SIMPLIFICATION AS A TREND IN SYNAPSID CRANIAL EVOLUTION
SIMPLIFICATION AS A TREND IN SYNAPSID CRANIAL EVOLUTION
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简化是联孔类颅骨进化的趋势
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
C. Sidor
中科院分区：
文献类型：
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作者：
C. Sidor
摘要 近年来，化石记录中形态趋势的普遍性和意义受到了重新审视。研究通常集中在身体尺寸进化的趋势上，这些趋势产生了相互矛盾的结果，并且很少解决其他形态特征是否在长时间尺度上表现出持续的方向性的问题。我研究了哺乳动物前突触类动物大约 1.5 亿年历史中头骨和下颌骨数量的减少（通过丢失或融合）。新的头骨简化度量（SSM）被定义为不同元素数量的函数，其结果表明，在时间（即地层）和系统发育尺度上都存在明显的简化。颅后进化也表现出类似的模式。相比之下，头骨的大小与存在的不同头骨的数量关系不大。在晚石炭世和早二叠世的大部分时间里，合弓动物头骨所携带的元素数量接近其观察到的最大数量。 SSM 在晚二叠世下降，但与早期兽孔类的辐射一致，观测到的 SSM 值范围在此期间扩大。从早三叠世的衍生非哺乳动物犬齿兽到早侏罗世最早的哺乳动物，最小和最大SSM均下降。来自三种代表性现代哺乳动物（鸭嘴兽、负鼠和人类）的数据表明，这种趋势在整个新生代持续存在。在系统发育的背景下，分类单元中存在的头骨元素的数量与从树根传递的分支事件的数量显示出显着的负相关关系。嵌入越深的类群具有较小的 SSM 分数。这种关系也适用于各种合弓亚群。尽管通常归因于长期选择的影响，但进化趋势也可以反映形态变化中潜在的内在偏差。就合弓动物的头骨（以及其他一些四足动物谱系的头骨）而言，新颖或新形态元素的罕见产生可能有助于观察到骨骼简化的趋势。通讯编辑：M. Zelditch
Abstract The prevalence and meaning of morphological trends in the fossil record have undergone renewed scrutiny in recent years. Studies have typically focused on trends in body size evolution, which have yielded conflicting results, and have only rarely addressed the question as to whether other morphological characteristics show persistent directionality over long time scales. I investigated reduction in number of skull and lower jaw bones (through loss or fusion) over approximately 150 million years of premammalian synapsid history. The results of a new skull simplification metric (SSM), which is defined as a function of the number of distinct elements, show that pronounced simplification is evident on both temporal (i.e., stratigraphic) and phylogenetic scales. Postcranial evolution exhibits a similar pattern. Skull size, in contrast, bears little relationship with the number of distinct skull bones present. Synapsid skulls carried close to their observed maximum number of elements for most of the Late Carboniferous and Early Permian. The SSM decreased in the Late Permian but, coincident with the radiation of early therapsids, the range of observed SSM values widened during this interval. From derived nonmammalian cynodonts in the Early Triassic through the earliest mammals in the Early Jurassic, both the minimum and maximum SSM decreased. Data from three representative modern mammals (platypus, opossum, and human) suggest that this trend continues through the Cenozoic. In a phylogenetic context, the number of skull elements present in a taxon shows a significant negative relationship with the number of branching events passed from the root of the tree; more deeply embedded taxa have smaller SSM scores. This relationship holds for various synapsid subgroups as well. Although commonly ascribed to the effects of long-term selection, evolutionary trends can alternatively reflect an underlying intrinsic bias in morphological change. In the case of synapsid skull bones (and those of some other tetrapods lineages), the rare production of novel, or neomorphic, elements may have contributed to the observed trend toward skeletal simplification. Corresponding Editor: M. Zelditch