Finarelli John A, Flynn John J
Committee on Evolutionary Biology, University of Chicago, Illinois 60637, USA.
Syst Biol. 2006 Apr;55(2):301-13. doi: 10.1080/10635150500541698.
A recent molecular phylogeny of the mammalian order Carnivora implied large body size as the ancestral condition for the caniform subclade Arctoidea using the distribution of species mean body sizes among living taxa. "Extant taxa-only" approaches such as these discount character state observations for fossil members of living clades and completely ignore data from extinct lineages. To more rigorously reconstruct body sizes of ancestral forms within the Caniformia, body size and first appearance data were collected for 149 extant and 367 extinct taxa. Body sizes were reconstructed for four ancestral nodes using weighted squared-change parsimony on log-transformed body mass data. Reconstructions based on extant taxa alone favored large body sizes (on the order of 10 to 50 kg) for the last common ancestors of both the Caniformia and Arctoidea. In contrast, reconstructions incorporating fossil data support small body sizes (< 5 kg) for the ancestors of those clades. When the temporal information associated with fossil data was discarded, body size reconstructions became ambiguous, demonstrating that incorporating both character state and temporal information from fossil taxa unambiguously supports a small ancestral body size, thereby falsifying hypotheses derived from extant taxa alone. Body size reconstructions for Caniformia, Arctoidea, and Musteloidea were not sensitive to potential errors introduced by uncertainty in the position of extinct lineages relative to the molecular topology, or to missing body size data for extinct members of an entire major clade (the aquatic Pinnipedia). Incorporating character state observations and temporal information from the fossil record into hypothesis testing has a significant impact on the ability to reconstruct ancestral characters and constrains the range of potential hypotheses of character evolution. Fossil data here provide the evidence to reliably document trends of both increasing and decreasing body size in several caniform clades. More generally, including fossils in such analyses incorporates evidence of directional trends, thereby yielding more reliable ancestral character state reconstructions.
最近一项关于食肉目哺乳动物的分子系统发育研究表明，依据现存分类单元中物种平均体型的分布情况，大体型是犬型亚目熊型总科的原始特征。像这样的“仅现存分类单元”方法忽略了现存类群化石成员的性状状态观察结果，并且完全忽视了已灭绝谱系的数据。为了更严谨地重建犬型亚目内部祖先形态的体型，我们收集了149个现存和367个已灭绝分类单元的体型及首次出现的数据。利用对数转换后的体重数据，通过加权平方变化简约法为四个祖先节点重建了体型。仅基于现存分类单元的重建结果显示，犬型亚目和熊型总科的最后共同祖先具有大体型（约10至50千克）。相比之下，纳入化石数据的重建结果支持这些类群祖先的体型较小（<5千克）。当与化石数据相关的时间信息被舍弃时，体型重建变得模糊不清，这表明纳入化石分类单元的性状状态和时间信息明确支持较小的祖先体型，从而证伪了仅从现存分类单元得出的假设。犬型亚目、熊型总科和鼬型总科的体型重建对已灭绝谱系相对于分子拓扑结构位置的不确定性所引入的潜在误差，或对整个主要类群（水生鳍足类）已灭绝成员缺失的体型数据不敏感。将化石记录中的性状状态观察结果和时间信息纳入假设检验，对重建祖先性状的能力有重大影响，并限制了性状进化潜在假设的范围。此处的化石数据为可靠记录几个犬型类群体型增大和减小的趋势提供了证据。更普遍地说，在此类分析中纳入化石包含了定向趋势的证据，从而产生更可靠的祖先性状状态重建结果。
