Sunday, 12 May 2013

Axial Skeleton

Recently, I have been asked to identify a number of pterosaur vertebrae.  Most of them were of small theropod dinosaur cervicals from the Kem-Kem formation in Morocco.  Pterosaur vertebrae in isolation requires a skilled eye to identify them, so I thought it would be a good idea to look at the axial skeleton of Pteranodon as the vertebrae are quite well documented.  I am going to gloss over the pectoral and pelvic girdles here.

This is a drawing from the website of the combined atlas and axis and the first cervical vertebra of pteranodon.  It is based on a drawing by Eaton.  Notice the pneumatic foramen (pn) where an air sac tube would pass to permit the interior of the bone to be filled with air.  This considerably lightens the skeleton for flight - pneumatic bones are also found in theropod dinosaurs like T-Rex and in the suropods.
The dorsal vertebrae of most pterosaurs are very short and have attachments for ribs.
Sacral vertebrae are fused and support the pelvis.  There are no articular surfaces and the vertebrae are joined by suture jointing, known as ankylosis.
Caudals are short and uninspiring and often look like the caudals of many other species, so in isolation they are indistinguishable from ichthyosaurs and many other large vertebrates.

The axial skeleton is seen to be arranged in functional sections.  The pectoral and pelvic girdles have been omitted here.

Ax is the combined atlas and axis which joins the spine to the skull.  This was quite a solid bone.

A shows the cervical vertebrae that make up the neck.  In Pteranodon, these were sturdy and had large dorsal spines to anchor the muscles that positioned the skull, with its large occipital head crest.  The joints had a good range of movement.

B is the Notarium.  This is a region of the spine where the thoraxic vertebrae are ankylose and support a fused region of bone that articulates with the scapulae.  The scapulo-coracoid is a strong bone that was anchored at the notarium and the sternum.  This forms the pectoral girdle which had to take the strain of the wings in flight.

C is a region where the dorsal vertebrae are not fused.  This region gave a small amount of flexion between two very solid and rigid regions of the back.  These few vertebrae held floating ribs.

D is the sacral region.  Like the thoraxic vertebrae attached to the notarium, the sacrals were fused and attached to the pelvis.

E In pterodactyloid pterosaurs, the caudal vertebrae were simple and very lightly muscled.

Half of the length of the axial skeleton in pteranodon is neck.  The neck is of a similar length to the skull.  The spine behind this region is mostly rigid, forming a solid box which gives the skeleton a sturdy structure to stabilise the wings in relation to the body, when the animal is in flight.  This was a very well adapted animal for flight.

Most large pterosaur axial skeletons show close similarities to pteranodon.  Most of the small pterosaurs also have these overall features, with minor variations.  This type of spine is essential for flying animals and this principle structure is also seen in all flying birds.