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 Oligocene–Miocene 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.

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. Zusätzlich behält der Schädel einige Plesiomorphien bei, wie sie auch bei Trigonostylops vorkommen, während die Gehör-Region und die Schädelbasis bereits jenen von Parastrapotherium, Astrapotherium und Granastrapotherium deutlich ähnlicher sind. Auf der anderen Seite unterscheidet sich der Schädel von Astraponotus von jenen aller anderen Astrapotheren durch den ungewöhnlich hohen und schmalen Hirnschädel, die extreme Reduktion der Nasalia und Prämaxillaria, das Fehlen eines anteorbitalen Randes und die Reduktion der Frontal-Region. Diese Merkmale erscheinen erstaunlich fortschrittlich für einen eozänen Astrapotheren und deuten darauf hin, dass extreme Spezialisierungen des Schädels unabhängig in der Evolution der Ordnung entstanden sind und dass Astraponotus eine eigene Entwicklungslinie, getrennt von jener von Astrapotherium und anderer Miozäner Formen, darstellt.

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Acknowledgments

We thank E. Ruigomez, for allowing us to study the materials under his care, and the technical staff of the MPEF for kindly preparing the material included in this study. We are grateful to J. Flynn (AMNH), P. Makovicky (FMNH), and M. Reguero (MLP) for access to materials under their care, for comparisons. We would also like to thanks two anonymous reviewers for their very helpful comments and corrections, Tania Brenes (MHNSR) for her help with the English version, and Michele Arnal (MACN) for her aid with the Zusammenfassung. Drawings were made by the artists M. Sansibieri and J. González. This work was partially financed by a grant from the Consejo Nacional de Investigaciones Científicas y Técnicas (Res. Nro.1227/05) to Alejandro Kramarz.

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Correspondence to Alejandro G. Kramarz.

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Kramarz, A.G., Bond, M. & Forasiepi, A.M. New remains of Astraponotus (Mammalia, Astrapotheria) and considerations on Astrapothere cranial evolution. Paläontol Z 85, 185–200 (2011). https://doi.org/10.1007/s12542-010-0087-4

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Keywords

  • Astraponotus
  • Astrapotherium
  • Trigonostylops
  • Eocene
  • Patagonia

Schlüsselwörter

  • Astraponotus
  • Astrapotherium
  • Trigonostylops
  • Eozän
  • Patagonien