Quaternary megafauna extinctions altered body size distribution in tortoises

Abstract

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.

Turtle body size evolution is determined by lineage‐specific specializations rather than global trends

“…The Eocene–Oligocene witnessed a peak of diversity, related to the origin of crown‐group Testudinidae (Lourenço et al, 2012 ; Vlachos & Rabi, 2018 ), after which the group spread from Eurasia to most of the world. Therefore, the recent high variation in body size within tortoises might be also related to the expansion of occupied habitats (as in freshwater turtles), also associated with specific diversification and extinction dynamics within the group (e.g., Joos et al, 2022 ).…”

The latest freshwater giants: a new Peltocephalus (Pleurodira: Podocnemididae) turtle from the Late Pleistocene of the Brazilian Amazon

“…Previous evidence indicated that podocnemidids attained gigantic sizes (SCL > 150 cm [ 3 ]) at least twice: Late Palaeocene [ 33 , 34 ] and Miocene [ 6 , 35 ]. This is consistent with the periods in which extreme sizes evolved in other groups of turtles [ 5 ], as well as in other reptiles, e.g.…”

“…Titanochelon schaefferi and Megalochelys atlas [ 32 ]). Furthermore, those giant reptiles seem to have disappeared after 50–40 kyr ago [ 40 ], with testudinids showing body size decrease by the end of the Pleistocene [ 3 ].…”

“…Although the Amazonian Miocene is famous for its gigantic reptiles [ 2 ], large Quaternary species are also known and might have been affected by anthropogenic action. Human predilection for giant turtles, for example, has been linked to body size decline in tortoises (Testudinidae) over the Late Neogene and Quaternary, and the overexploitation of island species leading to their extinction is well documented [ 3 ]. Freshwater turtles are usually overlooked, as giant species (straight carapace length >150 cm [ 3 ]) are absent from the Quaternary record.…”

“…Human predilection for giant turtles, for example, has been linked to body size decline in tortoises (Testudinidae) over the Late Neogene and Quaternary, and the overexploitation of island species leading to their extinction is well documented [ 3 ]. Freshwater turtles are usually overlooked, as giant species (straight carapace length >150 cm [ 3 ]) are absent from the Quaternary record. The largest freshwater turtles nowadays, Chitra chitra (Trionychidae) and Podocnemis expansa (Podocnemididae), reach no more than 140 and 110 cm, respectively [ 4 ], and only a handful of them have crossed the 150 cm threshold in the past [ 5 ], most recently and prominently during the Miocene [ 6 ].…”