Eat, heat and listen: on becoming a mammal

哺乳动物有着共同的独特特征，如温血动物（内温动物）、只更换一次的前牙（二齿动物）、耳朵中有三个高频声音检测骨骼和一个新的下颌关节、增强的感官、毛发或毛皮以及哺乳。哺乳动物的出现预示着“古代”动物群被我们今天认识的动物（包括鸟类，蜥蜴和蛇）所取代，以及现代陆地生态系统的起源。传统的观点认为，这些特征是以一种综合的方式进化的，然而新的数据和方法表明特征的逐步或镶嵌进化，挑战了我们对哺乳动物起源的进化驱动力的观点。这项提议的目的是解决这个问题，并确定哺乳动物的三个关键特征是如何、为什么以及何时进化的：内温动物、二齿动物和颌关节-中耳复合体。骨骼特征，如牙齿、颌骨和耳骨都记录在化石记录中。“软”性状，如感觉能力或恒温性，并不排除，并从其他证据推断。在过去的10年里，新的化石和技术丰富了我们的理解，但呈现出一幅复杂的画面。目前的研究将恒温动物的起源追溯到从石炭纪到早白垩纪的任何地方--大约2亿年的时间。我们的研究小组质疑了传统的、教科书上关于耳朵和下巴进化的叙述，以及哺乳动物最接近的祖先是否已经是温血动物。这些问题远未解决。此外，我们缺乏一个全面的视角，因为与哺乳动物祖先密切相关的南美洲化石尚未被纳入定量分析。此外，尽管活的哺乳动物的颌骨和耳朵的发育数据是几十年来哺乳动物起源争论的关键，但我们仍然缺乏真实的整合发育和古生物学数据来理解哺乳动物的颌骨，耳朵和牙齿进化。在这个建议中，我们试图通过多学科的方法来解决哺乳动物起源中颌骨，耳朵，牙齿和内温动物进化的系统发育和时间基础，包括：a，与合作伙伴和来自南美洲的新化石数据合作，这是理解哺乳动物起源的关键; B，整合化石和发育序列数据的研究; c.应用新的方法推导“软”性状、生长和吸热潜力。我们将生成跨越哺乳动物起源的化石的高分辨率X射线断层扫描数据集。我们将使用我们团队开创的新技术来计算非细胞牙骨质中的年轮，就像树木的年轮一样，保留了个体化石及其牙齿的年龄。这将告诉我们（a）牙齿长出时的年龄，以及哺乳动物起源前后牙齿替换的模式和速度，从而为早期哺乳动物牙齿进化提供一个机械模型。然后，利用牙骨质带的数量和宽度，我们将（B）确定相对生长速率，并利用与活动物的相关性来确定过渡化石是否具有哺乳动物或“爬行动物”的生理机能，解决恒温动物的争论。最后，我们将使用鸭嘴兽，针鼹，负鼠和小鼠的发育序列，包括在出生时保留后齿骨的突变c-Fos小鼠，沿着新的化石数据集，以i。建立发育和进化解剖学; ii.利用有限元分析方法，将发育中关节的功能与哺乳动物起源时期颌骨关节化石的解剖和功能进行比较。这将告诉我们，在哺乳动物颌骨的进化过程中，是否有类似的功能限制和适应。这种新颖的，多学科的和尖端的方法和数据集的组合从未被应用于哺乳动物或脊椎动物进化的问题。该项目将揭示有关哺乳动物起源的基本新见解，并将为未来的此类研究开创先例。

Mammals share unique characters such as warm bloodedness (endothermy), anterior teeth that replace only once (diphyodonty), three high-frequency sound detecting bones in the ear and a novel jaw joint, enhanced senses, hair or fur, and lactation. The appearance of mammals heralded the replacement of 'ancient' faunas with animals we recognise today including birds, lizards and snakes and the origins of modern terrestrial ecosystems. The traditional view holds that these traits evolved in an integrated fashion, however new data and methods suggest either stepwise or mosaic evolution of traits, challenging our perspective of the evolutionary drivers of mammalian origins. The aim of this proposal is to resolve this problem and determine how, why and when three key mammalian traits evolved: endothermy, diphyodonty, and the jaw joint - middle ear complex. Bony traits such as teeth, jaws and ear bones are recorded in the fossil record. 'Soft' traits such as sensory abilities or endothermy do not fossilize and are inferred from other evidence. In the past 10 years, new fossils and technologies have enriched our understanding but present a complex picture. Current studies date the origins of endothermy to anywhere from the Carboniferous to the Early Cretaceous - a range of nearly 200 million years. Our team have questioned the traditional, textbook narrative of ear and jaw evolution, and whether the closest ancestors of mammals were already warm-blooded. These questions are far from resolved. Furthermore, we lack a thorough perspective as South American fossils closely related to mammal ancestors are yet to be included in quantitative analysis. Moreover, despite data on the development of the jaw and ear in living mammals being pivotal to the mammalian origins debate for decades, we still lack real integration of developmental and palaeontological data in understanding mammalian jaw, ear and tooth evolution. In this proposal we seek to resolve the phylogenetic and temporal basis of the evolution of jaws, ears, teeth and endothermy across the origin of mammals via a multidisciplinary approach that involves: a, working with partners and new fossil data from South America key to understanding mammalian origins; b, integrating the study of fossils and developmental sequence data; c, applying novel methods to deduce 'soft' traits, growth and endothermic potential. We will generate high-resolution X-ray tomography datasets of fossils that span mammalian origins. We will use novel techniques pioneered by our team to count rings in acellular dental cementum, that, like tree rings, preserve the age of an individual fossil and its teeth. This will tell us (a) how old teeth were when they erupted and the pattern and rate of tooth replacement pre- and post mammalian origins, producing a mechanistic model for early mammalian dental evolution. Then, using the number and width of cementum bands, we will (b) determine relative growth rates and use correlates with living animals to ascertain whether transition fossils possess a mammalian or 'reptilian' physiology, resolving the endothermy debate. Finally, we will use developmental sequences of platypus, echidna, opossum and mouse, including mutant c-Fos mice that retain postdentary bones at birth, along with new fossil datasets, to i. establish developmental and evolutionary anatomy; ii. using finite element analysis compare the function of developing joints to the anatomy and function of fossil jaw joints across the origin of mammals. This will tell us whether similar functional constraints and adaptation are at play in the evolution of the mammalian jaw as occur during development. This novel, multidisciplinary and cutting-edge combination of methods and datasets has never been applied to questions of mammalian, or indeed vertebrate evolution. The project will uncover fundamental new insights on the origin of mammals and will set a precedent for future studies of this kind.