Megatheropods’ ‘grow fast, die young’ approach altered dinosaur community structure and diversity

By filling ecological niches that would otherwise host smaller carnivores, young megatheropods like tyrannosaurs shaped dinosaur communities throughout the Mesozoic era, a new study reports. The results help explain why species diversity among smaller dinosaurs was so much lower than that among their much larger contemporaries during the Mesozoic. Despite dominating the biodiversity on land for more than 150 million years, dinosaurs were not very rich in species. This is particularly true for smaller dinosaurs weighing less than 60 kilograms (kg). This pattern, however, differs from that of other vertebrate communities; because small- and medium-sized animals are often able to share a wide range of ecological niches and prey sources, smaller-bodied species typically harbor the highest diversity across regions. Their communities are generally represented by species that span the spectrum of body sizes. Yet, contrary to these assumptions, during the Mesozoic, large-bodied dinosaur species weighing 1,000 kg and more – including massive, meat-eating megatheropods like the Tyrannosaurus rex – were the most diverse groups of dinosaurs. Since even the most gigantic of dinosaurs begin their lives as relatively tiny hatchlings, it’s been thought that the large disparity in size between babies and adults, as well as the rapid growth of juveniles, may have led to the use of different resources at each life stage and that this may have influenced the diversity and unusual body size distribution of dinosaur communities observed in the fossil record. To test this theory, Katlin Schroeder analyzed a dataset representing 43 dinosaur communities across seven continents spanning 136 million years and including more than 550 species of dinosaurs. Schroeder et al.’s findings reveal the impact of megatheropods’ “grow fast, die young” approach to life on dinosaur communities. The results show that communities with megatheropods worldwide were largely devoid of mesocarnivores, or medium-sized carnivorous species weighing 100 to 1,000 kg. This gap in body size was apparent in carnivorous dinosaur communities regardless of where or when they existed. According to the authors, this suggests that the predominance of quickly growing megatheropod juveniles flooded ecological niches that might have otherwise been available to smaller taxa, filing both the morphological and functional role of mesocarnivoires, and thus, explaining their absence in the fossil record.

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