Monday, May 31, 2010

Attaching the cervical vertebrae to the skull
"Note how in Archaeopteryx the attachment of the cervical vertebrae to the skull is at the back of the head as in dinosaurs, not to the base of the skull as is found in modern birds."

Before discussing size further, let me note that the maniraptoran identification is pretty sound. On first seeing the vertebra, I was struck by the X-like shape of the neural arch (as seen in dorsal view), the large hypapophysis (a prominent keel located on the midline of the ventral surface) and the presence of fossae on the sides of the neural arch. It looked immediately like a maniraptoran cervical vertebra, especially that of an oviraptorosaur. This was confirmed by other details, like the shapes of the articular surfaces, the positions of the parapophyses, the shapes of the zygapophyses and so on. The presence of a rather large hypapophysis, combined with the position of the parapophyses (low down on the centrum), shows that the Ashdown maniraptoran vertebra is a posterior cervical: that is, a vertebra from near the base of the neck.
"In sharp contrast to the derived morphology of the skull and neck, the tail of Darwinopterus is identical to that of most basal clades, consisting of more than 20 caudals which, apart from the first three or four vertebrae, are long and rod-like and enclosed by a sheath of bony filaments composed of highly elongated ossified extensions of the zygapophyses and hypapophyses (figure 2e) (Wellnhofer 1975, 1978)."
They [hypapophyses] have not been reported in Archaeopteryx or any of the dinosaurs proposed as ancestors of the birds, but they appear in some fossils of modern birds.
We can only guess at the respiratory system of dinosaurs, and the appearance of well-developed thoracic hypapophyses would support the notion of something birdlike. None have been reported, however.
Supplementary information
Unambiguous synapomorphies for major coelurosaurian clades:
Maniraptora: 46.1, 54.1, 57.1, 102.1, 105.1, 158.1, 161.0, 173.1, 175.1, and 263.1.
102. Anterior trunk vertebrae without prominent hypapophyses (0) or with large hypapophyses (1).

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