Getting out of a mammalian egg: the egg tooth and caruncle of the echidna

In the short-beaked echidna, Tachyglossus aculeatus, after an initial period of in utero development, the egg is laid in the pouch and incubated for 10 days. During this time, fetuses develop an egg tooth and caruncle to help them hatch. However, there are only a few historical references that describe the development of the monotreme egg tooth. Using unprecedented access to echidna pre- and post-hatching tissues, the egg tooth and caruncle were assessed by micro-CT, histology and immunofluorescence, to map the changes at the morphological and molecular level. Unlike mammalian tooth germs that develop by invagination of a placode, the echidna egg tooth developed by evagination, similar to that of the first teeth in some reptiles. The egg tooth ankylosed to the premaxilla, rather than forming a mammalian thecodont attachment, with loss of the egg tooth post-hatching associated with high levels of odontoclasts, and apoptosis. The caruncle formed as a separate mineralisation from the adjacent nasal capsule, and as observed in birds and turtles, the nasal region epithelium expressed markers of cornification. Together, this highlights that the monotreme egg tooth shares many similarities with reptilian teeth, suggesting that this tooth is conserved from a common ancestor of mammals and reptiles.

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.

Osteology of the archosauromorph Teyujagua paradoxa and the early evolution of the archosauriform skull

Archosauriformes are a major group of fossil and living reptiles that include the crown group Archosauria (birds, crocodilians and their extinct relatives) and closely related taxa. Archosauriformes are characterized by a highly diagnostic skull architecture, which is linked to the predatory habits of their early representatives, and the development of extensive cranial pneumaticity associated with the nasal capsule. The evolution of the archosauriform skull from the more plesiomorphic configuration present ancestrally in the broader clade Archosauromorpha was, until recently, elusive. This began to change with the discovery and description of Teyujagua paradoxa, an early archosauromorph from the Lower Triassic Sanga do Cabral Formation of Brazil. Here, we provide a detailed osteological description of the holotype and, thus far, only known specimen of T. paradoxa. In addition to providing new details of the anatomy of T. paradoxa, our study also reveals an early development of skull pneumaticity prior to the emergence of the antorbital fenestra. We use these new data to discuss the evolution of antorbital openings within Archosauriformes. Reappraisal of the phylogenetic position of T. paradoxa supports previous hypotheses of a close relationship with Archosauriformes. The data presented here provide new insights into character evolution during the origin of the archosauriform skull.

Ontogeny of the skull of the Black Caiman (Melanosuchus niger) (Crocodylia: Alligatoridae)

We describe the formation of the chondrocranium and the ossification pattern of the skull of the Black Caiman (Melanosuchus niger (Spix, 1825)). The embryos were cleared and double-stained with Alizarin Red S and Alcian Blue 8GX. Additionally, they were visualized by histological hematoxylin and eosin staining and computed tomography imaging. The chondrocranium of M. niger comprised the nasal capsule, orbitotemporal, and optic–occipital regions. Its development began at stage 9, with the chondrification of the acrochordal cartilage, trabeculae, and mandibular cartilage. The optic capsule was formed in the caudolateral portion of the chondrocranium at stage 13. The basal plate appeared at stage 14, with foramina for the hypoglossal. The chondrocranium was completely formed at stage 16. The first osteogenic events were noted at stage 13, in the bones, maxilla, jugal, postorbital, and pterygoid. The quadratojugal, prefrontal, frontal, and squamosal began their ossification at stage 14. The parietal bone began to ossify only at stage 20. The basisphenoid began at stage 15 and the parasphenoid began at stage 16. The jaw bones ossified between stages 13 and 16. The dermal elements started their ossification prior to the endochondral bones.

Signals from the brain and olfactory epithelium control shaping of the mammalian nasal capsule cartilage

Facial shape is the basis for facial recognition and categorization. Facial features reflect the underlying geometry of the skeletal structures. Here, we reveal that cartilaginous nasal capsule (corresponding to upper jaw and face) is shaped by signals generated by neural structures: brain and olfactory epithelium. Brain-derived Sonic Hedgehog (SHH) enables the induction of nasal septum and posterior nasal capsule, whereas the formation of a capsule roof is controlled by signals from the olfactory epithelium. Unexpectedly, the cartilage of the nasal capsule turned out to be important for shaping membranous facial bones during development. This suggests that conserved neurosensory structures could benefit from protection and have evolved signals inducing cranial cartilages encasing them. Experiments with mutant mice revealed that the genomic regulatory regions controlling production of SHH in the nervous system contribute to facial cartilage morphogenesis, which might be a mechanism responsible for the adaptive evolution of animal faces and snouts.

The first virtual cranial endocast of a porolepiform fish and the evolution of the Dipnomorpha

The Dipnomorpha include the extinct Porolepiformes, in addition to Powichthys and Youngolepis and the extant Dipnoi (lungfish). As sister group to the Tetrapodomorpha, the Dipnomorpha hold a valuable place in our understanding of early sarcopterygian evolution. With complete cranial endocasts now known from most other stem sarcopterygian groups including actinistians and tetrapodomorphs (which bracket the Dipnomorpha), a thorough understanding of the porolepiform endocranium remains one of the last pieces in the puzzle towards understanding evolution of the neurocranium in stem sarcopterygians. We present the first virtual cranial endocast of a porolepiform fish (Glyptolepis paucidens) demonstrating that it displays predominantly primitive sarcopterygian endocast characters including: lack of a bifurcation of the olfactory nerves, separate pineal and parapineal recesses, circumvention of the nasal capsule by the profundus nerve, and sessile olfactory bulbs. The union of the orbitonasal canal and the nasal capsule is consistent with that seen in Powichthys, Youngolepis and other porolepiforms, however circumvention of the nasal capsule by the profundus resembles the dipnoan Dipterus, contrasting the state observed in other porolepiforms and Powichthys. Inconsistent with the general primitive nature of the endocast seen in Glyptolepis is the large, curved hypophyseal recess, a feature shared with Powicthys but unlike Youngolepis. Furthermore, Glyptolepis displays ventral expansion of the telencephalon - a character thought to be derived within the Dipnoi. This new data enhances our understanding of brain and sensory evolution in sarcopterygians. In particular, it shows conservatism in stem Porolepiformes contrary to the disparate morphology of their sister-group the Dipnoi.

The first virtual cranial endocast of a porolepiform fish and the evolution of the Dipnomorpha

The Dipnomorpha include the extinct Porolepiformes, in addition to Powichthys and Youngolepis and the extant Dipnoi (lungfish). As sister group to the Tetrapodomorpha, the Dipnomorpha hold a valuable place in our understanding of early sarcopterygian evolution. With complete cranial endocasts now known from most other stem sarcopterygian groups including actinistians and tetrapodomorphs (which bracket the Dipnomorpha), a thorough understanding of the porolepiform endocranium remains one of the last pieces in the puzzle towards understanding evolution of the neurocranium in stem sarcopterygians. We present the first virtual cranial endocast of a porolepiform fish (Glyptolepis paucidens) demonstrating that it displays predominantly primitive sarcopterygian endocast characters including: lack of a bifurcation of the olfactory nerves, separate pineal and parapineal recesses, circumvention of the nasal capsule by the profundus nerve, and sessile olfactory bulbs. The union of the orbitonasal canal and the nasal capsule is consistent with that seen in Powichthys, Youngolepis and other porolepiforms, however circumvention of the nasal capsule by the profundus resembles the dipnoan Dipterus, contrasting the state observed in other porolepiforms and Powichthys. Inconsistent with the general primitive nature of the endocast seen in Glyptolepis is the large, curved hypophyseal recess, a feature shared with Powicthys but unlike Youngolepis. Furthermore, Glyptolepis displays ventral expansion of the telencephalon - a character thought to be derived within the Dipnoi. This new data enhances our understanding of brain and sensory evolution in sarcopterygians. In particular, it shows conservatism in stem Porolepiformes contrary to the disparate morphology of their sister-group the Dipnoi.

New information on Titanichthys (Placodermi, Arthrodira) from the Cleveland Shale (Upper Devonian) of Ohio, USA

The placoderm Titanichthys from the Late Devonian (Famennian) is based on incomplete and fragmentary specimens that have hindered understanding of its overall anatomy and phylogenetic relationships. A new, nearly complete, articulated specimen from the Upper Devonian Cleveland Shale provides new information about the previously undescribed rostral, postmarginal, postsuborbital, submarginal, posterior superognathal plates, and the nasal capsule. A revised diagnosis is provided for the genus. Three new diagnostic characters are identified, including a transversely elliptical rostral plate that does not contact adjacent plates, a reduced posterior superognathal, and a median dorsal plate that inserts into the posterior dorsal lateral plate. The first comprehensive phylogenetic analysis of Titanichthys indicates that the genus is a basal aspinothoracid arthrodire closely related to the enigmatic taxa Bungartius and Tafilalichthys.