New remains of Astraponotus (Mammalia, Astrapotheria) and considerations on Astrapothere cranial evolution

Abstract

Astraponotus Ameghino, 1901, the only valid Mustersan (late Eocene) astrapothere, typifies the Ameghino's ''Capas Astraponotenses''. This taxon is traditionally interpreted as structurally ancestral to all the OligoceneMiocene astrapotheriids. However, it was imperfectly known: only isolated teeth and very partial mandibles have hitherto been described. In this contribution we provide the first description of the skull, mandible, and complete dentition of Astraponotus based on new materials from the Gran Hondonada and other Mustersan localities in central Patagonia, Argentina. The features observed in the dentition of Astraponotus are intermediate between the Casamayoran (middle Eocene) and the Oligocene-Miocene astrapotheres in the degree of hypsodonty, reduction of the dental formula, and development of accessory occlusal elements. Concordantly, the skull retains some plesiomorphies, also observed in Trigonostylops, whereas the auditory region and the basicranium are much closer to those of Parastrapotherium, Astrapotherium, and Granastrapotherium. On the other hand, the skull of Astraponotus differs from all known astrapotheres by the disproportioned height and narrowness of the braincase, the extreme reduction of the nasals and the premaxillaries, the absence of anteorbital rim, and the reduction of the frontal region. Some of these features represent cranial specializations exactly opposite to that of Astrapotherium. These characters look astonishingly derived for an Eocene astrapothere, suggesting that extreme cranial specializations occurred independently during the evolution of the order, and that Astraponotus represents a distinctive lineage from that of Astrapotherium and other Miocene forms.Keywords Astraponotus Á Astrapotherium Á Trigonostylops Á Eocene Á Patagonia Kurzfassung Astraponotus Ameghino, 1901, der einzige valide Astrapothere aus dem Mustersan (spätes Eozän), diagnostiziert die ''Capas Astraponotenses'' von Ameghino. Dieses Taxon wurde traditionell als strukturell ursprünglich im Vergleich zu allen Oligozänen-Miozänen Astrapotheriiden angesehen. Allerdings war es nur unzulänglich bekannt: nur isolierte Zähne und teilweise erhaltene Dentalia wurden bisher beschrieben. Hier beschreiben wir zum ersten Mal den Schädel, Unterkiefer und die gesamte Bezahnung von Astraponotus auf der Basis von neuen Materialien aus der Gran Hondonada und anderen Mustersanen Lokalitäten im zentralen Patagonien. Die in der Bezahnung beobachteten Merkmale von Astraponotus sind intermediär zwischen Astrapotheren des Casamayor (mittleres Eozän) und des Oligozän-Miozän in Hinsicht auf Hypsodontie, Reduktion der Zahnformel und der Entwicklung zusätzlicher Okklusionsstrukturen.Electronic supplementary material The online version of this article (

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