The apex of amphicyonid hypercarnivory: solving the riddle of Agnotherium antiquum Kaup, 1833 (Mammalia, Carnivora)

Since its first description, only a single undisputed specimen, an isolated, highly abraded m1, was known from the enigmatic amphicyonid Agnotherium antiquum Kaup, 1833. Here, we describe the second specimen of A. antiquum from the type locality of Eppelsheim (early late Miocene, MN 9/10 of Germany), unearthed 184 years after the first description of A. antiquum. The specimen consists of a partial left juvenile mandible with dp3, m1 in eruption, i3, canine, p4, and a highly reduced m2 still in crypt. The p1-3 and m3 are completely reduced, as are the metaconids of m1 and m2. We confirm previous assignments of late Miocene specimens from the localities of Charmoille, Switzerland, and Pedregueras 2A, Spain, to A. antiquum, whereas a maxilla from Bled Douarah, Tunisia, can only tentatively be assigned to the species. We confirm that the monotypic genus Agnotherium is a thaumastocyonine amphicyonid, discuss other supposed thaumastocyonine specimens, and briefly review the evolutionary history of the subfamily. Agnotherium was a 275 kg, extremely powerful, strictly carnivorous ambush hunter that represents the apex of amphicyonid evolution toward carnivory. Its tooth eruption pattern resembles recent Ursus more than recent Canis, but it shows a considerable delay of eruption timing like in the ursid Agriotherium. Agnotherium antiquum is one of six carnivoran species from the Eppelsheim Formation that is heavier than 200 kg. The co-occurrence of Agnotherium (or its relatives Tomocyon and Thaumastocyon), Machairodus, and a smaller saber-toothed carnivoran is typical for early late Miocene faunas of Europe and tentatively North Africa.