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。在接下来的大灭绝事件中，曾经是陆地脊椎动物中占主导地位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，它们不仅仅是生存下来，它们反过来又成为大多数生态系统中占主导地位的脊椎动物。然而，科学界仍然不明白哺乳动物是如何在灭绝后的世界中蓬勃发展的。许多哺乳动物化石从灭绝事件后的最初几百万年被发现，包括第一个大型哺乳动物食草动物。然而，目前还不清楚这些不寻常的化石与今天活着的哺乳动物有什么关系-它们是直系祖先还是仅仅是远亲？该项目将侧重于小行星撞击后最初1 200万年这一被忽视的化石记录，这一时期的特点是全球气温高，全球迅速变暖。通过了解这些灭绝的哺乳动物是什么，该项目将探索这次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快速变化的气候中恢复和进化。今天，地球正在经历一场现代灭绝事件，因为环境变化和人类行为正在威胁许多物种。尽管目前的环境与6600万年前的环境不同，但了解后灭绝世界中潜在的生物学过程将指导我们的未来。争议围绕着胎盘哺乳动物的起源以及古近纪灭绝事件在其进化中的作用，一方面来自形态学的证据，另一方面来自DNA的证据不一致。基于DNA的研究认为白垩纪的胎盘起源，K-Pg事件几乎没有影响，而基于形态学的研究则认为K-Pg事件驱动了古近纪的起源。以前的胎盘动物系统发育分析大量地从较老（白垩纪）和较年轻（始新世，5600万至3400万年前或更早）的类群中取样，但古新世（6600万至5400万年前）相当大的哺乳动物多样性在很大程度上被忽视了。该项目将纠正这一缺陷，包括在一个全面的系统发育，旨在直接解决有关的K-PG事件的影响的争议古新世血统。来自262个灭绝和现存分类群的1,500多个形态特征，包括一些用计算机断层扫描（CT）研究的特征，将与来自现存哺乳动物的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进行分析。重建的系统发育树将用于使用分歧时间分析来解决胎盘动物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的进化率。进化和多样性率将与古生态数据进行比较，以评估早第三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.