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ScienceWeek
PALEONTOLOGY: ON THE OLDEST BIRD BRAIN
The following points are made by Lawrence M. Witmer (Nature 2004 430:619):
1) Combining the feathered wings and wishbone of birds with the toothed jaws and long bony tail of reptiles, Archaeopteryx is the near-perfect transitional form. Since its discovery shortly after the publication of Charles Darwin's _Origin of Species_ in 1859, it has been a compelling example in the case for evolution.
2) It has also been the central player in the debate on the origin of birds and avian flight. Although discoveries of feathered dinosaurs and archaic birds from China have advanced our understanding of the transition to birds[1,2], the Archaeopteryx skeletons collected from Jurassic limestones in southern Germany remain (at 147 million years old) the oldest undisputed avian fossils, and the most primitive[3]. The fossils have been scrutinized by so many scientists over the past 140 years that it might seem that nothing new could be learned. But a landmark study by Dominguez Alonso et al[4] goes back to the first skeleton ever found to present data on the brain and sense organs. The results have implications for both the biology of Archaeopteryx and the evolutionary transition to birds.
3) Researchers at the Natural History Museum in London isolated the part of the skull that in life encased the brain. The braincase is so tiny -- smaller than the last segment of your little finger -- that Angela Milner, the team leader, safely carried it in a box in her shirt pocket from London to the University of Texas at Austin, where it could be analyzed with high-resolution X-ray computed tomography. Using X-rays, the team "sliced" the braincase so finely that they were able to peer inside the thin bone at the brain cavity and inner ear (the organ of balance and hearing), which was then digitally reconstructed.
4) Obtaining an understanding of the brain and sense organs is a top priority for palaeontologists, because such knowledge can offer insight into the behavior of extinct organisms not otherwise provided by the skeleton. In the case of Archaeopteryx, from the beginning the question has been -- could the oldest-known bird fly? In the past, answers have been sought from aerodynamics, justifiably focusing on the structure of the wings and feathers[3,5]. But flight isn't just about wings, rudders and flaps. It's also about the pilot and on-board computer, and those are the missing elements that this new study[4] provides for Archaeopteryx.
5) The brain of Archaeopteryx was much like that of birds today, albeit of a primitive sort. It was larger than the brain of an average reptile of equivalent body size but smaller than any similarly sized modern bird brain. Its organization was also basically avian, with enhancement of those areas concerned with movement. Moreover, the visual centers are enlarged, suggesting that Archaeopteryx was a visually oriented animal. The new findings relating to the delicate inner-ear canals are particularly important, because recent studies have associated canal architecture with behavior and mode of life. The canals of Archaeopteryx are again much more like those of birds than modern-day reptiles, suggesting that agility and coordination of head and eye movements were critical.
References (abridged):
1. Xu, X. et al. Nature 421, 335-340 (2003)
2. Zhou, Z., Barrett, P. M. & Hilton, J. Nature 421, 807-814 (2003)
3. Elzanowski, A. in Mesozoic Birds (eds Chiappe, L. M. & Witmer, L. M.) 129-159 (Univ. California Press, Berkeley, 2002)
4. Domİnguez Alonso, P., Milner, A. C., Ketcham, R. A., Cookson, M. J. & Rowe, T. B. Nature 430, 666-669 (2004)
5. Rayner, J. M. V. in Biomechanics in Evolution (eds Rayner, J. M. V. & Wootton, R. J.) 183-212 (Cambridge Univ. Press, 1991)
Nature http://www.nature.com/nature
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Related Material:
FOSSILS OF TWO NEW FEATHERED DINOSAURS
Notes by ScienceWeek:
In paleontology, the theropods are dinosaurs with four or fewer toes on the hind feet, a suborder of bipedal reptiles that first appeared in the Upper Triassic period (about 215 million years ago) and culminated in the Upper Cretaceous period (about 70 million years ago). A difficult problem in paleontology has been the tracing of the evolution of birds, with most paleontologists believing birds evolved from dinosaurs. The first feathered bird-like fossils are classified as Archaeopteryx and were found in Upper Jurassic deposits dating at about 150 million years ago, but there have been no feathered unequivocal intermediates yet discovered between these fossils and any dinosaur ancestors. Birds are rather unique in several aspects: feathers, toothless beaks, hollow bones, perching feet, etc., with a combination of skeletal features unknown in other living animals.
The following points are made by Kevin Padian (Nature 1998 393:729):
1) Among all living creatures, only birds have feathers. The discovery of a single isolated feather in the mid 19th century in Late Jurassic rocks of Bavaria was enough to demonstrate the remote ancestry of birds (approximately 150 million years ago). The discovery of the skeleton of Archaeopteryx in the same area in 1861 confirmed the idea of a feathered ancestor of birds, but little else in the skeleton appeared related to living birds.
2) Ji et al (1998) describe two small theropod dinosaurs from geological beds in the Liaoning province of China, the age of the beds disputed but apparently Late Jurassic/Early Cretaceous (about 145 million years ago). These theropods have down-like and vaned barbed feathers on the body, arms, legs, and tail. But these animals were clearly not birds, and they were clearly not capable of flight.
3) If these are true feathers -- and the evidence indicates they are indeed true feathers -- we are forced to revise our idea of the association of feathers with the animals we call birds.
4) By admitting that plumage did not first spring full-blown on the wings of Archaeopteryx, we are free to examine how feathers evolved in the first place.
5) The evolution of carnivorous dinosaurs through basal (relatively "primitive") *coelurosaurs into birds shows some unmistakable trends in the morphology of wishbones, breastbones, hollow bones, long arms, hands, etc.
6) Padian concludes: "The work of Ji et al should lay to rest any remaining doubts that birds evolved from small coelurosaurian dinosaurs. These new discoveries will excite the public and scientists alike by showing that down-like and later vaned body feathers evolved before flight feathers, and that a full complement of feathers was present in coelurosaurs before birds were invented."
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
coelurosaurs: These are a group of relatively small and lightly built dinosaurs in the suborder Theropoda having long necks and narrow pointed skulls.
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Related Material:
THE EVOLUTION OF DINOSAURS
Notes by ScienceWeek:
The Mesozoic Era is divided into the *Triassic, Jurassic, and Cretaceous periods, with the entire Mesozoic occupying the time-frame from approximately 230 million years ago to 70 million years ago. Existing evidence indicates that during the Mesozoic Era the reptiles diversified into many terrestrial habitats and became the dominant vertebrate group. *Therapsid reptiles, ancestors of the mammals, appear to have had temperature-regulating mechanisms (i.e., to have been "endothermic") and may have been the first endotherms. Special limb bones allowed the *archosaur reptiles to become bipedal, increasing their speed and size. Their descendants, the dinosaurs, *radiated into almost every terrestrial habitat, and some became so enormous as to approach theoretical terrestrial size limits.
Some researchers propose that on the basis of bone structure, posture, and biogeographical distribution, the dinosaurs were endothermic, and that endothermy, in combination with their size and other favorable attributes, may have enabled dinosaurs to dominate all other land vertebrates.
At the end of the Cretaceous period (which extended from approximately 145 million years ago to 70 million years ago), the dinosaurs, along with other large marine reptiles and various other groups, became extinct, an extinction whose cause has been the subject of considerable debate.
Because of their immense size and fierce appearance, the dinosaurs have long captured the public imagination. The Mesozoic Era, the time during which the dinosaurs thrived, has its own intriguing peculiarities. Until well into the age of the dinosaurs there were no birds and there was no grass, and no flowering plants existed until the Mesozoic was half completed. The arrangement of continents was quite different from that existing today, and throughout much of the Mesozoic, the central part of the US was apparently covered by an ocean.
The following points are made by Paul C. Serano (Science 1999 284:2137):
1) During the past 30 years, intensified paleontological exploration has doubled the recorded diversity of dinosaurs and extended their geographic range into polar regions. Exceptional fossil preservation has revealed eggshell microstructure, nesting patterns, and brooding posture among predators, and skin structures such as downy filaments and feathers. Analysis of bone microstructure and isotopic composition has shed light on embryonic and post-hatching growth patterns and on the thermophysiology of these animals. Footprint and track sites have yielded new clues regarding posture, locomotion, and herding among large-bodied herbivores. And the main lines of dinosaurian descent have been charted, placing the new discoveries in phylogenetic context.
2) The most important impact of this enriched perspective on dinosaurs may be its contribution to the study of large-scale evolutionary patterns. What triggers or drives major replacements in the history of life? How do novel and demanding functional capabilities, such as powered flight, first evolve? And how does the breakup of a supercontinent affect land-based life? The critical evidence resides in the fossil record -- in the structure, timing, and geography of evolutionary radiations such as that of dinosaurs.
3) The ascendancy of dinosaurs on land near the close of the Triassic now appears to have been as accidental and opportunistic as their demise and replacement by *therian mammals at the end of the Cretaceous. The dinosaurian radiation, launched by 1-meter-long bipeds, was slower in tempo and more restricted in adaptive scope than that of therian mammals. A notable exception was the evolution of birds from small-bodied predatory dinosaurs, which involved a dramatic decrease in body size. Recurring phylogenetic trends among dinosaurs include, to the contrary, an increase in body size. There is no evidence for co-evolution between predators and prey or between herbivores and flowering plants. As the major land masses drifted apart, dinosaurian biogeography was molded more by regional extinction and intercontinental dispersal than by the breakup sequence of *Pangaea.
Science http://www.sciencemag.org
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Notes by ScienceWeek:
Triassic, Jurassic, and Cretaceous periods: The approximate time-frames of these subdivisions of the Mesozoic Era are as follows: Triassic 230-180 million years ago; Jurassic 180-145 million years ago; Cretaceous 145-70 million years ago.
Therapsid reptiles: The therapsids were a group of mammal-like reptiles that became extinct at the end of the Triassic. The group contained both sluggish herbivores and active carnivores, and it is believed that small Mesozoic therapsids gave rise to the earliest mammals.
archosaur reptiles: (literally, "ruling reptiles") The parent group of the dinosaurs. Birds are descendants; crocodiles and alligators are living representatives.
radiated: In this context, the term "radiation" refers to the spread of a group of biological entities into new environments with consequent diversification.
therian mammals: The Theria is a subclass of mammals containing those mammals that bear live young (as opposed to mammals that lay eggs).
Pangaea: (Pangea) Some 375 million years ago, two large supercontinent called Laurasia and Gondwana comprised most or all of the present continental land masses, Laurasia in the north and Gondwana in the south. These two supercontinents merged into the single supercontinent Pangaea about 250 million years ago, and subsequent fragmentation of Pangaea began about 180 million years ago and the fragmentation eventually produced the continental land masses we know today. During this fragmentation there existed the second phase of Gondwana, incorporating what is now South America, Africa, Antarctica, Australia, and India. This historical picture, which is the present general consensus, is the result of a large number of geological and paleobiological studies.
ScienceWeek http://scienceweek.com
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