Unique Turbinal Morphology in Horseshoe Bats (Chiroptera: Rhinolophidae)

Xenorhinos bhatnagari sp. nov., a new, nasal-emitting trident bat (Rhinonycteridae, Rhinolophoidea) from early Miocene forests in northern Australia

A new Old World trident bat (Rhinonycteridae) is described from an early Miocene cave deposit in the Riversleigh World Heritage Area, northwestern Queensland, Australia. Living rhinonycterids comprise a small family of insect-eating, nasal-emitting rhinolophoid bats from Africa, Madagascar, Seychelles, the Middle East, and northern Australia. The new fossil species is one of at least 12 rhinonycterid species known from the Oligo-Miocene cave deposits at Riversleigh. We refer the new species to the genus Xenorhinos (Hand, Journal of Vertebrate Paleontology, 18, 430-439, 1998a) because it shares a number of unusual cranial features with the type and only other species of the genus, X. halli, including a broad rostrum, very wide interorbital region, pronounced ventral flexion of the rostrum, very constricted sphenoidal bridge, and, within the nasal fossa, reduced bony division, and relatively well developed turbinals. Xenorhinos species lived in northern Australia during the global Miocene Climatic Optimum, in closed wet forests, unlike the drier habitats that trident bats largely inhabit today. Our phylogenetic analysis suggests that more than one dispersal event gave rise to the Australian rhinonycterid radiation, with two lineages having sister-group relationships with non-Australian taxa.

Ecomorphology and sensory biology of bats

This special issue of The Anatomical Record is inspired by and dedicated to Professor Kunwar P. Bhatnagar, whose lifelong interests in biology, and long career studying bats, inspired many and advanced our knowledge of the world's only flying mammals. The 15 articles included here represent a broad range of investigators, treading topics familiar to Prof. Bhatnagar, who was interested in seemingly every aspect of bat biology. Key topics include broad themes of bat development, sensory systems, and specializations related to flight and diet. These articles paint a complex picture of the fascinating adaptations of bats, such as rapid fore limb development, ear morphologies relating to echolocation, and other enhanced senses that allow bats to exploit niches in virtually every part of the world. In this introduction, we integrate and contextualize these articles within the broader story of bat ecomorphology, providing an overview of each of the key themes noted above. This special issue will serve as a springboard for future studies both in bat biology and in the broader world of mammalian comparative anatomy and ecomorphology.

An updated synthesis of and outstanding questions in the olfactory and vomeronasal systems in bats: Genetics asks questions only anatomy can answer

The extensive diversity observed in bat nasal chemosensory systems has been well-documented at the histological level. Understanding how this diversity evolved and developing hypotheses as to why particular patterns exist require a phylogenetic perspective, which was first outlined in the work of anatomist Kunwar Bhatnagar. With the onset of genetics and genomics, it might be assumed that the puzzling patterns observed in the morphological data have been clarified. However, there is still a widespread mismatch of genetic and morphological correlations among bat chemosensory systems. Novel genomic evidence has set up new avenues to explore that demand more evidence from anatomical structures. Here, we outline the progress that has been made in both morphological and molecular studies on the olfactory and vomeronasal systems in bats since the work of Bhatnagar. Genomic data of olfactory and vomeronasal receptors demonstrate the strong need for further morphological sampling, with a particular focus on receiving brain regions, glands, and ducts.

Rhinolophus mehelyi scythotauricus subsp. nov. (Rhinolophidae, Chiroptera) from the Lower Pleistocene of the Taurida Cave in Crimea

A new extinct subspecies of the Mehely's horseshoe bat, Rhinolophus mehelyi scythotauricus subsp. nov., is described on the base of an incomplete skull from the Lower Pleistocene deposits of the Taurida cave in the central Crimea. It is the largest member of the R. euryale group. In terms of the evolutionary level, it is intermediate between Plio-Pleistocene R. mehelyi birzebbugensis Storch, 1974 and recent members of the species, but its large size and relatively narrow upper molars may indicate belonging to a separate phylogenetic lineage within R. mehelyi Matschie, 1901. R. mehelyi scythotauricus subsp. nov. is the first fossil record of the species in the Crimea; it is also one of the northernmost finds of R. mehelyi.

Ecological constraints on highly evolvable olfactory receptor genes and morphology in neotropical bats

Although evolvability of genes and traits may promote specialization during species diversification, how ecology subsequently restricts such variation remains unclear. Chemosensation requires animals to decipher a complex chemical background to locate fitness-related resources, and thus the underlying genomic architecture and morphology must cope with constant exposure to a changing odorant landscape; detecting adaptation amidst extensive chemosensory diversity is an open challenge. In phyllostomid bats, an ecologically diverse clade that evolved plant visiting from a presumed insectivorous ancestor, the evolution of novel food detection mechanisms is suggested to be a key innovation, as plant-visiting species rely strongly on olfaction, supplementarily using echolocation. If this is true, exceptional variation in underlying olfactory genes and phenotypes may have preceded dietary diversification. We compared olfactory receptor (OR) genes sequenced from olfactory epithelium transcriptomes and olfactory epithelium surface area of bats with differing diets. Surprisingly, although OR evolution rates were quite variable and generally high, they are largely independent of diet. Olfactory epithelial surface area, however, is relatively larger in plant-visiting bats and there is an inverse relationship between OR evolution rates and surface area. Relatively larger surface areas suggest greater reliance on olfactory detection and stronger constraint on maintaining an already diverse OR repertoire. Instead of the typical case in which specialization and elaboration are coupled with rapid diversification of associated genes, here the relevant genes are already evolving so quickly that increased reliance on smell has led to stabilizing selection, presumably to maintain the ability to consistently discriminate among specific odorants-a potential ecological constraint on sensory evolution.

Early Pleistocene Horseshoe Bat Rhinolophus macrorhinus cimmerius subsp. nov. (Rhinolophidae, Chiroptera) from the Taurida Cave in Crimea

Numerous remains (incomplete skull, cranial and mandibular fragments, and isolated teeth) of a large horseshoe bat of the Rhinolophus ferrumequinum group are described from the Lower Pleistocene depo-sits of the Taurida cave in the central Crimea. They are assigned to Rhinolophus macrorhinus cimmerius subsp. nov. In dental characters the new subspecies is less specialized than R. m. anomalidens Topál, 1979 from the Late Villafranchian of Central Europe, which implies the origin of the former from an earlier form morphologically close to R. m. macrorhinus Topál, 1963. The perfect preservation of the cranial structures made it possible to observe the remnants of the palatal ridges and the morphology of the nasal turbinals of R. macro-rhinus cimmerius subsp. nov.

The nasopharynx as a window to half a billion years of evolutionary change to the upper respiratory tract

The nasopharynx is a region at the nexus of several vital physiological systems, including the nasal cavity, oral cavity, braincase, middle ear, and cervical vertebrae. It has undergone pronounced morphological change over the course of tetrapod, mammalian, and human evolution. However, despite its place in evolutionary history, the nasopharynx has received relatively little attention. This special issue focuses on "the evolution, development, and functional morphology of the nasopharynx and its boundaries." Topics covered here include evolutionary developmental biology (or evo-devo), nasopharyngeal adaptions in bats, the importance of the nasopharynx and adjacent structures over the course of human evolution, normal development, middle ear morphology, clinical importance, and the study of the nasopharynx throughout history. Contributions to this special issue range among reviews and syntheses, descriptive analyses, phylogenetic analysis, traditional morphometrics, three-dimensional geometric morphometrics, and computational fluid dynamics. Here, we discuss the central importance of the nasopharynx as can be seen through vertebrate paleontology and comparative morphology. It is via the composite evolutionary history of the nasopharyngeal boundaries that our origins may be better understood, starting with the derivation of the choanae from the median olfactory pit of jawless fish nearly half a billion years ago to the basicranial flexion and facial reduction that distinguish Homo sapiens from all other living mammals. Indeed, the nasopharynx must be acknowledged for its importance in the processes of encephalization and acquisition of speech that have become the hallmark of our species.

On the Embryonic Development of the Nasal Turbinals and Their Homology in Bats

Multiple corrugated cartilaginous structures are formed within the mammalian nasal capsule, eventually developing into turbinals. Due to its complex and derived morphology, the homologies of the bat nasal turbinals have been highly disputed and uncertain. Tracing prenatal development has been proven to provide a means to resolve homological problems. To elucidate bat turbinate homology, we conducted the most comprehensive study to date on prenatal development of the nasal capsule. Using diffusible iodine-based contrast-enhanced computed tomography (diceCT), we studied in detail the 3D prenatal development of various bat species and non-bat laurasiatherians. We found that the structure previously identified as “maxilloturbinal” is not the true maxilloturbinal and is only part of the ethmoturbinal I pars anterior. Our results also allowed us to trace the evolutionary history of the nasal turbinals in bats. The turbinate structures are overall comparable between laurasiatherians and pteropodids, suggesting that pteropodids retain the ancestral laurasiatherian condition. The absence of the ethmoturbinal I pars posterior in yangochiropterans and rhinolophoids has possibly occurred independently by convergent evolution.

A test for rate-coupling of trophic and cranial evolutionary dynamics in New World bats

Morphological evolution is often assumed to be causally related to underlying patterns of ecological trait evolution. However, few studies have directly tested whether evolutionary dynamics of-and major shifts in-ecological resource use are coupled with morphological shifts that may facilitate trophic innovation. Using diet and multivariate cranial (microCT) data, we tested whether rates of trophic and cranial evolution are coupled in the radiation of New World bats. We developed a generalizable information-theoretic method for describing evolutionary rate heterogeneity across large candidate sets of multirate evolutionary models, without relying on a single best-fitting model. We found considerable variation in trophic evolutionary dynamics, in sharp contrast to a largely homogeneous cranial evolutionary process. This dichotomy is surprising given established functional associations between overall skull morphology and trophic ecology. We suggest that assigning discrete trophic states may underestimate trophic generalism and opportunism, and that this radiation could be characterized by labile crania and a homogeneous dynamic of generally high morphological rates. Overall, we discuss how trophic classifications could substantively impact our interpretation of how these dynamics covary in adaptive radiations.

Three-Dimensional and Histological Observations on Male Genital Organs of Greater Horseshoe Bat, Rhinolophus ferrumequinum

Anatomy of bat genital organs has been conventionally studied by gross and microscopic observations to date. Here, we employ both histological observation and diceCT (diffusible iodine-based contrast-enhanced computed tomography) to study the detailed three-dimensional morphological structure of the male genital organs in bats, using the greater horseshoe bat, Rhinolophus ferrumequinum. This is the first study to three-dimensionally describe the whole reproductive organs of bats in detail. Our highly resolved three-dimensional reconstruction reveals that the male organs of R. ferrumequinum consist of paired testes, epididymides, deferent ducts, and five accessory genital glands. The boundary between the ampullary and vesicular glands has been difficult to identify in previous observations, but our diceCT imaging allowed us to clearly differentiate the two. We found that the ampullary gland is located at the terminal part of the deferent ducts, and the vesicular gland lies distal to the ampullary glands. This species possesses a single and carrot-shaped urethral gland, which is not found in most chiropteran families. The presence of the urethral gland in this species and its secretions suggest that after copulation this species is capable of forming a vaginal plug, which can seal the female’s vaginal orifice to block the entrance of spermatozoa from other males. The presence of the urethral gland and elongated epididymal tail and the fact that some individuals can terminate their hibernation and reactivate imply forced copulation on hibernating females can occur in R. ferrumequinum.

Convergent evolution of olfactory and thermoregulatory capacities in small amphibious mammals

Olfaction and thermoregulation are key functions for mammals. The former is critical to feeding, mating, and predator avoidance behaviors, while the latter is essential for homeothermy. Aquatic and amphibious mammals face olfactory and thermoregulatory challenges not generally encountered by terrestrial species. In mammals, the nasal cavity houses a bony system supporting soft tissues and sensory organs implicated in either olfactory or thermoregulatory functions. It is hypothesized that to cope with aquatic environments, amphibious mammals have expanded their thermoregulatory capacity at the expense of their olfactory system. We investigated the evolutionary history of this potential trade-off using a comparative dataset of three-dimensional (3D) CT scans of 189 skulls, capturing 17 independent transitions from a strictly terrestrial to an amphibious lifestyle across small mammals (Afrosoricida, Eulipotyphla, and Rodentia). We identified rapid and repeated loss of olfactory capacities synchronously associated with gains in thermoregulatory capacity in amphibious taxa sampled from across mammalian phylogenetic diversity. Evolutionary models further reveal that these convergences result from faster rates of turbinal bone evolution and release of selective constraints on the thermoregulatory-olfaction trade-off in amphibious species. Lastly, we demonstrated that traits related to vital functions evolved faster to the optimum compared to traits that are not related to vital functions.

3D Digitization in Functional Morphology: Where is the Point of Diminishing Returns?

Modern computational and imaging methods are revolutionizing the fields of comparative morphology, biomechanics, and ecomorphology. In particular, imaging tools such as X-ray micro computed tomography (µCT) and diffusible iodine-based contrast enhanced CT allow observing and measuring small and/or otherwise inaccessible anatomical structures, and creating highly accurate three-dimensional (3D) renditions that can be used in biomechanical modeling and tests of functional or evolutionary hypotheses. But, do the larger datasets generated through 3D digitization always confer greater power to uncover functional or evolutionary patterns, when compared with more traditional methodologies? And, if so, why? Here, we contrast the advantages and challenges of using data generated via (3D) CT methods versus more traditional (2D) approaches in the study of skull macroevolution and feeding functional morphology in bats. First, we test for the effect of dimensionality and landmark number on inferences of adaptive shifts during cranial evolution by contrasting results from 3D versus 2D geometric morphometric datasets of bat crania. We find sharp differences between results generated from the 3D versus some of the 2D datasets (xy, yz, ventral, and frontal), which appear to be primarily driven by the loss of critical dimensions of morphological variation rather than number of landmarks. Second, we examine differences in accuracy and precision among 2D and 3D predictive models of bite force by comparing three skull lever models that differ in the sources of skull and muscle anatomical data. We find that a 3D model that relies on skull µCT scans and muscle data partly derived from diceCT is slightly more accurate than models based on skull photographs or skull µCT and muscle data fully derived from dissections. However, the benefit of using the diceCT-informed model is modest given the effort it currently takes to virtually dissect muscles from CT scans. By contrasting traditional and modern tools, we illustrate when and why 3D datasets may be preferable over 2D data, and vice versa, and how different methodologies can complement each other in comparative analyses of morphological function and evolution.

Maxilloturbinal Aids in Nasophonation in Horseshoe Bats (Chiroptera: Rhinolophidae)

Horseshoe bats (Family Rhinolophidae) show an impressive array of morphological traits associated with use of high duty cycle echolocation calls that they emit via their nostrils (nasophonation). Delicate maxilloturbinal bones inside the nasal fossa of horseshoe bats have a unique elongated strand-like shape unknown in other mammals. Maxilloturbinal strands also vary considerably in length and cross-sectional shape. In other mammals, maxilloturbinals help direct respired air and prevent respiratory heat and water loss. We investigated whether strand-shaped maxilloturbinals in horseshoe bats perform a similar function to those of other mammals, or whether they were shaped for a role in nasophonation. Using histology, we studied the mucosa of the nasal fossa in Rhinolophus lepidus, which we compared with Hipposideros lankadiva (Hipposideridae) and Megaderma lyra (Megadermatidae). Using micro-CT scans of 30 horseshoe bat species, we quantified maxilloturbinal surface area and skull shape within a phylogenetic context. Histological results showed horseshoe bat maxilloturbinals are covered in a thin, poorly vascularized, sparsely ciliated mucosa poorly suited for preventing respiratory heat and water loss. Maxilloturbinal surface area was correlated with basicranial width, but exceptionally long and dorsoventrally flat maxilloturbinals did not show enhanced surface area for heat and moisture exchange. Skull shape variation appears to be driven by structures linked to nasophonation, including maxilloturbinals. Resting echolocation call frequency better predicted skull shape than did skull size, and was specifically correlated with dimensions of the rostral inflations, palate, and maxilloturbinals. These traits appear to form a morphological complex, indicating a nasophonatory role for the strand-shaped rhinolophid maxilloturbinals. Anat Rec, 2018. © 2018 American Association for Anatomy.

Digitizing extant bat diversity: An open-access repository of 3D μCT-scanned skulls for research and education

Biological specimens are primary records of organismal ecology and history. As such, museum collections are invaluable repositories for testing ecological and evolutionary hypotheses across the tree of life. Digitizing and broadly sharing the phenotypic data from these collections serves to expand the traditional reach of museums, enabling widespread data sharing, collaboration, and education at an unprecedented scale. In recent years, μCT-scanning has been adopted as one way for efficiently digitizing museum specimens. Here, we describe a large repository of 3D, μCT-scanned images and surfaces of skulls from 359 extant species of bats, a highly diverse clade of modern vertebrates. This digital repository spans much of the taxonomic, biogeographic, and morphological diversity present across bats. All data have been published to the MorphoSource platform, an online database explicitly designed for the archiving of 3D morphological data. We demonstrate one potential use of this repository by testing for convergence in skull shape among one particularly diverse group of bats, the superfamily Noctilionoidea. Beyond its intrinsic utility to bat biologists, our digital specimens represent a resource for educators and for any researchers seeking to broadly test theories of trait evolution, functional ecology, and community assembly.