THUNDER LIZARD SONG AND DANCE: SKULLS FROM BRONTOSAURUS AND APATOSAURUS SHOW MARKED DIVERGENCE IN VOCALIZATION AND UPRIGHT COMBAT

Apatosaurines are giant Late Jurassic herbivorous dinosaurs, defined by short, massive forelimbs, curiously delicate triangular muzzles, and exceptionally wide, air-filled neck vertebrae (the largest among vertebrates). Best known are Apatosaurus ajax,  discovered in 1877, and Brontosaurus excelsus, discovered in 1879.  Since 1905, most scholars have merged the two genera -- but recent analysis of postcrania has reaffirmed their separation. Our new material shows that the head-neck mechanics differed markedly. The head-neck joint on the skull - the occipital condyle -- is far wider relative to skull size in Apatosaurus. The attachments for muscles that flex the head-neck joint laterally -- the basal tubera -- are also much wider in Apatosaurus, and so the leverage for turning the head and the anterior neck was greater. For a given body mass, Apatosaurus had wider middle and posterior neck vertebrae than did Brontosaurus. Therefore the outer sheath of neck muscles had greater leverage for lateral flexure. The greater power at the head-neck joint and all along the neck increased the potential energy transmitted by a blow of the entire cranial-cervical unit. The posture differed between the two genera. The maximum leverage for the head-neck flexor occurs at the mid range of movement regularly employed by the animal. Average head-neck posture in Apatosaurus was less flexed downward than in Brontosaurus.

Apatosaurine maxillary and jugal bones are very thin with narrow zones of contact, allowing vibration when air was expelled forcefully. Apatosaurus snouts were lighter than those of Brontosaurus because of expansion of the antorbital fenestra, so the call made by snout vibration should have been higher in pitch. Apatosaurine footprints, identified by hind claws with distinctive square-tips, show a hind paw area eight times greater than fore paw area, corroborating the theory that apatosaurs could rear up easily. In such posture the neck could be used to deliver blows during intra-specific conflict. Thus Brontosaurus and Apatosaurus probably diverged markedly in posture, vocalization and collision behavior.