Therian Petrosals From the Oldman and Milk River Formations (Late Cretaceous), Alberta, Canada

ABSTRACT

Therian petrosals from the Milk River and Oldman formations of Alberta are described. The Milk River specimen is the earliest known ear structure of a therian from North America and has a cochlea with one and a quarter turns. Two of the Oldman specimens are from marsupials, probably Eodelphis. These specimens provide significant new evidence about the earliest known otic structures in marsupials, including: absence of the promontory and stapedial grooves on the promontorium, presence of a prominent rostral tympanic process of the petrosal, achievement of a fully coiled cochlear canal, and development of the radial innervation system of the cochlea. Implications of the new findings are: 1) absence of the stapedial artery is consistently distributed in marsupials; 2) the rostral tympanic process of the petrosal has evolved more than once in marsupials; 3) a fully coiled cochlear canal had evolved in the therian ear by the Late Cretaceous; 4) a radial innervation complex of the cochlea evolved along with the coiled cochlea in the therian ear during the Late Cretaceous.