The late Quaternary is characterized by the extinction of many terrestrial megafauna,
which included tortoises (Family: Testudinidae). However, limited information is available
on how extinction shaped the phenotype of surviving taxa. Here, based on a global
dataset of straight carapace length, we investigate the temporal variation, spatial
distribution and evolution of tortoise body size over the past 23 million years, thereby
capturing the effects of Quaternary extinctions in this clade. We found a significant
change in body size distribution characterized by a reduction of both mean body size
and maximum body size of extant tortoises relative to fossil taxa. This reduction
of body size occurred earlier in mainland (Early Pleistocene 2.588-0.781 Ma) than
in island tortoises (Late Pleistocene/Holocene 0.126-0 Ma). Despite contrasting body
size patterns between fossil and extant taxa on a spatial scale, tortoise body size
showed limited variation over time until this decline. Body size is a fundamental
functional trait determining many aspects of species ecologies, with large tortoises
playing key roles as ecosystem engineers. As such, the transition from larger sized
to smaller sized classes indicated by our findings likely resulted in the homogenization
of tortoises' ecological functions and diminished the role of tortoises in structuring
the vegetation community.
