Abstract:

Tooth reduction and loss was an important evolutionary process in Mesozoic birds. Analysis of evolutionary trends in the total mass of the dentition, a function of tooth size and tooth number, has the potential to shed light on the evolutionary pattern of tooth reduction and loss, and on the causes of this pattern. Because modern birds lack teeth, however, they cannot provide the basis for a model that would allow estimation of tooth masses in their Mesozoic counterparts. We selected the teeth of crocodilians as analogues of those in Mesozoic birds because the former are the closest living relatives of the latter, and the two groups are similar in tooth morphology, tooth implantation, and tooth replacement pattern. To estimate tooth masses in Mesozoic birds, we formulated four regression equations relating tooth mass to various linear dimensions, which were measured in 31 intact isolated teeth from eight individual crocodiles (Crocodylus siamensis). The results for Mesozoic birds show that dental mass as a proportion of body mass was negligible, at least from the perspective of flight performance, suggesting that selection pressure favoring body mass reduction was probably not the primary driver of tooth reduction or loss. Variations in dental mass among Mesozoic birds may reflect the different foods they ate, and the different types of feeding behavior they displayed.

Key words: Mesozoic birds, dental reduction, tooth mass, body mass, feeding behavior

摘要：

牙齿退化是中生代鸟类演化的一个重要过程,牙齿总重综合了牙齿大小和数量的信息,研究这一特征的演化趋势,有助于深入地分析中生代鸟类牙齿退化的原因和方式。然而,现生鸟类均不具齿,无法为研究中生代鸟类的牙齿重量提供参考。除鸟类外的现生脊椎动物中,鳄类与鸟类的亲缘关系最近,且牙齿形态、着生和替换方式与后者相似,因此可为估算中生代鸟类牙齿重量提供参考模型。对从8件现生暹罗鳄标本采得的31枚牙齿进行了形态和重量测量,基于缩放比例原理建立多组回归方程,依此方程对牙齿和齿列保存较完整的中生代鸟类标本进行了牙齿总重估计。结果表明多数中生代鸟类牙齿总重普遍占体重比例极小,据此推测其对飞行的影响可忽略不计,减轻体重的自然选择压力可能不是造成中生代鸟类牙齿退化的主要原因。中生代鸟类牙齿总重的多样性可能反映了其食性和取食行为的差异。

关键词: 中生代鸟类, 牙齿退化, 牙齿重量, 体重, 取食行为