Nasal cavity of Sipalocyon externus (Metatheria, Sparassodonta)

GAILLARD, C.; FORASIEPI, A.M.

Buenos Aires

Congreso; 12 Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

The nasal cavity of Sipalocyon externus (Hathliacynidae; MACN-Pv-CH 1911, housed at the Museo Argentino de Ciencias Naturales) from Sacanana, Chubut, Sarmiento Formation, Colhuehuapian age, is studied through μCT-scanning. The preservation of the thin bones of the nasal cavity is exceptional in this specimen compared to other studied sparassodonts.The general organization of the elements in the nasal cavity is similar to extant marsupials studied by T. Macrini and T. Rowe. The maxilloturbinal is treelike with at least four branches. The nasoturbinal extends along the entire cavity. The rostral nasoturbinal is small and thick, similar to Dromiciops gliroides. The caudal nasoturbinal occupies a large portion of the cavity and has one pneumatic cavity resembling Dasyurus viverrinus. The vomer is fused caudally to the ethmoid plate and is very similar in shape to the one of Dasyurus hallucatus. The nasal septum is less ossified rostrally than the one of marsupials. The posterior transverse lamina does not contribute to the cribriform plate, a common condition for extant marsupials. The encranial surface of the cribriform plate is concave with five pairs of large foramina and several smaller size foramina for the passage of the olfactory nerve (CNI). The cribroethmoidal foramen, crossed in life by a division of the ethmoidal nerve (CNV1), is the largest foramen. The cribriform plate surface area is 83.2 mm2. Considering the body mass estimation of 2.3 kg for the specimen (using the equations of Myers, Gordon and Ekdale), the number of functional olfactory receptor genes are estimated between 516 to 623. These values are half the ones of the extant didelphid Monodelphis domestica and slightly lower than the number of functional olfactory receptor genes of Felis catus. The olfactory bulbs cast represents 5.8% of the total volume of the endocranial cavity, similar to the value of Sipalocyon gracilis, but smaller than the percentages recorded for the borhyaenid Borhyaena tuberata and the didelphid Monodelphis domestica. Overall, although the architecture of the nasal cavity of Sipalocyon externus resembles extant marsupials, its cribriform plate resembles more the one of extant cats with similar functional olfactory receptor genes values and likely similar olfactory sensitivity.