Jurassic Gliding Mammal with Unique Middle Ear Discovered in China

Posted on 11/14/2017 12:42:12 PM

Researchers have discovered a gliding euharamiyidan mammal from the early Late Jurassic strata in Hebei province, northeastern China. The specimens preserved impressions of hair details of the gliding membrane and the earliest known definitive mammalian middle ear. The research paper “A Jurassic gliding euharamiyidan mammal with an ear of five auditory bones” reporting the discovery appears in the journal Nature. 

The research team was led by Jin Meng of American Museum of Natural History. In addition to Shundong Bi of Indiana University of Pennsylvania, other collaborators include Gang Han of Bohai University, and Fangyuan Mao and Yuanqing Wang of Chinese Academy.

The new animal, Arboroharamiya allinhopsoni, is mouse-sized but has a long tail. Like other euharamiyidans, it had adapted to arboreal life and gliding locomotion. This was evidenced by their gracile skeleton, proportionally elongated limbs, grasping hand and foot, and development of the gliding membrane.

As the soft tissue, gliding membrane is difficult to be preserved in fossils, but the new specimens reported in the study preserved clear impressions of gliding membrane and hairs, unequivocal evidence for the gliding capability of the animal. The size of the gliding membrane, the long tail, and the hair pattern are highly similar to those of gliding species in extant marsupials and placental rodents. These animals were probably nocturnal, as in all extant gliding mammals. The research shows that arboreal experiment in mammals had already started very early, perhaps in the late Triassic Haramiyavia-like ancestor that has a gracile skeleton. Eventually gliding capability evolved in at least some haramiyidan species lived in the Jurassic forests.

Even more importantly in terms of mammalian evolution, the new creature possesses a unique auditory apparatus consisting of the stapes, incus, malleus, ectotympanic and surangular. Formation of a chain consisting multiple bones in the middle ear for sound transmission is unique for mammals among vertebrates. Some of the ear bones are transformed jaw elements, and this transformation represents a key evolutionary innovation in mammalian evolution. Fossils and developmental studies have long shownArboroharamiya allinhopsoni that the malleus, ectotympanic, and incus in the middle ear of modern mammals are homologous to the articular, prearticular, angular, and quadrate bones in reptiles, respectively. But the surangular is previously little known in any mammalian middle ear.

The new study provides the first evidence for the fate of the surangular during the transformation from the lower jawbones to the middle ear bones, at least in haramiyidans. In addition to the unique surangular, other middle ear elements are distinctly different from corresponding bones in known mammals.

The research concluded that such a new type of middle ear may have co-evolved with their special jaw joint that is capable of posterior move during chewing. It also shows that the middle ear of the new animal evolved independently from those of extant monotreme and therian mammals. With acquisition the definitive mammalian middle ear, it can be inferred that the new animal, presumably a nocturnal creature, had a keen sense of hearing. These small ear bones represent a big discovery that will likely reset research interests for evolution of mammalian middle ear and stimulate further discussions on mammalian evolution in general.

The species was named as Arboroharamiya allinhopsoni, in honor of Drs. Edgar F. Allin and James A. Hopson for their contribution to the study on mammalian middle ear evolution.

The haramiyidans were among the oldest mammals, which existed in the late Triassic period about 210 million years ago, although the phylogenetic position of haramiyidans is not yet universally accepted. The troublesome phylogenetic placement of the extinct group was partly stemmed from the fact that fossils of haramiyidans have been known primarily from isolated teeth since at least 1847. Recent discovery of Restoration of Arboroharamiya allinhopsoni.well-preserved skeletons from the Jurassic of China add fuel to the on-going debates: haramiyidans were mammals or just close relatives of mammals. Because of their antiquity, it will extend the diversification of mammals into the Late Triassic if haramiyidans are indeed mammals; otherwise, uncontested mammals may have been known from the Middle Jurassic.

The new research favors an early origin of mammals and reiterates that haramiyidans and multituberculates, another extinct but long-lived mammalian group, are closely related and fell within mammals, suggesting that that mammals originated some 208 million years ago during the Late Triassic Period.

The poor fossil record of haramiyidans was perhaps due to their gracile skeleton and their living in the forestry condition; both were unfavorable for fossil preservation. As tree dwellers, the extinction of euharamiyidans could be attributed to changes from gymnosperms dominant forests to angiosperms flourished ecosystem during the Jurassic-Cretaceous transition.

Department of Biology