Comment on "Nocturnality in dinosaurs inferred from scleral ring and orbit morphology"

Scelidosaurus harrisonii (Dinosauria: Ornithischia) from the Early Jurassic of Dorset, England: biology and phylogenetic relationships

A layer of keratinous scutes encased the skull of Scelidosaurus. The neurocranium and the associated principal sensory systems of this dinosaur are described. The cranial musculature is reconstructed and a subsequent functional analysis suggests that jaw motion was orthal, allowing pulping of vegetation and some high-angle shearing between opposing teeth. Wishboning of the lower jaw was enabled by transverse displacement of the quadrates, and the long-axis mandibular torsion that occurred during the chewing cycle was permitted by flexibility at the dentary symphysis. Limb proportions and pectoral and pelvic musculature reconstructions suggest that Scelidosaurus was a facultative quadruped of ‘average’ locomotor ability. It retained some anatomical features indicative of a bipedal-cursorial ancestry. Hindlimb motion was oblique-to-parasagittal to accommodate the girth of the abdomen. Scelidosaurus used a combination of costal and abdominally driven aspiration. The hypothesis that respiration was an ‘evolutionary driver’ of opisthopuby in all dinosaurs is overly simplistic. A critical assessment of datasets used to analyse the systematics of ornithischians (and thyreophoran subclades) has led to a revised dataset that positions Scelidosaurus as a stem ankylosaur, rather than a stem thyreophoran. The value of phylogenetic definitions is reconsidered in the light of the new thyreophoran cladogram.

Convergent evolution of bird-mammal shared characteristics for adapting to nocturnality

The diapsid lineage (birds) and synapsid lineage (mammals), share a suite of functionally similar characteristics (e.g. endothermy) that are considered to be a result of their convergent evolution, but the candidate selections leading to this convergent evolution are still under debate. Here, we used a newly developed molecular phyloecological approach to reconstruct the diel activity pattern of the common ancestors of living birds. Our results strongly suggest that they had adaptations to nocturnality during their early evolution, which is remarkably similar to that of ancestral mammals. Given their similar adaptation to nocturnality, we propose that the shared traits in birds and mammals may have partly evolved as a result of the convergent evolution of their early ancestors adapting to ecological factors (e.g. low ambient temperature) associated with nocturnality. Finally, a conceptually unifying ecological model on the evolution of endothermy in diverse organisms with an emphasis on low ambient temperature is proposed. We reason that endothermy may evolve as an adaptive strategy to enable organisms to effectively implement various life-cycle activities under relatively low-temperature environments. In particular, a habitat shift from high-temperature to relatively low-temperature environments is identified as a common factor underlying the evolution of endothermy.

Assembly of modern mammal community structure driven by Late Cretaceous dental evolution, rise of flowering plants, and dinosaur demise

The long-standing view that Mesozoic mammaliaforms living in dinosaur-dominated ecosystems were ecologically constrained to small size and insectivory has been challenged by astonishing fossil discoveries over the last three decades. By studying these well-preserved early mammaliaform specimens, paleontologists now agree that mammaliaforms underwent ecomorphological diversification during the Mesozoic Era. This implies that Mesozoic mammaliaform communities had ecological structure and breadth that were comparable to today's small-bodied mammalian communities. However, this hypothesis remains untested in part because the primary focus of most studies is on individual taxa. Here, we present a study quantifying the ecological structure of Mesozoic mammaliaform communities with the aim of identifying evolutionary and ecological drivers that influenced the deep-time assembly of small-bodied mammaliaform communities. We used body size, dietary preference, and locomotor mode to establish the ecospace occupation of 98 extant, small-bodied mammalian communities from diverse biomes around the world. We calculated ecological disparity and ecological richness to measure the magnitude of ecological differences among species in a community and the number of different eco-cells occupied by species of a community, respectively. This modern dataset served as a reference for analyzing five exceptionally preserved, extinct mammaliaform communities (two Jurassic, two Cretaceous, one Eocene) from Konservat-Lagerstätten. Our results indicate that the interplay of at least three factors, namely the evolution of the tribosphenic molar, the ecological rise of angiosperms, and potential competition with other vertebrates, may have been critical in shaping the ecological structure of small-bodied mammaliaform communities through time.

The sclerotic ring of squamates: an evo-devo-eco perspective

The sclerotic ring consists of several bones that form in the sclera of many reptiles. This element has not been well studied in squamates, a diverse order of reptiles with a rich fossil record but debated phylogeny. Squamates inhabit many environments, display a range of behaviours, and have evolved several different body plans. Most importantly, many species have secondarily lost their sclerotic rings. This research investigates the presence of sclerotic rings in squamates and traces the lineage of these bones across evolutionary time. We compiled a database on the presence/absence of the sclerotic ring in extinct and extant squamates and investigated the evolutionary history of the sclerotic ring and how its presence/absence and morphology is correlated with environment and behaviour within this clade. Of the 400 extant species examined (59 families, 214 genera), 69% have a sclerotic ring. Those species that do not are within Serpentes, Amphisbaenia, and Dibamidae. We find that three independent losses of the sclerotic ring in squamates are supported when considering both evolutionary and developmental evidence. We also show that squamate species that lack, or have a reduced, sclerotic ring, are fossorial and headfirst burrowers. Our dataset is the largest squamate dataset with measurements of sclerotic rings, and supports previous findings that size of the ring is related to both environment occupied and behaviour. Specifically, scotopic species tend to have both larger inner and outer sclerotic ring apertures, resulting in a narrower ring of bone than those found in photopic species. Non-fossorial species also have a larger sclerotic ring than fossorial species. This research expands our knowledge of these fascinating bones; with further phylogenetic analyses scleral ossicles could become an extremely useful character trait for inferring the behaviour of fossil squamates.

Eye shape and the nocturnal bottleneck of mammals

Most vertebrate groups exhibit eye shapes that vary predictably with activity pattern. Nocturnal vertebrates typically have large corneas relative to eye size as an adaptation for increased visual sensitivity. Conversely, diurnal vertebrates generally demonstrate smaller corneas relative to eye size as an adaptation for increased visual acuity. By contrast, several studies have concluded that many mammals exhibit typical nocturnal eye shapes, regardless of activity pattern. However, a recent study has argued that new statistical methods allow eye shape to accurately predict activity patterns of mammals, including cathemeral species (animals that are equally likely to be awake and active at any time of day or night). Here, we conduct a detailed analysis of eye shape and activity pattern in mammals, using a broad comparative sample of 266 species. We find that the eye shapes of cathemeral mammals completely overlap with nocturnal and diurnal species. Additionally, most diurnal and cathemeral mammals have eye shapes that are most similar to those of nocturnal birds and lizards. The only mammalian clade that diverges from this pattern is anthropoids, which have convergently evolved eye shapes similar to those of diurnal birds and lizards. Our results provide additional evidence for a nocturnal 'bottleneck' in the early evolution of crown mammals.