Relationships of the dog-like marsupials, deltatheroidans and early tribosphenic mammals

A cladistic analysis of the major groups of early metatherian mammals shows that the Deltatheroida and South American dog-like Borhyaenoidea are closely related and are placed in a new supercohort Deltatheralia. while all other metatherians are placed in the supercohort Marsupialia. A reassessment of Early Cretaceous mammals with tribosphenic molars shows that metatherians and eutherians apparently evolved independently from a peramuran-like ancestor, and an entoconid. distinctly basined talonid and large protocone were acquired separately in each group. Consequently. the Tribosphenida ( sensu McKenna) is apparently paraphyletic. Aegialodontia are regarded as structurally ancestral to Metatheria but not Eutheria. The known fossil record supports the view that differentation of the stocks which gave rise to metatherians and eutherians apparently occurred in Late Jurassic or Early Cretaceous time.     

Cells of the photoreceptor line in the pineal organ of an adult marsupial,Didelphis albiventris

We report the presence of atypical pinealocytes as components of epiphyseal follicles in the adult South American opossum Didelphis albiventris. Their main characteristic is a bulbous-shaped apical cytoplasmic extension which protrudes towards the follicular lumen among the microvilli and cilia of neighbouring ependymal cells. They resemble the photoreceptor-like pinealocytes of sauropsids and developing photoreceptors in the retina of newborn mammals. Morphological characteristics enable us to classify them as cells of the receptor line.This work was supported by the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET)     

Patterns of evolutionary transformation in the petrosal bone and some basicranial features in marsupial mammals, with special reference to didelphids

Twelve petrosal and four nonpetrosal characters were coded for representatives of all 15 extant genera of Didelphidae and for 16 additional genera of marsupials representing all extant orders. Three basal metatherians were used as outgroup comparison. Histological sections of a subset of the data were examined. An intermediate position of the hiatus Fallopii supports the monophyly of Didelphidae. Several basicranial regions support different clades within the Didelphidae that recent molecular work has identified, including a sister group relationship of Caluromys and Caluromysiops , the monophyly of large opossums, a Lestodelphys-Thylamys clade, and a Lestodelphys-Thylamys-Gracilinanus-Marmosops clade. Glironia lacks petrosal and jaw synapomorphies of Caluromys and Caluromysiops. The transverse canal, a synapomorphy of the crown-group Marsupialia, opens as a single foramen anterior to the carotid foramen in most marsupials or as numerous foramina in the pterygoid fossa in diprotodontians. It is either intramural (most marsupials) or simply endocranial (most diprotodontians excluding koalas and wombats). Loss of a deep sulcus in the anterior pole of the promontorium for the internal carotid artery and a rostral tympanic process of the petrosal also characterize the groundplan of the crown group Marsupialia. Pouch-young wombats show a groove in the anterior pole of the petrosal for the internal carotid artery. The absence of a prootic canal foramen in the tympanic side of the petrosal of adults supports the monophyly of Australidelphia. Some pouch-young marsupials possess a prootic canal that is later lost in ontogeny. A rather flat promontorium and a crest running medio-distally in the middle of the promontorium characterize Macropodidae.