Biostratigraphy and biochronology of the Late Miocene of central Argentina: Evidence from rodents and taphonomy

Biostratigraphy and biochronology of the Late Miocene of central Argentina is analyzed through evolutionary patterns of octodontoid rodents. The studied faunas were found in continental sediments assigned to the Cerro Azul and Saldungaray formations (La Pampa and Buenos Aires provinces, respectively). Taphonomic attributes of recovered mammals, linked mostly to the development of paleosols or to predators’ activities, constraint the deposition lapse to short intervals. A biostratigraphic scheme based mainly on the anagenetic pattern of change in the octodontoid rodent lineage Chasichimys-Xenodontomys is proposed. Since there is no stratigraphic superposition among the studied levels, relationships among deposits are established through the polarity evidenced by the stage of evolution of chronomorphs of this lineage. Five new biozones for the Late Miocene of central Argentina are recognized. Evidence of other anagenetic sequences, and general trends of increasing hypsodonty in octodontoids, support this proposal. The results enhance the value of the “stage of evolution” concept as a correlation tool in biostratigraphic and biochronologic studies, mainly when applied through phyletic sequences.     

Dental evolution in Xenodontomys and first notice on secondary acquisition of radial enamel in rodents (Rodentia, Caviomorpha, Octodontidae)

Rodents of the subfamily Ctenomyinae differentiated during the Late Miocene in relation to the development of open biomes in southern South America. This subfamily displays a peculiar and derived dental morphology characterized by euhypsodont molars with simplified occlusal figure. We analyze both adaptive and evolutionary significance of the gross molar morphology and the enamel microstructure of ctenomyines. In accordance with the basal position of the Xenodontomys lineage, dental changes experimented by this lineage illustrate a probable evolutionary pattern of acquisition of molar design for the subfamily. We propose that morphological trends in the lower molars of the Xenodontomys lineage would include changes arisen as a by-product from hypsodonty, and other adaptive ones. These latter comprise the acquisition of a crescent-shaped occlusal morphology, and the secondary acquisition of an external layer of radial enamel in the leading edge, which would favor the development of cutting edges. Such a secondary acquisition of radial enamel had not been found so far in other rodents. The evolutionary pattern of the dental changes in Xenodontomys reinforces the idea that anagenesis is frequent in the adaptive evolution of rodents.     

A New Early Miocene Chinchilloid Hystricognath Rodent; an Approach to the Understanding of the Early Chinchillid Dental Evolution

Chinchilloidea is an emblematical group of caviomorph rodents characterized by euhypsodont, laminated cheek teeth. Recent molecular analyses proposed that the extant Dinomys (and implicitly its fossil allies) is also part of this group. Their relationships with fossil caviomorphs with less derived dental features are still obscured by the deficiency of the fossil record documenting its early dental evolution. The new genus and species Garridomys curunuquem, from the early Miocene deposits of the Cerro Bandera Formation, northern Patagonia, is here described. It is represented by numerous mandible and maxillary remains with dentition. This species has protohypsodont cheek teeth with three transverse crests in all ontogenetic stages arranged in a transitory S-shaped pattern, resembling putative early dinomyids. Garridomys curunuquem is here interpreted as the sister group of the clade including the living and fossil chinchillids; both chinchillas and viscaccias would have diverged from a Garridomys-like ancestor and acquired hypsodonty independently. Garridomys and other chinchilloids would have diverged from the lineage leading to chinchillids in pre-Oligocene times, suggesting a very early, still poorly documented chinchilloid radiation.     

Deciduous dentition and eruption pattern in late Miocene Pachyrukhinae (Hegetotheriidae,Notoungulata) from La Pampa Province,Argentina

The study of juvenile remains of Paedotherium Burmeister from Cerro Azul Formation (La Pampa Province, Argentina; late Miocene) is presented. Upper and lower deciduous dentition (or permanent molars supposed to be associated with non-preserved deciduous teeth) are recognised. Several ontogenetic stages are distinguished among juveniles, according to the degree of wear and the replaced deciduous teeth. Besides, some morphological and metrical differences are observed along the crown height. Deciduous cheek teeth are high-crowned and placed covering the apex of the corresponding permanent tooth. The height of the crown and the degree of wear allow establishing the pattern of dental replacement of deciduous and permanent premolars in a posterior–anterior direction (DP/dp4–2 and P/p4–2), as well as the eruption of M/m3 before DP/dp4 is replaced. Some of the studied remains are recognised as young individuals of Tremacyllus Ameghino, but with complete permanent dentition, which leads to propose a different timing in the dental replacement with respect to Paedotherium; they also allow the establishment of an opposite premolar eruption pattern, from P/p2 to P/p4. This knowledge of the deciduous dentition of Paedotherium suggests the need of revising the morphological and metrical characters previously used for defining species within this taxon.     

First complete skull of an octodontoid (Rodentia, Caviomorpha) from the Early Miocene of South America and its bearing in the early evolution of Octodontoidea

Octodontoidea is the most diverse group of caviomorph rodents. The systematics of most of the fossil representatives has been essentially based upon dental characters. Described here is an almost complete skull with dentition assigned to Prospaniomys Ameghino based upon its dental morphology. The specimen comes from the Sarmiento Formation at Pampa de Gan Gan (central Patagonia, Argentina), assigned to the Colhuehuapian SALMA (early Miocene). The most remarkable features are in the posterior portion of the skull, some of them shared with the modern octodontids and interpreted as specialized by previous authors, which contrast with the generalized dental morphology. These combined features were not previously known in other octodontoids. The comparisons with other fossil and extant members of the superfamily suggest that the characters traditionally used to associate Prospaniomys with the echimyids are very probably plesiomorphies. Prospaniomys would represent an early diverging lineage more closely related to modern octodontids than to echimyids, in which cranial structures evolved more rapidly than dental and mandibular ones.     

Chronological implications of the nothrotheriid ‘Xyophorus’ (Mammalia,Xenarthra) from the Collón Curá Formation (Miocene of Patagonia,Argentina)

The specimen described herein and assigned to ‘Xyophorus’ sp. (Mammalia, Xenarthra, Tardigrada) was collected in the locality Cerro Zeballos, northwestern Chubut Province, Argentina. The fossiliferous sediments bearing the specimen are correlated with Collón Curá Formation. The specimen has the features described for other members of ‘Xyophorus’ (e.g. shape and size of the molariforms, relationship between diastema length, m1 and m2 length) and has a Diastema Length/Tooth Row Length index (DL/TRL index) of ca. 14, between that of ‘X.’ villarroeli (12.07) from the Mauri Formation, Bolivia (ca. 10.3 Ma) and that of ‘X.’ bondesioi (16.45) from Arroyo Chasicó Formation, Argentina (ca. 10–8.7 Ma). The relationship between DL/TRL index and age of the bearing sediments, would suggest a Tortonian age (late Miocene) for the deposits of Collón Curá Formation at Cerro Zeballos, which results in a ‘younger age’ compared to the middle Miocene age traditionally accepted for the Collón Curá Formation bearing the Colloncuran fauna sensu stricto. Although no absolute ages for Cerro Zeballos are available yet, the geographic proximity of Cerro Zeballos to Cushamen River (with levels dated at ca. 11.2 Ma) supports the tentative Tortonian age indicated by the presence of ‘Xyophorus’ sp.     

ADDITIONAL SPECIMENS OF SUDAMERICID (GONDWANATHERIA) MAMMALS FROM THE EARLY PALEOCENE OF ARGENTINA

Abstract: The extinct, Cretaceous–Paleogene Gondwanatherians have previously been considered to be early xenarthrans, multituberculates and more recently Mammalia incertae sedis. However, the phylogenetic relationships of Gondwanatheria have yet to be resolved. In this paper, additional dental specimens of the gondwanatherian Sudamerica ameghinoi from the Early Paleocene Salamanca Formation of Argentina are described. These specimens provide additional information on Gondwanatheria affinities, sudamericid morphology and help support earlier hypotheses on Sudamerica dental formula and tooth categories. Sudamericid dental functional morphology and body mass estimates, based on measurements of isolated teeth, are inferred. Dental morphology such as hypsodonty, enamel microstructure and crown features do support a robust clade for Sudamericidae.     

Leftover prey remains: a new taphonomic mode from the Late Miocene Cerro Azul Formation of Central Argentina

The Cerro Azul Formation (La Pampa Province, Argentina) comprises a rich vertebrate fauna of small mammals dominated by notoungulates and rodents. The fauna pertains to the Late Miocene specifically to the Huayquerian Stage/Age. Taphonomic analysis of micro‐mammals from Estancia Ré locality evidenced that the faunal assemblage was accumulated by the activity of a predator. This assemblage was compared with others from the Cerro Azul Formation in Telén and Caleufú localities (La Pampa Province), previously interpreted as products of predator activities. These microfossil accumulations differ from assemblages attributed to pellets and faeces produced by modern predators (nocturnal and diurnal bird raptors and carnivore mammals). However, due to their anatomical representation, degree of bone breakage and presence of tooth marks, they are interpreted as accumulations of uneaten prey remains discarded by the predator. The predator involved could not be determined with certainty, although the presence of tooth marks in some skeletal remains and the presence of coprolites in Telén and Caleufú suggest that it could be a carnivore mammal. Similarities in the accumulation mechanism, patterns of preservation and sedimentary contexts in the three assemblages support the recognition of a new taphonomic mode, termed ‘leftover prey remains’.     

Hypsosteiromys (Rodentia, Hystricognathi) from the Early Miocene of Patagonia (Argentina), the only Erethizontidae with a tendency to hypsodonty

Hypsosteiromys is the only New World porcupine that shows a tendency to hypsodonty. It is recorded exclusively from the Colhuehuapian Age (Early Miocene) of central Patagonia (Argentina). In addition to the type species, a second one, Hypsosteiromys nectus(AMEGHINO, 1902), is recognized, from the southern cliff of Colhuehuapi Lake (Chubut province). It differs from the type species in the lesser development of the anterolabial and posteroflexid notches of the m1-3, shorter p4 and dp4, and more slender incisors. Dental morphology suggests that the species of Hypsosteiromys lived in more open areas than most fossil and living Erethizontidae.     

Vertebrate taphonomy in loess-palaeosol deposits: A case study from the late Miocene of central Argentina

This paper deals with the taphonomic analysis of the late Miocene bone assemblage from the Cerro Azul Formation at Telén (La Pampa Province, Argentina). The faunal assemblage was assigned to the Huayquerian mammal age (late Miocene). The fossiliferous section shows a homogeneous lithology, and is interpreted as a loess deposit with two similar and slightly developed palaeosols, classified as calcic vertisols. The studied sample comprises 5598 remains anatomically and taxonomically determined. They were collected from an area of about 48,000 m², appearing randomly distributed through the section and with low density. Most remains are small- to very small-sized, disarticulated, and very fragmented. Different taphonomical histories are inferred for microvertebrates and macromammals. The microvertebrate assemblage is interpreted as the result of predator activities. After a brief period of pre-burial exposure, remains were dispersed from the original depositional area. On the other hand, a natural and gradual death process is envisaged for macromammals, followed by a long period of exposure to weathering and dispersal by physical agents. Remains of both groups, once buried, suffered the diagenetic processes of the host rock. Consequently, the fossil assemblage from Telén would represent a condensed assemblage corresponding to two distinct time spans, i.e., the accumulation of microvertebrates took place in a short time interval whereas that of macromammals occurred over a longer period, coincident with the development of both soils.     

Systematics,phylogeny and evolutionary pattern of the hystricognath rodent Eumysops (Echimyidae) from the Plio–Pleistocene of southern South America

?Eumysops is a peculiar representative of the currently tropical family Echimyidae, which evolved in increasingly dry and cold Plio–Pleistocene environments of southern South America. The results of a systematic and stratigraphic review of the genus, and of phylogenetic analyses based on both morphology and a combined morphological–molecular dataset in the context of extant representatives, are presented here. Recognised diversity includes four previously described species plus a new one from the late Pliocene. These species form a well-supported monophyletic clade, sister to the late Miocene ?Pampamys and the extant Thrichomys. The position of ?Eumysops–?Pampamys–Thrichomys in a major clade including non-‘eumysopine’ echimyids constrains the traditional taxon Eumysopinae only to these three genera. Phylogeny and stratigraphic distribution of ?Eumysops species suggest an essentially cladogenetic evolutionary pattern. Beyond this, a gradual directional change, involving increase in size and in molar hypsodonty, is shown by ?Eumysops chapalmalensis as part of a late Pliocene faunal turnover interpreted as a local representation of the 2.5-Ma cooling global event. Distinctive skeletal and dental anatomy of ?Eumysops, including large orbits, shortened braincase, marked hypsodonty and postcranial specialisations, would be a result of its southern history related to a particular palaeoclimatic context.     

Oldest record of the Great White Shark (Lamnidae,Carcharodon; Miocene) in the Southern Atlantic

Lamnid teeth close to but more plesiomorph than Carcharodon carcharias were collected in the late Miocene beds of the Paraná Formation in the central eastern Argentina. We propose, as other authors, that some species formerly assigned to Isurus or to Cosmopolitodus should be included in Carcharodon. Some workers suggested that C. carcharias originated by phyletic evolution in the Pacific basin. The teeth from Paraná could pertain to a new species of Carcharodon already known from Perú. In contrast with the Pacific basin, Carcharodon sp. was sympatric with the wide toothed species Carcharodon plicatilis in central Argentina. This is the oldest record of Carcharodon in the southern Atlantic.     

A new species of Gaimanophis (Serpentes,Boidae) from the Miocene of northwestern Argentina with remarks on the Neogene boids of South America

Gaimanophis is an extinct boid genus represented so far by a single species (Gaimanophis tenuis) known by isolated vertebrae from the early Miocene of Patagonia. In this paper, a new species of Gaimanophis is described from the India Muerta Formation (late Miocene) of Tucumán province (Argentina). Gaimanophis powelli sp. nov. distinguishes itself from G. tenuis mainly in its larger size, prezygapophyses less slanting, neural spine shorter dorsally than ventrally, and zygosphene straight bearing an anteriorly protruding tongue. This record indicates a wider temporal and geographical distribution of the genus from the early Miocene of Patagonia to the late Miocene of northwestern Argentina. The recognition of a new species of boid in South America increases the known diversity of this group. Although boids have inhabited in this territory since the Paleocene, fossils belonging to the group only show a glimpse of the real past diversity in the continent.     

Morphological abnormalities in Late Cretaceous and early Paleocene foraminifer tests (northern Patagonia, Argentina)

Morphological abnormalities are common in Late Cretaceous and early Paleocene foraminifer tests at two localities in northern Patagonia, Argentina. Protelphidium sp. in the Auca Mahuevo section (late Campanian–early Maastrichtian) exhibit abnormal size or shape of the later chambers, with the last chamber commonly larger than normal or inflated and variably extending onto one of the lateral sides of the test; modification of the coiling plane; protuberances near the proloculus or on one or more chambers; a double last chamber, and complex forms. Protelphidium hofkeri Haynes in the Cerro Azul section (Danian) exhibit abnormal size or shape of one or more chambers, producing peripheral irregularities. In addition, there are rare multiple tests in planktic species from the Cerro Azul section, probably teratological specimens. The sedimentology of the sections and the character of the accompanying faunas indicate that the abnormalities in the two benthic foraminiferal taxa were most probably caused by hypersalinity and/or fluctuations in salinity. The fossil occurrence of assemblages with abundant deformed specimens suggests that investigators should carefully look at many aspects of the environment before concluding that anthropogenic pollution is the only cause of deformations of living benthic foraminifera.     

The most ancient caviine rodent (Hystricognathi,Cavioidea) comes from the late Miocene of Northwest Argentina (South America)

The family Caviidae is one of the most diverse groups among South American hystricognath rodents and is represented by three main living lineages: Caviinae (cavies), Dolichotinae (maras) and Hydrochoerinae (capybaras). Caviinae includes the smaller forms of caviids represented by the extant Microcavia, Cavia and Galea. They are distributed in a wide range of environments throughout South America. In addition, three other genera from the late Miocene–Pliocene (Dolicavia, Palaeocavia and Neocavia) are recognised in high latitudes. In northwestern Argentina, the fossil forms of Caviinae have been poorly studied and for most of them there is no precise stratigraphic information. We describe and evaluate the phylogenetic affinities of the most ancient caviine from the Chiquimil Formation, Catamarca province, northwestern Argentina (9.14–7.14 ma). According to the morphological analysis of the mandibular and dental morphology and the results of the phylogenetic analysis, we assigned the new species tentatively to genus Palaeocavia. The phylogenetic position of the new species suggests an earlier origin for the lineage Palaeocavia + Cavia and for the entire clade Caviinae.     

First fossil record of the catfish genus Sorubim (Siluriformes,Pimelodidae), from the Late Miocene of Paraná, Northeastern Argentina

The aim of the present contribution is to describe a partial fossil skull belonging to the catfish genus Sorubim. The specimen originates from the Late Miocene Ituzaingó Formation, at Paraná locality, Entre Ríos Province, Argentina. The material described here (MACN Pv-14224) was determined just to genus because the living species of Sorubim are morphologically similar. Presence of Sorubim is in concordance with the hypothesis indicating that most extant pimelodid genera were already present by the Late Miocene. Present finding constitutes the first fossil record for the genus.     

<Emphasis Type="Italic">Gryposuchus</Emphasis> (Crocodylia,Gavialoidea) from the early Miocene of Venezuela

Here, a fragment of a mandible recently discovered in the Cerro Zamuro site (Castillo Formation, Lara State, northwestern Venezuela) is assigned to the giant gavialoid Gryposuchus. This specimen, recovered from putative brackish environments of the early Miocene (~18 Ma) age, is unequivocally the earliest record of the genus in South America. Gryposuchus, together with the other gryposuchine previously recognized from the Castillo Formation, Siquisiquesuchus venezuelensis, increases the early Miocene taxonomic diversity of the group in the northern Neotropics. This new information from the Castillo Formation supports the conclusion that early gryposuchine evolutionary stages were in coastal, shallow marine or brackish environments, while the presence of some genera, such as Gryposuchus, in middle to late Miocene freshwater environments, is secondary habitat colonization late in the evolution of the clade. Freshwater colonization is probably the result of the gradual adaptation of early marine-adapted gryposuchines to the extensive estuarine-like environments of northern South America lowlands associated with marine transgressions that systematically occurred during the middle Eocene to early Oligocene. This new record is evidence of the wide chronological distribution of Gryposuchus in northern South America, highlighting the importance of this area as the center of origin and radiation of this successful Miocene gavialoid.     

The southernmost record of a Neuryurini Hoffstetter, 1958 (Mammalia,Xenarthra, Glyptodontidae)

In this contribution we document the southernmost and probably the oldest record of the tribe Neuryurini (Glyptodontidae). The southernmost record of this tribe outside of Patagonia is Neuryurus sp. from Lujanian sediments (Luján Formation, late Pleistocene–early Holocene) at Quequén Grande River (Buenos Aires Province, Argentina). The first and only previous record of a specimen of the Neuryurini in Patagonia was reported from the upper levels of the Puerto Madryn Formation (Huayquerian SALMA, late Miocene), exposed in Península Valdés (Chubut Province, Argentina). The new specimen described here and assigned to an indeterminate Neuryurini was recovered at Estancia La Peninsular (Santa Cruz Province, Argentina). This locality has been recognized in geological studies as part of the Santa Cruz Formation (Santacrucian SALMA, late early Miocene), but there have been no fossils described from this site until now. Therefore, the new specimen represents, with the two confirmed records of Glyptodon Owen and Panochthus Burmeister (both from the Quaternary), the only non-Propalaehoplophorinae glyptodontids reported for Santa Cruz Province.