Evolution of the Tetrapod Vertebral Column

Some of the most important clues to the lineage of early tetrapods can be traced in the evolutions of the vertebral column. While all vertebrates have a backbone, the number of vertebrae and the shape and design of the individual bones vary significantly from taxon to taxon. Such morphological features as size, shape, and volume provide excellent clues to the lifestyle of a given extinct creature. The design of vertebrae can also serve as pointers to link related taxa.

To understand the vertebral column, one must first understand the language used to describe the individual components of the backbone. The fundamental purpose of the backbone is to protect the spinal cord and notochord and to provide a supportive frame for an animal. A single vertebra (plural: vertebrae) consists of two basic parts. The centrum (plural: centra) is the central part of the bone, through which the notochord runs; the centrum provides a resting place for the spinal cord. On top of the centrum is the neural arch, which rests on top of the spinal cord. In addition, a variety of riblike structures may also be present, depending on the kind of animal. Ribs along the trunk of the animal may be joined to the vertebrae at both the centra and the neural arches. A tail vertebra may have another extension called the haemal arch joined to the underside of the centrum.

The designs of centra and arches sometimes provide distinguishing feature of individual vertebrate taxa. Along with clues in the skull, the limbs, and the pectoral and pelvic girdles, these clues in the bones tell much of the story of how the animal lived and what other creatures it was related to. The vertebral column governs an animal's structural posture, mobility, and flexibility. In the continuing story of the prehistoric Earth, the vertebral column will join the skull, jaws, teeth, and limb elements as a key tool to use in diagnosing the evolutionary relationship of extinct groups.

The design of the backbone of the first reptiles provided support as well as lightness and flexibility; this differed in significant ways from the skeletal features of lumbering amphibians from the same time. The lifestyle of swamp-bound amphibians was not demanding when it came to hunting prey. Acanthostega and Ich-thyostega probably rested quietly in pools of shallow water, waiting for unwary fish to swim close enough to be snapped up and gobbled down.

Land-based prey in the Carboniferous included arthropods, such as insects, and tetrapods. Reptiles undoubtedly required a more mobile lifestyle than that of amphibians if the reptiles were to catch such prey. The lightly built backbones of reptiles provided structural strength and flexibility of movement. The cervical, or neck, vertebrae were highly modified over those of the reptiles' amphibian ancestors and included a ball-and-socket connection with the skull. This flexible neck allowed the head to rotate, an advantage when catching prey as well as a means of providing improved flexibility while the animal was walking and running.

The vertebral columns of these early reptiles also provided increasingly strong attachments to the forelimbs and hind limbs of the animals. Although the length of their legs was not yet significantly different from that of their amphibian precursors, the earliest reptiles were the first tetrapods to show an important modification to the ankle bones. Basal amphibian ankles were made up of many small bones that provided improved flex over the fins of fish; this made walking on land more feasible. Reptiles such as Hylonomus and Paleothyris showed an additional improvement by fusing three of the ankle bones into a single unit called the astragalus. This change provided a better orientation of the foot for walking on land.

prelude to diversification

By the end of the Carboniferous Period, the amniote solution for reproducing on land began to pay dividends. Fully terrestrial land vertebrates were diversifying rapidly and taking advantage of the seemingly unlimited opportunities of greening habitats and generous food supplies. Reptiles began to diversify, leaving fossil evidence of 25 or more taxa of such insect-eating and plant-eating creatures. Most were small, like Hylonomus and Paleothyris, measuring about eight inches (20 cm) long; however, larger taxa were beginning to appear.

As reptiles diversified into many ecological niches, they began new branches of the vertebrate family tree. Hylonomus and Paleothyris, the most basal reptiles, represent the first radiation of these animals. Going into the Permian Period, the lineage of terrestrial vertebrates exploded in several directions that eventually led to the rise of dinosaurs, other reptile groups, birds, and mammals. The Permian Period introduces these new forms of vertebrates and their evolutionary foundation they provided for the Mesozoic Era—the Age of Reptiles.

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