any reader who has made it this far, the explanation will be no surprise: the history of dinosaurs was largely dictated by changing oxygen levels.

HYPOTHESES FOR THE OBSERVED HISTORY OF DINOSAUR DIVERSITY, DISPARITY, AND SIZES

Here we can propose the following hypothesis based on dinosaur morphological and taxon abundance in the context of a long-term rise in Mesozoic oxygen levels. These explanations can answer the questions posed by Sereno about the observed history of dinosaurs. They are as follows.

Hypothesis 9.1: The Carnian through Hettangian interval (late Triassic to earliest Jurassic) was a time of low-oxygen levels and this coupled with very high carbon dioxide levels and hydrogen sulfide poisoning—not asteroid impact—was the major cause of the Triassic-Jurassic mass extinction.

Support for low-oxygen levels over this interval comes from marine stratal evidence of progressive anoxia (black shales, laminated beds, and trace fossils [such as chondrites] characteristic of low-oxygen sea bottoms), from the modeling of GEOCARBSULF, and from studies of fossil plants across this time interval by University of Chicago paleobotanist Jenny McElwaine, who showed morphological evidence of rising carbon dioxide in her fossil plants collected over this time interval. The combination of low oxygen, high global temperatures, and, based on the recovery of new biomarkers, perhaps hydrogen sulfide was the killing mechanism. While there were successive asteroid impacts over this time interval, one of which (the Manicouagan event of 214 million years ago, or 14 million years before the Triassic mass extinction) was large, the impacts played little or no role in the extinctions according to scientific observations of many stratal sections of this time interval.

Hypothesis 9.2: Ornithischian dinosaurs did not possess as effective a respiratory system as did saurischians. However, they were competitively superior to herbivorous saurischians with



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