Discoveries of tree-dwelling and subterranean beasts suggest earliest mammals’ incredible diversity

Early mammal fossils reveal
remarkable diversity
Discoveries of tree-dwelling and
subterranean beasts suggest
earliest mammals’ incredible
diversity
Presented by

Michelle Douglass
The fossils of two extinct shrew-sized
animals that existed about 160 million
years ago – one that lived in trees and
one that burrowed underground –
reveal wide-ranging ecological diversity
among the earliest mammals, similar to
that found in modern mammals.
The new fossils have been described as
two previously unknown
mammaliaforms (extinct relatives of
modern mammals) by scientists in the
US and in China. They include a tree-
dwelling creature, Agilodocodon
scansorius, and a mole-like
subterranean animal Docofosser
brachydactylus.
Before the 2000s, it was
generally thought Mesozoic
mammals couldn't diversify much
in the dinosaur-dominated
ecosystem
They represent new docodonts – an
order of extinct early ancestral
mammals without any living
descendants – which co-existed with
dinosaurs in the Mesozoic era.
The discovery is the latest evidence to
suggest parallels between modern
mammals and mammaliaforms in terms
of their adaptations to diverse
ecological habitats and their genes.
Surprising specialisation
Modern mammals are extraordinarily
diverse. But whether such a range of
adaptations was found as early as the
mammaliaforms has been debated.
“Before the 2000s, it was generally
thought Mesozoic mammals could not
diversify much in the dinosaur-
dominated ecosystem,” said Dr Zhe-Xi
Luo from the University of Chicago’s
Department of Organismal Biology and
Anatomy, who co-authored the papers
analysing Agilodocodon scansorius and
Docofosser brachydactylus.
But fossil discoveries in recent years
have built a different picture, he said.
“In the last 10-15 years,
palaeontologists found many Mesozoic
mammals with very interesting
functional and ecological
specialisations.”
The new research suggests docodonts
adapted to a very broad range of
environments such as arboreal and
subterranean ones, despite competition
from dinosaurs.
Researchers from the University of
Chicago in the US and the Beijing
Museum of Natural History in China
described the extinct beasts in two
separate studies published in the
journal Science.
A team had been studying the fossils
kept at the Beijing Museum of Natural
History since 2013.
The remarkably preserved 165 million-
year-old Agilodocodon scansorius from
the Middle Jurassic period showed
many skeletal adaptations for living in
trees. Its horny claws would have been
perfect for climbing and its incisors
were adapted to gnaw on bark and feed
on tree sap, providing the earliest
evidence of “gumnivorous” feeding in
mammaliaforms which, in modern
mammals, is seen in some New World
monkeys.
Its limb proportions, flexible wrist,
elbow and ankle joints for greater
mobility are all typical of climbing
animals. The researchers estimated
Agilodocodon scansorius would have
measured about 14cm (5.5 inches)
from head to tail.
Docofosser brachydactylus, preserved
in a 160 million-year-old fossil from the
late Jurassic period, had a skeletal
structure and proportions much like
modern-day golden moles
(Chrysochloridae). The creature had
shovel-like fingers and short, wide
molars – suggesting it was adapted to a
subterranean lifestyle.
Some skeletal features of Docofosser
and Agilodocodon resemble patterns
shaped by genes identified in living
animals, according to the research. For
example Docofosser’s short and wide
digits are very similar to the adaptation
found in golden moles, which is
influenced by genes.
The structure of Docofosser’s and
Agilodocodon’s ribs and spines
resemble developmental patterns
shaped by genes also identified in living
animals. This suggests some genetic
mechanisms found in living animals
operated long before modern mammals
appeared.
The new findings come after the
discovery in 2006 of a beaver-like,
swimming early mammal
Castorocauda lutrasimilis that lived 164
million years ago, which was described
by Dr Luo and colleagues. The fossil
was surprising because it challenged
the thinking at the time that mammals
might have been primitive creatures
that were confined to land.
“We consistently find with every new
fossil that the earliest mammals were
just as diverse in both feeding and
locomotor adaptations as modern
mammals,” Dr Luo said of the new
findings.
He added: “These fossils help
demonstrate that early mammals did
indeed have a wide range of ecological
diversity. It appears dinosaurs did not
dominate the Mesozoic landscape as
much as previously thought.”