The evolution of birds from theropod dinosaurs was one of the great evolutionary transitions in the history of life. The macroevolutionary tempo and mode of this transition is poorly studied, which is surprising because it may offer key insight into major questions in evolutionary biology, particularly whether the origins of evolutionary novelties or new ecological opportunities are associated with unusually elevated "bursts" of evolution. We present a comprehensive phylogeny placing birds within the context of theropod evolution and quantify rates of morphological evolution and changes in overall morphological disparity across the dinosaur-bird transition. Birds evolved significantly faster than other theropods, but they are indistinguishable from their closest relatives in morphospace. Our results demonstrate that the rise of birds was a complex process: birds are a continuum of millions of years of theropod evolution, and there was no great jump between nonbirds and birds in morphospace, but once the avian body plan was gradually assembled, birds experienced an early burst of rapid anatomical evolution. This suggests that high rates of morphological evolution after the development of a novel body plan may be a common feature of macroevolution, as first hypothesized by G.G. Simpson more than 60 years ago.

In post-Cambrian time, five events-the end-Ordovician, end-Frasnian in the Late Devonian, end-Permian, end-Triassic, and end-Cretaceous-are commonly grouped as the ''big five'' global intervals of mass extinction. Plotted by magnitude, extinction intensities for all Phanerozoic substages show a continuous distribution, with the five traditionally recognized mass extinctions located in the upper tail. Plotted by time, however, proportional extinctions clearly divide the Phanerozoic Eon into six stratigraphically coherent intervals of alternating high and low extinction intensity. These stratigraphic neighborhoods provide a temporal context for evaluating the intensity of extinction during the ''big five'' events. Compared with other stages and substages in the same neighborhood, only the end-Ordovician, end-Permian, and end-Cretaceous extinction intensities appear as outliers. Moreover, when origination and extinction are considered together, only these three of the ''big five'' events appear to have been generated exclusively by elevated extinction. Low origination contributed more than high extinction to the marked loss of diversity in the late Frasnian and at the end of the Triassic. Therefore, whereas the ''big five'' events are clearly times when diversity suffered mass depletion, only those at the end of the Ordovician, Permian, and Cretaceous periods unequivocally qualify as globally distinct mass extinctions. Each of the three has a unique pattern of extinction, and the diversity dynamics of these events differ, as well, from the other two major diversity depletions. As mass depletions of diversity have no common effect, common causation seems unlikely.

Quantifying rates of morphological evolution is important in many macroevolutionary studies, and critical when assessing possible adaptive radiations and episodes of punctuated equilibrium in the fossil record. However, studies of morphological rates of change have lagged behind those on taxonomic diversification, and most authors have focused on continuous characters and quantifying patterns of morphological rates over time. Here, we provide a phylogenetic approach, using discrete characters and three statistical tests to determine points on a cladogram (branches or entire clades) that are characterized by significantly high or low rates of change. These methods include a randomization approach that identifies branches with significantly high rates and likelihood ratio tests that pinpoint either branches or clades that have significantly higher or lower rates than the pooled rate of the remainder of the tree. As a test case for these methods, we analyze a discrete character dataset of lungfish, which have long been regarded as "living fossils" due to an apparent slowdown in rates since the Devonian. We find that morphological rates are highly heterogeneous across the phylogeny and recover a general pattern of decreasing rates along the phylogenetic backbone toward living taxa, from the Devonian until the present. Compared with previous work, we are able to report a more nuanced picture of lungfish evolution using these new methods. K E Y W O R D S :Adaptive radiation, evolutionary rates, lungfish, morphological evolution, paleontology, punctuated equilibrium.

The maximum size of organisms has increased enormously since the initial appearance of life >3.5 billion years ago (Gya), but the pattern and timing of this size increase is poorly known. Consequently, controls underlying the size spectrum of the global biota have been difficult to evaluate. Our period-level compilation of the largest known fossil organisms demonstrates that maximum size increased by 16 orders of magnitude since life first appeared in the fossil record. The great majority of the increase is accounted for by 2 discrete steps of approximately equal magnitude: the first in the middle of the Paleoproterozoic Era (Ϸ1.9 Gya) and the second during the late Neoproterozoic and early Paleozoic eras (0.6 -0.45 Gya). Each size step required a major innovation in organismal complexity-first the eukaryotic cell and later eukaryotic multicellularity. These size steps coincide with, or slightly postdate, increases in the concentration of atmospheric oxygen, suggesting latent evolutionary potential was realized soon after environmental limitations were removed.body size ͉ Cambrian ͉ oxygen ͉ Precambrian ͉ trend D espite widespread scientific and popular fascination with the largest and smallest organisms and numerous studies of body size evolution within individual taxonomic groups (1-9), the first-order pattern of body size evolution through the history of life has not been quantified rigorously. Because size influences (and may be limited by) a broad spectrum of physiological, ecological, and evolutionary processes (10-16), detailed documentation of size trends may shed light on the constraints and innovations that have shaped life's size spectrum over evolutionary time as well as the role of the body size spectrum in structuring global ecosystems. Bonner (17) presented a figure portraying a gradual, monotonic increase in the overall maximum size of living organisms over the past 3.5 billion years. The pattern appears consistent with a simple, continuous underlying process such as diffusion (18), but could also reflect a more complex process. Bonner, for example, proposed that lineages evolve toward larger sizes to exploit unoccupied ecological niches. For decades, Bonner's has been the only attempt to quantify body size evolution over the entire history of life on Earth, but the data he presented were not tied to particular fossil specimens and were plotted without consistent controls on taxonomic scale against a nonlinear timescale. Hence, we have lacked sufficient data on the tempo and mode of maximum size change to evaluate potential first-order biotic and abiotic controls on organism size through the history of life.Here, we document the evolutionary history of body size on Earth, focusing on the upper limit to size. Use of maximum size allows us to assess constraints on the evolution of large body size and avoids the more substantial empirical difficulties in determining mean, median, or minimum size for all life or even for many individual taxa. For each era within the Archean Eon (4,000-2,500 Mya) and ...

Cope's rule proposes that animal lineages evolve toward larger body size over time. To test this hypothesis across all marine animals, we compiled a data set of body sizes for 17,208 genera of marine animals spanning the past 542 million years. Mean biovolume across genera has increased by a factor of 150 since the Cambrian, whereas minimum biovolume has decreased by less than a factor of 10, and maximum biovolume has increased by more than a factor of 100,000. Neutral drift from a small initial value cannot explain this pattern. Instead, most of the size increase reflects differential diversification across classes, indicating that the pattern does not reflect a simple scaling-up of widespread and persistent selection for larger size within populations.