A reanalysis of Osborn's titanothere data indicates that extrapolative growth of a constant allometric relationship alone ("hypermorphosis") does not account for the trend toward body and horn size increase. Empirically, we also observe possible positive changes in the y-intercept ("predisplacement" or early onset of development), and possibly, changes in the slope ("acceleration/neoteny" or changes in growth rate) between the Oligocene and Eocene groups. Theoretically, these may be responses to more massive body shapes, perforce accompanying size increase, which increase the amount of force to which the horns were subjected.
© 1985 The Society for the Study of Evolution.