No abstract

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.

Middle Jurassic dinosaur fossils are exceedingly rare, but new discoveries from the Isle of Skye, Scotland, are beginning to fill this gap. We here describe a new dinosaur tracksite found in the Lealt Shale Formation (Bathonian) of the Great Estuarine Group at Rubha nam Brathairean (Brothers' Point) on Skye. The site preserves an abundance of small sauropod manus and pes prints and several isolated and broken medium-to-large tridactyl footprints. The main site occurs on a single horizon of shaley limestone that formed in a lagoonal environment. The sauropod tracks are tentatively assigned to the ichnotaxon Breviparopus due to the narrow gauge of the trackways, the digital characteristics of the pes, and the ratio of heteropody observed between the manus and the pes. A theropod trackmaker is inferred for some of the tridactyl impressions with several indicative of the ichnotaxon Eubrontes . This new site strengthens the inference, originally based on a previously discovered locality near Duntulm Castle (Duntulm Formation) in northern Skye, that sauropods habitually spent time in lagoons during the Middle Jurassic. Supplementary material : The photogrammetric model of track BP2_40 and associated metadata and photographs are available at https://doi.org/10.6084/m9.figshare.c.4046390

After the end-Cretaceous extinction, placental mammals quickly diversified 1 , occupied key ecological niches 2,3 , and increased in size 4,5 , but the latter was not true of other therians 6 .The uniquely extended gestation of placental young 7 may have factored in their success and size increase 8 , but reproduction style in early placentals remains unknown. Here, using palaeohistology and geochemistry, we present the earliest record of a placental life history, 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 mapping 9,10 by sixty million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (~7 months), rapid dental development, and short suckling interval (~30-75 days) show Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate P. bathmodon reproduced like a placental, and lived at a fast pace for its body size. Assuming P. bathmodon reflects close placental relatives, our findings suggest 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.Placentals are the most diverse group of mammals, comprising >6,000 extant species 11 and the largest animals ever. Their success may relate to their derived life history 8,12 , with maternal investment shifted prenatally through extended gestation 7,13 . This adaptation allows placentals the unique capability among mammals to produce highly precocial young: typically single offspring born at larger masses with well-developed dentition, fur, and open eyes 13,14 . Extended 3 gestation may have released placentals from developmental constraints associated with prolonged lactation in other mammals 8,15,16 , enabling experimentation with new locomotor modes and habitats 17,18 . However, when extended gestation evolved in mammals remains unclear: Mesozoic eutherians (mammals more closely related to placentals than marsupials) did not grow like living placentals [19][20][21] and it has been hypothesized that ancestral placentals gave birth to altricial young 21 . Nonetheless, immediately after the end-Cretaceous extinction, early Palaeocene placentals emerged from a 100-Ma lineage of small-bodied ancestors and quickly achieved much greater masses as they diversified into a variety of niches 4 . Thus, the early Palaeocene was likely an important interval in the eutherian transition to placental-like growth strategies, but the life histories of these mammals remain unknown.Among early placental clades, the Palaeocene-Eocene Pantodonta are a key group, because they were among the first large mammalian herbivores, becoming the largest mammals ever up to that point in time 22 . The early Palaeocene (~62 ma) Pantolambda bathmodon (~42 kg) is represented by multiple skel...