Cerro Azul Formation Research Articles

Dinomyid (Rodentia, Caviomorpha) diversity from the Late Miocene (Chasicoan Stage/Age) Cerro Azul Formation at the classical Arroyo Chasicó locality (Argentina)

ABSTRACT Dinomyidae is a South American group of caviomorph rodents with a single extant species (Dinomys branickii), but with a rich fossil record including medium-sized to giant species, which reached its peak diversity during the Late Miocene. The dinomyids from the Cerro Azul Formation (late Middle Miocene to Early Pliocene) have been studied in detail from localities in La Pampa Province, Argentina. In this contribution, dinomyids from Chasicoan Stage/Age (Late Miocene) levels of the Cerro Azul Formation at Arroyo Chasicó locality (Buenos Aires Province, Argentina) are revised, including previously published material and unpublished ones. We confirm the presence of Tetrastylus sp. Diaphoromys gamayensis, and define two new taxa, Bondesiomys chasiquensis gen. et sp. nov. and Gyriabrus sokka sp. nov. The diversity of dinomyids from the Cerro Azul Formation at Arroyo Chasicó is higher than other Chasicoan faunas at the same latitude, identified in localities of La Pampa Province, since in Cerro Patagua there are not Dinomyidae records and in Cerro la Bota Tetrastylus sp. and Dinomyidae indet. were the only recorded taxa.

Atypical holocene ctenomyid rodent burrows related to eolian dune deposits from central Argentina

This contribution aims to interpret the producers and the paleoenvironmental context of Holocene rodent burrows (and associated ichnofossils) from the Late Pleistocene-Holocene Meaucó Formation, central La Pampa Province, Argentina. Fieldwork included a detailed sedimentary logging, a ground penetrating radar survey and documentation of the fossil burrow systems. The Meaucó Formation rest on erosive unconformity over the Upper Miocene-Lower Pliocene Cerro Azul Formation and display a vertical change from eolian dunes that become sparser up section (as imaged by ground penetrating radar) and were capped by a paleosol. The paleosol is covered by recent eolian dune and interdune deposits. The fossil rodent burrows occur up to 1.7 m below the paleosol and share several morphologic features with modern Ctenomyidae burrows, except the high slope of entrance and internal ramps, and the maximum depth of burrows. To explain these differences, it is proposed that the burrows were excavated by ctenomyids in ancient sloping surfaces of eolian dunes, which resulted in increased inclination and apparent maximum depth of the systems. This is an example of influence of environmental conditions on the architecture of rodent burrows, and can be of help when interpreting other similar fossil examples. The paleosol community associated with the fossil vertebrate burrows also included other organisms recorded by their trace fossils: horses (Hippipeda isp.), a scrubby vegetation (rhizoliths), coleoptera or spiders (Skolithos linearis), cicada nymphs (Feoichnus challa) and ants (Daimoniobarax isp.).

Vertebrates from the Late Miocene of Salinas Grandes de Hidalgo (central Argentina). New faunal data: biochronological and paleoenvironmental considerations

ABSTRACT Salinas Grandes de Hidalgo (La Pampa Province) is a relevant locality of the Cerro Azul Formation in central Argentina because of its abundant and diverse vertebrate remains, including representatives of Reptilia, Aves and mostly Mammalia. Concerning previous records, the present faunal list adds one Argyrolagidae marsupial, one Ctenomyidae and two Caviidae rodents, as well as the first detailed analysis and description of glyptodonts from the Cerro Azul Formation in La Pampa Province. New geological information is added about the two fossiliferous outcrops included in Salinas Grandes de Hidalgo area. The classically studied outcrop located in the saline lake and the nearby one in the so-called ex-Autódromo, both herein correlated based on lithostratigraphy, particularly by the presence of paleosols in the intermediate section of the sedimentary succession. These sandy silts deposits in the base of the outcrops indicate the development of poorly confined flows and intervals of surface stabilisation by paedogenesis. We offer a biochronological interpretation based on a recently published U-Pb detrital zircons dating (5.9 ± 0.4 Ma) and the presence of significant taxa such as octodontoid rodents recovered from the dated levels and below it. From a paleoenvironmental viewpoint, we infer variations from woodlands to open environments.

Cyonasua zettii sp. nov. (Procyonidae, Mammalia) from the Late Miocene of Central Argentina and a review of the fossil record of Cerro Azul Formation

ABSTRACT A detailed taxonomic study of all known fossil specimens assigned to Procyonidae from sediments of Cerro Azul Formation (early Huayquerian Stage/Age, Late Miocene) outcroping at La Pampa and Buenos Aires provinces (Argentina) is presented here. Our study led us to propose a new species, Cyonasua zettii sp. nov. coming from La Pampa Province. In addition another six specimens studied, coming from La Pampa and Buenos Aires provinces, are assigned to Cyonasua longirostris and four specimens are assigned to Cyonasua sp. because its fragmentary nature prevents a determination at species level. The specimens described here are among the oldest records for the Order Carnivora (Mammalia) in South America.

Late Miocene–Early Pliocene mammals from the Tupungato–Piedras Coloradas Anticlines, Mendoza, Argentina

New research in the petroliferous area of Tupungato, Mendoza Province, central-western Argentina, has revealed a much richer fossil assemblage than previously known for this zone. First mammal remains from this area go back to the 1930s, but only three taxa were recognized since then, two notoungulates, the mesotheriid Typotheriopsis silveyrai and the hegetotheriid Hemihegetotherium achathaleptum, and the caviid rodent Cardiatherium paranense. A recent project, focused on geological and paleontological aspects of the Tupungato–Piedras Coloradas Anticlines, has allowed us to improve the geological knowledge of the area and increase its faunal diversity. This is one of the best localities for the study of the uppermost section of the Cacheuta foreland basin, and we present the first detailed section of the Río de los Pozos Formation (including part of previously considered Mogotes Formation). Concerning fossils, xenarthrans (sloths and armadillos), pachyrukhine hegetotheriids, a toxodontid, and different families of rodents (Chinchillidae, Octodontidae, Caviidae Dolichotinae, and Abrocomidae) are recognized for the first time in this locality, together with new remains of T. silveyrai, H. achathaleptum, and C. paranense. The comparison with Late Miocene–Pliocene Argentinean faunas reveals general similarities with most assemblages assigned to the Huayquerian Stage/Age, somewhat closer to those from the Cerro Azul Formation, and there are also shared taxa with Early Pliocene faunas. The biostratigraphic data presented here agree with published ages obtained for the Río de los Pozos Formation in the La Pilona Anticline.

New light on the late Middle Miocene–Early Pliocene representatives of Lagostomus (Chinchillidae, Rodentia) from Cerro Azul Formation, Argentina

Lagostominae (Rodentia, Caviomorpha, Chinchillidae) include a single living species, the plains vizcacha Lagostomus maximus, but representatives of this subfamily are registered since the Early Miocene. The oldest record of this genus was recently extended to the late Middle Miocene in one locality of center of La Pampa Province. Previously, from levels of the Late Miocene Cerro Azul Formation in Buenos Aires and La Pampa provinces, two species of Lagostomus were recognized based on specimens from two localities, L. telenkechanum from Arroyo Chasicó (Buenos Aires Province) and L. pretrichodactyla from Salinas Grandes de Hidalgo (La Pampa Province). In the present work, new materials from these two and other localities of the Cerro Azul Formation are analyzed. This study supports the presence of both species, which had coexisted in several of the analyzed assemblages. The studied sample includes several incomplete specimens and others interpreted as juvenile individuals, which can only be determined as Lagostomus sp. Also, a similar behavior to that of the extant L. maximus (i.e., gregarious populations with fossorial habits) is proposed to explain the different representation of Lagostomus in each assemblage. In Cerro Azul Formation the presence of representatives of Lagostomus is mainly linked with levels with paleosols and accumulations by predator activity. Finally, we conclude that this genus has less biostratigraphic value than previously thought, at least for central Argentina.

Neogene communal rodent burrow systems from central Argentina

ABSTRACT Fossil tetrapod burrows from the Cerro Azul Formation (Late Miocene-Early Pliocene) of southwestern Buenos Aires Province, Argentina, are described and interpreted. The local sedimentary sequence is composed of massive siltstone, with two interbedded calcareous palaeosols, that were deposited in a loessic plain under a semiarid climate. The burrow systems are recognised as Maneraichnus pampeanum igen. and isp. nov., an ichnotaxon that is typified by long ramps that lead to a main chamber located at bifurcation, with chambers forming at least one-third of the total tunnel/ramp volume. A rodent producer is indicated by the presence of interconnected burrow systems with a subcircular cross-section (average RDI = 0.98) and by paired, blunt, incisor traces. The large ratio of chamber volume to tunnel/ramp volume (0.45) is indicative of a communal species and a body mass ranging from 0.9 to 4.6 kg is inferred for the producer. Considering the mentioned constrains and the fossorial mammals of the Cerro Azul Formation, it is proposed that the burrow systems belonged to Lagostomus (Late Miocene-Recent). The fossil burrows also match extant L. maximus (‘vizcacha’) burrow systems in overall architecture, passive fill, and similar size of claw traces. Miocene species of Paedotherium (Notoungulate) is discussed as an alternative potential digger.http://zoobank.org:act:C3588BE1-BAAF-46D7-92FA-0ADEF295AED3 http://zoobank.org:act:2A9F3301-9EC8-4FD1-B875-078CD9324049

The oldest Andean synorogenic fossiliferous deposits of the Miocene Cerro Azul Formation, central Argentina

Cerro Patagua is a locality with fossiliferous outcrops of the Cerro Azul Formation in La Pampa Province (central Argentina). First fossil remains were known in 1999 and assigned to the Chasicoan Stage/Age (Late Miocene, Tortonian) after the presence of the Octodontoidea rodent Chasichimys bonaerense. Later references to vertebrate remains were scarce and mostly limited to some mentioned taxa in broader faunal contexts. Now, we present a detailed taxonomic study of the entire mammal assemblage from Cerro Patagua, which evidences that most taxa are typical of the Chasicoan Stage/Age. A peculiar enterolith-like structure, recovered from loess levels, is also described, and several taphonomic aspects observed on fossils are related to their level of provenance. In addition, we offer stratigraphical interpretations of the Cerro Azul Formation and a geomorphological analysis of the Cerro Patagua area, and provide a detrital zircon dating of 12.3 ± 1.8 Ma. This result reflects the time interval 14.1–10.5 Ma, a temporal range that includes the Serravalian Stage (late Middle Miocene) and could indicate that the oldest synorogenic deposits could have begun to accumulate at the end of this stage. Even considering the minimum age of this interval (Tortonian Stage), this dating is the oldest obtained for the Cerro Azul Formation, compared with previous published data from Arroyo Chasicó locality (9.43–9.07 Ma). This assignment could explain that taxa from Cerro Patagua show affinities with Late Miocene mammal assemblages from central Argentina, as well as with older faunas from Patagonia and Bolivia.

On the validity of the Macrochorobates scalabrinii Biozone (early Huayquerian Stage/Age, Late Miocene). Multi-proxy analysis of the enigmatic Arenas Blancas site (Buenos Aires Province, Argentina)

Arenas Blancas is a poorly known fossiliferous site located in the lower reach of the Chasicó creek (Buenos Aires Province, Argentina), with great relevance from a biostratigraphic viewpoint. The Macrochorobates scalabrinii Biozone was defined in this site, proposed as the biostratigraphic basis of the early Huayquerian Stage/Age (Late Miocene); however, the geological context and faunal record of this site have never been studied in detail. In this work, we perform a multi-proxy analysis of the Arenas Blancas site, as well as a nearby site here called Curva de la Vaca, and provide new interpretations on their origin, age, and biostratigraphy. Sedimentological, stratigraphic, and geomorphological characteristics of both sites suggest that the sequences include fluvial/alluvial deposits that represent Quaternary terraces. The taxonomic analysis of the Arenas Blancas faunal assemblage evidences the presence of 14 mammal taxa, together with some fishes, reptiles, and birds; the assemblage is correlated with the assemblage from the Cerro Azul Formation assigned to the Chasicoan Stage/Age (Late Miocene), also recovered from the lower reach of the Chasicó creek. The use of the taxa proposed in previous works to characterize the Macrochorobates scalabrinii Biozone (including this species) is not supported due to their taxonomic status and/or temporal distribution. Taphonomic evaluation indicates that the assemblage is constituted by reworked specimens; in this frame, it is proposed that fluvio/alluvial events, occurred during the Late Pleistocene-Holocene, eroded the Upper Miocene substrate and reworked fossil remains. Based on the present evidence, the Macrochorobates scalabrinii Biozone is here rejected as a valid biostratigraphic unit.

Updated data on Litopterna from the Huayquerian Stage/Age (Late Miocene-Early Pliocene) of central-east Argentina

Hitherto, only the Proterotheriidae Ameghino, 1887 litopterns from the Cerro Azul Formation (Chasicoan-­Huayquerian Stages/Ages; Late Miocene-Early Pliocene) in La Pampa Province had been studied, recognizing Proterotheriidae indet. at Cerro La Bota, Diplasiotherium pampa Soria, 2001 at several localities,

New cranio-dental remains of Nothrotheriinae (Mammalia, Xenarthra, Folivora) from the Late Miocene of Central Argentina

ABSTRACT Among extinct sloths, Nothrotheriidae nothrotheriines are characterised by caniniforms (when present) separated by a diastema from the molariform tooth row and quadrangular, rectangular or trapezoidal molariforms with longitudinal grooves on the lingual and labial surfaces. The subfamily Nothrotheriinae is recorded from the Middle Miocene of Bolivia and Argentina to the Pleistocene of different regions of South America, Central America and North America. Neogene well-known representatives include Pronothrotherium, Huilabradys, Mionothropus, Aymaratherium, Lakukullus and the recently defined genus Mcdonaldocnus, whose remains were found from Argentina, Colombia, Uruguay, Peru, Brazil and Bolivia. Mcdonaldocnus includes materials from Argentina and Bolivia previously assigned as ‘Xyophorus’. New records from Late Miocene levels of the Cerro Azul Formation cropping out in the Chasicó creek locality, Buenos Aires Province, and in the localities of Telén and Loventué, La Pampa Province, allow us to describe cranio-dental remains of Nothrotheriinae Mcdonaldocnus bondesioi and Mcdonaldocnus sp., respectively. The identification of Mcdonaldocnus sp. in the Cerro Azul Formation at La Pampa Province constitutes the first record of Nothrotheriinae for this province.

A LATE MIOCENE MAMMAL ASSEMBLAGE REVEALS A COMPLEX TAPHONOMIC HISTORY IN FLUVIAL DEPOSITS OF CENTRAL ARGENTINA

ABSTRACT Micro- and macromammal remains from Cerro La Bota (late Miocene; La Pampa Province, Argentina) are taphonomically analysed. All remains form a single sample recovered from Chasicoan sediments (facies FA2) of the Cerro Azul Formation, constituted by a lateral and vertical erosive superposition of sandstone and conglomeratic bodies, formed by episodic development of perennial streams without flow division on a loessic substrate. The analysis reveals that micro- and macromammal remains originally accumulated independently and differently. Once each type of accumulation was buried in a loessic context, the development of streams gave place to a reworking process of fossils that were finally incorporated together into the stream channel. Taphonomic characteristics of micromammals are interpreted as the result of the leftover prey accumulated by predators, probably at a den site. Instead, the macromammals respond to an attritional accumulation of individuals inhabiting the area. Even considering the reworking as an important process that affected this assemblage, the taxonomic and taphonomic evidences suggest a relatively short time for its formation. The different taphonomic histories of the remains evidence a complex association that differs from other assemblages from the Cerro Azul Formation previously analysed, and it is here assigned to the channel-lag taphonomic mode.

Taxonomic and ontogenetic diversity of Dinomyidae (Rodentia) from the late Miocene-early Pliocene of La Pampa province (Argentina) based on cranio-dental remains

Dinomyidae is a clade of caviomorph rodents that include medium to gigantic forms. They are represented by a single living species, Dinomys branickii, distributed from Venezuela to Bolivia in forested regions. However, in the past, they were very diverse. The oldest confident fossil records come from the early and middle Miocene, and during the late Miocene they reached their greatest diversity, reducing since then. In this contribution, the diversity of late Miocene-early Pliocene dinomyids from the Cerro Azul Formation in La Pampa Province (Argentina) is studied. New craniodental remains of at least three species are reported: Tetrastylus laevigatus, T. intermedius, and Diaphoromys gamayensis. Tetrastylus laevigatus is reported for this region based on abundant material, including a portion of the skull associated with the mandible. The presence of juvenile and adult individuals permitted us to analyze the ontogenetic variability of some traits. The specimens assigned to T. intermedius and D. gamayensis consist of mandibular and lower cheek teeth remains. Based on these new remains, we propose that D. gamayensis and D. fiegi are synonymous, with the priority being D. gamayensis. Considering the findings in each faunal association from the Cerro Azul Formation, representatives of T. laevigatus were registered in Salinas Grandes de Hidalgo, Algarrobo del Águila, Victorica, Bajo Giuliani, Laguna Chillhué, and Telén; T. intermedius was recorded in Caleufú. One new specimen of D. gamayensis from Guatraché (species originally described from Gamay) add information about its distribution. Finally these records are added to that of T. araucanus, previously described for Toay. This association suggests the existence of biogeographic connections with regions from northwestern Argentina during the late Miocene.

The Scelidotheriinae Proscelidodon gracillimus (Xenarthra: Mylodontidae) from the Cerro Azul Formation (late Miocene), Argentina: Ontogenetic variability and taxonomy

The subfamily Scelidotheriinae (Xenarthra, Folivora) is a clade of extinct sloths recorded from the middle Miocene to the Pleistocene/Holocene transition in South America, namely Argentina, Bolivia, Brazil, Colombia, Chile, Peru, Ecuador, Uruguay, and Paraguay. Quaternary taxa are the best known, but the knowledge of the late Neogene representatives has been greatly improved in recent years, particularly in Argentina. New materials of Scelidotheriinae recovered from the Cerro Azul Formation (late Miocene; Huayquerian) in La Pampa and Buenos Aires provinces are here described and identified as Proscelidodon gracillimus Rovereto, and ontogenetic variations are established for the first time in this group of sloths. The comparative study has led to the taxonomical revision of other species such as P. almagroi Rovereto (from Catamarca Province) and Elassotherium altirostre Cabrera (from western Buenos Aires Province), whose type specimens are reinterpreted as subadult and juvenile individuals, respectively, of P. gracillimus; thus, the former two become synonymous names of the latter. As the type material from Mendoza, the remains of P. gracillimus from Buenos Aires and La Pampa provinces have been recovered from Huayquerian levels, being absent from the older Chasicoan levels, within the late Miocene. In turn, the materials from Catamarca (type of P. almagroi) have less chronological precision, and could range from late Miocene to early Pliocene.

Nuevos registros de Lagostomus Brookes, 1828 (Rodentia, Chinchillidae) en el Mioceno tardío de Argentina y su importancia bioestratigráfica

In this work, we present two new records of Lagostomus from the late Miocene of Argentina. A right mandible from the Huachipampa Formation (Loma del Camello, San Juan Province) is assigned to Lagostomus telenkechanum, previously recorded in the Arroyo Chasicó Formation (Chasicoan Stage/Age, late Miocene; Arroyo Chasicó, Buenos Aires Province). A skull fragment from the Cerro Azul Formation (Salinas Grandes de Hidalgo, La Pampa Province) is referred to L. pretrichodactyla, previously recorded in several units assigned to the Huayquerian Stage/Age (late Miocene) in Catamarca and Mendoza provinces, and sediments of uncertain age in Buenos Aires Province. The presence of L. telenkechanum in the Huachipampa Formation supports a Chasicoan age for this unit, which corroborates previous hypotheses, and extends the area of distribution of this species. The presence of L. pretrichodactyla in the Cerro Azul Formation suggests a late Miocene age (probably Messinian) for this unit, at least at Salinas Grandes de Hidalgo, supporting previous hypothesis. These new records allow recognizing the biostratigraphic value of Lagostomus to a higher geographic scale than previously considered.

Updated knowledge on the Notoungulata (Mammalia) from the late Miocene Cerro Azul Formation, La Pampa Province, Argentina

ABSTRACT We describe new remains of notoungulates from the Cerro Azul Formation (late Miocene) in La Pampa Province and reassign others previously studied, helping to precise the taxonomic diversity of 25 fossiliferous localities of this formation. Geomorphological context previously described for the area provides data to place some localities in a temporal sequence. The following Notoungulata are recognised: Protypotherium minutum (Interatheriidae), Paedotherium minor, Tremacyllus impressus, Hemihegetotherium achataleptum, H. torresi, and Hemihegetotherium sp. (Hegetotheriidae), Typotheriopsis silveyrai, T. chasicoensis, and Pseudotypotherium subinsigne (Mesotheriidae), and Pisanodon nazari, Hemixotodon chasicoensis, and cf. Stenotephanos plicidens (Toxodontidae). This study allows adding new localities to the biochronological sequence previously proposed based on the evolutionary pattern of the octodontoid rodents. Chasicoan and early Huayquerian (as herein considered) faunal assemblages present higher diversity of notoungulates than those assigned to late Huayquerian. Some localities lack a representation of taxa significant enough to precise their biochronological position within the late Miocene. The difference in the notoungulates representation along the Chasicoan–Huayquerian ages may be due to climatic changes that affected the centre of Argentina from late Miocene to early Pliocene.

A Chasicoan (late Miocene) vertebrate assemblage from Cerro Azul Formation, central Argentina. Geomorphological and biochronological considerations

The Cerro Azul Formation, mainly extended in the center and east of La Pampa Province (Argentina), has provided interesting late Miocene faunal assemblages mostly assigned to the Huayquerian. However, other assemblages recovered from several localities were preliminarily assigned to the Chasicoan. One of them, based on the presence of the Ctenomyidae rodent Chasichimys bonaerense, was Cerro La Bota, placed at the southwestern limit of the Pampa Central Block. The revision of the vertebrate assemblage from this locality, as well as new geomorphologic and sedimentological interpretations of the Cerro Azul Formation in the area, confirms the previous assignation to the Chasicoan age. Cerro La Bota shares 18 taxa with Arroyo Chasicó Formation in Buenos Aires Province; several of them are mentioned for the first time for Cerro Azul Formation in La Pampa Province. Geomorphological data indicate that Cerro La Bota is included in a topographic step linked to the Valle Daza–Lonco Vaca fault that exposed the older levels of the Cerro Azul Formation in the area.

Large mammal burrows in late Miocene calcic paleosols from central Argentina: paleoenvironment, taphonomy and producers.

Large cylindrical sediment-filled structures interpreted as mammal burrows occur within the loess-paleosol sequence of the late Miocene Cerro Azul Formation of central Argentina. A total of 115 burrow fills from three localities were measured. They are typically shallowly dipping, subcylindrical, unbranched structures with rounded ends and lacking enlargements. The horizontal diameter of the burrows range between 0.15 and 1.50 m, with most of the burrows in the interval of 0.39 to 0.98 m. Geometric morphometric analysis of transverse cross-sections support their distinct subcircular and elliptical (horizontally flattened) shapes. Burrow fills are typically laminated in the lower part and massive in the upper part. The laminated intervals reflect pulses of flowing water entering the abandoned burrow during moderate rains, whereas massive intervals reflect mass flow input of dense sediment-water mixtures during heavy rains that produced sheet floods. Approximately 1% of the burrows contained fragmentary, disarticulated and weathered mammal bones that were introduced in the open burrow by currents along with other sedimentary particles. Analysis of the tetrapod burrow fossil record suggests that Miocene burrows, including those studied herein, reflect a remarkable increase in the average size of the fossorial fauna. We conclude that large late Miocene mammals dug burrows essentially as a shelter against environmental extremes and to escape predation. The simple architecture of the burrows suggests that the producers essentially foraged aboveground. Several mammal groups acquired fossorial habits in response to cold and seasonally dry climatic conditions that prevailed during the late Miocene in southern South America. The considerable range of horizontal diameters of the studied burrows can be attributed to a variety of producers, including dasypodids, the notoungulate Paedotherium minor, Glyptodontidae and Proscelidodon sp.

Proterotheriidae (Mammalia, Litopterna) from the Cerro Azul Formation (late Miocene), La Pampa Province, Argentina

Remains of Proterotheriidae (Litopterna) recovered from different localities in La Pampa Province, Argentina, are described. The fossiliferous levels in these localities correspond to the Cerro Azul Formation (late Miocene), where Chasicoan and Huayquerian faunal associations are known. The specimens of Proterotheriidae herein studied are assigned to Diplasiotherium pampa, Eoauchenia primitiva, cf. Brachytherium cuspidatum, Neobrachytherium sp. and Proterotheriidae indet. Although the holotype of D. pampa (left mandibular fragment with p3-m2) was defined in a previous paper, it is figured and fully described here, and its m3 is now known among the new collected specimens. Eoauchenia, Brachytherium and Neobrachytherium are registered in La Pampa Province for the first time, and the record of E. primitiva extends back to late Miocene its temporal distribution. Proterotheriids from the Cerro Azul Formation gather exclusive elements and others related to taxa represented in different Argentinean geological units corresponding to late Miocene-early Pliocene period.

Taxonomic Implications of a Large Sample of Tremacyllus (Hegetotheriidae: Pachyrukhinae) from the Late Miocene Cerro Azul Formation of La Pampa, Argentina

More than 40 remains of Tremacyllus Ameghino (Notoungulata, Hegetotheriidae, Pachyrukhinae) from the Province of La Pampa (Argentina) are analyzed. They were recovered from different outcrops of the late Miocene Cerro Azul Formation at Laguna Chillhue, Quehue, Telen, Salinas Grandes de Hidalgo, Estancia Re, Bajo Giuliani, Barrancas Coloradas and Caleufu. During the late Miocene, pachyrukhines are represented by two genera, Tremacyllus and Paedotherium Burmeister. The present sample of Tremacyllus allows for the dental morphological variation to be evaluated, especially in lower and upper premolars. This study shows high variability in the diagnostic characters of two recognized species, T. impressus Ameghino (Pliocene–Pleistocene of the Pampean Region) and T. incipiens Rovereto (late Miocene of Catamarca). We interpret these differences to actually correspond to intraspecific variation and, consequently, propose their synonymy. T. impressus has priority over T. incipiens, and an emended diagnosis is provided. This nomenclatural action implies, in turn, the extension of the biochron of Tremacyllus impressus from the late Miocene to the Pleistocene. T. impressus differs from Paedotherium in its smaller size, post-incisive impressions extended beyond the level of P2 (even reaching the anterior margin of P3), M3 shorter or equal to M2, relatively shorter mandibular symphysis, and lower premolars not molariform and more imbricated. Within the studied sample, we identify several juvenile specimens that confirm the dental eruption pattern previously proposed for Tremacyllus based on fewer remains.