The Nasal Complex of a Semiaquatic Artiodactyl, the Moose (Alces alces): Is it a Good Evolutionary Model for the Ancestors of Cetaceans?

Among Cetartiodactyla, cetaceans are the only obligate aquatic dwellers. Given morphological similarities between cetacean relatives such as Indohyus (the best represented Eocene raoellid artiodactyl) with other, later artiodactyls, any crown artiodactyl that engages in aquatic behaviors is of interest as an evolutionary model for the adaptations that accompanied the origins of cetaceans. The American moose (Alces alces) is the only non-cetacean artiodactyl to engage in aquatic foraging and, other than Hippopotamus, is distinctive in its diving behaviors. This study surveyed the soft and hard tissue nasal morphology of Alces alces to assess phylogenetic polarity and the presence of adaptations for diving and feeding in fresh water habitats. A fresh dissection of the facial musculature and nasal cavity was performed on one subadult male individual and osteological analyses were also performed on dry crania. This species was analyzed alongside fossil crania of Cervalces (its presumed ancestor), other cervids (e.g., Odocoileus virginianus, the white tail deer; Dama dama, the fallow deer), a bovid (Bos taurus, domestic cattle), and a carnivoran (Ursus americanus, the American black bear). A fresh dissection of the facial musculature and nasal anatomy of one fallow deer specimen was also performed for comparison with the moose. Results indicate that Alces alces exhibited a primitive configuration of maxillolabial muscles and, like Dama, exhibited a series of subcutaneous fibrous tissues connecting these muscles to skin. Alces and Dama, however, both exhibited autapomorphies in the soft tissue anatomy of the external nares. The former possessed a series of muscles that act to constrict the anterior nares, likely during diving. Extremely large fibrofatty pads that were perforated by muscle tendon supported their alar fold. Internally, a double-scrolled maxilloturbinal occupied nearly the entire volume of the anterior nasal cavity and protruded beyond the rim of the piriform aperture in dry crania. Dama had long, thin muscles taking origin on their nasal conchae and inserting onto the alar fold. Yet, despite these anterior nasal autapomorphies, the ethmoturbinal patterns of all observed cervids and the one bovid all appeared primitive with a posteroinferiorly oriented array of ethmoturbinals in close contact with a relatively straight cribriform plate, a macrosmatic condition. These differed from the curved cribriform plate of Ursus whose posterior nasal anatomy appeared hyper-macrosmatic. Indohyus exhibits no skeletal sign of a fleshy proboscis such as an enlarged piriform aperture or shortened nasal bones. Thus, there is little evidence that the early ancestors of cetaceans engaged in prolonged bouts of diving for aquatic foods but more probably were surface swimmers traveling between terrestrial food sources or fleeing predators. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:667-692, 2019. © 2018 Wiley Periodicals, Inc.