Morphology of the Semicircular Canals and Locomotion of Zionodon satanus

Abstract

Zionodon satanus is a relatively large “insectivore” from the Uinta Formation of Utah, known from associated fragmentary dental, cranial, and postcranial elements. Features of the postcrania, particularly the knee and ankle joints, suggest that this species primarily engaged in swift terrestrial locomotion, but with some climbing and digging capability. Among the cranial fragments of Z. satanus are two petrosal bones preserving portions of the bony labyrinth, particularly the semicircular canals, which are known to vary among extant mammals according to locomotor agility, with more agile animals having semicircular canals with a larger radius of curvature than slower ones. Thus, semicircular canal dimensions can provide insight into locomotion independent of postcranial morphology. Here we compare measurements of the semicircular canals of Z. satanus to other extant and extinct mammals to assess the degree of locomotor agility of this species and compare those results to those obtained by functional analysis of the postcrania.The petrosal bones of the type specimen of Z. satanus were microCT scanned and virtual models of the semicircular canals were extracted with AMIRA 6.5.0 software. One petrosal includes a complete posterior and partial lateral semicircular canal, whereas the other preserves a complete lateral, damaged but complete posterior, and most of the anterior semicircular canal. The height and width of each semicircular canal was measured from the extracted surface models and the radius of curvature was calculated from height and width measurements. The average radius of curvature of the semicircular canals of Z. satanus overlaps with extant mammals of similar body size of “fast” or “medium” agility and lies well outside the range of slow‐moving small mammals. Compared to other fossil “insectivores”, Z. satanus has relatively smaller semicircular canals than Leptictidium from the middle Eocene of Messel, Germany, which has been reconstructed as a bipedal runner and saltator, but larger ones than the North American leptictids Paleictops and Leptictis. These results corroborate observations from the postcranial skeleton, suggesting that Z. satanus likely engaged in frequent, rapid locomotion.