New observations on the holotype of the Maastrichtian aristonectine elasmosaurid Aristonectes parvidens Cabrera from Chubut Province, Argentina, reveals features that were not previously described while resulting in new interpretations. Quantitative comparison with non-aristonectine elasmosaurids demonstrates that the increase in the number of alveoli in the premaxilla, maxilla and mandible is not a consequence of skull size increase. Instead, decrease in alveolar size and interalveolar space, compared with that observed in non-aristonectine elasmosaurids, is at least as important. Increase in skull length compared with non-aristonectine elasmosaurids is not as marked as typically considered: skull length is equivalent to the summatory of length of the first 8–10 cervical vertebrae in non-aristonectines and the first 10–12 in aristonectines. The cervical region is characterized by a rapid increase of the relative broadness of the vertebral centra, which indicates a reduction in lateral mobility of the neck. Increase in alveoli number is achieved as a result of a number of changes that seem to indicate the importance of the biological role of the length of the alveolar row, this is probably related to changes in prey size and capture strategy such as trading for smaller fishes or invertebrates and/or a upgrading from ambushing one individual prey to simultaneously ambushing multiple prey individuals.

Since its original description nearly two hundred years ago, the late Pleistocene (Ensenadan—Lujanian SALMA) ground sloth genus Mylodon from South America has had a variety of taxonomic issues. The end result is that diagnostic characters for the genus and its only species, M. darwinii, include only skull, tibia, and astragalus features. While other postcranial remains have been found, they have been too fragmentary to provide any significant taxonomic characters. The rediscovery of a specimen of M. darwinii (previously listed as Glossotherium robustum) collected in the mid-1920s near Orient in Buenos Aires Province, Argentina and housed in the Field Museum of Natural History, provides an opportunity to expand the diagnosis for this taxon based on diagnostic skull and astragalus remains that are associated with an atlas and associated vertebrae, and most of the left forearm, carpus, and other elements of the manus. These bones all show features that distinguish M. darwinii from contemporaneous mylodontids (e.g., Catonyx, Glossotherium, Lestodon, Scelidotherium), as well as some simple differences in size. The suite of currently known morphological characters and reconstructed muscle attachments also indicate a general functional niche for M. darwinii. They suggest that its body weight was borne on the lateral side of the manus rather than the knuckles or unguals, and that Mylodon may have had a relatively high degree of manual dexterity or digital strength.

Although sexual size dimorphism is an important component of the biology of many living New World metatherians, little is known about sexual dimorphism in extinct members of this group. Here, we describe morphometric variation in two traditionally recognized species of the paucituberculatan Acdestis from the early Miocene Santa Cruz Formation of Patagonia, A. owenii and A. spegazzinii (=A. lemairei). The results of this morphometric study show that specimens of Acdestis from the Santa Cruz Formation form a single morphological continuum, with no features that clearly distinguish the two recognized morphotypes besides overall body size and the depth of the mandibular ramus, both features which are sexually variable in extant marsupials. This suggests that the two species of Acdestis from the Santa Cruz Formation are better regarded as gender morphs of a single dimorphic species rather than two highly similar monomorphic species. The degree of sexual dimorphism inferred here for Acdestis is comparable to that seen in living sexually dimorphic marsupials. The identification of sexual dimorphism in Acdestis suggests that extinct paucituberculatans may have exhibited much greater variation in their ecology and life history than currently predicted based on extant members of this group.

Calmoniid trilobites of the “Metacryphaeus tuberculatus group” occur in many Devonian localities within the Malvinokaffric Realm. In the Parnaíba Basin, in northeastern Brazil, the group is represented by Metacryphaeus tuberculatus Kozlowski, 1923, and M. meloi Carvalho, Edgecombe and Lieberman, 1997, from the late Eifelian—early Givetian Pimenteira Formation in the environs of the city of Picos and the early Givetian Passagem Member (Pimenteira Formation) from the vicinities of Picos and Pimenteiras in Piauí State. Based on specimens found in the region of João Costa, southeastern Piauí State, the present study reinforces data on the occurrence of M. tuberculatus in the Pimenteira Formation and reports the presence of genal spines in some individuals of M. meloi, indicating a feature previously unknown for this species.

Rare nepiomorph bivalves from the Silurian and the Devonian beds of the Central Andes Basin of Western Gondwanaland are herein presented. They were collected in four scattered outcrops in the Eastern Andes Cordillera and southern Interandean Bolivia. A total of 58 specimens of medium preservation were studied. Almost all of them are Ludlowian to Pridolian in age: Praecardium sp. A, Cardiola fluctuans Barrande, Dualina secunda Barrande, and one species is of Eifelian age: Praecardium sudamericanum (Kozlowski). This is both, the first report of the genus PraecardiumBarrande of Ludlowian to Pridolian age in this part of Gondwanaland and the first occurrence of the species Cardiola fluctuans Barrande in Bolivia. Through findings in the centre of the Silurian—Devonian Central Andes Basin (Pongo and Chaquimayu localities), this paper confirms the cosmopolitism of Dualina secunda Barrande. This research contributes to the as yet inaccurately known Silurian and Devonian paleogeography of the Central Andes Basin and to update old systematic descriptions of nepiomorph bivalves from late 19^th and early 20^th centuries.

The bivalve Retrotapes antarcticus, from the Eocene of Antarctica, includes two different morphotypes of which one presents flat valves while the other one exhibits globoid valves. Previous authors have identified the same two morphotypes in the extant species Retrotapes exalbidus. This paper assesses the flat versus globoid morphotypes via comparing the two species by analyzing Elliptic Fourier Contours of 17 valves of R. antarcticus and 40 valves of R. exalbidus. A principal component analysis was performed and the age of both morphotypes was estimated by way of counting growth lines. Several authors have argued that genetic differences between the two forms of R. exalbidus are not sufficient to separate them into two distinct species and suspected that the globoid morphotype may correspond to gerontic specimens. Our results demonstrate that the morphological transformation detected in R. antarcticus is similar to that observed in R. exalbidus. The analysis of growth lines confirms that the globoid morphotype corresponds to gerontic specimens in both species. This constitutes an example of gerontic intraspecific variation. Additionally, this contribution allows us to further understand the life-history of the fossil bivalve R. antarcticus for it presents a growth pattern akin to that of the living species.

This paper reports on the biostratigraphic, paleobiogeographic and paleoecologic aspects of palynoflora recovered from sedimentary rocks preliminarily assigned to the Lago Colhué Huapi Formation, in the Golfo San Jorge Basin. The present palynological assemblage includes Maastrichtian marker species of the austral Proteacidites/Nothofagidites Province such as Quadraplanus brossus and Tubulifloridites lilliei. Typical species of the Maastrichtian paleotropical Palmae Province, Buttinia andreevi and Gabonisporis vigourouxii, are also recognized in the palynoflora. These facts indicate the mixed character of the paleoflora and the prevalence of transitional climatic conditions at the paleolatitude of the Chubut Province in the uppermost Cretaceous. The palynoflora composition suggests the local existence of a warm and low energy freshwater body.