Introduction
Context
New fieldwork
Since 2008, a multidisciplinary international team has made annual expeditions to the Contamana area with the aim of conducting a comprehensive study of the Cenozoic deposits along the Quebrada Cachiyacu. This long-term study resulted in the discovery of scattered outcrops on mud paths and on the new road to Quebrada Agua Caliente (Fig. 1A-2). In total, at least 34 fossil-producing localities have been identified, which have yielded fossil remains of vertebrates (fish, amphibians, sauropsids and mammals) as well as molluscs, arthropods (ostracods and crustaceans), plant fossils (palynomorphs, charophytes, seeds, fruits and silicified wood), microorganisms (foraminiferans, poriferans and algae) and dinoflagellates, as indicated in the supplementary data.
In a single continuous section on the SW flank of the Maquia anticline we recognize twenty successive fossil-bearing levels, ranging from middle Eocene to late Miocene-Pliocene, in the SW flank of the Maquia anticline (Fig. S1) . Two recent studies by our team have focused on a fossil-rich level from the Quebrada Cachiyacu (namely CTA-27; Figures 2-3), i.e. in the late middle Eocene, around the Lutetian-Bartonian transition. Yet, the thanatocoenoses of other levels from the same area remain to be described and characterized.
The CTA-54 locality (Huchpayacu Formation; Early Paleocene) is off the map: for location purposes, refer to fig. Based on this unparalleled paleontological corpus, the current work documents ~60 million years of life history in the Neotropical Lowlands (early Paleocene to ?Pliocene), focusing on the biotas and their physical environment spanning the final stages of Proto-Amazonia and the beginning of modern Amazonia.
Material and methods
Material
- Institutional Abbreviation
- Other abbreviations
FAD, Date of First Appearance; FLO, first local event; Fm., formation; GABI, Major American Biotic Exchange; LAD, Last Date Published; LLO, last local event; NPFB, Northern Peruvian Foreland Basin; PMWS, Pebas Mega-wetland system; SALMA, South American Land Mammal Age; SPFB, Southern Peruvian Foreland Basin.
Methods
- Lithostratigraphy
- Chronostratigraphy
Results
Lithostratigraphic units and fossil assemblages
- Huchpayacu Formation (Paleocene)
- Pozo Formation, Lower Member; ‘Pozo Sands’ (middle and ?late Eocene interval)
- Chambira Formation (Oligocene–?early Miocene)
- Pebas Formation (late early to early late Miocene interval)
- Ipururo Formation (late Miocene–?early Pliocene)
- Ucayali Formation (?late Pliocene–Pleistocene)
Palaeontological content (Fig. 6). In the Quebrada Cachiyacu section, 11 fossiliferous outcrops are referred to the Lower Pozo Fm. Fig. 6R-Z), astegotheriine dasypodid Stegosimpsoniacf.chubutana, five caviomorphic rodents, nooungulates, an astrapother, a litoptern and a pyrothere (Fig. Depositional environment. The exposed part of the Pozo Fm. 5) consists of 50 m-0th-5 only of thin lenses of reddish silt and yellowish sandstone, of fluvial origin, alternating with thick intervals of mottled palaeosols and distal floodplain deposits (Fig. 5A).
Upstairs in the Pozo Fm. a single ostracod recognized in CTA-27 (Neocyprideis apostolescui; Figure 6G) may indicate euryhaline-mesohaline conditions, but this taxon is known to tolerate changes in salinity and is associated with a strictly freshwater fauna and flora with no evidence of marine influence (Antoine et al., 2012; Adnet et al., 2014). This interval is not clearly documented in the SPFB (see Louterbach et al., 2014 for an overview). A remote and species-poor vertebrate locality, located on the road to Agua Caliente (CTA-72; Fig. Supplementary data), is correlated with an unidentified term of the Chambira Fm., based on both its extrapolated stratigraphic position and the compatible facies. (red paleosol with anhydrite veins).
In the Contaman area, most of the available approximations (ash CTA-08SA and fossil groups CTA-32 and CTA-61) consistently indicate a late Oligocene age for the base of the Chambira Fm (Supplementary Data). Conglomerates and sand beds observed in the lower part on the SW flank of the Maquía anticline indicate the occurrence of channelized fluvial environments (Fig. 7C). Paleontological content (Figure 10). In contrast to the underlying sediments of the Huchpayacu, Pozo and Chambira formations, the fossil record of Pebasian deposits is much better documented at the scale of western Amazonia (e.g. Hoorn and Wesselingh, 2010; Salas-Gismondi et al., 2015).
Mollusks recovered from the Cachiyacu section are typical Pebasian endemics (sensuWesselingh et al., 2006; Fig. 10R-T; Supplementary data). Supplementary data), the latter consisting mainly of representatives of the Cyprid species herd (Fig. 10H–I; Wesselingh and Ramos, 2010; Gross et al., 2013). Most levels yield chondrichthyans (Fig. 10U–W), with maximum diversity in CTA-44 Top (two potamotrygonids, a dasyatid and a pristid) and CTA-43 (same association, plus a myliobatid; Supplementary Data).
Ground sloths, closely related to Pseudoprepoterium and Tetodontobradys from the Middle Miocene of Colombia and Late Miocene of Brazil (dos Santos et al., 1993), occur in CTA-63 and CTA-57, respectively (isolated teeth in view eight and occ-shaped a claw; Fig. 10 BC). 10O), a late representative of Acarechimys (CTA-43 and CTA-75; Fig. 10AF), and several taxa of unidentified affinities (Supplementary Data). Chinchilloids are the most diverse of the rodents ( Fig. 10P–Q, AH–AL ), with seven taxa occurring throughout the Pebas Fm. in CTA-57), and neoepiblemidNeoepiblemasp.
On the other hand, cavioids and erethizontoids are only observed in the upper part of the formation, with an unidentified dasyproctid (CTA-44 Top) and a new caviine (CTA-44 Top and CTA-43; Figure 10AG), and the erethizontid Steiromys . Bats are documented by an emballonurid in CTA-63 and CTA-75 (Fig. 10.00) and a molossid in CTA-44 Top (Supplementary Data).
Discussion
Taxonomic diversity trends (Eocene–Miocene)
There is no exposure on the right bank of the Río Ucayali, at least in the Contamana area. The lower locality is CTA-42, located ~ 160 m above CTA-75 (i.e., the upper level attributed to the Pebas Fm.), in the same section. The lack of preservation of organic matter in the levels sampled for palynology (CTA-42, CTA-59, and CTA-70; Figs. 3, S1) may suggest a fully fluvial environment with seasonal fluctuation of the water level.
Available data do not indicate marine episodes in the Contamana area during the time interval in question. Ucayali Fm., ie. the uppermost formation, which extends along the right bank of the Río Ucayali in the Contamana region, has not been thoroughly investigated, neither from a paleontological perspective nor for stratigraphy. To our knowledge, no fossil content has been described in the localities in question, except for some vertebrate specimens excavated in a remote area of the Ucayali Basin (Río Pachitea; . Spillmann, 1949).
Order-level diversity fluctuates to a greater extent, with a decrease in the Early and Middle Miocene (15 and 17 co-occurring orders, respectively) compared to earlier intervals (28 in the Middle Eocene; 23 in the Late Oligocene) and the Late Miocene (20 co-occurring orders; Figure 11C). Nevertheless, this first-order analysis does not take into account the replacement of terrestrial vertebrates by aquatic organisms with marine or fluo-lacustrine affinities in the “Pebasian” interval (early and middle Miocene). This enormous diversity is maintained during the late Oligocene (56 families comprising 73 species), but with a stronger vertebrate component (Fig. 11D–E).
Species richness is higher during the middle Miocene (76) than the late Miocene (60), due to the occurrence of polytypic/plurispecific ostracod and foraminiferan genera in the former interval (Fig. 11A, E). Overall, overall family and species diversity declines during the interval in question, with a drastic drop in the early Miocene (32 families/38 species). From a taxonomic perspective, charophytes occur only in the early Paleocene + middle Eocene interval, with a comparable diversity (13–14 species; Table 2 ; Fig. 11B ).
The amphibian (anuran) record of the Contaman area is limited to the Paleogene interval (Supplementary data; Fig. 11B–E): no Neogene representative has been excavated so far. Overall, mammal diversity appears to have peaked during the Paleogene in the Contaman area (Fig. 11), with 14/28/. 11. Taxonomic diversity as shown in the Contamana section throughout the Eocene-Miocene interval, excluding palynomorphs.
Paleoenvironmental inferences
- Depositional environments
- Paleoenvironments as inferred by organisms
- Correlation with other Amazonian sub-basins
Paleobiogeography
- Gondwanan faunal and floral components
- Panamerican heritage (and Holarctic outliers)
- South American endemics
- Neotropical markers — no evidence for the Great American Biotic Interchange
Due to the closure of the Central American seaway in Pliocene or perhaps even in Miocene times as suggested by Bacon et al. 2015), these groups subsequently spread into Central and North America. Arnal, M., Kramarz, A.G., Vucetich, M.G., Vieytes, E.C., 2014. A new early Miocene octodontoid rodent (Hystricognathi, Caviomorpha) from Patagonia (Argentina) and a reassessment of the early evolution of Octodontoidea. Cadena, E.A., 2014. The fossil record of turtles in Colombia; an overview of the discoveries, research and future challenges.
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Hoorn, C., Wesselingh, F.P., Hovikoski, J., Guerrero, J., 2010b. The evolution of the Amazonian megawetland (Miocene; Brazil, Colombia, Peru, Bolivia). Jaramillo, C.A., Pardo-Trujillo, A., Rueda, M., Harrington, G., Bayona, G., Torres, V., Mora, G., 2007. The Palynology of the Cerrejón Formation (Upper Paleocene), Northern Colombia. Muñoz-Torres, F., Whatley, R., Van Harten, D., 1998. The endemic non-marine Miocene ostracod fauna of the upper Amazon Basin.
Muñoz-Torres, F.A., Whatley, R.C., Van Harten, D., 2006. Miocene ostracod (Crustacea) biostratigraphy of the upper Amazon basin and evolution of the genus Cyprideis. Pujos, F., Albino, A.M., Baby, P., Guyot, J.-L., 2009. Presence of the extinct lizard Paradracaena (Teiidae) in the middle Miocene of the Peruvian Amazon. Implication of the presence of Megathericulus (Xenarthra: Tardigrada: Megatheriidae) in the Laventan of the Peruvian Amazon.
Sanjuan, J., Martín-Closas, C., 2012. Charophyte palaeoecology in the Upper Eocene of the Eastern Ebro Basin (Catalonia, Spain). Sanjuan, J., Martín-Closas, C., 2014. Taxonomy and paleobiogeography of charophytes from the Upper Eocene-Lower Oligocene of the Eastern Ebro Basin (Catalonia, NE Spain). He is a research associate in the Vertebrate Paleontology Department at the San Marcos University Natural History Museum in Lima, Peru.
In the last decade he has also worked with biologists and paleontologists on the tectonic control of the diversification of biota in the western Amazon. His works focus on the taxonomy, evolution and diversity of the Cenozoic ichthyofauna from western Amazonia.
