Departamento de Paleontología, Facultad de Ciencias, Universidad de la República, Iguá 4225, 11400 Montevideo, Uruguay.
Bureau of Land Management, Utah State Office, 440 West 200 South, Salt Lake City, UT 84101 USA.
Syst Biol. 2019 Mar 1;68(2):204-218. doi: 10.1093/sysbio/syy058.
Sloths, like other xenarthrans, are an extremely interesting group of mammals that, after a long history of evolution and diversification in South America, became established on islands in the Caribbean and later reached North America during the Great American Biotic Interchange. In all three regions, they were part of the impressive Pleistocene megafauna. Most taxa became extinct and only two small, distantly related tree-dwelling genera survived. Here, we incorporate several recently described genera of sloths into an assembled morphological data supermatrix and apply Bayesian inference, using phylogenetic and morphological clock methods, to 64 sloth genera. Thus, we investigate the evolution of the group in terms of the timing of divergence of different lineages and their diversity, morphological disparity and biogeographical history. The phylogeny obtained supports the existence of the commonly recognized clades for the group. Our results provide divergence time estimates for the major clades within Folivora that could not be dated with molecular methods. Lineage diversity shows an early increase, reaching a peak in the Early Miocene followed by a major drop at the end of the Santacrucian (Early Miocene). A second peak in the Late Miocene was also followed by a major drop at the end of the Huayquerian (Late Miocene). Both events show differential impact at the family level. After that, a slight Plio-Pleistocene decline was observed before the marked drop with the extinction at the end of the Pleistocene. Phenotypic evolutionary rates were high during the early history of the clade, mainly associated with Mylodontidae, but rapidly decreased to lower values around 25 Ma, whereas Megalonychidae had lower rates at the beginning followed by a steady increase, peaking during the Late Miocene and the Pliocene. Morphological disparity showed a similar trend, with an early increase, followed by a slowly increasing phase through the Late Oligocene and Early Miocene, and ending with another increase beginning at the middle of the Miocene. Biogeographic analysis showed southern South America as the most probable area of origin of the clade and the main region in which the early diversification events took place. Both Megatheriinae and Nothrotheriinae basal nodes were strongly correlated with Andean uplift events, whereas the early history of Mylodontidae is closely associated with southern South America and also shows an early occupation of the northern regions. Within Megalonychidae, our results show Choloepus as a descendant of an island dispersing ancestor and a probable re-ingression to South America by a clade that originated in Central or North America.
树懒是异关节总目动物中一个非常有趣的群体，经过在南美洲漫长的进化和多样化历程后，它们在加勒比海的岛屿上建立了种群，并在后来的大美洲生物大交换期间到达了北美洲。在这三个地区，它们都是令人印象深刻的更新世巨型动物群的一部分。大多数树懒属灭绝了，只有两个亲缘关系较远的小型树栖属幸存下来。在这里，我们将最近描述的几个树懒属纳入一个组装的形态数据超矩阵中，并使用基于贝叶斯推断的系统发育和形态钟方法，对 64 种树懒属进行研究。因此，我们根据不同谱系的分化时间以及它们的多样性、形态差异和生物地理历史来研究该群体的进化。获得的系统发育树支持了该群体中常见的公认分支。我们的结果为 Folivora 中的主要分支提供了无法用分子方法确定的分化时间估计。谱系多样性显示出早期的增加，在中新世早期达到顶峰，随后在 Santacrucian 末期（中新世早期）大幅下降。中新世晚期的第二个高峰也紧随其后，在 Huayquerian 末期（中新世晚期）大幅下降。这两个事件都显示出在科一级的不同影响。之后，在更新世末期灭绝之前，古新世到更新世的下降趋势略有下降。形态进化率在分支的早期历史中很高，主要与磨齿兽科有关，但在 2500 万年前左右迅速下降到较低值，而巨爪地懒科的起始值较低，随后稳步上升，在中新世晚期和上新世达到峰值。形态差异也呈现出相似的趋势，早期增加，然后通过渐新世和中新世早期缓慢增加阶段，最后从中新世中期开始再次增加。生物地理分析表明，南美洲南部是该分支最有可能的起源地，也是早期多样化事件发生的主要地区。巨爪地懒亚科和南美地懒亚科的基础节点都与安第斯山脉隆升事件密切相关，而磨齿兽科的早期历史与南美洲南部密切相关，也显示出早期占据北部地区。在巨爪地懒科中，我们的结果表明，大地懒是一个岛屿扩散祖先的后裔，并且一个起源于中美洲或北美洲的分支可能重新返回南美洲。
