The Ornithosauria / An elementary study of the bones of Pterodactyles made from fossil remains found in the Cambridge Upper Greensand, and arranged in the Woodwardian Museum of the University of Cambridge

by H. G. Seeley

The neural arch is strong, compressed from back to front, and placed as usual on the anterior part of the centrum. In outline it is sub-rhomboid with the sides concave. There is a strong process on each side above the neural canal for a rib, and apparently a neural spine, but all are broken. The transverse processes for the ribs are directed outward, and a little forward, flattened in front and behind, the surfaces being sub-parallel, so that in front the neural arch is concave from side to side. Behind, the neural spine is directed between the transverse processes so as to over-hang the exposed part of the superior surface of the centrum. At the points where the neural spine bends back from the transverse processes are the posterior zygapophyses, high above the neural canal, and parted from each other by an interspace as wide as the canal is high. They look downward, outward, and backward. The lateral surface below the transverse process is narrow, flattened, bends at a right angle with the posterior surface, rounds into the anterior surface, is a continuous curve with the side of the centrum, and is concave from below upward. The superior surfaces of the neural arch have the sides sub-parallel, they are each concave from side to side; and these surfaces are excavated for pneumatic foramina.

Dorsal vertebræ of the type with the centrum flattened closely resemble cervical vertebræ with the centrum flattened, differing chiefly in the less length of the centrum. Occasionally (as in No. 3) the neural arch comes away from the body of the vertebra.

The centrum is very depressed, sub-quadrate, and wider than long; the base is flat, or slightly concave, with occasionally a slight longitudinal mesial ridge; the lateral outlines are concave, so that the bone is pinched in from side to side. The neural surface of the centrum is flat and parallel with the base, and, as usual, wider behind than in front, but the centrum is not there so high. The surfaces for the neural arch are flat, and extend nearly to the base of the centrum in front, so that they look upward, outward and a little forward.

The articular ends are remarkable for their depressed oblong character, still preserving the anterior concavity with a small mesial process below, as in cervical vertebræ, and similar but smaller processes at the inferior outer angles of the posterior sub-semicylindrical convexity. The middle third of the anterior cup is made by the trapezoidal anterior end of the centrum; sometimes the sutures between it and the neural arch are well marked.

The neural arch is large, commonly with a sub-circular neural canal. The neural spine is high, compressed so as to have the lateral surfaces sub-parallel and rounding into each other superiorly; and it has a less antero-posterior extent than the centrum. At its base behind it widens rapidly, and forms massive quadrate processes, extending outward and backward, which on the outside each have a flattened ovate zygapophysial facet, which also looks downward. Above the facet and separated from it by a groove is a tubercle. Between the zygapophyses behind the bone is concave from side to side; the facets are placed above the neural canal.

The posterior zygapophyses are placed considerably higher than the anterior zygapophyses, and the part of the neural arch between is rather constricted from front to back. The neural arch steadily widens in front down to the base of the anterior zygapophysial processes in such way that the more or less flattened lateral surface looks outward and is gently concave from above downward. A ridge commencing at the tubercle over the posterior zygapophysial facet descends in a curve forward and downward, to form the posterior border of the anterior zygapophysial process. This is separated by a groove from the anterior articular surface, and anterior part of the base of the centrum, and has the aspect of a compressed part of the neural arch, extending obliquely downward, and forward, over and beyond the articular surface of the centrum. The anterior zygapophysial facets are oblong, narrow from side to side, and long from front to back; they are directed forward and a little outward, and are flattened, make nearly a right angle behind with the front of the neural arch, and look upward and inward. They are sometimes placed as high as the top of the neural canal, but are commonly lower. Around the neural canal the bone is conically impressed.

Minute pneumatic foramina are in the usual position, between the centrum and the neural arch; and sometimes others behind the anterior zygapophysial process.

The largest specimen known has the centrum nearly an inch and a half long.

The dorsal vertebræ in Cambridge specimens would appear to make a nearer approximation in number to birds than to Mammals or Lizards or Crocodiles, though Chelonians have few vertebræ in the back. Among Reptiles the form of the vertebra makes some approach to that of the Monitor, Chameleon and Scink. In most Mammals the dorsal vertebræ have the centrum convex, though in the lumbar region its visceral surface often becomes flattened. But though very unlike there is a nearer resemblance to the lower dorsal vertebræ of a Struthious bird.