In non-mammalian amniotes, the parasphenoid is a midline dermal element with a narrow rostral portion, the cultriform process, linked to the interorbital septum and an expanded distal portion, the alae or wings, forming part of the ventral skull base. In mammals, the parasphenoid has not been found in extant monotremes and only a handful of reports of a reduced parasphenoid (a remnant of the cultriform process) have been made for extant marsupials and placentals. Most reports are in serially-sectioned perinatal specimens where the contrast between the intramembranous origin of the parasphenoid and the overlying endochondral basisphenoid facilitates delimiting the different elements forming the skull base. The only report of a parasphenoid remnant in adult marsupials is in the white-eared opossum, Didelphis albiventris, and it was published more than 100 years ago. Here, we report the results of a survey of 576 specimens of Didelphidae and 115 other Marsupialia in the extant collections of the Section of Mammals, Carnegie Museum of Natural History. We observed what we interpret as a parasphenoid remnant in some juveniles and adults from ten of the 27 didelphid species studied: Didelphis albiventris, Didelphis marsupialis, Didelphis virginiana, Marmosa murina, Monodelphis arlindoi, Monodelphis domestica, Philander opossum, Thylamys elegans, Thylamys pusilla, and Thylamys venustus. This element was variable in its presence within the collection, as well as in its size, form, and position. In our largest specific sample, the Virginia opossum, Didelphis virginiana, a parasphenoid was present in 55% of 238 specimens. It is uncertain if the variable occurrence reflects a true absence of the parasphenoid or its loss during specimen preparation. Outside of Didelphidae, we noted a substantial parasphenoid in the microbiothere Dromiciops gliroides, contributing to a midline septum that partially divides the nasopharynx into two channels, and a probable small one in the macropodid Thylogale sp.

In extinct mammals and non-mammalian cynodonts, a midline mesocranial ridge interpreted by prior authors as composed of or including a parasphenoid has a wide distribution, supporting the presence of this structure as primitive for Mammalia. It is suggested here that the Miocene platypus Obdurodon has a well-developed parasphenoid further supporting the presence of a parasphenoid as a plesiomorphic feature for Mammalia that is independently lost in some therians and apparently in extant monotremes.