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Fossil birds and feathered dinosaurs
Evidence that birds are feathered dinosaurs continued to accumulate in 2002.
Xu and colleagues (Nature, v. 421, p. 335-339) reported a fossil of Microraptor
gui, a feathered dinosaur from the Early Cretaceous of China, that apparently
had long flight feathers on its hind limbs as well as its front limbs. The authors
argue that this fossil shows that bird flight originated from gliding rather
than from ground-up flapping, as argued by some paleontologists. However, Dial
(Science, v. 299, p. 402-404) did experiments with chukar partridges
to show how ground birds use their wings to help propel themselves up steep
inclines, suggesting a preadaptive function for the flapping motion of wings
before birds used them in flight. Xu and others (Nature, v. 410, p. 200-203)
reported branched feather structures from the Cretaceous Chinese dinosaur, Sinornithosaurus
millenii, which further proves that dinosaurs had fully avian feathers before
they could fly. Prum and Brush (Scientific American, March 2003, p. 84-93)
review the evidence from all the recent feathered dinosaur and bird discoveries
and provide a new model for how and why feathers evolved.
Mammals
These same Lower Cretaceous Chinese deposits that yield so many beautifully
preserved fossil birds and feathered dinosaurs yield spectacular early mammal
specimens as well. Ji and others (Nature, v. 416, p. 816-822) report
the earliest known placental mammal, an exceptionally complete specimen that
even has hair impressions from the Lower Cretaceous Yixian Formation of northeastern
China. At 125 million years old, it is 40-50 million years older than the next
oldest skeletal remains of a placental mammal anywhere in the world. Dubbed
((Eomaia scansoria)), it is primitive in most features, except for its
limbs and toes, which are adapted for climbing trees.
Even older rocks in China yield other Mesozoic mammals. Luo and colleagues (Science,
v. 292, p. 1535-1540) describe Hadrocodium wui, a complete skull and
jaws from the Lower Jurassic Lufeng Formation of China, that had triconodont
teeth but an enlarged brain and a fully mammalian ear region over 195 million
years ago. Luo and co-workers (Nature, v. 409, p. 53-57; Acta Paleontologica
Polonica, v. 47, p. 1-78) provide a long-awaited update of the phylogeny
of Mesozoic mammals. They argue that the tribosphenic tooth condition with the
reversed triangle shearing between molar crowns may have evolved independently
twice — once in the more familiar tribosphenic mammals (such as the marsupials
and placentals of the Boreosphenida) but also within the newly recognized Australosphenida.
This clade includes several Gondwana Mesozoic forms, such as the enigmatic Ausktribosphenos,
and Steropodon, the oldest known monotreme. Both fossils are from the
Early Cretaceous of Australia. Added to these is the recently described Ambondro
from the Cretaceous of Madagascar and the first Jurassic mammal from South America,
Asfaltomys patagonicus (Rauhut and others, Nature, v. 416, p.
165-168). Their sister taxon appears to be Shuotherium from the Jurassic
of China.
Other important Asian Mesozoic mammal finds of 2002 include Kulbeckia,
an early relative of the rodents and rabbits from beds 85-90 million years old
in Kazakhstan (Archibald and colleagues, Nature, v. 414, p. 62-65). Reilly
and White (Science, v. 299, p. 400-402) have shown that the long-controversial
epipubic, or "marsupial" bones of the hip region in primitive mammals,
once thought to support the pouch in marsupials, apparently act as a lever that
stiffen the trunk between diagonal motions of the limbs during walking.
The remarkably complete primate skeletons from the late Paleocene lacustrine
limestones of the Bighorn Basin in Wyoming are now beginning to be described.
Bloch and Boyer (Science, v. 298, p. 1606-1610) describe the skeleton
of the plesiadapiform, Carpolestes simpsoni, which shows that this long-controversial
group is indeed closely related to primates. The skeleton shows that early primates
were terminal branch feeders, rather than specialized leapers or visually directed
predators. Bloch and others (Journal of Vertebrate Paleontology, v. 22,
p. 366-379) reported on Acidomomys hebeticus, another new plesiadapiform
that is remarkably primitive for the late Paleocene.
Whales and Amphibians
The story of the origin of whales from land animals continues to expand. Madar
and others (Journal of Vertebrate Paleontology, v. 22, p. 405-422) described
additional skeletal material of the walking/swimming whale, Ambulocetus natans,
which served to further reinforce the idea that whales are closer to artiodactyls
than to mesonychids. Their findings corroborate last year's announcement that
early whales had the characteristic "double-pulley" artiodactyl astragalus.
Spoor and colleagues (Nature, v. 417, p. 163-166) reported that the ear
regions of Eocene whales from Pakistan are already modified for aquatic equilibrium
maintenance, even if their skeletons are still transitional between terrestrial
and aquatic adaptations. Gatesy and O'Leary (Trends in Ecology and Evolution,
v. 16, p. 562-570) provide a comprehensive view of our current understanding
of whale origins.
Finally, Zhu and others (Nature, v. 420, p. 760) describe Sinostega,
the first Devonian tetrapod from Asia. It is more similar to the Euramerican
Acanthostega than the Australian genus, Metaxygnathus, but shows
that early tetrapods were evolving on several continents in the Late Devonian.
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