pterosaurs was relatively long and straight. In pterodactyloids, the neck is typically longer than the torso. This length is not caused by an increase of the number of vertebrae, which is invariably seven. Some researchers include two transitional "cervicodorsals" which brings the number to nine. Instead, the vertebrae themselves became more elongated, up to eight times longer than wide. Nevertheless, the cervicals were wider than high, implying a better vertical than horizontal neck mobility. Pterodactyloids have lost all neck ribs. Pterosaur necks were probably rather thick and well-muscled, especially vertically. The torso was relatively short and egg-shaped. The vertebrae in the back of pterosaurs originally might have numbered eighteen. With advanced species a growing number of these tended to be incorporated into the sacrum. Such species also often show a fusion of the front dorsal vertebrae into a rigid whole which is called the notarium after a comparable structure in birds. This was an adaptation to withstand the forces caused by flapping the wings. The notarium included three to seven vertebrae, depending on the species involved but also on individual age. These vertebrae could be connected by tendons or a fusion of their neural spines into a "supraneural plate". Their ribs also would be tightly fused into the notarium. In general, the ribs are double-headed. The sacrum consisted of three to ten sacral vertebrae. They too, could be connected via a supraneural plate that, however, would not contact the notarium. The tails of pterosaurs were always rather slender. This means that the caudofemoralis retractor muscle which in most basal Archosauria provides the main propulsive force for the hindlimb, was relatively unimportant. The tail vertebrae were amphicoelous, the vertebral bodies on both ends being concave. Early species had long tails, containing up to fifty caudal vertebrae, the middle ones stiffened by elongated articulation processes, the zygapophyses, and chevrons. Such tails acted as rudders, sometimes ending at the rear in a vertical diamond-shaped or oval vane. In pterodactyloids, the tails were much reduced and never stiffened, with some species counting as few as ten vertebrae. Shoulder girdle The shoulder girdle was a strong structure that transferred the forces of flapping flight to the thorax. It was probably covered by thick muscle layers. The upper bone, the shoulder blade, was a straight bar. It was connected to a lower bone, the coracoid that is relatively long in pterosaurs. In advanced species, their combined