Department of Earth Sciences, University of Oxford , 3 South Parks Road, Oxford OX1 3AN , UK.
Proc Biol Sci. 2019 Apr 10;286(1900):20190590. doi: 10.1098/rspb.2019.0590.
Large carnivorous mammals have been suggested to show a ratchet-like mode of morphological evolution. A limited number of specializations for hypercarnivory evolve repeatedly in multiple clades, with those lineages evolving such specialities being unable to retreat back along their evolutionary trajectory or jump between adaptive peaks. While it has been hypothesized that such mechanisms should have applied to the evolution of other terrestrial carnivores, the non-mammalian synapsid clade Therocephalia appears to defy this expectation. The earliest, basalmost members of this clade are large macropredators, and it is later that small carnivores appear, seemingly evolving from top-predator ancestors. In order to test this reading of therocephalian evolution, variation in rates of body size evolution were tested for and incorporated into an ancestral reconstruction. Similar studies were made of the evolution of discrete characters related to carnivory. All analyses indicate the ancestral therocephalian was a large macro-predator, with serrated teeth, elongated canines and robust lower jaws. Small sizes apparently evolve later. It is therefore suggested that the hypercarnivore ratchet is a feature of mammalian evolution.
大型食肉动物被认为表现出一种类似于棘轮的形态进化模式。在多个分支中，反复出现了针对超肉食性的有限数量的特化，那些进化出这种特化的谱系无法沿着它们的进化轨迹倒退，也无法在适应峰之间跳跃。虽然有人假设这种机制应该适用于其他陆地食肉动物的进化，但非哺乳动物合弓纲兽脚亚目似乎违背了这一预期。这个分支最早、最基础的成员是大型掠食者，后来才出现了小型肉食动物，似乎是从顶级掠食者的祖先进化而来的。为了验证兽脚亚目进化的这种解读，对体型进化的速率变化进行了测试，并将其纳入祖先重建中。还对与肉食性相关的离散特征的进化进行了类似的研究。所有的分析都表明，祖先的兽脚亚目是一种大型的掠食者，具有锯齿状的牙齿、长而尖锐的犬齿和强壮的下颚。较小的体型显然是后来进化而来的。因此，有人认为超肉食性棘轮是哺乳动物进化的一个特征。
