Evolution of high tooth replacement rates in theropod dinosaurs

Tooth replacement rate is an important contributor to feeding ecology for polyphyodont animals. Dinosaurs exhibit a wide range of tooth replacement rates, mirroring their diverse craniofacial specializations, but little is known about broad-scale allometric or evolutionary patterns within the group. In the current broad but sparse dinosaurian sample, only three non-avian theropod tooth replacement rates have been estimated. We estimated tooth formation and replacement rates in three additional non-avian theropod dinosaurs, the derived latest Cretaceous abelisaurid Majungasaurus and the more generalized Late Jurassic Allosaurus and Ceratosaurus. We created the largest dental histological and CT dataset for any theropod dinosaur, sectioning and scanning over a dozen toothed elements of Majungasaurus and several additional elements from the other two genera. Using this large sample, we created models of tooth formation time that allow for theropod replacement rates to be estimated non-destructively. In contrast to previous results for theropods, we found high tooth replacement rates in all three genera, with Allosaurus and Ceratosaurus rates of ~100 days and 56 days for Majungasaurus. The latter rate is on par with those of derived herbivorous dinosaurs including some neosauropods, hadrosaurids, and ceratopsians. This elevated rate may be a response to high rates of tooth wear in Majungasaurus. Within Dinosauria, there is no relationship between body mass and tooth replacement rate and no trends in replacement rate over time. Rather, tooth replacement rate is clade-specific, with elevated rates in abelisaurids and diplodocoids and lower rates in coelurosaurs.

换齿率是影响多生齿动物（polyphyodont）取食生态的重要因素。恐龙类群的换齿率差异显著，反映出其多样化的颅面特化特征，但目前学界对该类群内大范围的异速生长模式与演化规律仍知之甚少。在当前覆盖范围较广但样本量稀缺的恐龙研究样本中，仅估算过3种非鸟类兽脚类的换齿率。本研究额外估算了3种非鸟类兽脚类恐龙的牙齿形成时长与换齿率，分别是高度特化的晚白垩世晚期阿贝力龙科玛君龙（Majungasaurus），以及相对原始的晚侏罗世异特龙（Allosaurus）与角鼻龙（Ceratosaurus）。本研究构建了目前规模最大的兽脚类恐龙牙齿组织学与CT扫描数据集：对玛君龙的十余枚带齿颌骨标本进行切片与扫描，并对另外两个属的多件标本开展了相同处理。基于该大样本数据集，我们建立了牙齿形成时长模型，可用于非破坏性估算兽脚类恐龙的换齿率。与此前兽脚类的相关研究结果不同，我们发现这三个属的换齿率均较高：异特龙与角鼻龙的换齿周期约为100天，玛君龙则为56天。玛君龙的这一换齿率可与部分特化植食性恐龙比肩，包括部分新蜥脚类（neosauropods）、鸭嘴龙科（hadrosaurids）以及角龙类（ceratopsians）。该较高的换齿率可能是对玛君龙牙齿高磨损率的适应性响应。在整个恐龙类群中，体重与换齿率之间并无关联，换齿率也未随时间呈现出显著的演化趋势。相反，换齿率具有支系特异性：阿贝力龙科（abelisaurids）与梁龙超科（diplodocoids）的换齿率较高，而虚骨龙类（coelurosaurs）的换齿率则较低。