylum Chordata. This phylum is defined by shared anatomical features present during development, most notably the notochord. The notochord is a flexible rod that is eventually replaced by the vertebral column in the subphylum Vertebrata.
The Phylum Chordata includes all animals with backbones, such as fish, amphibians, reptiles, birds, and mammals. This highlights the shared evolutionary history of these diverse groups. Lizards are further categorized into the Class Reptilia, alongside snakes, turtles, and crocodiles. The presence of bony, segmented vertebrae confirms a lizard’s status as a chordate and a vertebrate, distinguishing it from invertebrates.