Advances in research on dinosaur gigantism

Abstract

Animal gigantism is an important evolutionary phenomenon. Cope’s rule postulates that organisms in evolving lineages tend to increase in body size over time, but many animals don’t show these tendencies. Many of the most well-known examples of animal gigantism are found amongst dinosaurs. The carnivorous theropod Spinosaurus aegyptiacus (~14–18 m in length, ~7–21 t in mass), the herbivorous sauropod Argentinosaurus huinculensis (~30–40 m in length, 60–100 t in mass) and the herbivorous ornithopod Shantungosaurus giganteus (~15–19 m in length, ~10–23 t in mass) are amongst the largest terrestrial animals ever to have walked the Earth. Studies on bone histology show that dinosaurs attained giant size using one of three growth strategies: accelerated growth, delayed maturity or a combination of these strategies. Previous studies have mostly focused on explaining why dinosaurs became so large in terms of environmental and biological factors. In the former aspect, latitude, habitat size, temperature conditions and oxygen levels were all found to be related to dinosaur gigantism. In the latter aspect, diet and selective advantages in biomechanics, respiration, digestion and bone development (osteocyte size) were all found to affect dinosaur gigantism. Significant progress has been made in these regards, but a unanimously agreed consensus has yet to be reached. This has been hampered by difficulties including those encountered when comparing giant non-dinosaurian living animals with dinosaurs as well as incomplete knowledge of some dinosaur palaeoenvironments. In recent years, research on sauropod gigantism has advanced more significantly compared to other dinosaur groups. An evolutionary cascade model (ECM) has been developed to understand the uniquely gigantic body size of sauropods. This model comprises of five evolutionary cascades with each one linked to at least one other: “Reproduction”, “Feeding”, “Head and neck”, “Avian-style lung”, and “Metabolism”. All cascades start with observed or inferred basal traits and end in the trait “very high body mass”. Future research that extends EMC-style approaches to all dinosaur groups and integrates them with additional palaeoenvironmental and biological information to produce more holistic evolutionary perspectives will help to bring consensus to our understanding of dinosaur gigantism. This would be especially welcome, particularly if this can deepen knowledge of herbivore-carnivore co-evolution and understanding of tetrapod gigantism more generally.