Resumen de Morphology and evolution of the oral shield inmarsupial neonates including the newborn monito delmonte (Dromiciops gliroides , MarsupialiaMicrobiotheria) pouch young
Nanette Y Schneider, Yamila Gurovich
Newborn marsupials can be arranged into three grades of developmental complexity based on their externalform, as well as based on their organ systems and their cytology. The dasyurids are considered the leastdeveloped marsupials at birth, while didelphids and peramelids are intermediate, and macropods are the mostdeveloped. 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 themost developed. Marsupials are also characterized by having an extremely developed craniofacial region atbirth compared with placentals. However, the facial region is also observed to vary in development betweendifferent marsupial groups at birth. The oral shield is a morphological structure observed in the oral region ofthe head during late embryological development, which will diminish shortly after birth. Morphologicalvariation of the oral shield is observed and can be arranged by developmental complexity from greatlydeveloped, reduced to vestigial. In its most developed state, the lips are fused, forming together with therhinarium, a flattened ring around the buccal opening. In this study, we examine the external oral shieldmorphology 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 orderMicrobiotheria). The adaptive value of the oral shield structure is reviewed, and we discuss if this structure maybe influenced by developmental stage of newborn, pouch cover, species relatedness, or other reproductivefeatures. We observe that the oral shield structure is present in most species of Marsupialia and appears to beexclusively present in this infraclass. It has never been described in Monotremata or Eutherians. It is present inunrelated taxa (e.g. didelphids, dasyurids and microbiotherids). We observe that a well-developed oral shieldmay be related to ultra altricial development at birth, large litter size (more than two), and is present in mostspecies that lack a pouch in reproductive adult females or have a less prominent or less developed pouch withsome exceptions. We try to explore the evolution of the oral shield structure using existing databases and ourown observations to reconstruct likely ancestral character states that can then be used to estimate theevolutionary origin of this structure and if it was present in early mammals. We find that a simple to developoral shield structure (type 2–3) may have been present in marsupial ancestors as well as in early therians, eventhough this structure is not present in the extant monotremes. This in turn may suggest that early marsupialsmay 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 oralshield and hindlimb development may be influenced by species size and reproductive strategy, and possibly byyet unknown species-specific adaptations.
