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

生活方式的转变是重大的进化事件，通常反映在极端的形态变化上。一个特别明显的例子是鲸鱼对水生生活方式的二次适应。有齿鲸和须鲸(鲸目)与偶趾有蹄目动物(“偶蹄目”)一起被包括在鲸类分支中。他们早期的化石记录记录了近古古生物中的物种，这些物种对于理解这一从陆地环境到水生环境的重大进化转变至关重要。已知的最早的一科--袋鼠科的成员大多是陆生的，对水有一定的适应。他们生活在始新世早期(约5000万年前)。主要是在始新世中晚期，四足鲸“原始鲸科”的物种多样化。它们代表了鲸目动物运动进化的关键阶段，展示了与现代水獭相似的游泳能力。该项目将特别关注来自秘鲁的一种新的中始新世原生生物，其具有保存极为完好的后颅骨成分--太平洋Peregoctus。将详细描述其颅后大体形态和显微解剖-结合外部形态观察和计算机断层扫描数据的测量-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。将用系统发育比较的方法进一步研究陆地向水过渡的重要变量。这将涉及重建这些变量最有可能的祖先状态，并将这些水生适应的进化映射到十六龙类的系统发育中。我们将通过进化模型比较的方法，确定最有可能导致灭绝鲸鱼进入水生生活方式的进化模式。最后，多变量数据集将被用于系统发育信息的判别分析，以重建太平洋Peregoctus和其他灭绝的鲸目动物的运动模式。

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.