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Shape disparity in the blade-like premolars of multituberculate mammals: functional constraints and the evolution of herbivory
Journal of Mammalogy ( IF 1.7 ) Pub Date : 2020-05-05 , DOI: 10.1093/jmammal/gyaa029
Lucas N Weaver 1 , Gregory P Wilson 1
Affiliation  

Cimolodontan multituberculates were a diverse and long-lived group of mammals characterized by large, blade-like lower fourth premolars (p4). Blade-like (plagiaulacoid) dentitions have evolved numerous times in distantly related mammalian lineages. Here we investigate how p4-shape disparity changed through time in the Cimolodonta. We address two hypotheses: (H1) blade-like dentitions constrain the ability of plagiaulacoid mammals to evolve novel dental morphologies, (H2) cimolodontan dental evolution proceeded gradually along a morphocline during the Late Cretaceous. We quantify functionally important aspects of p4 shape, including ratios reflecting height (H:L), symmetry (L1:L), and mesial-face height (H1:H), in a large sample of cimolodontans spanning the mid-Cretaceous through early Paleogene of North America (ca. 100–35 Mya). Our results do not support the morphocline hypothesis (H2) and, instead, show that cimolodontans evolved a wide range of p4 shapes by the mid-Cretaceous, and that p4-shape disparity remained stable through the Late Cretaceous. We hypothesize that the two-stage cimolodontan chewing cycle (slicing-crushing then grinding) imposed functional constraints on p4 morphology. After the Cretaceous-Paleogene boundary, p4-shape disparity increased sharply, driven by the appearance of the Taeniolabidoidea, Microcosmodontidae, and Eucosmodontidae, in the early Paleocene. We contend that the slicing-crushing functions of the p4 became less important in those taxa, relaxing functional constraints on p4 morphology. Cimolodontans that retained both the slicing-crushing and grinding function of the p4 had a more limited range of p4 morphologies, and probably were largely restricted to animal-dominated omnivory. Taxa that shifted the initial slicing-crushing function from the p4 to the incisors had fewer functional constraints on p4 morphology, and were able to increase their molar grinding capacity to exploit plant-dominated omnivory and herbivory. That the p4 was reduced in herbivorous taxa rather than modified into a broader, multi-cusped tooth lends support to the morphological constraint hypothesis (H1), and this relationship between p4 morphology and function suggests that retaining a large, blade-like p4 might have limited the range of herbivorous diets cimolodontans could exploit. These findings highlight the ecological and evolutionary limitations that specialized dentitions can impose on mammals by restricting their morphological and, in turn, functional diversification.

中文翻译:

多瘤哺乳动物叶片状前磨牙的形状差异:功能限制和食草动物的进化

Cimolodontan multituberculates 是一组多样化和长寿的哺乳动物,其特征是大而刀片状的下第四前磨牙 (p4)。刀片状(plagiaulacoid)牙列在远缘哺乳动物谱系中进化了很多次。在这里,我们研究了 Cimolodonta 中 p4 形状差异如何随时间变化。我们提出了两个假设:(H 1)刀片状牙列限制了斜齿哺乳动物进化新牙齿形态的能力,(H 2) cimolodontan 牙齿演化在晚白垩世期间沿着形态跃层逐渐进行。我们量化了 p4 形状在功能上重要的方面,包括反映高度 (H:L)、对称性 (L1:L) 和近中面高度 (H1:H) 的比率,在跨越中白垩纪到早期的大量 cimolodontans 样本中北美古近纪(约 100-35 Mya)。我们的结果不支持morphocline假设(H 2) 并且相反,表明 cimolodontans 在白垩纪中期进化出广泛的 p4 形状,并且 p4 形状差异在白垩纪晚期保持稳定。我们假设两阶段 cimolodontan 咀嚼周期(切片-粉碎然后研磨)对 p4 形态施加了功能限制。在白垩纪-古近纪边界之后,在古新世早期,受带绦虫科、微齿龙科和真齿龙科的出现驱动,p4 形状差异急剧增加。我们认为 p4 的切片粉碎功能在这些分类群中变得不那么重要,放松了对 p4 形态的功能限制。保留 p4 的切片-粉碎和研磨功能的 Cimolodontans 具有更有限的 p4 形态范围,并且可能在很大程度上仅限于以动物为主的杂食动物。将初始切片粉碎功能从 p4 转移到门牙的分类群对 p4 形态的功能限制较少,并且能够增加其磨牙研磨能力以利用植物主导的杂食性和食草性。p4 在食草类群中被减少而不是被修改成更广泛的多尖牙,这为形态学约束假设提供了支持(H1 ),p4 形态和功能之间的这种关系表明,保留一个大的、刀片状的 p4 可能限制了西莫罗丹斯可以利用的食草饮食范围。这些发现强调了特殊牙列可以通过限制哺乳动物的形态和功能多样化而对哺乳动物施加的生态和进化限制。
更新日期:2020-05-05
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