Science - USA (2020-01-17)

sciencemag.org SCIENCE

ILLUSTRATION: CHUANG ZHAO

By Julia A. Schultz

I

n rare cases, fossils function like snap-

shots, capturing key moments of evolu-

tion. One of these rare cases is Origolestes

lii, a stem therian mammal from the

Yixian Formation (China) described by

Mao et al. ( 1 ) on page 305 of this issue.

The study provides a glimpse of a fundamen-

tal moment in mammalian evolution—the

separation of middle ear elements from the

Meckelian cartilage of the lower jaw, which

provided freedom to the system to improve

both chewing function and hearing ability.

Chewing is specific to mammals; no other

vertebrates use elaborate repeated jaw move-

ments to process food before swallowing.

Theria, the group all living placentals and

marsupials descend from (see the figure), ini-

tially utilized upward but slightly transverse

(sideward) movements of the lower jaw for

processing food. Within this group, versatile

chewing movements evolved for efficient

PERSPECTIVES

INSIGHTS

Rheinische Friedrich-Wilhelms-Universität Bonn,

Institut für Geowissenschaften, Bonn, Germany.

Email: [email protected]

244 17 JANUARY 2020 • VOL 367 ISSUE 6475

PALEONTOLOGY

Eat and listen—how chewing and hearing evolved

Mammalian middle ear bones separated from the jaw of vertebrate ancestors

Published by AAAS