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Biological variation is often considered in a scalable hierarchy, e.g., within the individual, within the populations, above the species level. Morphological integration, the concept of covariation among constituent parts of an organism, is also hierarchical; the degree to which these “modules” covary is a matter of the scale of the study as well as underlying processes driving the covariation. Multilevel analyses of trait covariation are a valuable tool to infer the origins and historical persistence of morphological diversity. Here, we investigate concordance in patterns of integration and modularity across three biological levels of variation: within a species, within two genera-level radiations, and among species at the family level. We demonstrate this approach using the skull of mammalian family Leporidae (rabbits and hares), which is morphologically diverse and has a rare-among-mammals functional signal of locomotion adaptation. We tested three alternative hypotheses of modularity; from the most supported we investigated disparity and integration of each module to infer which is most responsible for patterns of cranial variation across these levels, and whether variation is partitioned consistently across levels. We found a common pattern of modularity underlies leporid cranial diversity, though there is inconsistency across levels in each module’s disparity and integration. The face module contributes the most to disparity at all levels, which we propose is facilitating evolutionary diversity in this clade. Therefore, the distinctive facial tilt of leporids is an adaptation to locomotory behavior facilitated by a modular system that allows lineages to respond differently to selection pressures.

Highly cursorial animals are specialised for fast, sustained running via specific morphological adaptations, notably including changes in limb segment length and mechanical advantage. Members of the order Lagomorpha (hares, rabbits and pikas) vary in cursorial ability; hares are generally highly cursorial, rabbits more frequently saltate, and pikas predominantly trot. Previous investigations of lagomorphs have identified anatomical trends correlated with this ‘cursoriality gradient’, however, the phylogenetic sampling of such investigations has been limited to three American species, namely the American pika (Ochotona princeps), brush rabbit (Sylvilagus bachmani), and black-tailed jackrabbit (Lepus californicus). Here, we expand the phylogenetic sample and body size range by including novel data from Australian samples of the European rabbit (Oryctolagus cuniculus) and European hare (L. europaeus), alongside unpublished data on the Eastern cottontail (S. floridanus). X-ray Computed Tomography and digital landmarking were used to capture proportions within the appendicular skeleton of ~ 40 specimens of each European species. In doubling the number of species studied, we find the previously-identified morphological gradients associated with cursorial behaviour are complicated when evaluated in the larger sample. The relative length and joint velocity of limbs was found to be lower than predicted in European rabbits and hares. Furthermore, we present a novel assessment of morphological integration in the lagomorph appendicular skeleton, finding between-limb covariation patterns that are generally similar to those of other mammals. Broadly, these results suggest cursoriality is only one of many selective forces driving lagomorph skeletal evolution, with variations in body size and fossoriality potentially having measurable impacts.

Studies on the evolution of brain size variation usually focus on large clades encompassing broad phylogenetic groups. This risks introducing ‘noise’ in the results, often obscuring effects that might be detected in less inclusive clades. Here, we focus on a sample of endocranial volumes (endocasts) of 18 species of rabbits and hares (Lagomorpha: Leporidae), which are a discrete radiation of mammals with a suitably large range of body sizes. Using 60 individuals, we test five popular hypotheses on brain size and olfactory bulb evolution in mammals. We also address the pervasive issue of missing data, using multiple phylogenetic imputations as to conserve the full sample size for all analyses. Our analyses show that home range and burrowing behaviour are the only predictors of leporid brain size variation. Litter size, which is one of the most widely reported constraints on brain size, was unexpectedly not associated with brain size. However, a constraining effect may be masked by a strong association of litter size with temperature seasonality, warranting further study. Lastly, we show that unreasonable estimations of phylogenetic signal (Pagel’s lamba) warrant additional caution when using small sample sizes, such as ours, in comparative studies.

The story of the discovery of late Miocene fossils within the Baynunah geological formation in Abu Dhabi’s western Al Dhafra region concerns a host of international paleontologists, many of whom have contributed to this volume, as well as the backing of their related scientific institutions. In particular, the first study of the Baynunah owes a lot to the late Peter Whybrow (1942-2004) and a co-editor of the present volume, the late Andrew Hill (1946-2015). The discovery of the majority of the important fossils described in this publication would have not happened without their personal contributions. From the early 1980s until today, the Abu Dhabi government has always been a supportive partner to this paleontological research and continues to document, research, and protect important fossil sites that are part of the late Miocene Baynunah Formation, as well as curate the important collection of fossil specimens resulting from this work. This present publication provides details of the latest comprehensive overview of the Baynunah fossils and updates the earlier volume, Fossil Vertebrates of Arabia (Whybrow and Hill, 1999). A large number of new fossil specimens were discovered during the newer field seasons of work carried out between 2003 and 2018. This has provided an opportunity to undertake major revisions and updates of some of the Baynunah fauna.

The Baynunah Formation of western Abu Dhabi Emirate includes the only record of late Miocene fossil mammals from the Arabian Peninsula. This study reports on new fossil rodents, recording three species previously unknown from the fauna. The first fossil squirrel from the Baynunah Formation is described here, although the limited material makes a detailed taxonomic assessment difficult. An additional indeterminate dendromurine species is also identified. Additionally, a new genus and species of an exceedingly large fossil gerbil, [gen et sp nov], is described. This species exhibits several primitive traits relative to the co-occurring Abudhabia baynunensis, yet is more derived than closely related (yet older) Ameuromys grandis. [gen et sp nov] most likely represents an intermediate species in the Ameuromys grandis — Abudhabia baynunensis group. The rodent fauna also supports previous biochronological estimates for the Baynunah Formation (8 to 6 Ma). The absence of leporids among the Baynunah micromammals suggests the fauna is older than 7 Ma. Lastly, although the Baynunah fauna shows a strong African biogeographic signal overall, the rodents represent a portion of the fauna with significant relationships with Asia.

In addition to skeletal remains that record the presence of a diverse vertebrate fauna, the Baynunah Formation also preserves fossil trackways. These are found on deflated surfaces of carbonate-rich beds, mainly at sites located inland from the coast. Footprints, like other trace fossils, may be difficult to assign to particular species, but provide a window onto ancient behavior that is not attainable from skeletal remains alone. Nine sites bearing fossil footprints have been identified to date in the Baynunah Formation. These are presented and described here, most for the first time. The large majority of footprints (and the most easily identifiable) were made by proboscideans, but three trackways belong to a large ungulate, probably a giraffid, and one print may be that of a hippopotamid. The site of Mleisa 1 is particularly important for its remarkable preservation of the passage of a proboscidean herd intersected by the trackway of a large solitary individual, showing that herding behavior, and possibly also sexual segregation, both hallmarks of modern elephants, were already present in late Miocene proboscideans. Given the large areas across which the carbonates of the Baynunah Formation are exposed inland, many more trackways likely remain to be discovered.

Fossil localities of the Baynunah Formation are here described, including previously known localities and those discovered by fieldwork since 2002. Site descriptions and coordinates are given, including updated coordinates for previously known localities. Many localities have been damaged or entirely lost to development activities.

GACR [GA205/09/0184]; Spanish Ministerio de Economia y Competitividad [CGL2011-28681]; NSF [EAR 0958178]

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