INTRODUCTION



Introduction

Our Approach

Historical Analysis

Growth Curve Reconstruction and Developmental Mass Extrapolation

Statistical Analysis of Evolutionary Trends

Findings to Date

Qualifications and Research Roles of the PI's

References

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The derivation of birds (avialans) from theropod (carnivorous) dinosaurs is one of the most intriguing and controversial evolutionary transitions. Paleontological evidence in the form of body fossils of feathered dinosaurs (e.g. Ji et al. 1998, Ji et al., 2001, Xu et al. 2001) and some of the earliest birds (e.g. Sanz 1988, Sereno et al. 1992, Chiappe 1995, 1997, Hou et al. 1995, Forster et al. 1998) has recently provided an overwhelming anatomical argument in support of the dinosaur-bird connection (Padian and Chiappe 1998; Fig. 1).
figure 1: derivation of birds (avialans)
Figure 1. Comparison between the first bird Archaeopteryx and its close dinosaurian relative, Velociraptor, showing some of the key anatomoical similarities pointing to birds having evolved from other dinosaurs. Notably besides these skeletal similarities, some of the non-avian dinosaurs have been shown to have had feathers. Figure courtesy of Haden McNeil Publishing Inc. from Erickson, 2001.
    However, the sequence and timing of acquisition of the suite of unique morphological, physiological, and behavioral avialan apomorphies (characteristics) remain poorly understood. Rapid growth rates are among the most puzzling pieces of the physiological mosaic of avian evolution (Chinsamy 2001, Erickson et al. 2001, Padian et al. 2001). In fact, birds are among the fastest growing vertebrates ever to inhabit the planet (Case 1978, Calder 1984, Starck and Ricklefs 1998; Fig. 2).
    <i>Standardized comparison Figure 2. Standardized comparison of exponential stage growth rates among living vertebrate groups demonstrates the elevated growth rates of birds with respect to the primitive "reptilian" condition. These regressions are based on hundreds of vertebrate taxa studied throughout development Maximal growth rates were assessed during the exponential stage of development as most body mass is accrued. Mass standardization negates the affects of morphological differences. Note that even the most primitive living avialans (precocial birds) grow at rates up to ten times faster than equably sized "reptiles." Regressions are redrawn from Case (1978) and Calder (1984).
   
Are these rapid growth rates an innovation unique to birds, or were they inherited from dinosaurian precursors?

If birds acquired their rapid growth rates uniquely, what exactly was the pattern of change?

Was it stepped as suggested by previous analyses of bone histology and body mass (Erickson et al., 2001). In other words could avian growth have been initially slower than growth rates in their nearest coelurosaurian relatives, only to become more rapid later in evolutionary history (Chinsamy et al. 1994, 1995, Chinsamy and Elzanowski 2001)?
   
In our research we seek answers to these questions. To address them we are: 1) rigorously sampling the fossilized bones of dinosaurs and birds spanning the critical, transitional interval (Erickson et al. 2001, Padian et al. 2001), 2) studying the gross morphology (size and shape) and microstructure of the bones to assess longevity (age), 3) using these data to make the first age-mass (~wt) growth curves for dinosaurs and early birds, finally 4) we are analyzing our findings in an evolutionary context to infer how modern avian growth rates came to be.
   
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