Thursday, May 27, 2010

Neuroanatomy (3)

This is an excellent source of info about pterosaur and modern bird similarities:
"Virtual endocasts derived from CT scans of nearly complete skulls of two pterosaurs (Fig. 1)—the more basal Rhamphorhynchus and the pterodactyloid Anhanguera—are the most complete to date.
They confirm some previous findings of birdlike attributes1–8: expansion of the cerebrum and cerebellum, displacing the enlarged optic tecta (lobes) ventrolaterally; small olfactory areas; and enlarged flocculi (cerebellar auricles) (Fig. 2). Despite these structural similarities, the brains of Rhamphorhynchus and Anhanguera, relative to body mass, do not fall [quite] within the range of extant birds, although they were enlarged relative to extant nonavian reptiles4,5,11 (Fig. 3; see Methods). Moreover, comparisons of total brain mass do not reveal differences in relative size of brain components (and hence underlying neural organization). For example, the enormous flocculi of pterosaurs probably outweighed the optic tecta, whereas the reverse is certainly true in birds.
Nevertheless, pterosaurs do possess a number of avian neuroanatomical traits that may well be associated with the sensory and coordination functions necessary for flight. Jerison4 suggested that avian brains were relatively larger than those of pterosaurs because birds evolved in the environmentally complex and neurologically challenging arboreal habitat that required greater neural processing and hence greater mass. That may be true, but another factor is that birds and pterosaurs had different phylogenetic starting points: pterosaurs evolved from relatively very smallbrained basal archosaurs, whereas birds evolved from theropod dinosaurs that had already initiated a substantial trend of brain expansion5,9. The virtual endocasts include the semicircular canals (Fig. 2), which had previously been only partially known for one pterosaur, Parapsicephalus1. The entire osseous labyrinth is preserved bilaterally in Anhanguera and the large majority of it is preserved in Rhamphorhynchus. The semicircular canal system is greatly expanded, with the long canals encircling the flocculus. Its general arrangement closely resembles that of birds and some other dinosaurs5, but, whereas it is relatively modest in these groups, the vestibular apparatus is relatively much larger in the pterosaurs." (Lawrence M. Witmer1, Sankar Chatterjee2, Jonathan Franzosa3 & Timothy Rowe3)

The article says:"the brains of Rhamphorhynchus and Anhanguera, relative to body mass, do not fall [quite] within the range of extant birds."
I have inserted the word "quite" because you must look at Figure 3 of the report to appreciate how very close the pterosaur is to being within the range of extant birds. And also review the Methods section to see the roughness of the calculation.
Also keep in mind that I am not claiming that pterosaurs were identical to modern birds. There was obviously a development of the brain in the transition from pterosaur to modern bird. But not much change was required (if any) in this respect.

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