Paleobiologists have discovered how aquatic archosauromorphs evolved their exceptionally long necks.

After the Permian-Triassic extinction, the causes of which scientists actively debate, the Earth's fauna experienced a significant enrichment with vertebrate forms. During the Mesozoic Era (251-66 million years ago), reptiles successfully evolved to occupy all available environments. On land, there were pseudosuchians and non-flying dinosaurs, while the air was dominated by pterosaurs and birds, and the sea was inhabited by ichthyosaurs, mosasaurs, and zupopterygia.

The diversity of ecological niches forced ancient animals to adapt in their own ways and develop distinctive structural features. One of the most unique representatives of the Mesozoic vertebrate fauna is Tanysauria, a clade of early archosauromorphs. Among them were both terrestrial and marine varieties, but their common and main characteristic was an extremely long neck. Some species (for example, Tanystropheus) developed necks so long that they made up half of the body or even more.

These lizards achieved such distinctiveness through the elongation of vertebrae or by increasing their number in the cervical region. The five-meter Tanystropheus longobardicus had the longest neck—up to three meters, which is half of its body—but it had only 13 elongated vertebrae.

Although the structure of these bones has been studied for a long time, little is known about their evolution. To fill this gap, paleobiologists from Poland, Germany, and Switzerland decided to investigate. Based on recently discovered remains, they examined the internal anatomy of cervical vertebrae from various Tanystropheus species (32 specimens) and the more primitive Protanystropheus antiquus (seven specimens) using computed tomography (CT). The results of their scientific work were published in the journal Zoological Journal of the Linnean Society.

Tomography allowed researchers to analyze the internal structure and wall thickness of the samples without destroying them. In the samples of P. antiquus, there is a cavity in the center of the vertebrae, and at both ends, the bone walls are relatively thin and thicken towards the center. These cavities diagonally intersect the plates of spongy bone tissue (trabeculae), and in some samples, the trabeculae separated the internal cavities into several pockets.

In all Tanystropheus vertebrae, the scientists also found a large cavity surrounded by a dense layer of bone. However, there were no trabecular plates inside it—only a few such structures were found in the front and rear sections of the vertebra. Furthermore, as noted by paleobiologists, in Tanystropheus, unlike P. antiquus, it was almost impossible to distinguish the vertebral canal. In cross-section, the vertebrae were nearly perfectly round—the tubular structure could provide additional rigidity.

Analysis of the bone tissue showed that there are more capillaries near the center of the cervical vertebrae. As one moves away forwards and backwards, the tissue composition changes to lamellar bone, which, due to its layered structure, has increased strength.

The researchers concluded that the cavities found in the center of the cervical vertebrae formed after the dissolution of spongy bone tissue. Interestingly, this structure somewhat contradicts the aquatic lifestyle of Tanystropheus, as overly lightweight vertebrae would reduce the animal's buoyancy. The unique design of the elongated cervical vertebrae, the paleobiologists concluded, is a result of an evolutionary compromise between length, mass, and strength. Their hollow cylindrical structure can be compared to the pneumaticity of bones in pterosaurs to facilitate flight.

In the first ten million years after the Great Extinction, archosauromorphs, particularly Tanysauria, evolved rapidly, and competitive pressure forced them to adapt to life in the water. Meanwhile, the species Dinocephalosaurus orientalis, from a group related to Tanystropheus, took a different path: its neck consists of more than twenty vertebrae.