New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China

Abstract

A new genus and species of eutherian mammal, Acristatherium yanensis gen. et sp. nov., is described from the Early Cretaceous Jehol biota, China. The new taxon is based on a partial skull that is preserved in three dimensions from the Lujiatun bed of the Yixian Formation and dated 123.2±1.0 Ma. Its right upper and lower dentitions are nearly complete and it has a dental formula 4.1.5.3/3.1.5.3. The new mammal reveals several craniodental characteristics of Early Cretaceous eutherians previously unknown in fossil records of therians, such as a possible vestige of the septomaxilla. The craniodental features of the new taxon are compared with those of relevant Early Cretaceous eutherians and therians. Phylogenetic analyses based on a data matrix containing 70 taxa and 408 characters place A. yanensis as the most basal eutherian in the selected group. The morphological differences between Acristatherium and Eomaia indicate that eutherians already had a significant degree of generic diversification ca 125 Ma.

A new Early Cretaceous eutherian mammal from the Sasayama Group, Hyogo, Japan

“…This indicates that the absence of the entoconid in S. kawaii can be interpreted as a primitive condition. However, the lower molars of Jurassic-Cretaceous eutherians known thus far including those of taxa older than S. kawaii, such as in Acristatherium, Eomaia, Juramaia and Murtoilestes, possess distinct entoconids [3][4][5]38]. Ausktribosphenids and henosferids also have entoconids [39][40][41][42], whereas Ambondro does not have a distinct entoconid [43].…”

“…These are features characteristic of early eutherians, such as in Acristatherium, Eomaia, Holoclemensia, Juramaia, Montanalestes, Murtoilestes and Prokennalestes [3][4][5]14,23,33,38]; thus, they can be regarded as primitive for eutherians. Within these early eutherians, the general configuration of lower molars of Sasayamamylos is similar to those of Holoclemensia, but the former genus is distinguished from the latter by the more open trigonid angle and the wider talonid of m2 [23].…”

“…Sasayamamylos also differs from Holoclemensia in having a non-molariform ultimate lower premolar [23], which also distinguishes Sasayamamylos from ausktribosphenids and some mid-Cretaceous eutherians, such as Bobolestes (including Otlestes, which is suggested as a junior synonym of Bobolestes by Averianov & Archibald [44]) and Montanalestes [33,39,40,[44][45][46]. On the other hand, Sasayamamylos shares a non-molariform ultimate lower premolar with Acristatherium, Eomaia, Juramaia, Prokennalestes and Sheikhdzheilia [3][4][5]14,44]; this characteristic is also thought to be plesiomorphic for eutherians.…”

“…The paraconid is also slightly procumbent anteriorly. The cusp e, which is present in the molars of Prokennalestes and Acristatherium [5,14], is absent. The talonid is well developed and basined; it is nearly as long as, slightly narrower than, and nearly half as tall as the trigonid.…”

“…The primitive eutherian premolar count is generally considered to be five [12,13], as shown by early eutherian taxa, such as Acristatherium, Eomaia and Juramaia [3][4][5]. Other Early Cretaceous eutherians with known premolar counts (Prokennalestes and unnamed eutherians from the Lower Cretaceous Shahai and Fuxin formations of northeastern China) also have five premolars [14][15][16].…”