Bringing up baby: preliminary exploration of the effect of ontogenetic niche partitioning in dinosaurs versus long-term maternal care in mammals in their respective ecosystems

Abstract

Mesozoic dinosaurs and Cenozoic mammals are often regarded as broadly ecologically equivalent, as they included the majority of medium-to-large-bodied terrestrial vertebrates of their respective eras. One of the most significant differences between them is their mode of reproduction: oviparity and large clutch size regardless of adult body size in the former; viviparity and litter size decreasing with adult body size in the latter. Furthermore, the disparity between hatchling and adult body size is much greater in dinosaurs than neonate and adult body size in mammals on average. The effects of these differences are examined with regards to the size distribution and species richness in fossil communities. Species lists and estimated adult body sizes were assembled for Jurassic and Cretaceous dinosaur and Cenozoic mammal communities based on the instantaneous diversity within wellsampled formations. The distribution of adult sizes within communities were compared to cases in which earlier growth stages were included: dinosaur hatchling size was estimated from known egg sizes of related taxa, while mammalian neonate size was estimated from those of extant relatives. The size distribution including the entire ontogenetic series results in a greater shift of average body size in dinosaurian communities than in mammals due to the much smaller dinosaur baby size. However, these two sets of plots may not reflect the ecological realities of their respective communities. In many mammals the young are provisioned via lactation and later by provisioning by mothers until they are a substantial fraction of adult body size: thus, the adult-only plots for mammals may be accurate reflections in terms of the realised feeding community structure. In contrast, evidence for long-term parental care in non-avian dinosaurs is scanty for most clades, with many juvenile dinosaurs living (individually or in small groups) independently for most of their lives. Thus, due to these ontogenetic niche shifts, plots in which different growth stages are counted as their own “taxa” might more accurately represent the trophic ecology within dinosaurian communities. In this scenario, effective “species” counts are higher in dinosaurian communities compared to mammalian ones: a reversal of the situation when only adult size is examined.