Metatherian petrosals from the Middle-Late Eocene of Northwestern Argentina

BABOT M.J.; GARCÍA LÓPEZ D.A

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Palaeontological Association

We present two incomplete petrosals (MHAS 062 and MHAS 063) collected by screen washing procedures in the Geste Formation exposed in Catamarca Province, Northwestern Argentina. The bones, assigned to the same morphotype, preserve the pars cochlearis almost complete and the most anterior portion of the pars canalicularis. The promontorium is globular and occupies nearly the entire tympanic aspect of the pars cochlearis; the rostral tympanic process is blunt; the epitympanic wing and the medial flange are absent. The stapedial ratio is 2.1 in MHAS 062 and 1.1 MHAS 063. The floor of the cavum supracochleare is present and the secondary facial foramen is located anterolateral to the fenestra vestibuli. The hiatus Fallopii opens in an intermediate (tympanic) position. The epitympanic recess, partially preserved, bears a small fossa incudis clearly demarcated from the facial sulcus by the crista parotica. The presence/absence of the prootic canal cannot be determined. The stapedius fossa is separated from the roof of the post-promontorial tympanic sinus by a faint crest. On the cerebellar aspect, the internal acoustic meatus (IAM) exhibits a kidney-shaped foramen acusticum inferius separated from a smaller foramen acusticum superius by a wide transverse crest. In posterior view, the pars canalicularis shows the anterior wall of a deep subarcuate fossa and the anterior parts of the semicircular canals. The petrosal crest is sharp. The cochlear canaliculus is located posteromedial to the IAM. Anterodorsally, an unnamed aperture penetrates the substance of the petrosal. The sulcus for the inferior petrosal sinus is barely visible. The presence of a rostral tympanic process, a sulcus for the inferior petrosal sinus, and the absence of promontorial grooves for the stapedial system allow us to refer these elements to Metatheria. Among this group, we found morphological similarities with the Itaboraian petrosal Type V and with the Peligran petrosal from Patagonia. In size, the new petrosals resemble the smallest Itaboraian metatherian petrosal (Type VIII). The estimated molar area for the auditory bones here studied falls within the range of Minusculodelphis and clearly differs from the larger teeth of the metatherian Regia punae, Bonapartherium serrensis, and Punadolops alonsoi recovered at Geste Formation. These findings reveal the presence of a wider diversity of small mammals than already known for Eocene beds of Northwestern Argentina. Additionally, these new records highlight the value of screen washing techniques as well as the need of considering different lines of evidence, when studying fragmentary records of fossil micromammals.