Grouping behavior in a Triassic marine apex predator.

Marine tetrapods occupy important roles in modern marine ecosystems and often gather in large aggregations driven by patchy prey distribution,1,2 social or reproductive behaviors,3,4 or oceanographic factors.5 Here, we show that similar grouping behaviors evolved in an early marine tetrapod lineage, documented by dozens of specimens of the giant ichthyosaur Shonisaurus in the Luning Formation in West Union Canyon, Nevada, USA.6,7 A concentration of at least seven skeletons closely preserved on a single bedding plane received the bulk of previous attention. However, many more specimens are preserved across ∼106 square meters and ∼200 stratigraphic meters of outcrop representing an estimated >105-6 years. Unlike other marine-tetrapod-rich deposits, this assemblage is essentially monotaxic; other vertebrate fossils are exceptionally scarce. Large individuals are disproportionately abundant, with the exception of multiple neonatal or embryonic specimens, indicating an unusual demographic composition apparently lacking intermediate-sized juveniles or subadults. Combined with geological evidence, our data suggest that dense aggregations of Shonisaurus inhabited this moderately deep, low-diversity, tropical marine environment for millennia during the latest Carnian Stage of the Late Triassic Period (237-227 Ma). Thus, philopatric grouping behavior in marine tetrapods, potentially linked to reproductive activity, has an antiquity of at least 230 million years.

The origin of placental mammal life histories.

After the end-Cretaceous extinction, placental mammals quickly diversified1, occupied key ecological niches2,3 and increased in size4,5, but this last was not true of other therians6. The uniquely extended gestation of placental young7 may have factored into their success and size increase8, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping9,10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30-75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals.

Novel track morphotypes from new tracksites indicate increased Middle Jurassic dinosaur diversity on the Isle of Skye, Scotland.

Dinosaur fossils from the Middle Jurassic are rare globally, but the Isle of Skye (Scotland, UK) preserves a varied dinosaur record of abundant trace fossils and rare body fossils from this time. Here we describe two new tracksites from Rubha nam Brathairean (Brothers' Point) near where the first dinosaur footprint in Scotland was found in the 1980s. These sites were formed in subaerially exposed mudstones of the Lealt Shale Formation of the Great Estuarine Group and record a dynamic, subtropical, coastal margin. These tracksites preserve a wide variety of dinosaur track types, including a novel morphotype for Skye: Deltapodus which has a probable stegosaur trackmaker. Additionally, a wide variety of tridactyl tracks shows evidence of multiple theropods of different sizes and possibly hints at the presence of large-bodied ornithopods. Overall, the new tracksites show the dinosaur fauna of Skye is more diverse than previously recognized and give insight into the early evolution of major dinosaur groups whose Middle Jurassic body fossil records are currently sparse.