ON THE SKULLOF ASTRAPONOTUS (MAMMALIA, ASTRAPOTHERIA) AND CONSIDERATIONS ON THE ASTRAPOTHERE CRANIAL EVOLUTION

KRAMARZ, A.; BOND, M.; FORASIEPI, A.M.

Neuquen

Congreso; III Congreso Latinoamericano de Paleontologia de Vertebrados; 2008

Universidad Nacional de Comahue

Astraponotus Ameghino, 1901, the only valid Mustersan (late Eocene) astrapothere, typifies the Ameghino’s “capas Astraponotenses” (Ameghino, 1906). This taxon is traditionally interpreted as structurally ancestral to all the Oligo-Miocene astrapotheriids (Scott, 1937; Soria, 1984; Cifelli, 1993); however, it was very imperfectly known: only isolated teeth and very partial mandibles have been described (Ameghino, 1901; Simpson, 1967). In this contribution, we provide the first description of the skull and complete upper dentition of Astraponotus based upon three partial skulls (MPEF PV 1084, 1279, and 1296) from the Mustersan Gran Hondonada locality, Chubut Province, Argentina (Cladera et al., 2004). The upper dentition of Astraponotus is structurally intermediate between the Casamayoran (middle Eocene) and the Oligo-Miocene astrapotheres in the degree of hypsodonty, reduction of the dental formula, and development of accessory occlusal elements. The skull retains some plesiomorphies, observed in the Casamayoran Trigonostylops (e.g. rostral and temporal regions not shortened, frontals not expanded forming a broad roof over the orbit, multiple anteorbital foramina far anterior to the orbit, and occiput not constricted), whereas those of the auditory region and the basicranium are much closer to that of Parastrapotherium, Astrapotherium, and other Miocene astrapotheriids (e.g., broad basicranium, deeply excavated tympanic cavity with the petrosal deep inside, and lack of ossified auditory bulla). However, the skull of Astraponotus differs from all known astrapotheres by the disproportioned height and narrowness of the braincase, the extreme reduction of the premaxillaries and nasals, the absence of anterorbital rim, and the reduction of the frontal region. The latter feature represents a cranial specialization exactly opposite to that of Astrapotherium. These features look astonishingly derived for an Eocene astrapothere, suggesting that Astraponotus represents a distinctive lineage from that of Astrapotherium and Parastrapotherium, and that extreme cranial specializations occurred independently during the evolution of the Order.