Arroyo Seco de la Frazada (Lower Section) (Miocene of Argentina)

Where: Mendoza, Argentina (34.9° S, 68.5° W: paleocoordinates 35.6° S, 63.2° W)

• coordinate estimated from map

• small collection-level geographic resolution

When: Aisol Formation, Colhuehuapian (21.0 - 17.5 Ma)

• Forasiepi et al 2015: This contribution presents a new radiometric dating of a 19.480 ± 0.025 Ma tuff bed, which constrains the absolute age of the mammalian assemblage at these latitudes to the Early Miocene.

•In preliminary studies, the Aisol Formation was con- sidered early Miocene (probably Santacrucian SALMA; e.g., Groeber 1951; Dessanti 1954, 1956; González Díaz 1972; Núñez 1976) based upon few fossil findings and stratigraphic correlations. Nonetheless, it is also worth mentioning that in one of those preliminary studies, Groeber (1952) suggested that the unit (informally called «Estratos Calchaquíes» by then) was late Miocene, and was included among the «Arauco-Entrerriano» levels. Later studies did not further the debate. Pascual and Odreman-Rivas (1973; see also Pascual and De la Fuente 1993) placed the Aisol Formation in the Miocene, in the temporal interval between the Santacrucian and Friasian SALMAs. It was after the analysis of new fossil findings that Soria (1983) specifically allocated the Aisol Formation to the Friasian s.l. (including Colloncuran, Friasian s.s., and Mayoan, following Bondesio et al. 1980), because of the occurrence of post-Santacrucian and pre- Chasicoan mammals. This hypothesis was also followed by later authors (Sepúlveda et al. 2001, 2007). The analysis of the new fossil material and the strati- graphic control of the findings led to recognize two dif- ferent fosiliferous sections in the profile, with faunal as- semblages of different ages. The LS is probably middle Miocene, as Soria suggested (1983), perhaps Friasian s.s. (older than 15 Ma; Marshall and Salinas 1990) or Collon- curan (15.5-14 Ma; Marshall et al. 1977; Marshall and Salinas 1990), following the Patagonian scheme. Unfor- tunately, there is no consensus about the time span cov- ered by some of the SALMAs, including the Friasian one. Specific studies on age and taxonomic composi- tion are needed before making any comparisons with more distantly located stratigraphic units.

•Three different sections are recognized within the Aisol Formation, with fossil vertebrates in the lower (LS) and middle (MS) sections. According to preliminary results of Forasiepi et al., 2011: Lower Section: Friasian s.s (not used anymore). or Colloncuran and Middle Section: Huayquerian.

Environment/lithology: terrestrial; sandstone

• The lower section (LS) of the profile is composed mainly of medium to fine-grained sandstones predominantly grey, reddish, yellow, and less frequently pale green; the entire bed is roughly sub-horizontal. The sandstones are calcite-cemented and the entire bed has strong cross- stratification (Fig. 2), with higher angles towards the middle and upper parts of the section. Between the sandstones, levels of reddish to brownish, fine siltstones are interbedded. The thickness of the siltstone levels is larger towards the lower part of the section. In contrast, towards the upper part, reddish to greenish siltstones intraclasts are frequently found within the sandstones (Fig. 2). The intraclasts are variable in size and their surfaces are mostly rounded. Fossil wood and vertebrate remains are common in different parts of the LS. With the exception of the bones found in the siltstone, other material is highly eroded, showing signals of transportation and corrosion of biological organisms. In the area of the profile, the LS is approximately 15 m thick. The base, however, was not observed; it is covered by recent deposits.

Size class: macrofossils

Collection methods: Museo de Historia Natural de San Rafael, Mendoza, Argentina

Primary reference: A. M. Forasiepi, A. G. Martinelli, M. S. Fuente, S. Dieguez, and M. Bond. 2011. Paleontology and stratigraphy of the Aisol Formation (Neogene), San Rafael, Mendoza. Cenozoic Geology of the Central Andes of Argentina 135-154 [E. Vlachos/E. Vlachos]more details

Purpose of describing collection: taxonomic analysis

PaleoDB collection 189519: authorized by Evangelos Vlachos, entered by Evangelos Vlachos on 25.10.2017, edited by Miranta Kouvari

Creative Commons license: CC BY (attribution)

Taxonomic list

Reptilia
 Testudines - Testudinidae
Chelonoidis sp. Fitzinger 1835 turtle
MHNSR- PV1064, femur of giant size
Mammalia
 Megatherioidea - Megatheriidae
Planopinae indet. Ameghino 1887 edentate
MHNSR-PV1028, partial skull
 Notoungulata - Interatheriidae
Protypotherium sp. Ameghino 1882 notoungulate
left dentary with dentition (MHNSR- PV1050)
 Notoungulata - Hegetotheriidae
Hegetotherium sp. Ameghino 1887 notoungulate
right M3 and broken pieces of the postcranial skeleton (MNHSR-PV 1052; Fig. 7B)
 Notoungulata - Mesotheriidae
cf. Eutrachytherus modestus2 Roth 1898 notoungulate
MHNSR- PV 1152. It'sa bit vague in which collection should this be.
 Notoungulata - Toxodontidae
Palyeidodon cf. obtusum Roth 1898 notoungulate
MHNSR-PV1021, partial skull
Nesodon taweretus n. sp.1 Forasiepi et al. 2015 notoungulate
MHNSR-PV1004 (Figs. 6C and 6D) and probably MACN-PV18603
 Panameriungulata - Macraucheniidae
cf. Theosodon sp. Ameghino 1887 placental
additional metapods (e.g., MHNSR-PV253; MHNSR- PV1032), astragalus (MHNSR-PV1005), portions of tibia (e.g., MHNSR-PV1016, MHNSR-PV1017), and femora (e.g., MHNSR-PV1025, MHNSR-PV1020)
 Rodentia - Chinchillidae
Chinchillidae indet. Bennett 1833 chinchilid
MHNSR-PV1056, an euhypsodont, left M3
Amphibia
 Salientia -
Anura indet. Fischer von Waldheim 1813 frog
MHNSR-PV1074, isolated vertebra