The Chiropteran Premaxilla: A Reanalysis of Morphological Variation and Its Phylogenetic Interpretation

Anatomical and Morphological Structure of the Skull of a Juvenile Specimen of Myotis myotis (Chiroptera: Vespertilionidae)

Few studies analyze the morphology and anatomy of the bat skull, and most of them are incomplete. Some of the difficulties stem from the fact that, in the representatives of the order Chiroptera, the interosseous sutures disappear by fusing together before active flight begins, which takes place over only a few months. This study presents a detailed morphological and anatomical description of the skull of a juvenile specimen of Myotis myotis (Borkhausen, 1797). Juvenile skulls are difficult to preserve and often incomplete. Previously inconsistent terminology related to bones, sutures, and other cranial structures was unified, which will provide insight on the distribution of each structure in both juvenile and adult specimens to be investigated. The description fill in the gaps in knowledge about the cranial structures of Myotis myotis and the representatives of the family Vespertilionidae. This will allow for precise descriptions of the skulls of bats.

A 50-million-year-old, three-dimensionally preserved bat skull supports an early origin for modern echolocation

Bats are among the most recognizable, numerous, and widespread of all mammals. But much of their fossil record is missing, and bat origins remain poorly understood, as do the relationships of early to modern bats. Here, we describe a new early Eocene bat that helps bridge the gap between archaic stem bats and the hyperdiverse modern bat radiation of more than 1,460 living species. Recovered from ∼50 million-year-old cave sediments in the Quercy Phosphorites of southwestern France, Vielasia sigei's remains include a near-complete, three-dimensionally preserved skull-the oldest uncrushed bat cranium yet found. Phylogenetic analyses of a 2,665 craniodental character matrix, with and without 36.8 kb of DNA sequence data, place Vielasia outside modern bats, with total evidence tip-dating placing it sister to the crown clade. Vielasia retains the archaic dentition and skeletal features typical of early Eocene bats, but its inner ear shows specializations found in modern echolocating bats. These features, which include a petrosal only loosely attached to the basicranium, an expanded cochlea representing ∼25% basicranial width, and a long basilar membrane, collectively suggest that the kind of laryngeal echolocation used by most modern bats predates the crown radiation. At least 23 individuals of V. sigei are preserved together in a limestone cave deposit, indicating that cave roosting behavior had evolved in bats by the end of the early Eocene; this period saw the beginning of significant global climate cooling that may have been an evolutionary driver for bats to first congregate in caves.

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.

The history and homology of the os paradoxum or dumb-bell-shaped bone of the platypus Ornithorhynchus anatinus (Mammalia, Monotremata)

Abstract

The os paradoxum or dumb-bell-shaped bone is a paired bone occurring in the middle of the specialized bill of the platypus Ornithorhynchus anatinus. It has been variously considered as a neomorph of the platypus, as the homologue of the paired vomer of sauropsids, or as a part of the paired premaxillae. A review of the near 200-year history of this element strongly supports the os paradoxum as a remnant of the medial palatine processes of the premaxillae given its ontogenetic continuity with the premaxillae and association with the vomeronasal organ and cartilage, incisive foramen, and cartilaginous nasal septum. In conjunction with this hypothesis, homologies of the unpaired vomer of extant mammals and the paired vomer of extant sauropsids are also supported. These views are reinforced with observations from CT scans of O. anatinus, the Miocene ornithorhynchid Obdurodon dicksoni, and the extant didelphid marsupial Didelphis marsupialis. At the choanae, Obdurodon has what appears to be a separate parasphenoid bone unknown in extant monotremes.

The incisive foramen as character in distinguishing morphologically similar species of mammals

An analysis of the morphology and variability of the size and shape of a key morphological structure in the rostral part of the skull—the incisive foramen—has been carried out. It is shown that incisive foramina are variable morphological structures, the features of which are group-specific (at the level of genera and families), and in some cases also species-specific. At both these levels, the shape and size of the incisive foramen have features that can serve as criteria for species identification by osteological patterns. Their location is important for diagnosis because these structures are preserved in most specimens that have suffered various kinds of damage (e.g. in fodder residues of carnivorous mammals or in owl pellets), and their placement in the anterior part of the bony palate as well as them being protected from the sides with rows of teeth makes these structures invulnerable to trauma-related variation. It is shown that there is a specific structure (size, location, and shape) of incisive foramina at the level of taxonomic groups of all ranks, from orders to species. The analysis was performed mainly on the examples of different groups of rodents as an order, represented by the largest number of pairs of close species. Examples with several different groups, in particular with different pairs of species of voles, mice, mole rats, ground squirrels, and others are considered. Examples with differences in close pairs of species in other groups (white-toothed shrews, polecats, roe deer, etc.) are also known. In all pairs of related species, a pattern was found, according to which species that are restricted to steppe ecosystems have the smallest incisive foramina, while species from wetland habitats have large ones. In many cases, groups of genera and families well differ in the shape and location of incisive foramina, and close pairs of species differ well in the size of these structures (primarily in length), although it is important to always consider the ontogenetic age of specimens: in young individuals, the incisive foramina are naturally small, similar to incisive foramina in other species, which are characterized by small incisive foramina in general. Based on the known data on the role of incisive foramina and the Jacobson organ in the life of mammals, hypotheses have been considered that may explain the differences in species and genera by the structure (size, location, and shape) of incisive foramina.

Mind the gap: natural cleft palates reduce biting performance in bats

Novel morphological traits pose interesting evolutionary paradoxes when they become widespread in a lineage while being deleterious in others. Cleft palate is a rare congenital condition in mammals in which the incisor-bearing premaxilla bones of the upper jaw develop abnormally. However, ∼50% of bat species have natural, non-pathological cleft palates. We used the family Vespertilionidae as a model and linear and geometric morphometrics within a phylogenetic framework to (1) explore evolutionary patterns in cleft morphology, and (2) test whether cleft morphological variation is correlated with skull shape in bats. We also used finite element (FE) analyses to experimentally test how presence of a cleft palate impacts skull performance during biting in a species with extreme cleft morphology (hoary bat, Lasiurus cinereus). We constructed and compared the performance of two FE models: one based on the hoary bat's natural skull morphology, and another with a digitally filled cleft simulating a complete premaxilla. Results showed that cleft length and width are correlated with skull shape in Vespertilionidae, with narrower, shallower clefts seen in more gracile skulls and broader, deeper clefts in more robust skulls. FE analysis showed that the model with a natural cleft produced lower bite forces, and had higher stress and strain than the model with a filled cleft. In the rostrum, safety factors were 1.59-2.20 times higher in the model with a filled cleft than in the natural model. Our results demonstrate that cleft palates in bats reduce biting performance, and evolution of skull robusticity may compensate for this reduction in performance.

Creating diversity in mammalian facial morphology: a review of potential developmental mechanisms

Mammals (class Mammalia) have evolved diverse craniofacial morphology to adapt to a wide range of ecological niches. However, the genetic and developmental mechanisms underlying the diversification of mammalian craniofacial morphology remain largely unknown. In this paper, we focus on the facial length and orofacial clefts of mammals and deduce potential mechanisms that produced diversity in mammalian facial morphology. Small-scale changes in facial morphology from the common ancestor, such as slight changes in facial length and the evolution of the midline cleft in some lineages of bats, could be attributed to heterochrony in facial bone ossification. In contrast, large-scale changes of facial morphology from the common ancestor, such as a truncated, widened face as well as the evolution of the bilateral cleft possessed by some bat species, could be brought about by changes in growth and patterning of the facial primordium (the facial processes) at the early stages of embryogenesis.

Patterns of orofacial clefting in the facial morphology of bats: a possible naturally occurring model of cleft palate

A normal feature of the facial anatomy of many species of bat is the presence of bony discontinuities or clefts, which bear a remarkable similarity to orofacial clefts that occur in humans as a congenital pathology. These clefts occur in two forms: a midline cleft between the two premaxillae (analogous to the rare midline craniofacial clefts in humans) and bilateral paramedian clefts between the premaxilla and the maxillae (analogous to the typical cleft lip and palate in humans). Here, we describe the distribution of orofacial clefting across major bat clades, exploring the relationship of the different patterns of clefting to feeding mode, development of the vomeronasal organ, development of the nasolacrimal duct and mode of emission of the echolocation call in different bat groups. We also present the results of detailed radiographic and soft tissue dissections of representative examples of the two types of cleft. The midline cleft has arisen independently multiple times in bat phylogeny, whereas the paramedian cleft has arisen once and is a synapomorphy uniting the Rhinolophidae and Hipposideridae. In all cases examined, the bony cleft is filled in by a robust fibrous membrane, continuous with the periosteum of the margins of the cleft. In the paramedian clefts, this membrane splits to enclose the premaxilla but forms a loose fold laterally between the premaxilla and maxilla, allowing the premaxilla and nose-leaf to pivot dorsoventrally in the sagittal plane under the action of facial muscles attached to the nasal cartilages. It is possible that this is a specific adaptation for echolocation and/or aerial insectivory. Given the shared embryological location of orofacial clefts in bats and humans, it is likely that aspects of the developmental control networks that produce cleft lip and palate in humans may also be implicated in the formation of these clefts as a normal feature in some bats. A better understanding of craniofacial development in bats with and without clefts may therefore suggest avenues for research into abnormal craniofacial development in humans.

Phylogeny of Molossidae Gervais (Mammalia: Chiroptera) inferred by morphological data

Molossidae is a large (roughly 100 species) pantropically distributed clade of swift aerially insectivorous bats for which the phylogeny remains relatively unknown and little studied compared with other speciose groups of bats. We investigated phylogenetic relationships among 62 species, representing all extant molossid genera and most of the subgenera, using 102 morphological characters from the skull, dentition, postcrania, external morphology, tongue, and penis, based on direct observation and literature reports. Both parsimony and Bayesian analyses were used in phylogenetic reconstruction. Our analysis supports two main clades of molossids, both of which mingle Old World and New World taxa. One clade is comprised of Mormopterus,Platymops, Sauromys, Neoplatymops, Molossops, Cynomops, Cheiromeles, Molossus, and Promops. The other clade includes Tadarida, Otomops, Nyctinomops, Eumops, Chaerephon, and Mops. The position of Myopterus with respect to these two groups is unclear. As in other recent analyses, we find that several genera do not appear to be monophyletic (e.g. Tadarida, Chaerephon, and Molossops sensu lato). We recommend that the subgenera of Molossops sensu lato and Austronomus be recognized at the generic level. We conclude that much more data are needed to investigate lower level problems (generic monophyly and relationships within genera) and to resolve the higher-level branching pattern of the family.

The placental mammal ancestor and the post-K-Pg radiation of placentals

To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria.