Morphology and evolution of the oral shield in marsupial neonates including the newborn monito del monte (Dromiciops gliroides, Marsupialia Microbiotheria) pouch young.

Newborn marsupials can be arranged into three grades of developmental complexity based on their external form, as well as based on their organ systems and their cytology. The dasyurids are considered the least developed marsupials at birth, while didelphids and peramelids are intermediate, and macropods are the most developed. Currently there is still little information on caenolestid and microbiotherid development at birth. Developmental stages can be graded as G1, G2 and G3, with G1 being the least developed at birth, and G3 the most developed. Marsupials are also characterized by having an extremely developed craniofacial region at birth compared with placentals. However, the facial region is also observed to vary in development between different marsupial groups at birth. The oral shield is a morphological structure observed in the oral region of the head during late embryological development, which will diminish shortly after birth. Morphological variation of the oral shield is observed and can be arranged by developmental complexity from greatly developed, reduced to vestigial. In its most developed state, the lips are fused, forming together with the rhinarium, a flattened ring around the buccal opening. In this study, we examine the external oral shield morphology in different species of newborn marsupials (dasyurids, peramelids, macropods and didelphids), including the newborn monito del monte young (Dromiciops gliroides - the sole survivor of the order Microbiotheria). The adaptive value of the oral shield structure is reviewed, and we discuss if this structure may be influenced by developmental stage of newborn, pouch cover, species relatedness, or other reproductive features. We observe that the oral shield structure is present in most species of Marsupialia and appears to be exclusively present in this infraclass. It has never been described in Monotremata or Eutherians. It is present in unrelated taxa (e.g. didelphids, dasyurids and microbiotherids). We observe that a well-developed oral shield may be related to ultra altricial development at birth, large litter size (more than two), and is present in most species that lack a pouch in reproductive adult females or have a less prominent or less developed pouch with some exceptions. We try to explore the evolution of the oral shield structure using existing databases and our own observations to reconstruct likely ancestral character states that can then be used to estimate the evolutionary origin of this structure and if it was present in early mammals. We find that a simple to develop oral shield structure (type 2-3) may have been present in marsupial ancestors as well as in early therians, even though this structure is not present in the extant monotremes. This in turn may suggest that early marsupials may have had a very simple pouch or lacked a pouch as seen in some living marsupials, such as some dasyurids, didelphids and caenolestids. The study's results also suggest that different morphological stages of the oral shield and hindlimb development may be influenced by species size and reproductive strategy, and possibly by yet unknown species-specific adaptations.

Persistence of a Mesozoic, non-therian mammalian lineage (Gondwanatheria) in the mid-Paleogene of Patagonia.

We describe two isolated molariforms recovered from early-middle Eocene (early Lutetian) levels of northwestern Patagonia, Argentina. Comparisons with major lineages of therian and non-therian mammals lead us to refer them to a new genus and species of Gondwanatheria (Allotheria). There is a single root supporting each tooth that is very short, wide, rounded, and covered by cementum; the steep sidewalls, lack of a neck between the crown and root, and the heavily worn stage in both molariforms suggest that they were of a protohypsodont type. Both teeth are strongly worn at their centers, all along their length, with the labial edge less worn than the lingual; they show strong transverse crests that alternate with lingual grooves. The protohypsodont aspect of the teeth, as well as the strong, transverse crests, are suggestive of sudamericid affinities; on the other hand, the thin enamel layer and the occlusal pattern formed by the crests and grooves shows more similarities to molariform teeth of the Ferugliotheriidae. The new taxon adds evidence regarding the (1) extensive radiation of the Gondwanatheria throughout the Southern Hemisphere, (2) persistence of several lineages well after the Cretaceous/Paleogene boundary, and (3) early evolution of hypsodont types among South American herbivorous mammals.

Quantitative Analysis of the Timing of Development of the Cerebellum and Precerebellar Nuclei in Monotremes, Metatherians, Rodents, and Humans.

We have used a quantitative statistical approach to compare the pace of development in the cerebellum and precerebellar systems relative to body size in monotremes and metatherians with that in eutherians (rodents and humans). Embryos, fetuses, and early postnatal mammals were scored on whether key structural events had been reached in the development of the cerebellum itself (CC-corpus cerebelli; 10 milestones), or the pontine and inferior olivary precerebellar nuclear groups (PC; 4 milestones). We found that many early cerebellar and precerebellar milestones (e.g., formation of Purkinje cell layer and deep cerebellar nuclei) were reached at a smaller absolute body length in both metatherians and eutherians together, compared to monotremes. Some later milestones (e.g., formation of the external granular layer and primary fissuration) were reached at a smaller body length in metatherians than eutherians. When the analysis was performed with proportional body length expressed as a natural log-transformed ratio of length at birth, milestones were reached at a much smaller proportional body length in rodents and humans than in the metatherians or monotremes. The findings are consistent with the slower pace of metabolic activity and embryonic development in monotremes. They also indicate slightly advanced maturation of some early features of the cerebellum in some metatherians (i.e., early cerebellar development in dasyurids relative to body size), but do not support the notion of an accelerated development of the cerebellum to cope with the demands of early birth. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1998-2013, 2020. © 2019 American Association for Anatomy.

Numerical Analysis of the Cerebral Cortex in Diprotodontids (Marsupialia; Australidelphia) and Comparison with Eutherian Brains.

Diprotodontids are a diverse group of Australian metatherians, which occupy a range of ecological niches from nectar and pollen-feeders to grazers and folivores. The group encompasses small-brained nectar-feeding species (Tarsipes) and large-brained grazing and browsing species (macropods). This group of Australian metatherians therefore represents an opportunity to examine how the cerebral cortex has expanded in an adaptive radiation quite independent of that occurring among eutherians. We have used the Nelson Brain Collection and online resources to perform a quantitative analysis of the isocortex, hippocampal formation and olfactory structures in diprotodontids. We found that the scaling relationship between iso- and periallocortical grey matter and brain size, and between subcortical white matter and iso- and periallocortex grey matter, are both almost identical among diprotodontids and eutherians. By contrast, the relationship between gyrification and brain size is strikingly different between diprotodontids and eutherians, with gyrification being much lower for a given brain size among the diprotodontids, although gyrification is much more varied among macropods than other diprotodontids. The scaling of iso- and periallocortical volume with dorsal striatal and dorsal thalamic volume is almost identical among the diprotodontids and eutherians, but the claustrum is smaller, and amygdala larger, for a given brain size among diprotodontids than eutherians. The hippocampal formation and central olfactory areas (anterior olfactory region and piriform cortex) both scale more steeply with brain size among diprotodontids compared to eutherians. Our findings suggest that, although white matter scaling is identical among all therians, there are significant differences between diprotodontids and eutherians in the way that cortical folding and expansion of allocortical structures occurs with brain enlargement.

Quantitative analysis of forebrain pallial morphology in monotremes and comparison with that in therians.

We have made quantitative volumetric analyses of cerebral cortical (pallial) structures in the brains of three species of monotreme (Ornithorhynchus anatinus, Tachyglossus aculeatus, Zaglossus bruijni) and compared the findings with similar measurements in a range of therian mammals (6 marsupials and 50 placentals). We have found that although the iso- and periallocortical grey matter volume of the monotremes is about what would be expected for their brain size, the proportion of iso- and periallocortical white matter in monotremes is substantially lower than that in the forebrains of therians. This suggests that the forebrains of the three monotremes have fewer association, commissural and/or projection connections than those of similarly sized forebrains of therian mammals. We also found that the iso- and periallocortex of the platypus is relatively smooth-surfaced compared to similarly sized brains of therian mammals, with a distinct caudal shift in the positioning of cortical white matter in the forebrain, consistent with expansion of the posterior thalamic radiation. Central laminated olfactory structures (anterior olfactory nucleus and piriform cortex) are large in the tachyglossid monotremes (Tachyglossus aculeatus and Zaglossus bruijni) and large in xenarthran placental mammals, suggesting convergence of the forebrain structure of monotreme formivores with that of similarly specialized therians like the xenarthrans Myrmecophaga tridactyla and Dasypus novemcinctus.