A Jurassic gliding euharamiyidan mammal with an ear of five auditory bones

Abstract

Gliding is a distinctive locomotion type that has been identified in only three mammal species from the Mesozoic era. Here we describe another Jurassic glider that belongs to the euharamiyidan mammals and shows hair details on its gliding membrane that are highly similar to those of extant gliding mammals. This species possesses a five-boned auditory apparatus consisting of the stapes, incus, malleus, ectotympanic and surangular, representing, to our knowledge, the earliest known definitive mammalian middle ear. The surangular has not been previously identified in any mammalian middle ear, and the morphology of each auditory bone differs from those of known mammals and their kin. We conclude that gliding locomotion was probably common in euharamiyidans, which lends support to idea that there was a major adaptive radiation of mammals in the mid-Jurassic period. The acquisition of the auditory bones in euharamiyidans was related to the formation of the dentary-squamosal jaw joint, which allows a posterior chewing movement, and must have evolved independently from the middle ear structures of monotremes and therian mammals.

A comparative study on auditory and hyoid bones of Jurassic euharamiyidans and contrasting evidence for mammalian middle ear evolution

“…The end of the medial process was a breakage, suggesting that the tip of the process was broken. The medial process was interpreted as homologous to the reflected lamina of the angular in non‐mammalian cynodonts (Han et al ), which is here reiterated. The lateral portion of the ectotympanic also extends posteriorly as a process, but it is smaller than the medial one.…”

“…The short process (crus) was not identified in the original study (Han et al. ) because it is difficult to see under a microscope or from the optic image (Fig. ).…”

“…Anterior to the manubrium is the robust medial process, a feature unknown in the malleus of mammals. This process has been homologized to the retroarticular process of the articular (Han et al ). If this interpretation is correct, the elongate medial process is unique in Arboroharamiya .…”

“…The reason to identify the bone as the surangular has been given by Han et al. (): supplementary information) and will not be repeated here. The surangular has a fan‐shaped body that gradually thins anteriorly to form a needle‐shaped anterior process.…”

“…The primary specimens used for this study are the holotype specimen of A. allinhopsoni (Han et al ; HG‐M017, the Paleontology Center, Bohai University, Jinzhou, Liaoning Province, China), the holotype of A. jenkinsi (Zheng et al ; STM33‐9, Tianyu Museum of Nature, Shandong Province, China), and the holotype of Q. jizantang (Mao & Meng, 2019a; JZT‐D061, the Jizantang Paleontological Museum, Chaoyang County, Liaoning Province, China). Additional data are collected from the holotype of Xianshou linglong (Bi et al ; IVPP V16707, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China).…”