Sidor C A
Department of Paleobiology, Smithsonian Institution, Washington, DC 20560, USA.
Evolution. 2001 Jul;55(7):1419-42. doi: 10.1111/j.0014-3820.2001.tb00663.x.
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.
近年来，化石记录中形态学趋势的普遍性和意义受到了新的审视。研究通常集中在体型进化趋势上，而这些研究结果相互矛盾，并且很少涉及其他形态特征在长时间尺度上是否显示出持续方向性的问题。我研究了在大约1.5亿年的前哺乳动物合弓纲历史中，头骨和下颌骨数量的减少（通过丢失或融合）情况。一种新的头骨简化度量（SSM）的结果表明，显著的简化在时间（即地层）和系统发育尺度上都很明显，该度量被定义为不同元素数量的函数。颅后进化呈现出类似的模式。相比之下，头骨大小与现存不同头骨骨头的数量几乎没有关系。在石炭纪晚期和二叠纪早期的大部分时间里，合弓纲头骨的元素数量接近其观察到的最大值。SSM在二叠纪晚期下降，但与早期兽孔类的辐射同时发生的是，在此期间观察到的SSM值范围扩大。从三叠纪早期的衍生非哺乳类犬齿兽类到侏罗纪早期的最早哺乳动物，SSM的最小值和最大值都下降了。来自三种有代表性的现代哺乳动物（鸭嘴兽、负鼠和人类）的数据表明，这种趋势一直持续到新生代。在系统发育背景下，一个分类单元中存在的头骨元素数量与从树的根部经过的分支事件数量呈显著负相关；嵌入更深的分类单元的SSM分数更小。这种关系在各种合弓纲亚群中也成立。尽管进化趋势通常被归因于长期选择的影响，但它们也可能反映了形态变化中潜在的内在偏差。就合弓纲头骨骨头（以及其他一些四足动物谱系的骨头）而言，新元素（即新形态元素）的罕见产生可能促成了观察到的骨骼简化趋势。
