Early Miocene Santa Cruz Formation Research Articles

Astrapotherium from the Middle Miocene Collón Cura Formation and the Decline of Astrapotheres in Southern South America

Astrapotherium is the better-known member of the placental Order Astrapotheria. This large herbivorous inhabited the Patagonian ecosystems during Early and middle Miocene times. The genus is widely represented in the Early Miocene Santa Cruz Formation, with a dozen of nominal species, of which A. magnum and A. burmeisteri are the only ones herein considered as valid. The middle Miocene record was limited to few fragmentary remains from the Rio Frias Formation and presumably from the Collon Cura Formation, described as A. hesperinum Cabrera 1940, here interpreted as nomen vanum. Here we describe an almost complete skull with associated mandible derived from the Collon Cura Formation in the vicinities of Comallo, in Rio Negro Province (Argentina). It is largely the most complete astrapothere material from middle Miocene rocks of high latitudes, and it is referred to a new species, Astrapotherium guillei. It differs from the remaining species essentially in lacking P3, achieved convergently with the latest diverging uruguaytheriines. The astrapotheriines diversified in high latitudes during the Early Miocene, but they retained an stereotyped morphotype and became progressively less diverse, unlike their Northern counterparts (the Uruguaytheriinae). Astrapotherium guillei is the latest known Patagonian astrapothere and represents the final expression of the Astrapotheriinae´s decline. doi: 10.5710/AMGH.15.07.2019.3258

Sedimentology and fossil vertebrates of the Santa Cruz Formation (early Miocene) in Lago Posadas, southwestern Patagonia, Argentina

Lago Posadas is located at the foot of the Southern Patagonian Andes, in southwestern Argentina, where the early Miocene Santa Cruz Formation (SCF) shows thick and laterally continuous exposures. This region has been scarcely explored for fossil vertebrates since the first efforts by J.B. Hatcher in 1898-99. In this contribution, we performed sedimentologic and paleontological studies in order to reconstruct depositional environments and the associated fossil vertebrate fauna. Sedimentologic data suggest that the sedimentary record begins with restricted marine-estuarine deposits grading upward to fluvial floodplains and fluvial channels. Extensive floodplains, occasionally interrupted by low-sinuosity, sand-dominated channels, show dominant reddish coloration, moderate to low paleosol development, abundant crevasse splay sandstones and lack of vegetal remains, suggesting deposition in a low gradient, oxygenated setting under elevated sedimentation rates. Vertical stratigraphic trends are subtle, suggesting little paleoenvironmental changes during deposition of the whole SCF in this region. Paleocurrent directions, sandstone composition and paleogeographic reconstructions all indicate that deposition of the SCF was strongly associated to the contemporaneous uplift of the Andes. Fossil vertebrates analyzed are the result of our collecting effort and revision of museum collections. The faunal assemblage includes 31 taxa: 28 mammals and three birds. Mammals belong to the main groups recorded in other areas of the SCF (metatherians, xenarthrans, notoungulates, litopterns, astrapotheres and rodents). The assemblage allows a Santacrucian Age sensu lato assignment for the fauna at Lago Posadas. Taxonomic revisions of several taxa are necessary to further adjust the biostratigraphic significance of this association. The combined record of arboreal, browser and frugivores, on one side, and grazer mammals and rheas, on the other, suggest the presence of both trees and open environments. Frugivores, among primary consumers, and the secondary consumers guild are under-represented due to sample and fossil remain size biases. The sedimentologic and paleontological record of the SCF in Lago Posadas suggests that the uplift of the Southern Patagonian Andes acted as a primary control on basin subsidence and sediment supply, providing a special signature for sub-andean localities. However, previously registered climatic changes are poorly recorded in this study.

First Miocene record of Akaniaceae in Patagonia (Argentina): a fossil wood from the early Miocene Santa Cruz formation and its palaeobiogeographical implications

Fil: Brea, Mariana. Provincia de Entre Rios. Centro de Investigaciones Cientificas y Transferencia de Tecnologia a la Produccion. Universidad Autonoma de Entre Rios. Centro de Investigaciones Cientificas y Transferencia de Tecnologia a la Produccion. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Santa Fe. Centro de Investigaciones Cientificas y Transferencia de Tecnologia a la Produccion; Argentina

The southernmost fossil record of Squamates

Squamates form a substantial part of the present-day South American herpetofauna, and their fossils constitute an indispensable evidence for understanding the origin and evolution of the main taxa. Squamates are relatively common in Miocene localities of Patagonia, especially in levels of the late early Miocene Santa Cruz Formation. In this contribution, remains of the three species of the extinct iguanidErichosaurusAmeghino 1899 (E. diminutus,E. bombimaxillaandE. debilis) are redescribed, and new squamate specimens are reported for first time. The genusErichosaurusis considered invalid.Erichosaurus debilis,E. diminutusand a new specimen are recognized as indeterminate species of the extant polichrotinePristidactylus, whereasE. bombimaxillaremains as an indeterminate iguanid. Snakes are represented by an indeterminate colubrid. All these specimens, together with a tupinambine teiid previously described for the same formation, represent the southernmost fossil record of squamates in South America and indicate the occurrence of the iguanidPristidactylus, the teiidTupinambisand the colubrid snakes south to their present distribution as back as during the early Miocene.

Sobre los objetivos y resultados de la expedición paleontológica de Handel T. Martin (1903-04) a la Formación Santa Cruz en Patagonia austral.

Entre enero y junio de 1904, Handel T. Martin (1862-1931), de la Universidad de Kansas (KU), colectó vertebrados fósiles de la Formación Santa Cruz (Mioceno temprano) a lo largo del Río Gallegos y la costa atlántica de Patagonia, como una iniciativa privada de Martin. En reporte de la expedición, Martin afirmó que cuando él y su compañero llegaron a Buenos Aires, visitaron a Florentino Ameghino en el (entonces) Museo Nacional. Su álbum de fotografías muestra que también visitó el Museo de La Plata y la casa de Ameghino en La Plata. Antes de dirigirse a la Patagonia, Martin organizó su equipo en Bahía Blanca, donde su hermano se había establecido. No hay certeza de si Martin recogió un total de 235 o 395 ejemplares. Claramente, su principal objetivo fue recoger fósiles para comercializar. Muchos especímenes fueron vendidos a instituciones en los Estados Unidos y Europa por Martin y Robert Ferris Damon, un comerciante de fósiles y minerales. Se han identificado cerca de 170 especímenes en colecciones formales. Gran parte de la colección - unos 130 de los mejores ejemplares - se mantuvo en KU, constituyendo una de las mayores colecciones de vertebrados santacrucenses fuera de Argentina (además de las del Museo Peabody de Yale en New Haven, el Museo Field de Chicago y el Museo Americano de Nueva York). Aunque la colección en KU es en gran medida ignorada por los paleontólogos y poco estudiada, contiene una buena representación de la fauna santacrucenses, con muchos ejemplares de calidad.

Evidence for Sexual Dimorphism in the Early Miocene Palaeothentid Acdestis owenii (Marsupialia: Paucituberculata)

Abstract. 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 compar...

U-Pb geochronology of the Santa Cruz Formation (early Miocene) at the Río Bote and Río Santa Cruz (southernmost Patagonia, Argentina): Implications for the correlation of fossil vertebrate localities

The early Miocene Santa Cruz Formation (SCF) in southern Patagonia hosts the Santacrucian South American Land Mammal Age (SALMA), whose age is known mainly from exposures along the Atlantic coast. Zircon U-Pb ages were obtained from intercalated tuffs from four inland sections of the SCF: 17.36 ± 0.63 Ma for the westernmost Río Bote locality, and 17.04 ± 0.55 Ma–16.32 ± 0.62 Ma for central Río Santa Cruz localities. All ages agree with the bounding age of underlying marine units and with equivalent strata in coastal exposures. New ages and available sedimentation rates imply time spans for each section of ∼18.2 to 17.36 Ma for Río Bote and 17.45–15.63 Ma for central Río Santa Cruz (Burdigalian). These estimates support the view that deposition of the SCF began at western localities ∼1 Ma earlier than at eastern localities, and that the central Río Santa Cruz localities expose the youngest SCF in southern Santa Cruz Province. Associated vertebrate faunas are consistent with our geochronologic synthesis, showing older (Notohippidian) taxa in western localities and younger (Santacrucian) taxa in central localities. The Notohippidian fauna (19.0–18.0 Ma) of the western localities is synchronous with Pinturan faunas (19.0–18.0 Ma), but older than Santacrucian faunas of the Río Santa Cruz (17.2–15.6 Ma) and coastal localities (18.0–16.2 Ma). The Santacrucian faunas of the central Río Santa Cruz localities temporally overlap Colloncuran (15.7 Ma), Friasian (16.5 Ma), and eastern Santacrucian faunas.

The Basicranium and Orbital Region of the Early MioceneEucholoeops ingensAmeghino, (Xenarthra, Pilosa, Megalonychidae)

New, exceptionally well preserved skulls of the basal megalonychid ground sloth Eucholoeops ingens Ameghino have been recovered through recent field work in the late early Miocene Santa Cruz Formation of southern Argentina (Patagonia, Santacrucian SALMA). These specimens have permitted detailed description of the anatomy of the basicranium, endocranium, and orbital wall, including descriptions of the morphology and sutural relationships of the bones from this portion of the skull, and their associated cranial foramina. Comparisons are made to other megalonychid and megatherioid sloths, including the extant two-toed sloth Choloepus Illiger (Megalonychidae). As the descriptions are based on multiple specimens of E. ingens, we have been able to examine intraspecific variation in the features described, and have found marked differences among individual specimens in sutural patterns (e.g., the bones that surround the foramen ovale aperture), cranial foramina (e.g., the presence/absence of a postglenoid foramen), and bony morphology (e.g., fusion of the alae of the vomer). The basicranial and orbital morphology of Eucholoeops ingens is shown to possess numerous plesiomorphic aspects, including the presence of a descending lamina of the pterygoid that is hemispherical in outline, and the absence of an alisphenoid/parietal contact, no doubt due to its position as the oldest megalonychid known from relatively complete material. The presence of distinct grooves on the promontorial surface directed toward the fenestra ovalis suggest that Eucholoeops ingens may be the only known xenarthran to have retained a functional stapedial artery Into adulthood.

EucholoeopsAmeghino, 1887 (Xenarthra, Tardigrada, Megalonychidae) from the Santa Cruz Formation, Argentine Patagonia: implications for the systematics of Santacrucian sloths

New well-preserved remains of the megalonychid sloth Eucholoeops Ameghino, 1887 recovered under strict stratigraphic control from late Early Miocene Santa Cruz Formation (c. 19 to 14 Ma; Santacrucian Age), together with analysis of older collections, consideration of intraspecific variation in extinct and extant sloths, and assessment of the validity of the early literature on Santacrucian sloths, permit revision of the status of the numerous species erected for this genus. The current contribution deals with the systematics of E. ingens Ameghino, 1887, but its methodology provides a basis for revision of other Eucholoeops species, as well as other sloth genera recovered from the Santa Cruz Formation. The failure to make progress on the systematics of the Santacrucian taxa since their first description is shown to be due mainly to a combination of the poor quality of many of the specimens, which are often fragmented and incomplete and from older collections, as well as inadequate stratigraphic and geographic control of their recovery, an overly rigid reliance on the early literature that accompanied their descriptions, and lack of consideration for intraspecific variation. A neotype is designated for E. ingens, as the original specimen is no longer available. The species E. latirostris Ameghino, 1891, E. externus Ameghino, 1891, and E. curtus Ameghino, 1894 are considered as junior synonyms of E. ingens.

What constitutes a fossil mammal community in the early Miocene Santa Cruz Formation?

ABSTRACT The late early Miocene Santa Cruz Formation of southern Argentina is one of the most productive fossil mammal-bearing formations in South America. Hundreds of species have been named from this formation, resulting in a clear overestimation of Santa Cruz mammal diversity. Recent paleoecological studies have attempted to minimize this problem by analyzing more restricted faunal lists based only on recently collected fossil samples. In some cases, this approach has excluded clearly recognizable Santa Cruz taxa. In this study, I use a family-level rarefaction analysis of the Yale Peabody Collection of Santa Cruz fossils to test whether such absences are likely due to smaller sample size or to other factors. I further attempt to reconcile these absences using distributional data from recent and historical Santa Cruz Formation collections. The rarefaction analysis indicates that most single-locality samples from the Santa Cruz Formation have lower familial diversity than expected based on the taxonomic distribution of specimens in the Princeton University Collection at Yale Peabody Museum (YPM-PU) as a whole. However, I conclude that single-locality samples are not necessarily more appropriate than multi-locality samples for paleoecological analyses of ancient Santa Cruz Formation mammal communities given the large regional extent of the Santa Cruz Formation and the large geographic area encompassed by most modern communities used for comparison. This study highlights the need for more precise stratigraphic correlations among Santa Cruz localities and integration of important historical collections into a modern stratigraphic framework.

A well-preserved partial skeleton of the poorly known early Miocene seriema Noriegavis santacrucensis (Aves, Cariamidae)

Seriemas (Cariamidae) include two extant species, Cariama cristata and Chunga burmeisteri, which live in semi-open plains of South America and have a poorly documented evolutionary history. One of the earliest fossil representatives of Cariamidae is the recently described Noriegavis santacrucensis from the early Miocene Santa Cruz Formation (Argentina). So far, however, this species was only known from a cranium and tentatively referred distal tibiotarsi, and its phylogenetic assignment has been questioned. Here we describe a well-preserved partial skeleton from the Santa Cruz Formation, which substantiates the classification of Noriegavis in Cariamidae. Plesiomorphic features show N. santacrucensis to be outside crown group Cariamidae, but the species is nevertheless very similar to its modern relatives and documents that the osteology of seriemas underwent only few changes during the past 16 million years.

Mass estimation of Santacrucian sloths from the Early Miocene Santa Cruz Formation of Patagonia, Argentina

Miocene deposits of the Santa Cruz Formation, Patagonia, comprise a diverse and excellently preserved vertebrate fauna, allowing detailed paleobiological and paleoecological studies based on three ecological parameters: body mass, diet, and substrate preference. In contrast to the small and arboreal extant sloths, Bradypus and Choloepus, Santacrucian sloths were much more diverse and larger, and comprised 11 genera previously characterized as arboreal or climbing forms. Here, we focus on body mass estimation based on measurements of postcranial elements. We present a morphometric database comprising 64 linear, base-ten logged variables applied to Santacrucian sloths and a wide sample of extant mammals, as well as the body mass of the extant taxa as reported in the literature. To detect any potential phylogenetical bias, we performed a variance decomposition test on our sample of extant mammals. Based on four orthogram statistics, logged body mass was found not to be dependent on phylogenetic tree topology...

Diets of notoungulates from the Santa Cruz Formation, Argentina: new evidence from enamel microwear

ABSTRACT The late early Miocene Santa Cruz Formation of Patagonian Argentina is remarkable for its well-preserved and diverse mammalian fauna. This study evaluates the feeding preferences of three notoungulates from Santa Cruz (Nesodon imbricatus, Adinotherium ovinum and Protypotherium), typically reconstructed as open habitat grazers. Diets are inferred from enamel microwear features using a low-magnification microscopy technique. Analysis indicates that these notoungulates were primarily feeding on browse, a result that contrasts sharply with previous interpretations based on their hypsodont dentitions. All three notoungulates exhibit a low average number of scratches per taxon, indicative of browsing; high scratch averages are characteristic of grazers. Furthermore, these notoungulates were not primarily feeding on either hard objects (e.g., fruits with seeds) or abrasives (e.g., grasses). Nesodon imbricatus, however, exhibits microwear, suggesting it infrequently fed on hard objects such as bark. The association of tall tooth crowns with browsing diets as demonstrated by the Santa Cruz notoungulates is evidence that hypsodonty does not necessarily indicate open habitat feeding or grazing and that the diets of hypsodont herbivores should be evaluated on a case-by-case basis.