http://www.sciencedaily.com/releases/2013/08/130807134008.htm
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130807134008.htm
New Proto-Mammal Fossil Sheds Light On Evolution of Earliest Mammals
Aug. 7, 2013 A newly discovered fossil reveals the evolutionary adaptations
of a 165-million-year-old proto-mammal, providing evidence that traits such as
hair and fur originated well before the rise of the first true mammals. The
biological features of this ancient mammalian relative, named Megaconus
mammaliaformis, are described by scientists from the University of Chicago in
the Aug 8 issue of Nature.
"We finally have a glimpse of what may be the ancestral condition of all
mammals, by looking at what is preserved in Megaconus. It allows us to piece
together poorly understood details of the critical transition of modern mammals
from pre-mammalian ancestors," said Zhe-Xi Luo, professor of organismal biology
and anatomy at the University of Chicago.
Discovered in Inner Mongolia, China, Megaconus is one of the best-preserved
fossils of the mammaliaform groups, which are long-extinct relatives to modern
mammals. Dated to be around 165 million years old, Megaconus co-existed with
feathered dinosaurs in the Jurassic era, nearly 100 million years before
Tyrannosaurus Rex roamed Earth.
Preserved in the fossil is a clear halo of guard hairs and underfur residue,
making Megaconus only the second known pre-mammalian fossil with fur. It was
found with sparse hairs around its abdomen, leading the team to hypothesize
that it had a naked abdomen. On its heel, Megaconus possessed a long keratinous
spur, which was possibly poisonous. Similar to spurs found on modern egg-laying
mammals, such as male platypuses, the spur is evidence that this fossil was
most likely a male member of its species.
"Megaconus confirms that many modern mammalian biological functions related to
skin and integument had already evolved before the rise of modern mammals,"
said Luo, who was also part of the team that first discovered evidence of hair
in pre-mammalian species in 2006.
A terrestrial animal about the size of a large ground squirrel, Megaconus was
likely an omnivore, possessing clearly mammalian dental features and jaw hinge.
Its molars had elaborate rows of cusps for chewing on plants, and some of its
anterior teeth possessed large cusps that allowed it to eat insects and worms,
perhaps even other small vertebrates. It had teeth with high crowns and fused
roots similar to more modern, but unrelated, mammalian species such as rodents.
Its high-crowned teeth also appeared to be slow growing like modern placental
mammals.
The skeleton of Megaconus, especially its hind-leg bones and finger claws,
likely gave it a gait similar to modern armadillos, a previously unknown type
of locomotion in mammaliaforms.
Luo and his team identified clearly non-mammalian characteristics as well. Its
primitive middle ear, still attached to the jaw, was reptile-like. Its
anklebones and vertebral column are also similar to the anatomy of previously
known mammal-like reptiles.
"We cannot say that Megaconus is our direct ancestor, but it certainly looks
like a great-great-grand uncle 165 million years removed. These features are
evidence of what our mammalian ancestor looked like during the
Triassic-Jurassic transition," Luo said.
"Megaconus shows that many adaptations found in modern mammals were already
tried by our distant, extinct relatives. In a sense, the three big branches of
modern mammals are all accidental survivors among many other mammaliaform
lineages that perished in extinction," Luo added.
The fossil, now in the collections in Paleontological Museum of Liaoning in
China, was discovered and studied by an international team of paleontologists
from Paleontological Museum of Liaoning, University of Bonn in Germany, and the
University of Chicago.
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Story Source:
The above story is based on materials provided by University of Chicago
Medical Center.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal References:
Chang-Fu Zhou, Shaoyuan Wu, Thomas Martin, Zhe-Xi Luo. A Jurassic
mammaliaform and the earliest mammalian evolutionary adaptations. Nature, 2013;
500 (7461): 163 DOI: 10.1038/nature12429
Q. Ji. A Swimming Mammaliaform from the Middle Jurassic and
Ecomorphological Diversification of Early Mammals. Science, 2006; 311 (5764):
1123 DOI: 10.1126/science.1123026
Need to cite this story in your essay, paper, or report? Use one of the
following formats:
APA
MLA
University of Chicago Medical Center (2013, August 7). New proto-mammal fossil
sheds light on evolution of earliest mammals. ScienceDaily. Retrieved August 8,
2013, from http://www.sciencedaily.com /releases/2013/08/130807134008.htm
Note: If no author is given, the source is cited instead.
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