The entotympanic in late fetal Artiodactyla (Mammalia)

Evolution, conservatism and overlooked homologies of the mammalian skull

In the last decade, studies integrating palaeontology, embryology and experimental developmental biology have markedly altered our homological understanding of the mammalian skull. Indeed, new evidence suggests that we should revisit and restructure the conventional anatomical terminology applied to the components of the mammalian skull. Notably, these are classical problems that have remained unresolved since the ninteenth century. In this review, I offer perspectives on the overlooked problems associated with the homology, development, and conservatism of the mammalian skull, aiming to encourage future studies in these areas. I emphasise that ossification patterns, bone fusion, cranial sutures and taxon-specific neomorphic bones in the skull are virtually unexplored, and further studies would improve our homological understanding of the mammalian skull. Lastly, I highlight that overlooked bones may exist in the skull that are not yet known to science and suggest that further search is needed. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Vergleichende Entwicklungsgeschichte — A Festschrift on the occasion of the 80th birthday of Prof. Dr. Wolfgang Maier, Tübingen

Following the traditional and holistic concept of Vergleichende Entwicklungsgeschichte, Wolfgang Maier studied different aspects of vertebrate morphology, including dentition, the locomotor apparatus, nasal and ear regions. His work comprises investigations on pre- to postnatal stages of extant species as well as fossils and is mainly based on histological serial sections, but also on µCT data in recent years. This resulted in an integrative research agenda on the evolutionary biology of mammals and other vertebrates. Most of his studies are designed around the interrelationship of ontogenetic and functional adaptations and evolutionary transformations. The present collection in Vertebrate Zoology collates a series of research articles related to and in honor of Wolfgang Maier’s work. Invited colleagues of Maier provide current insights to their own research, in many cases inspired by his scholarship, ranging from mammalian to reptilian and fish comparative anatomy. In total, this volume contains 19 publications. They apply modern and traditional techniques to investigate the diversity of biological form. In so doing, they integrate traditional conceptual frameworks from the earliest days of morphological research more than two centuries ago.

Morphology of the Bony Labyrinth Supports the Affinities of Paradolichopithecus with the Papionina

Discoveries in recent decades indicate that the large papionin monkeys Paradolipopithecus and Procynocephalus are key members of the Late Pliocene - Early Pleistocene mammalian faunas of Eurasia. However, their taxonomical status, phylogenetic relationships, and ecological profile remain unclear. Here we investigate the two latter aspects through the study of the inner ear anatomy, as revealed by applying micro-CT scan imaging techniques on the cranium LGPUT DFN3-150 of Paradolichopithecus from the lower Pleistocene (2.3 Ma) fossil site Dafnero-3 in Northwestern Greece. Using geometric morphometric methods, we quantified shape variation and the allometric and phylogenetic signals in extant cercopithecines (n = 80), and explored the morphological affinities of the fossil specimen with extant taxa. LGPUT DFN3-150 has a large centroid size similar to that of baboons and their relatives. It shares several shape features with Macacina and Cercopithecini, which we interpret as probable retention of a primitive morphology. Overall, its inner ear morphology is more consistent with a stem Papionini more closely related to Papionina than Macacina, or to a basal crown Papionina. Our results, along with morphometrical and ecological features from previous studies, call into question the traditional hypothesis of a Paradolichopithecus-Macacina clade, and provide alternative perspectives in the study of Eurasian primate evolution during the late Neogene-Quaternary.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10764-022-00329-4.

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 vertebrate middle and inner ear: A short overview

The evolution of the various hearing adaptations is connected to major structural changes in nearly all groups of vertebrates. Besides hearing, the detection of acceleration and orientation in space are key functions of this mechanosensory system. The symposium "show me your ear - the inner and middle ear in vertebrates" held at the 11th International Congress of Vertebrate Morphology (ICVM) 2016 in Washington, DC (USA) intended to present current research addressing adaptation and evolution of the vertebrate otic region, auditory ossicles, vestibular system, and hearing physiology. The symposium aimed at an audience with interest in hearing research focusing on morphological, functional, and comparative studies. The presented talks and posters lead to the contributions of this virtual issue highlighting recent advances in the vertebrate balance and hearing system. This article serves as an introduction to the virtual issue contributions and intends to give a short overview of research papers focusing on vertebrate labyrinth and middle ear related structures in past and recent years.

On the development of the chondrocranium and the histological anatomy of the head in perinatal stages of marsupial mammals

An overview of the literature on the chondrocranium of marsupial mammals reveals a relative conservatism in shape and structures. We document the histological cranial anatomy of individuals representing Monodelphis domestica, Dromiciops gliroides, Perameles sp. and Macropus eugenii. The marsupial chondrocranium is generally characterized by the great breadth of the lamina basalis, absence of pila metoptica and large otic capsules. Its most anterior portion (cupula nasi anterior) is robust, and anterior to it there are well-developed tactile sensory structures, functionally important in the neonate. Investigations of ossification centers at and around the nasal septum are needed to trace the presence of certain bones (e.g., mesethmoid, parasphenoid) across marsupial taxa. In many adult marsupials, the tympanic floor is formed by at least three bones: alisphenoid (alisphenoid tympanic process), ectotympanic and petrosal (rostral and caudal tympanic processes); the squamosal also contributes in some diprotodontians. The presence of an entotympanic in marsupials has not been convincingly demonstrated. The tubal element surrounding the auditory tube in most marsupials is fibrous connective tissue rather than cartilage; the latter is the case in most placentals recorded to date. However, we detected fibrocartilage in a late juvenile of Dromiciops, and a similar tissue has been reported for Tarsipes. Contradictory reports on the presence of the tegmen tympani can be found in the literature. We describe a small tegmen tympani in Macropus. Several heterochronic shifts in the timing of development of the chondocranium and associated structures (e.g., nerves, muscles) and in the ossification sequence have been interpreted as largely being influenced by functional requirements related to the altriciality of the newborn marsupial during early postnatal life. Comparative studies of chondocranial development of mammals can benefit from a solid phylogenetic framework, research on non-classical model organisms, and integration with imaging and sectional data derived from computer-tomography.

Prenatal growth stages show the development of the ruminant bony labyrinth and petrosal bone

Foetuses are a source of scientific information to understand the development and evolution of anatomical structures. The bony labyrinth, surrounding the organ of balance and hearing, is a phylogenetically and ecologically informative structure for which still little concerning growth and shape variability is known in many groups of vertebrates. Except in humans, it is poorly known in many other placentals and its prenatal growth has almost never been studied. Ruminants are a diversified group of placentals and represent an interesting case study to understand the prenatal growth of the ear region. We computed tomography -scanned five cow foetuses and an adult petrosal bone (Bos taurus, Artiodactyla, Mammalia), and describe the bony labyrinth when already ossified. The foetuses encompass the second half of the 9.3-month-long gestation period of the cow. They were sampled at different ontogenetic stages to understand how and when the petrosal bone and bony labyrinth ossify in ruminants. The petrosal bone and bony labyrinth ossify within about 20 days in the fourth month of gestation. The bony labyrinth is already fully ossified at least in the 6th month, while only the cochlea, most of the vestibule and the common crus are already ossified at the beginning of the 4th month. The pars canalicularis of the petrosal thus ossifies at last. The size and volume of the bony labyrinth stay similar from the 6th month (possibly even from the 5th). From the end of the 4th month of gestation, a progressive lengthening of the cochlear aqueduct and endolymphatic sac occurs, culminating in the adult form and partly explaining the larger volume of the later. The inner ear in the cow ossifies quickly during the gestation period, being fully ossified around mid-gestation time, as in humans. The adult size and most of its volume are reached by mid-gestation time while the petrosal bone and skull still grow. A negative ontogenetic allometry between the bony labyrinth and the petrosal bone and skull is thus observed. It matches the evolutionary negative allometry of the structure observed in earlier studies. Few changes occur after ossification is achieved; only open structures (i.e. cochlear aqueduct and endolymphatic sac) continue to grow after birth and reflect size increase of the petrosal bone.

Shape variation and ontogeny of the ruminant bony labyrinth, an example in Tragulidae

Despite its growing use in anatomical and ecological studies, the morphological variability and ontogenetic development of the bony labyrinth have very rarely been investigated in ruminants. Here we study its morphology in 15 adult and 10 juvenile specimens in the three extant tragulid ruminant genera. Intraspecific and interspecific variability is quantified using morphometric and 3D geometric morphometrics analyses. The bony labyrinth of Tragulus, Hyemoschus, and Moschiola is strikingly different, clustering in clearly different morphospaces despite similar ecological adaptations. Although the bony labyrinths within two species of the same genus cannot be distinguished from each other based on the chosen semi-landmarks, discrete interspecific differences exist. We were able to show for the first time that an artiodactyl mammal in a late fetal stage possesses an almost fully formed bony labyrinth similar to that of adults. No significant change either occurs in size or morphology after ossification of the petrosal bone. Some intraspecific variation is observed on the shape of the lateral semi-circular canal, the size and shape of the common crus, the coil of the cochlea or the stapedial ratio. Variable structures are expected to be highly informative characters for a large cladistic analysis. They can be used for phylogenetic studies in ruminants. Incorporating juvenile specimens in studies is not problematic, as they fall within the morphological range of adults.

The anterior process of the malleus in Cetartiodactyla

The anterior fixation of the anterior process of the malleus has been studied in a number of Cetartiodactyla. This anterior process, also known as processus gracilis, is provided by the prearticular (gonial), a dermal bone, whereas the cartilage of Meckel becomes resorbed in perinatal ontogenetic stages. Posteriorly, the prearticular fuses with the cartilaginous caput of the malleus; rostrally, the prearticular (= processus gracilis) is always fixed to the anterior crus of the ectotympanic by an extremely thin splint (thickness < 50 μm). From the rostral part of the processus gracilis all studied cetartiodactyls develop a processus internus of considerable size. This process was known as 'ossiculum accessorium mallei' in former times, and its homology has been disputed; from our microscopic-anatomical study we can definitely state that it is always a 'processus internus praearticularis'. This process contacts the fascia of the tensor tympani muscle. In non-ruminants, it articulates dorsally with the tegmen tympani and dorsolaterally with the ectotympanic; whereas the processus gracilis becomes more and more slender, the processus internus may grow to considerable size and normally is fused to the adjacent bones already in juveniles. However, in ruminants, the tegmen tympani tends to disappear and the processus internus praearticularis appears as relatively small bony knob at the floor of the medial cranial cavity, and it seems not to be fused to the surrounding bones; in later age stages, it may become secondarily overgrown by the petrosal. This dorsally exposed processus internus praearticularis seems to be a synapomorphy of the Ruminantia. The functional meaning of this internal process of the prearticular, which is also developed to a minor degree in Carnivora, remains unclear at the moment - but we present some speculations about this.

Evolution of the mammalian middle ear: a historical review

Here we present a brief, historical review of research into the mammalian middle ear structures. Most of their essential homologies were established by embryologists, notably including Reichert, during the 19th century. The evolutionary dimension was confirmed by finds of fossil synapsids, mainly from the Karroo of South Africa. In 1913, Ernst Gaupp was the first to present a synthesis of the available embryological and paleontological data, but a number of morphological details remained to be solved, such as the origin of the tympanic membrane. Gaupp favoured an independent origin of the eardrum in anurans, sauropsids, and mammals; we support most of his ideas. The present review emphasizes the problem of how the mammalian middle ear structures that developed at the angle of the lower jaw were transferred to the basicranium; the ontogenesis of extant marsupials provides important information on this question.