Contrasting terrestrial and marine ecospace dynamics after the end-Triassic mass extinction event.

Mass extinctions have fundamentally altered the structure of the biosphere throughout Earth's history. The ecological severity of mass extinctions is well studied in marine ecosystems by categorizing marine taxa into functional groups based on 'ecospace' approaches, but the ecological response of terrestrial ecosystems to mass extinctions is less well understood due to the lack of a comparable methodology. Here, we present a new terrestrial ecospace framework that categorizes fauna into functional groups as defined by tiering, motility and feeding traits. We applied the new terrestrial and traditional marine ecospace analyses to data from the Paleobiology Database across the end-Triassic mass extinction-a time of catastrophic global warming-to compare changes between the marine and terrestrial biospheres. We found that terrestrial functional groups experienced higher extinction severity, that taxonomic and functional richness are more tightly coupled in the terrestrial, and that the terrestrial realm continued to experience high ecological dissimilarity in the wake of the extinction. Although signals of extinction severity and ecological turnover are sensitive to the quality of the terrestrial fossil record, our findings suggest greater ecological pressure from the end-Triassic mass extinction on terrestrial ecosystems than marine ecosystems, contributing to more prolonged terrestrial ecological flux.

The Role of Locomotory Ancestry on Secondarily Aquatic Transitions.

Land-to-sea evolutionary transitions are great transformations where terrestrial amniote clades returned to aquatic environments. These secondarily aquatic amniote clades include charismatic marine mammal and marine reptile groups, as well as countless semi-aquatic forms that modified their terrestrial locomotor anatomy to varying degrees to be suited for swimming via axial and/or appendicular propulsion. The terrestrial ancestors of secondarily aquatic groups would have started off swimming strikingly differently from one another given their evolutionary histories, as inferred by the way modern terrestrial amniotes swim. With such stark locomotor functional differences between reptiles and mammals, we ask if this impacted these transitions. Axial propulsion appears favored by aquatic descendants of terrestrially sprawling quadrupedal reptiles, with exceptions. Appendicular propulsion is more prevalent across the aquatic descendants of ancestrally parasagittal-postured mammals, particularly early transitioning forms. Ancestral terrestrial anatomical differences that precede secondarily aquatic invasions between mammals and reptiles, as well as the distribution of axial and appendicular swimming in secondarily aquatic clades, may indicate that ancestral terrestrial locomotor anatomy played a role, potentially in both constraint and facilitation, in certain aquatic locomotion styles. This perspective of the land-to-sea transition can lead to new avenues of functional, biomechanical, and developmental study of secondarily aquatic transitions.

Spinosaurus is not an aquatic dinosaur.

A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur Spinosaurus aegyptiacus when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture that S. aegyptiacus was the first fully aquatic dinosaur. The 'aquatic hypothesis' posits that S. aegyptiacus was a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models of S. aegyptiacus. We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on land S. aegyptiacus was bipedal and in deep water was an unstable, slow-surface swimmer (<1 m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show that Spinosaurus ranged far inland. Two stages are clarified in the evolution of Spinosaurus, which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.