A macroevolutionary pathway to megaherbivory

Abstract

Several scenarios have been proposed to explain rapid net size increases in some early Cenozoic mammalian lineages: sustained and gradual directional change, successive occupation of adaptive zones associated with progressively larger body sizes, and nondirectional evolution associated with branching events in combination with higher diversification potential of the larger lineages. We test these hypotheses in brontotheres, which are among the first radiations of mammals that consistently evolved multitonne sizes. Body-mass evolution in brontotheres mainly occurred during speciation and had no preferential direction. Long-term directional change stemmed from the higher survival of larger lineages in less-saturated herbivore guilds. Our study emphasizes the role of differential species proliferation in explaining the long-term phenotypic trends observed in the fossil record, which are more than an accumulation of steady microevolutionary changes.

Estimation of maximum body size in fossil species: A case study using Tyrannosaurus rex

“…For one, it may be informative about the total niche space occupied by that species (Peters, 1983). For another, if body size increase within a lineage is driven by directional selection, following some formulations of Cope's Rule (Sanisidro et al, 2023), then it would be useful to be able to consider the upper tail of the body size distribution on which natural selection is purported to have acted. Similar considerations would be informative concerning the absolute limits of large body size.…”

Imbalanced speciation pulses sustain the radiation of mammals

“…Myr −1 , Welch’s t test, p = 0.055). This sorting process is expected under a model of “speciational evolution” (i.e., speciation-driven phenotypic change), where clades undergoing frequent speciation have an evolutionary advantage ( 40 , 41 ). At mass extinction events in particular, higher diversity and variability among descendant species increases the chances that at least one of them survives.…”

Trait-mediated speciation and human-driven extinctions in proboscideans revealed by unsupervised Bayesian neural networks

“…Crucially, speciation and extinction processes also play a leading role in fashioning biological disparity. When the pruning and sprouting of the tree of life are selective and favor certain properties of the species over others [i.e., species sorting or species selection; (10)], speciation and extinction become effective builders of evolutionary active trends that spur lineages into unexplored regions of the morphospace [ (11)(12)(13) see "note added in proof "]. Thus, it does not come as a surprise that evolutionary research has long sought to link patterns of diversity to evolutionary processes (14)(15)(16).…”

“…For example, species with larger body size may be more likely to disappear during severe extinction events (45). However, as productivity and ecosystem functioning recover, niches related to larger body sizes could have greater diversification potential once smaller-sized guilds become saturated (13,46). While some of the models allow for a joint analysis of combined multiple factors (e.g., multiple traits or time variables), they typically assume that their effects are additive, independent, and constrained by monotonic (often linear) functions (47)(48)(49)(50)(51).…”