-Fossil birds, and other fossils with bird-like features, allow us to determine the age of birds, the ancestors of birds, the series of changes that birds underwent in evolution, whether modern birds are related to the radiation of fossil birds in the Mesozoic, and what makes a bird a bird. -Evolution proceeds when a new heritable trait emerges in some organism and is passed genetically to its descendants. The idea indicates that two groups of animals that share a set of these new or "derived" traits are more closely related to each other than they are to groups that display only the original traits but not the derived ones. Of course these traits can be lost subsequently, and similar traits can evolve more than once, but tracking these traits is the best way to work out the phylogenetic relationships among organisms. May track more than one trait, such as both feathers and skeletons. Results of these analytical tracks is a "cladogram," a treelike diagram that describe the order in which new traits and new creature evolves. Each branching point reflects the emergence of a ancestor that founded a group having derived characters that were not present in the group from which it evolved. The ancestor and its descendants constitute a branch in the tree; root word "clade" = branch. This style of reasoning is cladistics. The groups are hierarchial in a historical sense, a branch includes all animals that shared a recent common ancestor. "What is a bird" is a historical question. -Two theories of the age of diversity of modern bird orders. (1) orders may date from Eocene, 65 million years ago, massive extinction of earlier kinds of birds was associated with global changes brought about when an asteroid storm hit the earth from outer space. This idea is based on fossil birds, which show no clear match between the older fossils and the kinds of bird groups now on earth, all the major groups radiated around the Eocene (Feduccia, Science, 267:637, 1995). (2) molecular genetics studies, based on extrapolation of rates of 1-2% change / million years, as calibrated from a few fossil forms, suggest the major groups of birds go back even earlier, to > 80 million years ago, and there was no strong evidence for a K-T extinction (Padian & Chiappe 1998). The same debate has affected mammal paleontology. We need to work on gathering fossils and developing molecular techniques to narrow down this 15my discrepancy.

中文翻译：

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鸟类学

-化石鸟类，以及其他具有鸟类特征的化石，让我们能够确定鸟类的年龄、鸟类的祖先、鸟类在进化过程中所经历的一系列变化、现代鸟类是否与化石鸟类在地球上的辐射有关。中生代，以及什么使鸟成为鸟。- 当某种生物体中出现新的可遗传特征并通过遗传传递给其后代时，进化就会进行。这个想法表明，与仅显示原始特征而不显示衍生特征的群体相比，共享一组这些新的或“衍生”特征的两组动物彼此之间的关系更密切。当然，这些性状随后可能会丢失，类似的性状可能会进化不止一次，但追踪这些性状是找出生物之间系统发育关系的最佳方式。可能追踪不止一种特征，例如羽毛和骨骼。这些分析轨迹的结果是一个“分支图”，一种描述新特征和新生物进化顺序的树状图。每个分支点都反映了一个祖先的出现，该祖先建立了一个具有派生特征的群体，这些特征在其进化的群体中并不存在。祖先及其后代构成树中的一个分支；词根“clade”= 分支。这种推理方式是cladistics。这些群体在历史意义上是有等级的，一个分支包括所有最近有共同祖先的动物。“什么是鸟”是一个历史问题。-现代鸟类多样性时代的两种理论。(1) 订单可以追溯到 6500 万年前的始新世，早期鸟类的大规模灭绝与小行星风暴从外太空撞击地球所带来的全球变化有关。这个想法是基于鸟类化石，在较旧的化石和现在地球上的鸟类群体之间没有明显的匹配，所有主要群体都围绕始新世辐射（Feduccia，科学，267：637，1995）。(2) 分子遗传学研究，基于 1-2% 变化率/百万年的外推，根据一些化石形式的校准，表明主要鸟类群可以追溯到更早，到 > 8000 万年前，并且有不是 KT 灭绝的有力证据（Padian & Chiappe 1998）。同样的争论也影响了哺乳动物古生物学。我们需要致力于收集化石和开发分子技术来缩小这个 15my 的差异。