Collaborative Research: A Phylogeny of Placental Mammals based on Paleocene Taxa: Determining the Impact of the K-Pg Extinction on Mammalian Evolutionary History

6600万年前，一颗小行星撞击墨西哥湾，深刻地改变了地球上的生命。在随后的大规模灭绝事件中，曾经是占主导地位的陆地脊椎动物的非鸟类恐龙没有幸存下来。其中一个幸存的群体是哺乳动物；然而，它们不仅仅是存活下来，它们反过来成为大多数生态系统中的优势脊椎动物。然而，科学界仍然不理解哺乳动物是如何在后灭绝世界中蓬勃发展的。在物种灭绝事件后的最初几百万年里，已经发现了许多哺乳动物化石，其中包括第一种大型哺乳动物食草动物。然而，目前还不清楚这些不同寻常的化石与今天活着的哺乳动物之间的关系--他们是直系祖先还是仅仅是远亲？该项目将重点研究小行星撞击后最初1200万年被忽视的化石记录，这段时间的特点是全球气温很高，全球迅速变暖的时间很短。通过了解这些灭绝的哺乳动物是什么，这个项目将探索这种灭绝事件在哺乳动物进化中的真正作用，研究哪些种类的哺乳动物幸存下来，以及它们如何以及为什么在非常迅速变化的气候中恢复和进化。今天，地球正在经历一场现代的灭绝事件，因为环境变化和人类行动正在威胁着许多物种。尽管目前的情况与6600万年前不同，但理解后灭绝世界中潜在的生物过程将指导我们未来。控制围绕胎盘哺乳动物的起源和白垩纪-古近纪(K-PG)灭绝事件在它们进化中的作用，一方面来自形态的证据，另一方面是DNA的分歧。基于DNA的研究认为是白垩纪的胎盘起源，K-PG事件几乎没有影响，而基于形态的研究则认为是由K-PG事件驱动的古近纪起源。以前对胎盘动物的系统发育分析大量地从较老的(白垩纪)和较年轻的(始新世，5600万至3400万年前或更早)分类群中取样，但从古新世(6600万至5400万年前)开始的大规模哺乳动物多样性在很大程度上被忽视了。该项目将通过将大多数古新世谱系纳入全面的系统发展史来纠正这一缺点，旨在直接解决关于K-PG事件影响的争议。来自262个灭绝和现存分类群的1500多个形态特征，包括一些用计算机断层扫描(CT)研究的特征，将与现存哺乳动物26个核基因的35,000多个碱基对相结合，并使用最大简约法、最大似然法和贝叶斯方法一起分析。重建的系统发育树将用于通过分歧时间分析来解决胎座的起源和主要亚支的多样性，并使用基于模型的技术来估计身体大小和形态特征进化速率。进化和多样性比率将与古生态数据进行比较，以评估早古近纪气候变化对哺乳动物历史的影响。

Sixty-six million years ago, an asteroid impact in the Gulf of Mexico profoundly changed life on Earth. In the following mass extinction event, non-avian dinosaurs, which had been the dominant terrestrial vertebrates, did not survive. One surviving group was the mammals; yet, they did more than just survive, they in turn became the dominant vertebrates in most ecosystems. However, the scientific community still does not understand how mammals flourished in the post-extinction world. Numerous mammalian fossils from the first few million years after the extinction event have been discovered, including the first large mammalian herbivores. However, it is unclear how these unusual fossils relate to mammals alive today - are they direct ancestors or merely distant cousins? This project will focus on this neglected fossil record from the first 12 million years after the asteroid impact, a time characterized by high global temperatures and brief episodes of rapid global warming. By understanding what these extinct mammals were, this project will explore the true role of this extinction event in the evolution of mammals, examining which kinds of mammals survived, and how and why they recovered and evolved in a very rapidly changing climate. Today, Earth is experiencing a modern-day extinction event, as environmental change and human actions are threatening many species. Although the current circumstances differ from those 66 million years ago, understanding the underlying biological processes acting in a post-extinction world will guide us in the future.Controversy surrounds the origin of placental mammals and the role of the Cretaceous-Paleogene (K-Pg) extinction event on their evolution, with evidence from morphology on the one hand and DNA on the other at odds. DNA-based studies argue for a Cretaceous placental origin, with the K-Pg event having little impact, whereas morphology-based studies argue for a Paleogene origin driven by the K-Pg event. Previous phylogenetic analyses of placentals have sampled heavily from older (Cretaceous) and younger (Eocene, 56 to 34 million years ago or earlier) taxa, but the sizable mammalian diversity from the Paleocene (66 to 54 million years ago) has been largely ignored. This project will correct this shortcoming by including most Paleocene lineages in a comprehensive phylogeny designed to directly address the controversy regarding the impact of the K-Pg event. Over 1,500 morphological characters, including some studied with computer tomography (CT), from 262 extinct and extant taxa will be combined with over 35,000 base pairs from 26 nuclear genes from extant mammals, and analyzed together using maximum parsimony, maximum likelihood, and Bayesian methodology. The reconstructed phylogenetic trees will be used to resolve the origin of Placentalia and diversification of major subclades using divergence time analyses, and to estimate body size and morphological character evolution rates using model-based techniques. The evolutionary and diversity rates will be compared to paleoecological data to assess the impact of early Paleogene climate variation on mammalian history.