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ScienceWeek
PALEONTOLOGY: ON THE ANCESTOR OF TYRANNOSAURUS REX
The following points are made by Thomas R. Holtz Jr (Nature 2006 439:665):
1) Dinosaur research -- and indeed a whole swathe of palaeontology -- has been revolutionized by recent discoveries in China. Most famous are the feathered dinosaurs, early flowering plants, various mammals and other spectacular fossils from the 128- to 110-million-year-old lake deposits in Liaoning Province in the northeast[1]. Less well studied, however, are the many-colored badlands of Xinjiang on the far western side of the country. These rocks contain fossils dating to the beginning of the Late Jurassic epoch, roughly 161 million to 156 million years ago.
2) From the Wucaiwan locality in Xinjiang come fossils of a new carnivorous dinosaur[2]. A pair of skeletons show that Guanlong wucaii ("crowned dragon of the five-colored rocks") was a 3-meter-long predator. Details of the anatomy of this relatively small species indicate that it is the most ancient known member of the line that led to Tyrannosaurus rex and its giant kin, the Tyrannosauridae or "tyrant dinosaurs".
3) Tyrannosaurids were the dominant group of predators in eastern and central Asia and North America during the last 20 million years of the Late Cretaceous epoch. Their morphology (enlarged skulls with enormous, robust teeth; highly reduced arms ending in two-fingered hands; and elongated hindlimbs), and above all their great size (9-13 meters long for the most completely known species), have made them among the most recognizable of fossil groups[3]. This distinctiveness, as well as their relatively rich fossil record, both in completeness of skeletons and numbers of individuals, has made the tyrannosaurids the subject of numerous palaeobiological studies[4,5].
4) Unfortunately, the uniqueness of the Tyrannosauridae has obscured their origin within the larger evolutionary tree of Theropoda -- the clade (group) of carnivorous dinosaurs, including birds. The evolution of the distinctive adaptations present in the better-preserved Late Cretaceous forms such as Tyrannosaurus, Gorgosaurus, and Tarbosaurus has transformed their skulls, limbs, and vertebrae, thereby "overwriting" much of the anatomical traces of their ancestry. For most of the twentieth century, palaeontologists followed H.F. Osborn in regarding the tyrannosaurids as the last descendants of Jurassic and Early Cretaceous carnosaurian giants, such as Allosaurus and Acrocanthosaurus. But in the 1990s, an alternative hypothesis8 that tyrannosaurids arose among the swift small-bodied coelurosaurs was ultimately supported by analyses that invoked the principles of cladistics. (Because cladistics provides testable hypotheses, this is now accepted as the best approach to evolutionary analysis.)
References (abridged):
1. Zhou, Z. , Barrett, P. M. & Hilton, J. Nature 421, 807-814 (2003)
2. Xu, X. et al. Nature 439, 715-718 (2006)
3. Holtz, T. R. in The Dinosauria 2nd edn (eds Weishampel, D. B., Dodson, P. & Osmolska, H.) 111-136 (Univ. California Press, 2004)
4. Carr, T. D. J. Vert. Paleontol. 19, 497-520 (1999)
5. Currie, P. J. Can. J. Earth Sci. 40, 651-665 (2003)
Nature http://www.nature.com/nature
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DINOSAURS, DRAGONS, AND DWARFS: THE EVOLUTION OF MAXIMAL BODY SIZE
The following points are made by G.P. Burness et al (Proc. Nat. Acad. Sci. 2001 98:14518):
1) The size and taxonomic affiliation of the largest locally present species ("top species") of terrestrial vertebrate vary greatly among faunas, raising many unsolved questions. Why are the top species on continents bigger than those on even the largest islands, bigger in turn than those on small islands? Why are the top mammals marsupials on Australia but placentals on the other continents? Why is the world's largest extant lizard (the Komodo dragon) native to a modest-sized Indonesian island, of all unlikely places? Why is the top herbivore larger than the top carnivore at most sites? Why were the largest dinosaurs bigger than any modern terrestrial species?
2) A useful starting point is the observation of Marquet and Taper (1998), based on three data sets (Great Basin mountaintops, Sea of Cortez islands, and the continents), that the size of a landmass's top mammal increases with the landmass's area. To explain this pattern, they noted that populations numbering less than some minimum number of individuals are at high risk of extinction, but larger individuals require more food and hence larger home ranges, thus only large landmasses can support at least the necessary minimum number of individuals of larger-bodied species. If this reasoning were correct, one might expect body size of the top species also to depend on other correlates of food requirements and population densities, such as trophic level and metabolic rate. Hence the authors assembled a data set consisting of the top terrestrial herbivores and carnivores on 25 oceanic islands and the 5 continents to test 3 quantitative predictions:
a) Within a trophic level, body mass of the top species will increase with land area, with a slope predictable from the slope of the relation between body mass and home range area.
b) For a given land area, the top herbivore will be larger than the top carnivore by a factor predictable from the greater amounts of food available to herbivores than to carnivores.
c) Within a trophic level and for a given area of landmass, top species that are ectotherms will be larger than ones that are endotherms, by a factor predictable from ectotherms' lower food requirements.
3) The authors point out that on reflection, one can think of other factors likely to perturb these predictions, such as environmental productivity, over-water dispersal, evolutionary times required for body size changes, and changing landmass area with geological time. Indeed, the database of the authors does suggest effects of these other factors. The authors point out they propose their three predictions not because they expect them always to be correct, but because they expect them to describe broad patterns that must be understood in order to be able to detect and interpret deviations from those patterns.
Proc. Nat. Acad. Sci. http://www.pnas.org
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ON THE EVOLUTION OF SIZE IN LIVING SYSTEMS
Notes by ScienceWeek:
A long view of the evolutionary history of life on Earth suggests that living systems tend to evolve into larger and more complex forms. However, some of the most successful living systems are relatively small and have remained small. Is there a pattern in the evolution of size? And if there is a pattern, what are the forces responsible for the pattern and how do we explain the exceptions?
The following points are made by Sean B. Carroll (Nature 2001 409:1102):
1) For the first 2.5 billion years of life on Earth, most species rarely exceeded 1 millimeter in size and were generally much smaller. The earliest reported bacterial microfossils from approximately 3.5 billion years ago averaged approximately 5 microns in diameter. Early *eukaryotic microfossils (*acritarchs), while considerably larger, still ranged generally from approximately 40 to 200 microns in size (with a few larger exceptions) for much of their first 600 to 800 million year history. Organismal size increased appreciably with the evolution of multicellular forms. In bacterial and algal forms with cell walls, one of the simplest ways to become multicellular was for the products of cell division to remain together to form long filaments. Many early multicellular eukaryotes were millimeter-scale, linear or branched, filamentous forms.
2) The size and shape of life did not expand appreciably until the late *Proterozoic. Radially symmetric impressions and trace fossils indicate the presence of millimeter scale multicellular organisms (metazoans) around 550 million years ago. The puzzling *Ediacaran fauna comprised of tubular, frond-like, radially symmetric forms generally reached several centimeters in size (although some forms approached 1 meter in size), as did macroscopic algae. Organismal sizes expanded considerably in the *Cambrian, including *bilaterians up to 50 centimeters in size, as well as sponges and algae up to 5 to 10 centimeters. Maximal body lengths of animals increased subsequently by another 2 orders of magnitude, as did algal size (e.g., *kelp).
3) The largest existing organisms, giant fungi and trees, evolved from independent small ancestors. Land plants are believed to have evolved from *charophyte green algae, and both green algae and plants apparently evolved from a unicellular *flagellate ancestor. Fossil spores indicating the earliest evidence of plant life date from the *mid-Ordovician. The oldest plant-body fossil (Cooksonia) suggests that early land plants were small, and on the basis of molecular phylogenetic analyses are believed to be comparable in organization and life cycle to *liverworts. Many of the principal groups of land plants have evolved large (> 10 meters) species at some point in their history. Thus, increases in both mean and maximal organismal size apparently occurred in the evolution of bacteria, eukaryotes, and within the algal, fungal, and animal lineages.
4) There is a long history of support for the general notion of overall evolutionary trends toward increases in size, complexity, and diversity. However, there are two fundamentally distinct mechanisms that have been proposed to explain these trends. One proposed mechanism is a random and passive tendency to evolve away from the initial minima of organismal size, complexity, and diversity through an overall increase in variance ("there is no where to go but up"). The second proposed mechanism is a non-random, active or "driven" process that biases evolution towards increased size or complexity. What must be noted is that there are relationships between size and complexity and between complexity and diversity that are intuitive apparent. Increases in organismal size through increases in cell number create the potential for increases in diversity of cell type, and as a result, increases in anatomical complexity. Increases in morphological complexity then may lead to expansions into previously unoccupied "ecospace" and an accompanying expansion of species diversity.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
eukaryotic: Cells (or organisms composed of such cells) containing internal membrane-bound organelles such as a nucleus.
acritarchs: Unicellular microfossils of unknown or uncertain biological origin that occur abundantly in strata from the Precambrian and Paleozoic (see next note).
Proterozoic: The complete geological time-scale is as follows:
Time-Frame Starting Date (Millions of Years Ago)
---------- -------------------------------------
Hadean 4600
Archaean 4000
Proterozoic 2500
Cambrian 570
Ordovician 510
Silurian 439
Devonian 408.5
Carboniferous 362.5
Permian 290
Triassic 245
Jurassic 208
Cretaceous 145.6
Paleocene 65
Eocene 56.5
Oligocene 35.4
Miocene 23.3
Pliocene 5.2
Pleistocene 1.64
Holocene 0.01
Ediacaran: The term "Ediacaran" refers to an assemblage (until recently the oldest) of soft-bodied marine animals, the assemblage first discovered in the Ediacara Hills in Australia.
Cambrian: See time-scale above. The most outstanding aspect of the Cambrian was the rather sudden appearance of numerous invertebrate fossils, so numerous that some researchers have termed the Cambrian an explosion of evolutionary processes. Many of the life forms that existed during the Cambrian are long extinct, but their fossils are numerous, and through their fossils the various Cambrian species have been the subject of much study by paleobiologists. The Cambrian explosion of life forms has been a long-standing puzzle for paleobiologists, and at present there is apparently no single generally accepted explanation.
bilaterians: The "Bilateria" are a major division of the animal kingdom comprising all forms with bilateral symmetry, and the term "bilaterians" refers to the first such forms appearing after the emergence of protozoa.
kelp: A group of large brown "seaweeds", actually algae, growing in large structures that may be as long as 60 meters.
charophyte green algae: In general, "green algae" are algae in which chlorophyll is not masked by another pigment. Charophyte green algae (also known as "stoneworts"), are a type of green algae usually found in fresh or brackish water.
flagellate: Possessing one or more flagella. A flagellum is a long threadlike extension providing locomotion for a cell.
mid-Ordovician: See time-scale above.
liverworts: (Hepaticopsida) A group of lower plants in which the dominant generation is the sexual phase of the plant (gametophyte phase).
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