Secondary adaptation to aquatic life: an integrative morphofunctional analysis in Cetartiodactyla

生活方式的转变是主要的进化事件，通常反映在极端的形态改变上。一个特别明显的例子是鲸鱼对水生生活方式的二次适应。齿鲸和须鲸（鲸目动物）与偶蹄动物（“偶蹄动物”）一起被归入鲸目动物分支。他们的早期化石记录记录了“始祖鲸类”的物种，这些物种对于理解从陆地环境到水生环境的主要进化转变至关重要。已知最早的巴基蝶科的成员大多生活在陆地上，对水有一些适应。他们生活在始新世早期（约5000万年前）。四足鲸“原鲸科”的多样化主要发生在始新世中晚期。它们代表了鲸类动物运动进化的关键阶段，显示出接近现代水獭的游泳能力。该项目将特别关注来自秘鲁的一种新的始新世中原始动物，它的颅骨后元素保存得非常完好，名为Peregocetus pacificus。将结合外部形态学观察和计算机断层扫描数据的测量，对其颅后大体形态学和显微解剖学进行全面描述，并与其他已灭绝和现存的哺乳动物进行比较，以解释关键解剖特征的功能。对土地-水过渡的重要变量将用系统发育比较方法进一步研究。这将包括重建这些变量最可能的祖先状态，并绘制这些水生适应在鲸足动物系统发育中的进化图。我们将通过进化模型的比较，找出最可能导致灭绝鲸转向水生生活方式的进化模型。最后，利用多元数据集进行系统发育判别分析，重建太平洋鲸和其他已灭绝鲸类动物的运动模式。

Lifestyle transitions are major evolutionary events often reflected by extreme morphological modifications. One particularly obvious example is the secondary adaptation of whales to an aquatic lifestyle. Toothed and baleen whales (Cetacea) are included in the Cetartiodactyla clade with even-toed ungulates (“Artiodactyla”). Their early fossil record documents species within the paraphyletic “Archaeoceti” that are crucial to understand this major evolutionary transition from the terrestrial to the aquatic environment. Members of the earliest known family, the Pakicetidae, were mostly terrestrial with some adaptations to water. They lived during the early Eocene (ca. 50 million years ago). And it is mostly during the middle to late Eocene that the quadrupedal whales “Protocetidae” diversified. They represent a key stage of cetacean locomotor evolution, displaying swimming abilities close to modern otters. This project will focus in particular on a new middle Eocene protocetid from Peru, with exceptionally well-preserved postcranial elements, Peregocetus pacificus. Its postcranial gross morphology and microanatomy will be thoroughly described – combining external morphology observation and measurements to computed tomography data – and compared with other extinct and extant mammals to interpret the function of key anatomical traits. Variables of importance for the land-to-water transition will be further studied with phylogenetic comparative methods. This will involve reconstructing the most likely ancestral states for these variables and mapping the evolution of these aquatic adaptations on the phylogeny of cetartiodactyls. We will identify the most likely evolutionary model that led extinct whales to an aquatic lifestyle by means of evolutionary model comparison. Finally, a multivariate dataset will be used in a phylogenetically informed discriminant analysis to reconstruct the locomotion mode of Peregocetus pacificus and other extinct cetaceans.