Cimolestes is a genus of early eutherians with a full complement of teeth adapted for eating insects and other small animals. Paleontologists have disagreed on its relationship to other mammals, in part because quite different animals were assigned to the genus, making Cimolestes a grade taxon of animals with similar features rather than a genus of closely relatedones. Fossils have been found in North America, where they first appeared during the Late Cretaceous. According to some paleontologists, they died out at the start of the Paleocene, while others report the genus from the early Eocene. Most species have been described from teeth and isolated fragments. One complete articulated skeleton provisionally assigned to Cimolestes has been found. It shows a small, agile, tree-dwelling predator with long toes for grasping branches and a prehensile tail at least twice the length of its body. It has the largest number of tail vertebrae known in any mammal.
Classification
The genus was once considered to be marsupials; later it was reclassified with the placental mammals, as ancestors of the Carnivora and the extinct Creodonta. Recent researchers have agreed the species assigned to Cimolestes are primitive eutherian mammals, members of a Cimolestid clade, part of the larger clade Didelphodonta. Didelphodonts have been placed within the Ferae, as a sister group to Carnivora. However, consensus is emerging that modern placental mammals evolved later than previously thought, that other types of mammals had long, diversified, and successful histories, and that Cimolestes and many related genera are stem eutherians, more closely related to placentals than to marsupials but outside of placental mammals proper, and not closely related to any living animal. Cimolestes in particular follows as the direct outgroup to Taeniodonta, indicating that the latter evolved from forms similar to it.
Reassigned species
In order to make the genus reflect an actual group of most closely related species, three nominal species of Cimolestes, C. magnus, C. cerberoides, and C. propalaeoryctes, have been reassigned to their own genera, Altacreodus, Ambilestes, and Scollardius, respectively. Cimolestes incisus Marsh and Cimolestes stirtoni Clemens remain within the genus.
Fossil distribution
Fossils of Cimolestes have been found in: ;Cretaceous
