Allostratigraphy and paleontology of the lower Miocene Chilcatay Formation in the Zamaca area, East Pisco basin, southern Peru

Based on mapping of laterally traceable stratigraphic discontinuities, we propose a highresolution allostratigraphic scheme for one of the world’s foremost fossil marine vertebrate Lagerstätten: the lower Miocene strata of the Chilcatay Formation exposed along the Ica River near Zamaca, southern Peru. Measured sections combined with 1:10,000 scale mapping of a 24 km area provide an overview of the stratal architecture, as well as a general facies framework and interpretation of the various depositional settings. As a whole, the Chilcatay alloformation is bounded by the CE0.1 unconformity at the base and the PE0.0 unconformity at the top. An internal Chilcatay surface, termed CE0.2, splits the alloformation into two distinct allomembers (Ct1 and Ct2). The Ct1 allomember comprises three facies associations recording deposition in shoreface, offshore, and subaqueous delta settings. The Ct2 allomember comprises two facies associations, recording deposition in shoreface and offshore settings. Using these data, we place the rich marine vertebrate assemblage in a precise spatial and stratigraphic framework. The well-diversified vertebrate assemblage is dominated by cetaceans (mostly odontocetes) and sharks (mostly lamniforms and carcharhiniforms); rays, bony fish, and turtles are also present. Taxonomic novelties include the first records of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation. ARTICLE HISTORY Received 10 January 2019 Revised 2 April 2019 Accepted 3 April 2019

Here we present the second of two companion geological maps of the Chilcatay Formation exposed along the Ica River . This new map focuses on the area of Zamaca (Main Map), which has yielded a rich assemblage of fossil marine vertebrates. The primary objectives of this paper are: (i) to gain a better understanding of stratal geometry and depositional settings of each of the facies associations recognized within the study area; (ii) to place the local fossil assemblage within an accurate spatial and stratigraphic framework; and (iii) to provide a quantitative and qualitative evaluation of marine vertebrate distribution.
The basin fill is underlain by Mesozoic and older crystalline and metasedimentary basement rocks (Mukasa & Henry, 1990). Its lithostratigraphic framework was first established by Dunbar et al. (1990) and subsequently refined by DeVries (1998), who Figure 1. Location maps of the present study. (a) Regional geographic context. The red rectangle outlines location of the area shown in detail in Figure 1(b); (b) annotated air photo image showing locations of the study area (red box). The Chilcatay strata exposed in the area in the white frame (Ullujaya) has been mapped by Di . Abbreviations for the informal names here used for referring to these otherwise un-named locations: LDC, Los Dos Cerritos; RN, Roca Negra. divided the basin-fill succession into five formations: (i) the Eocene Caballas and Paracas formations, the latter including the Los Choros and Yumaque members (DeVries, 2017); (ii) the uppermost Eocene to lower Oligocene Otuma Formation (DeVries, Urbina, & Jud, 2017); (iii) the upper Oligocene to lower Miocene Chilcatay Formation (DeVries & Jud, 2018); and (iii) the (?)middle Miocene to lower Pliocene Pisco Formation (DeVries & Jud, 2018;Di Celma et al., 2017;Gariboldi et al., 2017).
Because both the base and the top of the Chilcatay Formation are defined by composite unconformities (namely, CE0 and PE0), this lithostratigraphic unit could be more properly regarded as an unconformity-bounded alloformation (NACSN, 2005). A recent regional paleogeographic reconstruction  shows that, along the Ica River, marine transgression on the bounding unconformities advanced broadly toward the northeast and north, with implied onlap of strata onto the composite CE0 and PE0 transgressive surfaces ( Figure 2).
Age constraints for the Chilcatay strata exposed within and nearby the study area are largely based on 40 Ar/ 39 Ar dating of volcanic ash layers and biostratigraphy. At Ullujaya, about 2.5 km to the north of the study area (Figure 2), an ash layer sampled 2 m above the lowermost available exposure of the Chilcatay Formation (14°35 ′ 2.70 ′′ S -75°38 ′ 24.80 ′′ W) and labeled UJA-T35 yielded an 40 Ar/ 39 Ar age of 19.00 ± 0.28 Ma (Bosio et al., in press). At Los Dos Cerritos (14°35 ′ 41 ′′ S-75°40 ′ 05 ′′ W), an informal name used here for this otherwise unnamed locality (Figure 2), a second ash layer (SOT-T3) located just 1 m below the erosional contact with the overlying Pisco Formation, yielded an 40 Ar/ 39 Ar age of 18.02 ± 0.07 Ma , which is consistent with a previous dating of the same tephra by Belia and Nick (2016). Finally, at Roca Negra ( Figure 2), a third ash layer sampled 4.5 m above the basal unconformity of the Chilcatay Formation (14°39 ′ 2.95 ′′ S -75°3 8 ′ 53.45 ′′ W) and labeled PN-T2 yielded an 40 Ar/ 39 Ar age of 19.25 ± 0.05 Ma (Bosio et al., in press).
A comprehensive micropaleontological study, including both silicoflagellates and diatoms, further constraints the deposition of the local Chilcatay strata between 19 and 18 Ma Lambert et al., 2018), thus providing a biostratigraphic framework that is consistent with the aforementioned radiometric ages.

Methods
To establish a robust stratigraphic framework for the Chilcatay strata exposed in the fossil-bearing locality of Zamaca, an area covering approximately 24 km 2 was geologically mapped at 1:10.000 scale and a 59 m-thick composite section was logged at decimetre scale. One intraformational unconformity (designated CE0.2) and two laterally persistent packages of coarse-grained beds (designated Ct1-1 and Ct1-2 in ascending order) constitute a basis for stratigraphic correlations throughout the study area and possibly beyond. Sedimentological data collected for paleoenvironmental and stratigraphic interpretations include bed thickness, grain size, grain composition, sedimentary structures, bedding contacts, trace fossils, and the identification of important stratal surfaces. The composite section, which is shown in the supplementary material along with the Main Map, was then used to place new and previous fossil finds into their proper stratigraphic positions.
Vertebrate fossil localities were recorded via a handheld geo-referencing device with a mean horizontal error of less than 10 m. Their stratigraphic position along the measured section was estimated with varying degrees of accuracy, ranging from ± 0.3 to ± 3 m. As for other Miocene fossil-bearing localities of the East Pisco Basin Di Celma et al., 2018b), most of the detected specimens were examined directly in the field. Only chondrichthyan fossils (teeth and spines) and some particularly notable cetacean specimens have been collected and are now housed at the Museo de Historia Natural de la Universidad Nacional Mayor de San Marcos (MUSM, Lima, Peru). Loose scales of clupeiform fish, which are common finds in some stratal packages at Zamaca, were neither collected nor geo-referenced.

The Chilcatay Formation
This study builds on previous stratigraphic work aimed to define the stratal architecture of the Chilcatay and Pisco formations along the western side of the Ica River (Bosio et al., 2019;Di Celma et al., 2016a, 2017. The Chilcatay strata encountered near Zamaca can be divided into two discrete units, informally termed allomember Ct1 and allomember Ct2 from oldest to youngest and defined at base by the CE0.1 and CE0.2 unconformities, respectively. These bounding surfaces converge and merge in marginal areas, i.e. toward the northeast, to form the CE0 composite unconformity.

CE0.1 unconformity
The base of the Chilcatay Formation is a distinctive angular unconformity (CE0.1) of regional extent that bevels the underlying Otuma Formation (Figure 3 (a)). This surface is delineated by a monospecific suite of the Glossifungites Ichnofacies expressed essentially by large Thalassinoides that penetrate downward to an average depth of 0.5 m below the discontinuity and are passively filled with medium-grained sand from the overlying Ct1c facies association.  Figure 3(e); (d) close-up view of Ct1-1 (the two blue lines mark its base and top) sandwiched between finer-grained offshore siltstones (14°37 ′ 39 ′′ S-75°38 ′ 42 ′′ W). The basal contact is sharp, extensively scoured and highly irregular; (e) close-up view of the Ct1-2 Marker Unit (the two yellow lines mark its base and top). The association of two different ash layers (a white one and a dark grey one) below and within the Ct1-2 marker bed has been confirmed at different localities by petrographic features.
Ct1c facies association: The CE0.1 unconformity is mantled by a lag of well-rounded pebble-to bouldergrade clasts (up to 0.5 m thick) that, in turn, is overlain by a 10 m-thick package of sandstones variably interbedded with lenticular conglomerate beds. Occasional volcanic ash layers occur as thin interbeds. Sandstones are structureless, fineto medium-grained and may contain varying amounts of granules and small pebbles either scattered or forming lenticular stringers. Conglomerate beds are up to 2.5 m thick and, in general, display poor organization with respect to grain size and bedding. Component clasts, which are locally associated with abundant oyster shells, are mostly derived from the pre-Cenozoic crystalline basement, but also comprise well-rounded clasts of biotite-rich pumice. Outsize clasts up to 2 m in diameter are occasionally observed protruding above the top of the bed into the overlying sandstones (Figure 3(b)). Conglomerates and sandstones typically occur in close vertical association, but locally the entire Ct1c consists of sandstones (Figure 3(a)).
The basal conglomerate of the Ct1c facies association rests directly above CE0.1 and is interpreted to represent a transgressive lag overlying a wave-ravinement surface (Zecchin, Catuneanu, & Caffau, 2019 and references therein). The overlying conglomeratic sandstones were likely deposited in a shoreface environment, with pervasive bioturbation and the presence of the interbedded conglomerates suggesting highly variable energy conditions (Leithold & Bourgeois, 1984;Zonneveld & Moslow, 2004).
Ct1a facies association: This facies association is 31 m-thick and comprises silty to sandy mudstones interbedded with occasional very fineto fine-grained sandstone beds up to 0.3 m thick, as well as a few 2-15 cmthick volcanic ash layers. Invertebrate macrofossils are mostly represented by barnacles, echinoids and bivalves. Within this fine-grained interval there are two laterally extensive packages of coarser grained beds that have been used as stratigraphic datums, namely Ct1-1 and Ct1-2 (Figure 3(c)). These two packages can be easily identified in the outcrops scattered across the study area as they tend to be emphasized by differential weathering of cliff-forming sandstones compared to the surrounding recessive siltstones.
The Ct1-1 marker is about 2.5 m thick and consists of multiple, partially amalgamated, sandstone layers that are arrayed in a fining-upward pattern ranging from medium-to coarse-grained sand at the base, locally containing abundant skeletal debris (most notably barnacles), to fine-grained sand at the top. In places, these sandstone layers pass laterally into discontinuous conglomerate beds that may constitute a substantial portion of the sediment. Conglomerates are composed of a variable mixture of basement-derived pebble-to boulder-grade clasts, and well-rounded pebble-to cobble-grade clasts of biotite-rich pumice. Outsize clasts are common and reach diameters of more than 3 m. Basement-derived clasts are dominantly angular to sub-angular, but a minor amount of subrounded to well-rounded elements also occur. The basal contact of the lower layer is sharp and continuous with broad (up to 1.5 m in cross section) and shallow (0.5 m deep) erosional scours into the underlying siltstones (Figure 3(d)). Beds of this package include a fauna of starfish, ophiuroids, sea urchins, and crinoids, as well as oysters described by DeVries and Jud (2018) and are anomalously rich in both articulated and disarticulated skeletal elements from sharks, marine mammals, and other marine vertebrates.
The Ct1-2 marker is about 2.5 m thick and consists of tabular, laterally extensive beds of light brown (weathering to yellow), fineto medium-grained sandstones ranging in thickness from 0.4 to 0.5 m, interbedded with a whitish, 0.5 m-thick siltstone bed and a gray-yellowish, 15 cm-thick volcanic ash layer (Figure 4(a)). The sandstone beds are massive and their tops intensely bioturbated by vertically spiraling Gyrolithes burrows.
Within Ct1a, ash layers are white in color except one being dark gray. They are composed of >95% of glass shards, few juvenile feldspar and occasional amphibole and biotite. Glass shards are colorless, fine-grained, with platy and minor stretched morphology, while in the dark grey tephra they are brownish.
We interpret the fine-grained sediments of Ct1a as background deposition in a low-energy offshore setting, where continuous accumulation of fine material via suspended fallout deposition was interrupted only by the deposition of occasional, thin sandstone beds likely related to offshore-directed, storm-generated currents (tempestites) (e.g. Aigner & Reineck, 1982).
We concur with DeVries and Jud (2018) in interpreting the coarse-grained layers of Ct1-1 as the result of tsunami backwash processes that entrained and transported in the offshore setting a mixture of skeletal sand and gravel from nearby shoreface, beach, and landward erosion areas (e.g. Cantalamessa & Di Celma, 2005;Einsele, 1998). Distinguishing characteristics include: (i) the highly irregular basal erosion surfaces with multiple asymmetric and symmetric scours; (ii) the unusually coarse grain-size in comparison with the encasing background sediments; (iii) the absence of internal bioturbation; (iv) the presence of multiple graded beds; and (v) the admixture of both wellrounded and angular clasts derived from nearshore and subaerial coastal areas, respectively. The preservation potential of tsunami deposits in offshore settings is facilitated by their emplacement below storm wave base, where they cannot be modified by typical waves. Alternative interpretations of these deposits as the result of extreme weather events are contradicted by the occurrence of outsize boulders too large to be transported seaward any significant distance by even the most violent storm surges. Based on the enormous abundance of vertebrate skeletal remains encased in these deposits relative to adjacent strata, we suggest that these animals fell victim of this catastrophic event, mirroring the way dead fish, turtles, and dolphins are often strewn across the landscape following modern tsunamis (Chagué-Goff, Schneider, Goff, Dominey-Howes, & Strotz, 2011).
The sandstone beds of Ct1-2 are interpreted as tempestites resulting from exceptionally severe storm events transporting shoreface-derived sand-sized sediment into the offshore environment. Upon deposition, these distal storm deposits were colonized and partially reworked during fair-weather conditions by opportunistic . The clinoforms show sigmoidal and less commonly tangential downlap onto the underlying Ct1a, whereas the tops are partially truncated by the CE0.2 unconformity; (b) close-up of the burrowed firmground (CE0.2) at the base of Ct2 (14°37 ′ 1 ′′ S 75°38 ′ 48 ′′ W), penetrated by large Thalassinoides (Th) and Gyrolithes (Gy) documenting a Glossifungites ichnofacies (30-cm-long hammer for scale). Note that the brownish, shark-tooth rich sand from the mantling Ct2a is piped down into these burrows and into the underlying light-gray weathering siltstones of Ct1a; (c) northwestward panoramic view (14°37 ′ 22 ′′ S-75°38 ′ 14 ′′ W), showing clinobeds toeing out asymptotically on the underlying Ct1a siltstones and rapidly wedging out basinwards (to the left); (d) panoramic view of the regularly-bedded upper portion of Ct1 and the lower portion of Ct2, which is cut by a southwest-dipping sharp surface overlain by variably deformed strata (14°37 ′ 4 ′′ S-75°38 ′ 48 ′′ W). Downwards, the surface becomes less distinct and passes into a sheared interval zone comprising numerous extensional faults with throws of 0.1-0.5 m in the immediately underlying undeformed sandstones of Ct2a. The concave shape and gradient of the basal surface indicate a position at the lateral (eastern) margin of a submarine landslide. Encircled geologist for scale. organisms favoring sandy substrates (e.g. Pemberton & MacEachern, 1997).
Ct1b facies association: This facies association is comprised of a southwesterly prograding wedge characterized by oblique and sigmoidal clinoforms downlapping onto the underlying sediments of Ct1a (Figure 4(a)). This wedge extends about 4 km downdip and its thickness decreases basinwards from about 20 m in the northeastern portion of the study area to a zero-edge in its central part. Updip, the foresets are truncated by the overlying CE0.2 unconformity and topsets are poorly preserved or absent, whereas downdip they pass asymptotically into bioturbated toesets and bottomsets (Figure 4(a)). Individual foresets are between 0.2 and 0.5 m thick, have dip angle ranging between 15°and 20°, and include a variable mixture of coarse-grained biogenic components, like barnacles, mollusks, echinoids, and calcareous tubes, as well as granule-and small pebble-sized terrigenous components.

CE0.2 unconformity
Seaward of the basinal pinch out of Ct1b, the CE0.2 unconformity is interpreted at an abrupt basinward dislocation of facies belts, with sediments of a shallower setting (Ct2a) resting directly on sediments of a deeper setting (Ct1a). This surface is commonly recognized through the presence of large, passively infilled Thalassinoides and Gyrolithes burrows extending into underlying silts of Ct1a (Figure 4(b)). When traced landward, CE0.2 directly overlies Ct1b (Figure 4(c)) and is typically penetrated by Thalassinoides. Despite its abrupt nature, considerable lateral extent, and the presence of locally derived rip-up clasts (see below), CE0.2 exhibits only minor erosional relief across the scale of the outcrop and is interpreted as cut during lowstand subaerial exposure and subsequently reworked by marine ravinement during transgression.

Ct2 allomember
Two genetically related facies associations have been identified in this allomember. They are designated Ct2a and Ct2b in ascending order and indicate deposition in nearshore to offshore settings during renewed relative sea level rise and transgression.
Ct2a facies association: This facies association is about 3.5 m thick and lies both sharply and unconformably above the fine-grained sediments of Ct1a in distal positions (Figure 4(b)), and the erosional top of Ct1b in proximal positions (Figures 4(a) and 4(c)). It is composed of highly fossiliferous, medium-to very coarse-grained sandstones (Figure 4(b)) containing sub-rounded to sub-angular pebbles derived from the immediately underlying siltstones, as well as basement derived clasts and rare coal fragments scattered along the base. Biogenic reworking is extremely pronounced and results in a generally massive bedding appearance. The fossil content comprises dispersed crustaceans (crabs and barnacles), shark teeth and vertebrae (Landini et al., 2019).
Considering the intensity of bioturbation and lack of primary sedimentary structures, we propose that Ct2a was deposited in a weak-energy shoreface setting (MacEachern & Pemberton, 1992), marking the local onset of a regionally extensive marine transgression.
Ct2b facies association: This facies association is more than 15 m-thick and comprises a heterolithic succession of weakly bioturbated, thinly-bedded silty mudstone intercalated with minor, laterally persistent, very fine-grained sandstone interbeds and occasional volcanic ash layers.
Over most of the study area, Ct2b comprises gently dipping to flat-lying beds. Along the western side of the study area, however, a package characterized by significant internal distortion and deformation of bedding is exposed. This internally deformed package is underlain by a distinct, scoop-shaped surface that cuts to varying degrees into the underlying undeformed succession. At its most eastward exposure (Figure 4(d)), this surface exhibits at least 15 m of incision into Ct2b. Towards the west it extends beneath the level of the CE0.2 unconformity, cutting through the Ct1-2 marker and attaining a total depth of incision of at least 30 m. Yet further west, it passes into the subcrop, such that the deepest point of erosion is not exposed.
Overall, the fine-grained nature of Ct2b reflects deposition in an offshore setting, where silty mudstones were deposited from suspension during fairweather periods, and sandstone tempestite beds were deposited by occasional storms (Aigner & Reineck, 1982); supporting this interpretation, clupeiform scales and diatoms are frequently found in the Ct2b strata, thus suggesting an epipelagic-neritic paleoenvironment (see also DeVries & Jud, 2018 at this respect). Because the internally deformed sediment wedge is underlain by undeformed strata and is separated from the laterally adjacent undeformed strata by a sharp concaveupward surface, we here interpret it as the result of submarine slumping with remobilization of primary bedding and ductile deformation of the strata.

Vertebrate paleontology
In total, we recorded 140 individual marine vertebrate fossils inside the study area, distributed throughout the 59 m-thick section of the Chilcatay Formation. Of the 129 specimens whose stratigraphic level could be determined, 25 were found in Ct1c, 95 in Ct1a (54, or 41.9% of the total assemblage, in Ct1-1), 2 in Ct1b, 6 in Ct2a, and 1 in Ct2b (see Main Map).
As at the nearby locality of Ullujaya , fossils are mostly incomplete and either partially or fully articulated, albeit with some notable exception (e.g. Lambert et al., 2018). Cetacean remains dominate the assemblage (79.9%). Curiously, there are just two baleen whales (Mysticeti; Figures 5(c) and 5 (d)), with all other specimens identified as belonging to either echolocating toothed cetacean (Odontoceti) or, in cases where only vertebrae and/or ribs were preserved, aff. Odontoceti.
Most of the kentriodontids (including 12 specimens now kept at MUSM) belong to Kentriodon sp., which also occurs at Ullujaya. Physeteroids include a wellpreserved skull with an unusually narrow and elongated rostrum bearing relatively small teeth. Together with other skulls observed at Zamaca and the two physeteroid specimens collected at Ullujaya , the new fossil reveals an unexpectedly high degree of sperm whale disparity in the Chilcatay assemblage. Squalodelphinids are represented by remains resembling Notocetus vanbenedeni ( Figure 5(a)) and Huaridelphis raimondii (including MUSM 599 and MUSM 603, described by Lambert et al., 2010). Platanistids are tentatively recorded for the first time in the Chilcatay Formation, and include a rostrum with associated mandibles (MUSM 631) and a partial skeleton including the skull. Similarly, eurhinodelphinids are also identified for the first time here, and comprise a partial skull (MUSM 632), as well as an uncollected skull with associated cervicals. The heterodont dolphin Inticetus vertizi is known only from the holotype (MUSM 1980; Figure 5(b)), an almost complete skeleton found at the base of the Chilcatay section at Roca Negra (Lambert et al., 2018). Finally, Chilcacetus cavirhinus is represented by a skull with associated postcrania (MUSM 2527), collected about 10 m above the base of the measured section (abs). This specimen is now under study, along with additional material from Ullujaya.
Bony fish represent 11.5% of the marine vertebrate assemblage, and are distributed between 10 and 22.4 m abs. They are represented by fragmentary skeletons, consisting of large cranial and postcranial bones, including some tuna-like vertebrae and two istiophorid-like billfish skulls (in addition to these records, it should be noted that loose clupeiform scales are widespread in the fine-grained sediments of Ct1a and Ct2b, suggesting the presence of schools of epipelagic fish in the early Miocene offshore paleoenvironments of Zamaca). Disarticulated elasmobranch vertebrae are present, but uncommon (2%). Marine turtles (6%) were found from 7 m to 47.2 m abs, and include an almost complete skeleton, some skulls (one of which collected), and some more fragmentary specimens. Most of these bones are large, and could plausibly represent the same dermochelyid as recorded at Ullujaya .
We collected more than four thousand elasmobranch teeth and spines from the northern portion of the study area, representing at least twenty-two species in five orders of sharks and rays (Landini et al., 2019).

Conclusions
We present a comprehensive overview of the stratigraphic architecture, depositional settings, and vertebrate paleontology of the lower Miocene Chilcatay strata exposed at Zamaca (East Pisco Basin, southern Peru). The allostratigraphic scheme originally established for the Chilcatay Formation at Ullujaya is extended southwards into the Zamaca area, where the intraformational CE0.2 unconformity subdivides the exposed succession into two allostratigraphic units (Ct1 and Ct2 allomembers). The Ct1 allomember comprises three facies associations recording deposition in shoreface (Ct1c), offshore (Ct1a), and a mixed siliciclastic-carbonate subaqueous delta (Ct1b) perched on the basin margin. Two coarse-grained packages, Ct1-1 and Ct1-2, have been traced across the study area within the fine-grained Ct1a facies association. The Ct2 allomember comprises two facies associations recording deposition in shoreface (Ct2a) and offshore (Ct2b) settings.
This work provides both a quantitative and qualitative account of marine vertebrate distribution in a fossil rich early Miocene marine locality, with individual fossils being located precisely on a high-resolution geological map and accompanying stratigraphic section. Fossils are unevenly distributed and especially concentrated within Ct1-1, a possible tsunami deposit. The Zamaca vertebrate assemblage is dominated by cetaceans (mostly toothed whales) and sharks (mostly lamniforms and carcharhiniforms); ray, bony fish and turtles are also present. Its overall composition resembles that of the coeval assemblage at Ullujaya, yet also provides the first record of baleen whales, platanistids, and eurhinodelphinids from the Chilcatay Formation.

Software
The geological map was compiled by scanning hand drafts as black and white TIF files, and then digitizing the line art using the Corel Draw X3 graphics package. We used the GIS Data processing application Global Mapper 12 to generate contour lines for the 1:10,000 scale topographic base map. To do so, we relied on digital elevation model (DEM) based on the Shuttle Radar Topography Mission 26 (SRTM), as released by the United States Geological Survey (SRTM3 USGS version 2.1). The background aerial imagery is from World Imagery (Esri, https://services.arcgisonline.com/ ArcGIS/rest/services/World_Imagery/MapServer). stays at the MUSM. Journal reviewers G. Coletti, T.J. DeVries, M. Murad-al-shaikh, and Associate Editor Monica Pondrelli are gratefully acknowledged for their thoughtful contribution and helpful criticism that sharpened the focus of this study. We also thank T.J. DeVries for providing invaluable guidance to new field sites and insightful discussions; responsibility for facts and interpretations nevertheless remains with the authors. The field prospections were made under the auspices of a collaborative program between the Museo de Historia Natural of the Universidad Nacional Mayor de San Marcos (Lima, Peru) and several research institutions. All specimens collected as part of this project have been deposited at the Museo de Historia Natural.

Disclosure statement
No potential conflict of interest was reported by the authors.