Geotimes
Highlights
Vertebrates
Donald Prothero

From early humans to early tetrapods, a number of important discoveries were reported in 2002. Perhaps the most spectacular find was Sahelanthropus tchadensis, a nearly complete skull of a small hominid from beds 6-7 million years old in northern Chad, which was duly reported by Brunet and others in Nature (v. 418, p. 145-151) and quickly picked up by many of the world's science and news publications. While the skull is chimp-like in its small size, small brain and large brow ridges (so large as to suggest the skull belonged to a male), it shows some remarkably precocious hominid features, including a short, flattened face, reduced canine teeth and enlarged cheek teeth with heavy crown wear. Most importantly, the hole through which the spinal cord exits the braincase is directly below the skull, proving that Sahelanthropus had an upright posture at the very beginning of hominid evolution. Its great age (at least a million years older than the next youngest hominid fossil) contradicts the molecular biologists' claim that humans diverged from the chimpanzee-gorilla lineage only 5-6 million years ago.

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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Prothero is professor of geology at Occidental College in Los Angeles and lectures in geobiology at Caltech in Pasadena. E-mail: prothero@oxy.edu.

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