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mamalia awal leluhur yang arboreal  dan di bawah tanah ditemukan

Fosil-fosil dari dua mamalia leluhur saling terkait, baru ditemukan di Cina , menunjukkan bahwa keragaman ekologi yang luas dari mamalia modern memiliki preseden lebih dari 160 juta tahun yang lalu ....read more 

Earliest-known
arboreal and subterranean ancestral mammals discovered

Date:

February 12, 2015

Source:

University of Chicago

Summary:

The fossils of two
interrelated ancestral mammals, newly discovered in China, suggest that the
wide-ranging ecological diversity of modern mammals had a precedent more than
160 million years ago.

..................

the fossils of two
interrelated ancestral mammals, newly discovered in China, suggest that the
wide-ranging ecological diversity of modern mammals had a precedent more than
160 million years ago.

With claws for climbing and teeth adapted for a tree sap diet, Agilodocodon
scansorius is the earliest-known tree-dwelling mammaliaform
(long-extinct relatives of modern mammals). The other fossil,Docofossor
brachydactylus, is the earliest-known subterranean mammaliaform, possessing
multiple adaptations similar to African golden moles such as shovel-like
paws. Docofossor also has distinct skeletal features that
resemble patterns shaped by genes identified in living mammals, suggesting
these genetic mechanisms operated long before the rise of modern mammals.

These discoveries are reported by international teams of scientists from
the University of Chicago and Beijing Museum of Natural History in two separate
papers published Feb. 13 in Science.

"We consistently find with every new fossil that the earliest mammals
were just as diverse in both feeding and locomotor adaptations as modern
mammals," said Zhe-Xi Luo, PhD, professor of organismal biology and
anatomy at the University of Chicago and an author on both papers. "The
groundwork for mammalian success today appears to have been laid long
ago."

Agilodocodon and Docofossor provide strong evidence that arboreal
and subterranean lifestyles evolved early in mammalian evolution, convergent to
those of true mammals. These two shrew-sized creatures -- members of the
mammaliaform order Docodonta -- have unique adaptations tailored for their
respective ecological habitats.

Agilodocodon, which lived roughly 165 million years ago, had hands and feet with curved
horny claws and limb proportions that are typical for mammals that live in
trees or bushes. It is adapted for feeding on the gum or sap of trees, with
spade-like front teeth to gnaw into bark. This adaptation is similar to the
teeth of some modern New World monkeys, and is the earliest-known evidence of
gumnivorous feeding in mammaliaforms. Agilodocodon also had
well-developed, flexible elbows and wrist and ankle joints that allowed for
much greater mobility, all characteristics of climbing mammals.

"The finger and limb bone dimensions of Agilodocodon match up with
those of modern tree-dwellers, and its incisors are evidence it fed on plant
sap," said study co-author David Grossnickle, graduate student at the
University of Chicago. "It's amazing that these arboreal adaptions
occurred so early in the history of mammals and shows that at least some
extinct mammalian relatives exploited evolutionarily significant herbivorous
niches, long before true mammals."

Docofossor, which lived around 160 million years ago, had a skeletal structure and
body proportions strikingly similar to the modern day African golden mole. It
had shovel-like fingers for digging, short and wide upper molars typical of
mammals that forage underground, and a sprawling posture indicative of
subterranean movement.

Docofossor had reduced bone segments in its fingers, leading to shortened but
wide digits. African golden moles possess almost the exact same adaptation,
which provides an evolutionary advantage for digging mammals. This
characteristic is due to the fusion of bone joints during development -- a
process influenced by the genes BMP and GDF-5. Because of the many anatomical
similarities, the researchers hypothesize that this genetic mechanism may have
played a comparable role in early mammal evolution, as in the case of Docofossor.

The spines and ribs of both Agilodocodon and Docofossor also
show evidence for the influence of genes seen in modern mammals. Agilodocodon has
a sharp boundary between the thoracic ribcage to lumbar vertebrae that have no
ribs. However,Docofossor shows a gradual thoracic to lumber transition.
These shifting patterns of thoracic-lumbar transition have been seen in modern
mammals and are known to be regulated by the genes Hox 9-10 and Myf
5-6. That these ancient mammaliaforms had similar developmental patterns is
an evidence that these gene networks could have functioned in a similar way
long before true mammals evolved.

"We believe the shortened digits of Docofossor, which is a
dead ringer for modern golden moles, could very well have been caused by BMP
and GDF," Luo said. "We can now provide fossil evidence that gene
patterning that causes variation in modern mammalian skeletal development also
operated in basal mammals all the way back in the Jurassic."

Early mammals were once thought to have limited ecological opportunities to
diversify during the dinosaur-dominated Mesozoic era. However, Agilodocodon, Docofossor and
numerous other fossils -- including Castorocauda, a swimming,
fish-eating mammaliaform described by Luo and colleagues in 2006 -- provide
strong evidence that ancestral mammals adapted to wide-ranging environments
despite competition from dinosaurs.

"We know that modern mammals are spectacularly diverse, but it was
unknown whether early mammals managed to diversify in the same way," Luo
said. "These new 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."







Story Source:

The above story is based on materials provided by University of Chicago. Note: Materials may be edited
for content and length.







Journal References:

1.    Q.-J. Meng, Q. Ji, Y.-G. Zhang, D. Liu,
D. M. Grossnickle, Z.-X. Luo. An arboreal docodont from the Jurassic
and mammaliaform ecological diversification.Science, 2015; 347
(6223): 764 DOI: 10.1126/science.1260879

2.    Z.-X. Luo, Q.-J. Meng, Q. Ji, D. Liu,
Y.-G. Zhang, A. I. Neander. Evolutionary development in basal
mammaliaforms as revealed by a docodontan.Science, 2015; 347 (6223):
760 DOI: 10.1126/science.1260880

http://www.sciencedaily.com/releases/2015/02/150212141447.htm

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