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 shown 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
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
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 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
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