New fossils of the early Miocene stem-cervid Acteocemas (Artiodactyla, Ruminantia) from the Iberian Peninsula shed light on the evolutionary origin of deer antler regeneration

Acteocemas, a very poorly documented early Miocene stem-cervid, is one of the first ruminants bearing antler-like appendages, which has provided a ground for discussion on the origin of antlers. We describe a new and very complete appendage from the site of Sant Andreu de la Barca (Spain) together with some other unpublished specimens from the nearby Costablanca attributed to Acteocemas aff. infans, compare with fossils from elsewhere in Europe (including the A. infans holotype), and perform micro-CT scans. The findings provide new empirical data that Acteocemas protoantlers were able to be cast and re-grown. However, microstructural analyses suggest that the protoantler lifespan could be longer than that of modern antlers, preventing it from assuming a similar cycle. Results support that increased seasonality associated with a drop in global temperatures played a role in the origin of antler regeneration, and that deciduousness (through bone shedding) was an efficient way for (male)deer to reduce the seasonal leftover of bone mass. The early evolution of deciduousness, as in the probable irregular protoanter cycle of Acteocemas, was limited by the warming ca. 17–15 Ma, whereas the emergence of antlers with coronet was concomitant with the second increase in seasonality associated to the cooling ca. 15–13 Ma.