María Guiomar Vucetich

Review and analysis of the radiation of the south American Hystricognathi (Mammalia, Rodentia)

The fossil record of the first genera of each family of South American Hystricognathi rodents is reviewed. On this basis the main events for the radiation of the infraorder on this continent are recognized. Two main events can be identified: the first occurs probably during the Eocene-Oligocene boundary, and the second at the Middle-Late Miocene boundary.

Main Pathways of Mammalian Diversification in South America

The most complete record of the continental mammal-bearing Cenozoic of South America comes from Argentina (Marshall et al., 1983a, 1984). This record, representing the southernmost part of the continent, became a unique example in which to study the evolutionary events affecting its mammal communities in relation to the outstanding climatic and environmental changes (Pascual, 1984). The main patterns of geological evolution in this southern part of the continent had a profound influence on its mammalian history. The chronostratigraphic diagrams (Figs. 1 and 2) show the geographical and chronological distribution of Cenozoic sedimentary and volcanic rocks in eastern and western Argentina. This lithostratigraphic distributional pattern makes evident the following historical facts:

Ontogeny and diversity of the oldest capybaras (Rodentia: Hydrochoeridae; late Miocene of Argentina)

Abstract According to our previous model for interpreting the fossil record of capybaras the cheek teeth grow in width and length throughout life; flexids (especially h.s.i. and h.t.i.) deepen allometrically resulting in diverse occlusal morphologies during ontogeny; in the more derived species the ‘onset’ of flexid development is pre-displaced, and the relative depth and development rate of the flexids increase. Consequently, we interpreted the different occlusal morphologies found in the fossil record as different ontogenetic stages, leading to a drastic diminution of latest Miocene-Pliocene taxonomic diversity. The analysis of the geologically oldest capybaras, which are from the Arroyo Chasico Formation (Chasicoan SALMA, late Miocene), is added. This study suggests a single species occurs in this formation that cannot be separated at the genus level from Huayquerian species. In the Arroyo Chasicó species, which is older and theoretically more primitive than that of the Huayquerian, the flexids are shallower as predicted. The analysis supports our model of capybara dental ontogeny and evolution and encourages revision of the whole family according to this model.

Paleoclimatic implications of the presence of Clyomys (Rodentia, Echimyidae) in the Pleistocene of central Argentina

The presence of the Echimyidae Eumysopinae Clyomys in Ensenadan (Early-Middle Pleistocene) sediments cropping out northeast of Mar del Plata and south of Necochea (costal region of Buenos Aires province, Argentina) is reported. This is the first citation of Clyomys in Argentina and the first post-Pliocene record of echymids at this latitude. These remains represent a species closely related to the extant representatives of the genus, which live at least 1600 km north of the new localities. These fossils suggest that a southward expansion of the Brazilian fauna occurred during a warm and humid pulse in the Pleistocene.

A New Extinct Genus of Cavioidea (Rodentia, Hystricognathi) from the Miocene of Patagonia (Argentina) and the Evolution of Cavioid Mandibular Morphology

The family Caviidae is represented in modern faunas by cavies and maras, whereas the family Hydrochoeridae is represented by capybaras. The evolutionary origin of these families has been related to a diversity of plesiomorphic fossil forms (recorded from the late Oligocene up to the middle Miocene) traditionally grouped in the family “Eocardiidae”. These fossil forms were included, together with Caviidae and Hydrochoeridae, within the Cavioidea s.s. (sensu stricto), because they share high crowned cheek teeth, double-hearted occlusal surface, short lower incisors, and moderate hystricognathy. Within Cavioidea s.s., caviids and hydrochoerids were interpreted as forming its crown group, because they have unique craniomandibular and dental features. In this contribution, a new taxon of Cavioidea s.s. from the middle Miocene of central Patagonia, Argentina, is described, and its phylogenetic position is determined on the basis of a morphological cladistic analysis in which “eocardiids” were included. The study permits the understanding of the sequence of appearance of characters that originated the highly divergent morphology of crown-group cavioids. The analysis of the sequence of appearance of the characters that traditionally diagnosed the crown group indicates that these changes did not occur at the same time. On the contrary, many of these features seem to have appeared at different nodes of the evolutionary history of Cavioidea s.s. The remarkably derived morphology of modern cavioids is the result of a stepwise appearance of a mosaic of evolutionary innovations that originated gradually along the history of Cavioidea during the late-middle Miocene.

NEW MIOCENE RODENTS FROM PATAGONIA (ARGENTINA) AND THEIR BEARING ON THE EARLY RADIATION OF THE OCTODONTOIDS (HYSTRICOGNATHI)

Abstract Three acaremyid rodents from the Miocene of Patagonia are described. Galileomys antelucanus, gen. et sp. nov. (Colhuehuapian SALMA, early Miocene) is the most plesiomorphic genus within the Acaremyidae. Galileomys? colloncurensis, sp. nov. is the first acaremyid recorded from the Colloncuran SALMA (middle Miocene). The generic referral is tentative because the known material does not display enough characters. Acaremys cf. murinus Ameghino, 1887 represents a second Colhuehuapian acaremyid. Cladistic analyses of dental and mandibular characters do not support previous hypotheses that Sciamys and Acaremys are the ancestors of the Octodontidae. On the contrary, these two genera and Galileomys constitute a monophyletic group united by the presence of a hypoflexus in P4, a well defined anterodorsal limit of mandibular masseteric fossa, and a figure-eight-shaped molar pattern. The inclusion of other genera in the Acaremyidae is not supported by cladistic analysis. A shared figure-eight-shaped dental pattern was the primary basis for a presumed close relationship between the Acaremyidae and Octodontidae, but this state evolved independently in the two taxa. The Acaremyidae is an extinct, early radiation of the Octodontoidea that persisted at least until the middle Miocene. The Octodontidae probably originated from an ancestor closer to the “echimyids” than to the Acaremyidae, with neither normal dental replacement nor mental foramen.

First record of Dasyproctidae (Rodentia) in the Pleistocene of Argentina. Paleoclimatic implication

Abstract Plesiaguti totoi gen. et sp. nov., the first member of the Dasyproctidae recorded in the Pleistocene of Argentina, is described. It comes from the coastal cliffs of Necochea, on the southern coast of Buenos Aires Province. Taking into account its tooth morphology, pentalophodont and almost brachyodont, it represents one of the most primitive dasyproctids. The morphology of the occlusal surface suggests that P. totoi is more closely related to the living representatives of the family than to extinct species of the Oligocene–Miocene of Argentina. This phylogenetic hypothesis, plus the fact that dasyproctids vanished from these latitudes by at least the early Pliocene, argues that P. totoi is an immigrant to Buenos Aires Province. Its presence reinforces the idea of the existence of one or more warm pulses during the Pleistocene that permitted the southward migration of Brazilian fauna.

A PECULIAR OCTODONTOID (RODENTIA, CAVIOMORPHA) WITH TERRACED MOLARS FROM THE LOWER MIOCENE OF PATAGONIA (ARGENTINA)

ABSTRACT The octodontoid Caviocricetus lucasi gen. et sp. nov. from the lower Miocene (upper levels of the Trelew Member, Sarmiento Formation) of Chubut Province (Argentina), is described. It represents the first recognized case of a caviomorph rodent with terraced cheek teeth. Such morphology suggests that small invertebrates could have been an important part of the diet of this species. Such a diet is not known within the Caviomorpha. This terraced design is understood as a primitive feature, but the upper molars of this species have a derived tetralophodont pattern with the posteroloph (or the metaloph?) absent or highly reduced. This pattern represents a type of occlusal design apparently different from those previously known, and could explain the molar structure of some controversial genera, such as Platypittamys. Caviocricetus lucasi may be related to a group of genera previously assigned to the Echimyidae, but such assignment is not accepted until a revision of the Octodontoidea is undertaken.

New rodents (Mammalia) from the late Oligocene of Cabeza Blanca (Chubut) and the first rodent radiation in Patagonia

Caviomorph rodents, the New World Hystricognathi, are one of the most characteristic groups of South American mammals. Although they have been in the continent at least since the middle Eocene, those of the Deseadan Land mammal Age (early–late Oligocene) are the best source to understanding their early history due of their good record, large geographic distribution and good temporal calibration of many of the local faunas. Here, we describe the following new taxa from the classical locality of Cabeza Blanca: Octodontoidea Acaremyidae Galileomys baiosn. sp., Octodontoidea incertae sedis Ethelomys loomisin. gen.,n. comb., Acarechimys leucotheaen. sp., Protacaremys? adilosn. sp., Chinchilloidea incertae sedis Loncolicu tretosn. gen., n. sp., Incamys menniorumn. sp., Caviomorpha incertae sedis Llitun notucan. gen., n. sp., Leucokephalos zeffiaen. gen., n. sp. and Cephalomyidae Cephalomys ceciaen. sp. The DP4 of an ‘eocardiid’, Asteromys punctus? is described for the first time. These new taxa allow us to reinterpret the relationships of some of the previously known Deseadan species and genera. They show a great early diversification at least for extra Andean Patagonia, involving at least three of the main caviomorph lineages: octodontoids, chinchilloids and cavioids.

Revision of the fossil rodent Acaremys Ameghino, 1887 (Hystricognathi, Octodontoidea, Acaremyidae) from the Miocene of Patagonia (Argentina) and the description of a new acaremyid

Within South American rodents, the Acaremyidae is an independent fossil lineage of octodontoids represented in the late Oligocene–middle Miocene of Patagonia. Acaremys is represented by six species recorded in the early Miocene, which have not been re-studied since their original description. Morphological and phylogenetic analyses suggest that Acaremys is paraphyletic. Three species are valid, Acaremysmurinus, Acaremysmajor, and Acaremysmessor. Acaremyskaraikensis is a junior synonym of Acaremysmurinus. ‘Acaremys’ tricarinatus is excluded from the genus being closely related to Sciamys. The new species, Pseudoacaremys kramarzi, is closely related to ‘Acaremys’ tricarinatus and Sciamys. ‘Acaremys’ preminutus is excluded from the family being closely related to the early Miocene Protacaremys prior, or living octodontoids. The phylogenetic analysis demonstrated that Acaremyidae includes Platypittamys, Galileomys, Acaremys, Pseudoacaremys and Sciamys. The new acaremyid increases the diversity of extinct octodontoids and added a new evolutionary lineage within Acaremyidae. The evolutionary history of the superfamily suggests that the hypsodonty and the consequently occlusal simplification evolved twice within Octodontoidea: in Acaremyidae and in Octodontidae. In addition, the cladistic analysis confirmed that most character ambiguities are due to missing data, and hence, it is essential to find better remains to elucidate the relationships among acaremyids.