The evolution of fossoriality and the adaptive role of horns in the Mylagaulidae (Mammalia: Rodentia)

Abstract

Ceratogaulus, a member of the extinct fossorial rodent clade Mylagaulidae, is the only known rodent with horns and the smallest known horned mammal. The function of the large, dorsally projecting nasal horns on this burrowing animal has been the subject of wide speculation among palaeontologists; suggested uses range from sexual combat to burrowing. Mammals have evolved adaptations for digging repeatedly; horns and other cranial appendages have also evolved numerous times. These two adaptations co-occur in mammals extremely rarely: only two fossil genera (Ceratogaulus and the xenarthran Peltephilus) and no extant mammals are both horned and fossorial. Tracing the evolution of fossoriality in aplodontoid rodents (the larger clade to which Ceratogaulus belongs) reveals that Ceratogaulus descended from ancestors who dug by head-lifting. Whereas this suggests an obvious explanation for the horns of this rodent, evidence from functional morphology, anatomy, phylogeny and geologic context indicates that the horns in Ceratogaulus were used for defence, rather than digging, and evolved to offset increased predation costs associated with spending more time foraging above ground as body size increased.

Functional Anatomy of Incisal Biting in <i>Aplodontia rufa</i> and Sciuromorph Rodents – Part 1: Masticatory Muscles, Skull Shape and Digging

“…Interestingly, in the same study, Aplodontia falls within the range of head-lift diggers in some of the analyses, and the authors note that the overall, triangular shape of the skull is similar to head-lift diggers. Although it is not certain that Aplodontia is a head-lift digger [Hopkins, 2005] in the wild, I have observed caged individuals pushing material with their heads.…”

AMylagaulus(Mammalia, Rodentia) with nasal horns from the Miocene (Clarendonian) of western Oklahoma

“…Only the dorsal-most 7 mm of the occiput is intact at the midline, but enough is intact to enable measurement of this angle (OA by the method of Korth, 2000a) as 49 degrees. As measured by Korth (2000a: (Hopkins, 2005). The shallower angle in OMNH 74630, lower than in any other mylagaulid, may be due to my measuring it along the upper end of the occiput rather than from the occipital condyles or foramen magnum.…”

“…One species of mylagauline originally named as Epigaulus minor by Hibbard and Phillis (1945) possesses horns on the nasals (although the cranium and horns were not illustrated by the authors). In subsequent works this species E. minor has been placed variously in the genus Ceratogaulus, e.g., by Korth (2000a) and Hopkins (2005Hopkins ( , 2008, largely due to the presence of the horns, or in the genus Mylagaulus, e.g., by Flynn and Jacobs (2008). Ceratogaulus rhinocerus Matthew, 1902, C. hatcheri Gidley, 1907, and C. anecdotus Korth, 2000a, are known to have horns (Korth, 2000a).…”

Geologic framework, age, and lithologic characteristics of the North Park Formation in North Park, north-central Colorado

“…Ages are corrected using conversion factors of Dalrymple (1979). 3 The Dry Union Formation contains fossil mammals as young as 6-5 Ma (Hopkins, 2005). 4 Sediments of the Dry Union Formation probably are younger than 30.4 or 28.9 Ma (Shroba and others, 2014, and references cited therein).…”

“…The range in age for the White River Formation is from Lillegraven (1993), Larson and Evanoff (1998), Prothero and Emry (2004), and Tedford and others (2004); that for the Bishop Conglomerate is from Kowallis and others (2005); and those of the Arroyo Ojito Formation and the Zia Formation are from Tedford and Barghoorn (1999). Minimum age for the Dry Union Formation is based on ages of fossil mammals (Hopkins, 2005 The Troublesome Formation probably is older than 28.4 Ma because a latite porphyry intrusion, with a zircon ission-track age of 28.4±2.3 Ma, probably intruded beds of the Troublesome Formation at the north end of the Granby basin (Izett, 1974). 2 Locally, beds of the Browns Park Formation in northwestern Colorado are cut by a lamprophyre dike and latite porphyry intrusion that yielded potassium-argon (K-Ar) ages on biotite of 11.4±0.5 Ma and 7.8±0.4 Ma, respectively (Buffler, 2003).…”