On the vestibular labyrinth of Brachiosaurus brancai

The extensive remains of large sauropods, excavated in the Upper Jurassic layers of the Tendaguru region of Tanzania, East Africa by Janensch [15], include an intact fossil cast of a vestibular labyrinth and an endocast of the large Brachiosaurus brancai. The approximately 150 million year old labyrinth cast demonstrates clearly a form and organisation congruent in detail to those of extant vertebrate species. Besides the near-orthogonal arrangement of semicircular canals (SCCs), the superior and inferior branches of the vestibulo-acoustic nerve, the endolymphatic duct, the oval and round windows, and the cochlea can be identified. The orientation of the labyrinth in the temporal bone is also equivalent to that of many extant vertebrates. Furthermore, the existence of the twelve cranial nerves can be identified from the endocast. The present study was initiated after the photogrammetric measurement of the skeleton volume of B. brancai [13] yielded a realistic estimate of body mass (74.42 metric tons). Dimensional analysis shows that body mass and average SCC dimensions of B. brancai generally fit with the allometric relationship found in previous studies of extant species. However, the anterior SCC is significantly larger than the allometric relationship would predict. This would indicate greater sensitivity, supporting the idea that the behavioural repertoire must have included much slower pitch movements of the head. These slower movements would most likely have involved flexion of the neck, rather than head pitching about the atlas joint. Pursuing the relationship between body mass and SCC dimensions further, the SCC frequency response is estimated by scaling up from the SCC dimensions of the rhesus monkey; this yields a range between 0.008-26 Hz, approximately one octave lower than for humans.