Quaternary megafauna extinctions altered body size distribution in tortoises

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

Feeding ecology has shaped the evolution of modern sharks

Sharks are iconic predators in today’s oceans, yet their modern diversity has ancient origins. In particular, present hypotheses suggest that a combination of mass extinction, global climate change, and competition has regulated the community structure of dominant mackerel (Lamniformes) and ground (Carcharhiniformes) sharks over the last 66 million years. However, while these scenarios advocate an interplay of major abiotic and biotic events, the precise drivers remain obscure. Here, we focus on the role of feeding ecology using a geometric morphometric analysis of 3,837 fossil and extant shark teeth. Our results reveal that morphological segregation rather than competition has characterized lamniform and carcharhiniform evolution. Moreover, although lamniforms suffered a long-term disparity decline potentially linked to dietary “specialization,” their recent disparity rivals that of “generalist” carcharhiniforms. We further confirm that low eustatic sea levels impacted lamniform disparity across the end-Cretaceous mass extinction. Adaptations to changing prey availability and the proliferation of coral reef habitats during the Paleogene also likely facilitated carcharhiniform dispersals and cladogenesis, underpinning their current taxonomic dominance. Ultimately, we posit that trophic partitioning and resource utilization shaped past shark ecology and represent critical determinants for their future species survivorship

Body dimensions of the extinct giant shark Otodus megalodon: a 2D reconstruction

Inferring the size of extinct animals is fraught with danger, especially when they were much larger than their modern relatives. Such extrapolations are particularly risky when allometry is present. The extinct giant shark †Otodus megalodon is known almost exclusively from fossilised teeth. Estimates of †O. megalodon body size have been made from its teeth, using the great white shark (Carcharodon carcharias) as the only modern analogue. This can be problematic as the two species likely belong to different families, and the position of the †Otodus lineage within Lamniformes is unclear. Here, we infer †O. megalodon body dimensions based on anatomical measurements of five ecologically and physiologically similar extant lamniforms: Carcharodon carcharias, Isurus oxyrinchus, Isurus paucus, Lamna ditropis and Lamna nasus. We first assessed for allometry in all analogues using linear regressions and geometric morphometric analyses. Finding no evidence of allometry, we made morphological extrapolations to infer body dimensions of †O. megalodon at different sizes. Our results suggest that a 16 m †O. megalodon likely had a head ~ 4.65 m long, a dorsal fin ~ 1.62 m tall and a tail ~ 3.85 m high. Morphometric analyses further suggest that its dorsal and caudal fins were adapted for swift predatory locomotion and long-swimming periods

Integrating deep-time palaeontology in conservation prioritisation

Halting biodiversity loss under growing anthropogenic pressure is arguably the greatest environmental challenge we face. Given that not all species are equally threatened and that resources are always limited, establishing robust prioritisation schemes is critical for implementing effective conservation actions. To this end, the International Union for Conservation of Nature (IUCN) Red List of Threatened Species has become a widely used source of information on species’ extinction risk. Various metrics have been proposed that combine IUCN status with different aspects of biodiversity to identify conservation priorities. However, current strategies do not take full advantage of palaeontological data, with conservation palaeobiology often focussing on the near-time fossil record (the last 2 million years). Here, we make a case for the value of the deep-time (over 2 million years ago), as it can offer tangible parallels with today’s biodiversity crisis and inform on the intrinsic traits that make species prone to extinction. As such, palaeontological data holds great predictive power, which could be harnessed to flag species likely to be threatened but that are currently too poorly known to be identified as such. Finally, we identify key IUCN-based prioritisation metrics and outline opportunities for integrating palaeontological data to validate their implementation. Although the human signal of the current extinction crisis makes direct comparisons with the geological past challenging, the deep-time fossil record has more to offer to conservation than is currently recognised

Diversification trajectories and paleobiogeography of Neogene chondrichthyans from Europe

Despite the rich fossil record of Neogene chondrichthyans (chimaeras, sharks, rays, and skates) from Europe, little is known about the macroevolutionary processes that generated their current diversity and geographical distribution. We compiled 4368 Neogene occurrences comprising 102 genera, 41 families, and 12 orders from four European regions (Atlantic, Mediterranean, North Sea, and Paratethys) and evaluated their diversification trajectories and paleobiogeographic patterns. In all regions analyzed, we found that generic richness increased during the early Miocene, then decreased sharply during the middle Miocene in the Paratethys, and moderately during the late Miocene and Pliocene in the Mediterranean and North Seas. Origination rates display the most significant pulses in the early Miocene in all regions. Extinction rate pulses varied across regions, with the Paratethys displaying the most significant pulses during the late Miocene and the Mediterranean and North Seas during the late Miocene and early Pliocene. Overall, up to 27% and 56% of the European Neogene genera are now globally and regionally extinct, respectively. The observed pulses of origination and extinction in the different regions coincide with warming and cooling events that occurred during the Neogene globally and regionally. Our study reveals complex diversity dynamics of Neogene chondrichthyans from Europe and their distinct biogeographic composition despite the multiple marine passages that connected the different marine regions during this time