De Esteban-Trivigno Soledad, Mendoza Manuel, De Renzi Miquel
Institut Català de Paleontologia, c/Escola Industrial, 23 Sabadell, Barcelona, Spain.
J Morphol. 2008 Oct;269(10):1276-93. doi: 10.1002/jmor.10659.
The Magnorder Xenarthra includes strange extinct groups, like glyptodonts, similar to large armadillos, and ground sloths, terrestrial relatives of the extant tree sloths. They have created considerable paleobiological interest in the last decades; however, the ecology of most of these species is still controversial or unknown. The body mass estimation of extinct species has great importance for paleobiological reconstructions. The commonest way to estimate body mass from fossils is through linear regression. However, if the studied species does not have similar extant relatives, the allometric pattern described by the regression could differ from those shown by the extinct group. That is the case for glyptodonts and ground sloths. Thus, stepwise multiple regression were developed including extant xenarthrans (their taxonomic relatives) and ungulates (their size and ecological relatives). Cases were weighted to maximize the taxonomic evenness. Twenty-eight equations were obtained. The distribution of the percent of prediction error (%PE) was analyzed between taxonomic groups (Perissodactyla, Artiodactyla, and Xenarthra) and size groups (0-20 kg, 20-300 kg, and more than 300 kg). To assess the predictive power of the functions, equations were applied to species not included in the regression development [test set cross validation, (TSCV)]. Only five equations had a homogeneous %PE between the aforementioned groups. These were applied to five extinct species. A mean body mass of 80 kg was estimated for Propalaehoplophorus australis (Cingulata: Glyptodontidae), 594 kg for Scelidotherium leptocephalum (Phyllophaga: Mylodontidae), and 3,550.7 kg for Lestodon armatus (Phyllophaga: Mylodontidae). The high scatter of the body mass estimations obtained for Catonyx tarijensis (Phyllophaga: Mylodontidae) and Thalassocnus natans (Phyllophaga: Megatheriidae), probably due to different specializations, prevented us from predicting its body mass. Surprisingly, although obtained from ungulates and xenarthrans, these five selected equations were also able to predict the body mass of species from groups as different as rodents, carnivores, hyracoideans, or tubulidentates. This result suggests the presence of a complex common allometric pattern for all quadrupedal placentals.
大目贫齿目包括一些奇特的已灭绝类群，如雕齿兽，类似于大型犰狳，还有地懒，是现存树懒的陆生近亲。在过去几十年里，它们引发了相当大的古生物学研究兴趣；然而，这些物种中大多数的生态习性仍存在争议或尚不为人所知。灭绝物种的体重估计对于古生物学重建具有重要意义。从化石估计体重最常见的方法是通过线性回归。然而，如果所研究的物种没有类似的现存近亲，回归所描述的异速生长模式可能与灭绝类群所显示的模式不同。雕齿兽和地懒就是这种情况。因此，开发了逐步多元回归，纳入现存的贫齿目动物（它们的分类学亲属）和有蹄类动物（它们的体型和生态亲属）。对案例进行加权以最大化分类学均匀度。得到了28个方程。分析了分类学类群（奇蹄目、偶蹄目和贫齿目）和体型类群（0 - 20千克、20 - 300千克和超过300千克）之间预测误差百分比（%PE）的分布。为了评估这些函数的预测能力，将方程应用于回归开发中未包含的物种[测试集交叉验证，（TSCV）]。只有五个方程在上述类群之间具有均匀的%PE。将这些方程应用于五个灭绝物种。估计南方原雕齿兽（有甲目：雕齿兽科）的平均体重为80千克，细头雕齿兽（叶食亚目：磨齿兽科）为594千克，武装恐齿兽（叶食亚目：磨齿兽科）为3550.7千克。塔里扬猫爪兽（叶食亚目：磨齿兽科）和纳坦斯海懒兽（叶食亚目：大地懒科）体重估计的高离散度，可能是由于不同的特化，使得我们无法预测它们的体重。令人惊讶的是，尽管这些方程是从有蹄类动物和贫齿目动物得出的，但这五个选定的方程也能够预测来自不同类群的物种的体重，如啮齿动物、食肉动物、蹄兔目动物或管齿目动物。这一结果表明所有四足胎盘动物存在一种复杂的共同异速生长模式。
