Cranial and Postcranial Morphology of the Insectivoran-Grade Mammals Hsiangolestes and Naranius (Mammalia, Eutheria) with Analyses of Their Phylogenetic Relationships

Abstract

ABSTRACT Early Cenozoic “insectivorans” possess some of the most primitive morphologies among eutherian mammals. Studies of these archaic mammals offer insights into the early diversifications of basal eutherians. Despite such importance, early fossil “insectivorans” from Asia are poorly known due to a scarcity of fossil remains, which often consist only of fragmentary jaws and teeth. Discoveries of remarkably well-preserved fossil “insectivorans”, including complete skulls and articulated postcranial skeletons, from the early Eocene Hengyang Basin in south-central Hunan Province, China, offer a rare opportunity to thoroughly study two taxa belonging to different families. Fine-grained red beds from Hengyang Basin preserve extraordinary fossils with morphological structures rarely seen elsewhere. Thin sections of a skull of Hsiangolestes youngi Zheng and Huang, 1984, for example, reveal the extremely delicate nasal and maxillary turbinates, which, as far as we are aware, are the first known from fossils of this age. We thus take this opportunity to document in detail the cranial and dental morphology, as well as postcranial skeletons, of the Hengyang “insectivorans.”. In this monograph, we describe several complete skulls and serial sections of a skull, as well as many partial skulls, mandibles, and postcranial skeletons of Hsiangolestes youngi, an Asian early Eocene insectivoran-grade mammal. We also report a new species of Naranius Russell and Dashzeveg, 1986—N. hengdongensis—an Asian early Eocene cimolestid and describe its well-preserved skulls and mandibles. Hsiangolestes is endemic to Asia. It is currently known only from the earliest Eocene Lingcha Formation, Hengyang Basin, Hunan Province, China. Naranius closely resembles Cimolestes Marsh, 1889, the type genus of the family Cimolestidae. It is mainly distributed in Asia and known from the earliest Eocene deposits in the Bumban Member of the Naran Bulak Formation, Nemegt Basin, of Mongolia, and the Lingcha Formation, Hengyang Basin, Hunan Province, China. The only record of Naranius reported outside of Asia is N. americanus from the early Wasatchian Red Hot Local Fauna, Mississippi, United States. Using PAUP and TNT search algorithms, we place these Hengyang taxa within phylogenetic context of other fossil “insectivorans” from the Mesozoic and early Cenozoic of Asia together with some well-known Holarctic taxa. A phylogenetic analysis of 290 cranial and dental characters from 36 fossil and modern insectivoran-grade taxa is presented, focusing on new materials of Hsiangolestes youngi and Naranius hengdongensis. Based on the results of our phylogenetic analyses, we propose that (1) Hsiangolestes, Prosarcodon, Sarcodon, and Sinosinopa, form a monophyletic group, for which we propose the family name Sarcodontidae; (2) the family Cimolestidae should be restricted to Naranius and Cimolestes, which are sister taxa; (3) the systematic position of Naranius americanus is uncertain; and (4) the family Micropternodontidae should be restricted to Micropternodus and its allies in North America.