Panochthus frenzelianus was described by Ameghino in 1889 based on a skull, mandible, cephalic shield, osteoderms from the posteriormost rows of carapace, and caudal rings. Unfortunately, the type specimen was not specified and the provenance and repository of the described specimens were not informed. In 1903, Osborn stated that the holotype of this species was part of a collection of fossil mammals that Cope bought in France in 1878 and later deposited in the AMHN of New York. In 1942, Castellanos published a photograph of the purported holotype of P. frenzelianus (a mounted skeleton) and confirmed that it was deposited at the AMNH, as previously stated by Osborn. However, the specimen presents some differences with the material described in 1889 by Ameghino. Here we attempt to determine the true type specimen of P. frenzelianus and its repository based on information available on the literature, the AMNH collection, archives (e.g., notes, catalogs), and on a morphological analysis of some elements assigned to Panochthus housed in that institution. We found that: (1) the specimen figured by Castellanos and exhibited at the AMNH is a composite specimen formed by remains of at least three individuals; (2) these remains do not bear the diagnostic features mentioned by Ameghino and, therefore, they cannot be assigned to the holotype of P. frenzelianus; (3) a cephalic shield and a mandible belonging to another specimen at the AMNH bear the diagnostic characters mentioned by Ameghino, and are the only parts that we could confidently assign to the holotype of P. frenzelianus.

“XYOPHORUS” SP. FROM THE MIDDLE MIOCENE OF CHUBUT PROVINCE: SYSTEMATIC, BIOGEOGRAPHIC, AND BIOCHRONOLOGIC IMPLICATIONS OF THE RECORD OF A NOTHROTHERIINAE FROM THE NEOGENE OF ARGENTINA. Xyophorus Ameghino was described based on mandibular fragments of Xyophorus rostratus Ameghino from the Early Miocene of Patagonia. The specimen herein described was collected in the locality “El Petiso”, located in northwestern Chubut Province, in sediments referred to the Middle Miocene and it is represented by a mandibular fragment that is characterized by: (1) posterior margin of the mandibular symphysis anterior to the plane of the caniniform; (2) posterolateral opening of mandibular canal on the lateral side of the coronoid process and nearly to the oclusal plane; (3) oclusal surface of the molariforms with two crests separated by a “V” valley; and (4) molariforms 1–2 presents vertical grooves on the labial and lingual sides, and molariform 3 with a groove on the labial one. The features present in this specimen and in the species herein referred to “Xyophorus” bondesioi Scillato-Yané, and “Xyophorus” villarroeli Saint-André allow distinguishing them from the species of the genus Xyophorus (e.g., X. andinus Ameghino, X. atlanticus Ameghino, X. crassissimus Ameghino) justifying the erection of a new genus and its inclusion in Nothrotheriinae. The record of “Xyophorus” sp. from the Middle Miocene of Chubut Province represents the oldest and australmost for Nothrotheriinae from Argentina, whereas, the record of “Xyophorus” cf. bondesioi from the locality of Cerdas, in Bolivia, would correspond to the oldest one for the subfamily.

PALEOECOLOGY OF BONAPARTHERIOIDEA (MARSUPIALIA, POLYDOLOPIMORPHIA, BONAPARTHERIIFORMES). The wear pattern, occlusal movement, body mass, and diversity of Bonapartherioidea families are analyzed. Hypothesis for the dietary habits and possible extinction causes are developed. Bonaparteroids are divided into three groups according to the development of Phase II in the chewing cycle. Grinding facet 5 is not developed in Prepidolopidae, is slightly developed in Rosendolopidae, and is broadly developed in Bonapartheriidae and Gashterniidae. Lateral movements characterize the occlusion of Prepidolopidae and Rosendolopidae, while oblique movements are typical of Bonapartheriidae and Gashterniidae. During the Peligran age—Vacan subage, species of Bonaparteroids occupied frugivorous, insectivorous, and folivorous niches. From the Barrancan age to La Cantera fauna, they mostly occupied the granivorous and insectivorous niches. Bonaparteroids became extinct during the Eocene/ Oligocene boundary, probably associated with an abrupt fall in temperature and precipitations.

The presence of the oyster Pycnodonte (Phygraea) vesicularis (Lamarck) is recorded in the Maastrichtian mudstones of the Jagüel Formation. This species was free-lying and inhabited muddy bottoms in a shallow marine shelf with a low sedimentation rate. Their shells supported a diverse community including fungi, algae, sponges, cirripedes, bryozoans, phoronids, polychaetes and mollusks. The most conspicuous components of this community are the boring polychaetes. The aim of this work is to assess possible paleoecological relationships between the oysters and polychaetes. Eighty-nine specimens of P. (Ph.) vesicularis were studied and presence/absence data were recorded. Most of the boring openings are located on the ventral margins and are perpendicular to the growth lines. Preferential distribution of borings on the external surface of valves and on the ventral margins suggests a syn-vivo colonization. The high density of polychaete traces on the valves is an evidence of aggregative behavior of these worms. Although the borings weakened the valve and reduced shell strength, this does not seem to have been lethal for the oysters because there is no evidence of borings opening on the external surface reaching the internal surface of the valve. This suggests that a symbiotic relationship probably existed between P. (Ph.) vesicularis and polychaetes and that this relationship could be classified as commensalism.

A palynological analysis of the section of La Colonia Formation exposed at Estancia San Miguel yielded conspicuous assemblages of organic-walled dinoflagellate cysts (dinocysts) and green algae. The monotypic palaeoperidinioid dinocyst assemblage of ?Ginginodinium sp. in the basal beds of this section indicates low-salinity water conditions in a restricted shallow marine paleoenvironment. The green-algaedominated assemblages together with specimens of ?Morkallacysta spp., Dinocysts type P and ?Vesperopsis sp. recorded in the middle and upper part of the San Miguel section indicate brackish to freshwater and freshwater depositional conditions, respectively. The changes in the composition of the palynological assemblages, in agreement with the analysis of the sedimentary facies, reflect a salinity-drop in the water bodies and a progressive upward-shallowing trend. The occurrence of specimens of the neritic open marine Cribroperidinium spp., Apteodinium sp., Circulodinium sp. and Areoligera sp. cf. A. circumsenonensis Fensome et al. is here considered as reflecting contemporaneous transported material from the adjacent shelf. The marine part of the La Colonia Formation is associated to Late Cretaceous and Paleocene ages. Nevertheless, palynomorphs together with a stratigraphical criterion suggest an age non older than Paleocene for the deposits of the unit at the San Miguel section.