Increased sampling reveals the complex evolution of sauropod dinosaur tooth replacement rates.

Abstract

Unlike most herbivores, sauropod dinosaurs evolved simple teeth that were replaced rapidly. Sauropod craniodental morphology is conserved relative to that of many archosaur clades, but tooth breadth and replacement rate vary substantially. Two neosauropod clades, Titanosauria and Diplodocoidea, independently evolved both narrow-crowned teeth and high tooth replacement rates among a suite of other convergent features. Brachiosaurids also evolved somewhat narrower-crowned teeth, but the two brachiosaurids whose tooth replacement rate has been examined to date have low replacement rates. Poor and uneven sampling across Sauropoda limits our understanding of the evolution of tooth replacement rate and related ecological inferences. To better understand the evolution of tooth replacement rate and tooth breadth, we integrated histological and tomographic data to nearly double the number of examined sauropod genera, focusing on improved sampling through the Cretaceous. We provide histological descriptions of the dentine and enamel of two Early Cretaceous taxa, Abydosaurus and Moabosaurus. The former has unusually thin daily increments in its dentine, indicating prolonged tooth formation times. The dentine of the latter is typical of what is observed in most sauropods, but it has enigmatic banding in its enamel. We performed ancestral state reconstruction on a time-calibrated phylogeny to show that the earliest macronarians and brachiosaurids retain the ancestral sauropod condition of relatively low tooth replacement rates (2-3 months to replace a tooth in each alveolus), whereas diplodocoids evolved much higher rates (2-5 weeks to replace a tooth in each alveolus). Early diverging somphospondylans had a broad range of tooth replacement rates. Broad-crowned teeth exhibit some correlation with low tooth replacement rates, whereas narrow-crowned teeth display a more variable relationship with replacement rate.