Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina

. The early-middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The lower levels of the CBF consist of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork provided fossil vertebrates from these levels with an estimated age between ~16.5 Ma and 15.1 Ma (late Burdigalian-early Langhian). The studied section temporally overlaps with the middle or upper sections of the Santa Cruz Formation (SCF) in the Austral-Magallanes Basin of southern Patagonia, the Río Frías Formation in Chile, and the lower Collón Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality, including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato . We also recorded a testudine, which constitutes the southernmost record of tortoises in South America

particularly for the SCF along Río Santa Cruz (Fernicola et al., 2019a;Kay et al., 2021) and Río Chalía (Cuitiño et al., 2021) in the central region of the province, and in the northwestern part of the province at Lago Posadas, on the eastern slope of the Patagonian Andes (Cuitiño et al., 2019a).In the northwestern Santa Cruz, the early-middle Miocene record still is much less known than that from east of the Andes in the widely distributed SCF (Cuitiño et al., 2019b(Cuitiño et al., , 2021)).
The Cerro Boleadoras and Cerro Plomo region, very close to the border with Chile, ~50 km south to the Lago Buenos Aires-General Carrera (Fig. 1), is promising for understanding the evolution of the Miocene continental biota in northwestern Santa Cruz Province (~47° S, 71° W).In this area the most fossiliferous levels of the early-middle Miocene continental Cerro Boleadoras Formation (CBF) crop out.co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both forested and open environments for the area occupied by the CBF rocks.However, it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other.Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification during the Miocene in southern Patagonia, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.

Introduction
The early-middle Miocene (Burdigalian-early Langhian) fossil record from Austral Patagonia provides insights on the evolution of continental environments, climates, and biota of the Southern part of South America when it was essentially isolated from other continental landmasses (Kay et al., 2012(Kay et al., , 2021)).In a volume published a decade ago (Vizcaíno et al., 2012a), we launched a research program to evaluate the ecological changes that occurred during this time interval, integrating the information from well-studied outcrops and biota of the Santa Cruz Formation (SCF) along the Atlantic coast of the Santa Cruz Province (Argentina) with other much less studied and more distant inland localities.As part of that endeavour, in recent years we have produced a valuable body of knowledge, FIG. 1. Maps of the study area.A. Regional geographic location of the Santa Cruz Province.B. Fossiliferous localities mentioned in the text.1. Cerro Boleadoras and Cerro Plomo area this work; 2. Lago Posadas (Cuitiño et al., 2019a); 3. Río Chalía (Cuitiño et al., 2021); 4. Río Santa Cruz (Fernicola et al., 2019a); 5. Atlantic coastal localities (Vizcaíno et al., 2012a).C. Geological map of the study area; location of the Cerro Boleadoras and Cerro Plomo sections integrated for the stratigraphic section of figure 2. C-GB: Cosmelli-Guadal Basin; MLBA: Meseta Lago Buenos Aires.
So far there are few reports of the vertebrate fossils from CBF, consisting exclusively of mammals.Ugarte (1956) recorded the presence of South American native ungulates Nesodon? and Astrapotherium magnum?, the glyptodont Propalaehoplophorus and the megatheriid ground sloth Prepotherium.The present repository of these specimens is unknown.In 1990 and 1991 Austral summers, joint expeditions of the Museo de La Plata (Argentina) and Duke University (NC, USA) collected some 220 specimens from CBF now housed in the collection of the División Paleontología Vertebrados of that museum.Of this collection, only some xenarthrans (Carlini et al., 1993;Scillato-Yané and Carlini, 1998;Carlini et al., 2013) caviomorph rodents (Vucetich, 1994), and typotheres (Vera et al., 2021), have been examined.Although none of the taxa is exclusive of middle Miocene faunas, some authors have considered a Friasian s.l.South American Land Mammal Age (SALMA; middle Miocene) for the fauna from CBF (Ugarte, 1956;Scillato-Yané et al., 1993;Scillato-Yané and Carlini, 1998).Instead, based on the rodents, Vucetich (1994) referred the CBF to the early Miocene and established correlations with the Santacrucian age.Recently, a radiometric date near the base of the CBF provided a maximum U-Pb age from detrital zircons of ca.16.5, i.e., Burdigalian (early Miocene;Folguera et al., 2018; see below), suggesting a most probable Santacrucian age for the fauna.
During recent fieldwork (February 2020), we made a new collection of fossil vertebrates from the CBF, particularly from the lower levels of Cerro Boleadoras and Cerro Plomo (Fig. 1).The aim of this contribution is to report this new collection of fossil vertebrates, to compile an integrated faunal list for this locality including other specimens from previous collections when possible, and to discuss its affinities and chronological and paleoenvironmental implications.
as fluvial channel deposits.Interspersed are fluvial floodplain mudstone deposits showing incipient soil-horizonation, slickensides and cuneiform blocky structures (Fig. 3).The thickness of the fluvial channel deposits increases upwards from 10 m to nearly 25 m, whereas the floodplain deposits decrease their thickness upwards from 3 to 1 m.Both sedimentary facies show increased grain-size up section, from mudstones and fine to medium sandstones to fine sandstones and fine conglomerates.In most of its exposures, the Río Zeballos Group is covered by late Miocene to Quaternary basalts of the Meseta del Lago Buenos Aires Formation (Gorring et al., 1997;Kay et al., 2002).

Age of the fossil-bearing succession
Based on a U-Pb detrital zircon maxium age, the lower part of the CBF in the Río Jeinemeni area is estimated at ca. 16.5 Ma,i.e.,Burdigalian FIG. 2. Integrated stratigraphic section of Cerro Boleadoras Formation (Río Zeballos Group) in the study area.Maximum Depositional Age (MDA) from Folguera et al. (2018) and ages estimated from the sedimentary rate are indicated (see text for detail).(Folguera et al., 2018;Fig. 2).This is consistent with the recently obtained early Miocene detrital U-Pb ages for the underlying Río Jeinemeni Formation as well as middle-late Miocene for the overlying Río Correntoso Formation (Aramendía et al., 2022).The fossil-bearing fluvial deposits of the SCF exposed near Lago Posadas (Fig. 1B, C) are temporally equivalent to the Río Jeinemeni and Cerro Boleadoras formations, with an age constrained between 18 and 14 Ma based on 40 Ar/ 39 Ar tuff ages (Blisniuk et al., 2005), tephrochronology (Perkins et al., 2012) and 87 Sr/ 86 Sr ages for the underlying marine deposits (Cuitiño et al., 2015(Cuitiño et al., , 2019a)).
Although no radiometric ages are available for the Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation...
Cerro Boleadoras and Cerro Plomo area, based on facies similarity, the studied stratigraphic interval is assigned to the lower part of the CBF, whose ~16.5 Ma age is extrapolated from that of the Río Jeinemeni area.Facies similarity is also evident with the nearby SCF in Lago Posadas, allowing us to hypothesize that the average sedimentation rate value of 114 m/Myr calculated for that area (Blisniuk et al., 2005) could be extrapolated to the 150 m-thick surveyed succession of the lower part of the CBF, which would range between ca.16.5-15.1Ma (Fig. 3).

Materials and methods
The new fossil collection consists of approximately 100 specimens of vertebrates, including frogs, turtles, birds and mammals that are housed in the Museo Regional Provincial Padre M.J. Molina (MPM-PV) of Río Gallegos, Argentina.The collection comes from the CBF at Cerro Boleadoras and Cerro Plomo between 1,290 and 1,450 m a.s.l., representing a ~150 m-thick section, integrated on the basis of lateral correlation between both hills (Figs. 2, 3).Most of the specimens were found loose on the surface, so they were collected in lots, except for one specimen found in situ.
Unfortunately, due to the restrictions related to the COVID-19, the MLP and Duke University (MLP-DU) collections from CBF housed at MLP have been virtually unreachable since April 2020.However, we had access to the tortoises (two specimens) and rodents (44 specimens) that were studied herein and information from colleagues that possess other taxa specimens on loan from MLP.

Paleontology
In this section we provide a systematic list and general descriptions of the specimens from CBF collected by us (MPM-PV collection) and those from the MLP-DU collection that we had access to (see Material and methods; Table 1).The geographic and stratigraphic occurrence of all the specimens listed here is Cerro Boleadoras and Cerro Plomo area, RP 41, 60 km South of Lago Buenos Aires, Santa Cruz Province, Argentina.Cerro Boleadoras Formation, Río Zeballos Group, early-middle Miocene.
Description.The material includes many bone fragments of the roof of the skull and some fragmentary maxillae (MPM-PV 19140; Fig. 4A).
The roofing bones are ornamented with discrete ridges and low conical tubercles that are distributed over the outer surface of each bone.A distal part of a left humerus of MPM-PV 19140 preserves the distal ball and the two epicondyles (Fig. 4B).There are several vertebrae, all disarticulated; most lack neural arches (Fig. 4C-D).The vertebral bodies are procoelic and range from 8.8 to 27.5 mm in length, 7 to 34.2 mm in transverse diameter, and 5.3 to 14.8 mm in vertical diameter.The largest is a sacral vertebra with a flattened, procoelous centrum and the characteristic posterior two condyles for the articulation with the cotyles of the urostyle (MPM-PV 19143).
Comments.The remains belong to several individuals.As a whole, they show the ornamentation, size range, and degree of ossification resembling Calyptocephalella, the most common among fossil anuran from Patagonia (Muzzopappa, 2019).
For those reasons, we tentatively assign them to Calyptocephalella (late Cretaceous-Miocene, Patagonia Argentina; present, Chile).Calyptocephalella sp. had been recorded in the Santa Cruz Formation (SCF) from the outcrops along the Atlantic coast (Tauber, 1999;Fernicola and Albino, 2012), while Calyptocephalella cf. C. canqueli Schaeffer, 1949 was reported for the Barrancas Blancas locality along the Río Santa Cruz (Muzzopappa, 2019).contacts the border.Based on the morphology of the anterior-most part of the anterior lobe, we can infer that this specimen belonged to a male individual.On the visceral side, the epiplastral lip is tall, but with a straight vertical border not overhanging the entoplastron (that is, there is no gular pocket).
Ventrally, the two gular scutes are narrow and the gularo-humeral sulcus creates a near-right angle, terminating on the epiplastra and at a considerable distance anteriorly to the entoplastron.Based on the part that can be reconstructed, the entoplastron was wide and placed towards the posterior part of the anterior lobe.A partial hyo/hypoplastron fragment from the near-bridge area also is preserved (MPM-PV 19149; Fig. 5P-Q).On its ventral side there is only a small part of a sulcus preserved, and there is no evidence of the presence of axillary/inguinal scutes, something that does not allow species-level identification.The posterior lobes of the left and right partial xiphiplastron are preserved as well (MLP 90-III-5-161; Fig. 5R-U).These parts exhibit rounded and flattened xiphiplastral extremities with narrow (Fig. 5R, S) or wide (Fig. 5T-U) anal notches.A partial left femur (MLP 90-III-5-161; Fig. 5V-W) preserves the rounded femoral head, which is as tall as wide, expanded perpendicularly to the diaphysis of the bone.The head has a similar width to the width of the trochanters.Ventrally, the two trochanters are fused, and do not project above the height of the femoral head.A deep and rounded intertrochanteric fossa is developed between the head and the trochanters.
Comments.The preserved specimens allow a clear allocation to Testudinidae, based on the presence of an alternating pattern of neural/costal associations, the good coincidence between the pleuro-marginal sulci and the costo-peripheral sutures on the peripherals, the presence of a well-developed epiplastral lip, and the presence of fused femoral trochanters.Moreover, the observed morphology of the most diagnostic elements of the new collection (e.g., the associated epiplastra in Fig. 5M-O 6A-B).Ungual phalanges of digit II are more strongly curved in dorsal and lateral aspects, higher dorsoventrally and more laterally compressed (i.e., subelliptic in section) than other ungual phalanges.The tuberculum flexorious is well developed and ends in a rounded plantar surface.The corpus phalangis exhibits well marked sulci neurovascularis on both sides (Jones, 2010).
Comments.The genus has been recorded in the early-middle Miocene of Patagonia, Argentina.Phorusrhacos longissimus is known by very abundant and well-preserved remains from outcrops of the SCF along the Atlantic coast (Degrange et al., 2012), from the Cerro de los fósiles locality at Lago Belgrano (Degrange et al., 2019) and from Río Chalía (=Sehuén) (Alvarenga and Höfling, 2003;Noriega et al., 2021).Although the type specimen comes from the Río Santa Cruz (Ameghino, 1887); however, the species is not recorded in more recent collections from the Río Santa Cruz (Diederle and Noriega, 2019).Abundant and well-preserved remains of Patagornis marshi are known in the SCF along the Atlantic coast (Alvarenga and Höfling, 2003;Degrange et al., 2012) and at Río Chalía (=Sehuén) (Noriega et al., 2021).It is not recorded in SCF outcrops along the Río Santa Cruz (Diederle and Noriega, 2019).(Moreno and Mercerat, 1891), but somewhat smaller than that of P. lemoinei, with flanges proximally convergent, more similar to that of P. lemoinei than to P. affinis (Ameghino, 1899).
The dimensions of the specimen together with the porous texture of its bony cortex suggest that it probably belongs to a subadult individual.
Comments.Psilopterus has been recorded in the early-middle Miocene of Patagonia, Argentina.
Recently, it was recorded from the SCF along the Atlantic coast (Degrange et al., 2012), from the Barrancas Blancas locality along the Río Santa Cruz (Diederle and Noriega, 2019), and along the Río Chalía (Noriega et al., 2021).Psilopterus lemoinei is one of the best-known phorusrhacid species.Carlini, 1998;Carlini et al., 2013;Brandoni et al., 2016).Calcanei of planopines are very rare in the collections.They were described only for Prepotherium potens (Scott, 1904), Planops martini (Hoffstetter, 1961), and for Prepoplanops boleadorensis (Carlini et al., 2013).This new specimen is clearly referable to a Planopinae because of its large size and the morphology of its proximal articular surface.The morphology of the astragalar (ectal and sustentacular) and cuboid facets resembles the holotype of Prepolanops boleadorensis (MLP 97-XI-3-1), so we refer our specimen provisionally to this species.
Comments.Sloths are common elements of the earlymiddle Miocene deposits of Patagonia.Skulls and mandibles are rare, and isolated postcranial elements are particularly abundant.However, associations between the two are rare, making the taxonomic allocation of isolated postcranial elements difficult.We refer the postcranial elements (e.g., pes and manus elements, fragments of long bones, vertebrae, etc.) reported here to Megatherioidea indet., a clade that includes the most common Santacrucian genera (see Bargo et al., 2019 and references therein).
Xenarthra/Cingulata/Peltephilidae Peltephilus Ameghino, 1887 Peltephilus sp (Fig. 7D) Referred material.See Appendix 1. Description.MPM-PV 19125 consists of four isolated osteoderms: two with irregularly polygonal shape and a very rugose surface, characteristic of the cephalic shield (Fig. 7D), and two of the caparace.The carapace osteoderms of Peltephilus have different sizes and shapes, with coarsely pitted surfaces and marginal pits.One of the new osteoderms belonging to the fixed part of the carapace is large, pentagonal, and thick (16.0 mm anteroposterior length; 9.0 mm transverse width; 5.25 mm thickness).It has an inverted u-shaped groove anteriorly.The other is a quadrangular movable osteoderm, small and thin   7E).The grooves and the figures reach the posterior margin of the osteoderm.
The grooves enclose four large foramina and other smaller ones.There are also small foramina on both lateral osteoderm margins, and on the posterior margin there are four large foramina with two or three alternate smaller ones.Fixed osteoderms of the referred specimens (MPM-PV 17517, 17522, 17526 and 19127) are variable in size (anteroposterior length from 14 to 17 mm, transverse width 9 mm and thickness from 4-8.5 mm).They have three to eight conspicuous foramina on the posterior margin.The lateral and anterior margins have continuous and well-defined rows of foramina.Four or five peripheral figures surround the central figure; most of the grooves that delimit these figures have foramina.
Comments.The genus Vetelia is currently considered to be phylogenetically related to the extant Tolypeutinae Priodontes, Cabassous, and Tolypeutes (Barasoain et al., 2021).It was recorded from the early-middle Miocene of Patagonia Argentina and Chile, and late Miocene of Buenos Aires, La Pampa, San Juan, La Rioja y Catamarca.According to González-Ruiz (2010) Vetelia is recorded in the "Nothippidian" fauna of the SCF at Karaiken, the enigmatic "la Cueva" locality (see Marshall, 1976), the SCF of Río Chalía, and the middle Miocene of Río Frías (at Río Cisnes) and Collón Curá formations.Cuitiño et al., (2021) confirmed the presence of Vetelia sp. at Río Chalía, but Fernicola and Vizcaíno (2019) did not report it for the SCF along the Río Santa Cruz.
According to González-Ruiz (2010) and Vizcaíno et al. (2012b), Vetelia puncta was first reported from the north bank of the Río Santa Cruz, in outcrops referred by Ameghino (1902) to the "Notohippidian" (the oldest Santacrucian faunal assemblage according to Marshall et al., 1983).Fernicola et al. (2009) reported a single osteoderm of the species from the coast of Santa Cruz Province, south of the mouth of the Rio Coyle.The species has previously been reported from the Cerro Boleadoras and the Río Frías Formations (González-Ruiz, 2010; Barasoain et al., 2021).
Comments.The genus has been recorded in earlymiddle Miocene of Patagonia Argentina.The species are distinguished mainly by cranial and upper dental elements (Sinclair, 1909).However, the genus is also characterized by molarized lower premolars, with the trigonid and talonid of p3-p4 exhibiting a similar development (Fernández and Muñoz, 2019), features present in our specimens.Thus, we are confident about the generic allocation but refrain from proposing a specific identification.Interatherium has been reported for the "Pinturan" levels of the Sarmiento Formation (Chubut Province), the SCF along the Río Santa Cruz, the Río Chalía, and South to the Río Coyle, and Lago Posadas, and the Collón Curá Formation (Río Negro Province) (Fernández and Muñoz, 2019;Cuitiño et al., 2019aCuitiño et al., , 2021;;Kay et al., 2021).
In labial view, the dp2 has a single cusp surrounded by a low cingulid and there is an incipient ectoflexid (Fig. 8M).On dp2, a short crest runs posterolabially to the base of the paralophid.Also on dp3, the paraconid and metaconid are well developed, the latter dividing the tooth into two lingual valleys, an anterior metaflexid, which is larger and deeper than a posterior entoflexid.Finally, a low cristid extends between the paraconid and the metaconid.
On the dp3 the protoconid and the hypoconid are separated by a deep ectoflexid.The labial cingulid is better developed than the lingual one and is more marked than in dp2.In the occlusal view, the trigonid is longer and narrower than the talonid, but the metaflexid and the entoflexid have the same width (Fig. 8O).Paraconid, metaconid, and hypoconulid are placed at the same lingual level.
Comments.The genus Thoatherium has been recorded in the SCF (early-middle Miocene) of Santa Cruz Province at Río Santa Cruz (Schmidt et al., 2019), Río Chalía (Cuitiño et al., 2021;Monsalvo et al., 2021), and the Atlantic coastal localities (Cassini et al., 2012), and in the Galera Formation (early Miocene), Pampa Castillo, Aisén Region of Chile (McGrath et al., 2020).MPM-PV 19150 is comparable in size to Thoatherium minusculum and Tetramerorhinus lucarius, but smaller than Anisolophus floweri.The dp2 of MPM-PV 19150 shares with Th. minusculum (Scott, 1910;Soria, 2001) the anterior bifurcation of the paralophid, a strong metaconid, a weak external groove, and a short and posteriorly directed hypolophid.The dp3 of MPM-PV 19150 is similar to Th. minusculum, with the trigonid longer and narrower than the talonid (Scott, 1910;Soria, 2001 et al., 2021;Monsalvo et al., 2021).The m1 trigonid of MPM-PV 19151 is shorter than the talonid, as in Anisolophus australis, A. minusculus (Soria, 2001) and A. floweri (Corona et al., 2019).Although its measurements fall in the range of size of A. australis (Soria, 2001), the poor preservation and degree of wear of MPM-PV 19151 led us to consider it as Anisolophus sp.Description.The assignment of the new specimens from CBF to Neoreomys australis is based on the highcrowned but rooted cheek teeth (the protohypsodont condition); this species has the highest crowns of the genus (Kramarz, 2006a).The hypoflexus/id has cementum in moderate stages of wear, and the flexus/ ids-fossetes/ids are more elongated and narrower than in Luantus, Chubutomys, or Phanomys.The teeth are larger than in the cavioids mentioned above, and the enamel is thicker (similar to Alloiomys and Australoprocta) and continuous around the perimeter of the crown (Fig. 9A-B).The dental series resembles the condition of Luantus in being more convergent towards the midline than in Dasyprocta but less so than the euhypsodont cavioids, such as Eocardia, Guiomys, or crown Caviidae.The mandible is robust and the notch for the tendon insertion of the masseter medialis pars infraorbitalis muscle (nMpi) is confluent with the masseteric crest (Fig. 9A').
Among the new specimens reported here, MPM-PV 19168 is a mandible of a juvenile individual with dp4, m1, and erupting m2 (Fig. 9A).The dp4 is similar in size and morphology to that of N. australis, and different from those of N. huilensis (La Victoria and Villavieja formations, La Venta, Colombia, middle Miocene) in that the type species is larger and with less persistent flexids and fossetids.Deciduous premolars of N. pinturensis (Pinturas Formation, early Miocene) are not known so far.Labially, the dentary has a delicate nMpi below of dp4-m1.Lingually, the incisor extends distally to m2.The specimens grouped in the lot MPM-PV 19175 are probably associated and, although they are in poor condition, can be easily assignable to N. australis.Comments.Several specimens (mainly isolated teeth and one mandibular fragment) from Cerro Boleadoras housed at MLP were assigned to Neoreomys sp. by Vucetich (1994: 368-369).The genus has a wide geographic distribution, including the early-middle Miocene of Patagonia, and early Miocene of San Juan Province, Argentina, the early-middle Miocene of Chile, and middle Miocene of Colombia.Neoreomys australis has been recorded in the Pinturas Formation (Kramarz, 2006a), and Santa Cruz Formation at the Atlantic coast (Candela et al., 2012), Río Santa Cruz (Arnal et al., 2019), and Río Chalía (Cuitiño et al., 2021), Santa Cruz Province, Argentina.
Rodentia/Hystricognathi/Caviomorpha/ Cavioidea Eocardia Ameghino, 1887 Eocardia montana Ameghino, 1887 (Fig. 9C-D) Referred material.See Appendix 1. Description.These specimens (see Appendix 1) of Eocardia montana are characterized by euhypsodont molars, with double-heart shaped occlusal profile and a constriction in the labial apex of each lobe, a funnel-shaped hypoflexid with cementum, enamel interrupted on the lingual wall, and absence of lingual flexids/fossetids in moderate stages of wear.In the mandibular fragment MLP 91-I-10-1 (Fig. 9C), the m2 has a small lingual flexid (metaflexid) that remains open throughout the entire crown height (Fig. 9C, C").The persistence of a flexid in euhypsodont taxa, although uncommon, is a variable condition in E. montana (e.g., MACN-A 4455).On MLP 91-I-10-1, the notch for the tendon insertion of nMpi of is located near the ventral margin of the dentary, between m1-m2, and connected to a poorly developed masseteric crest (Fig. 9C').MLP 90-III-5-12 (Fig. 9D) is a left dentary fragment of a juvenile, with a slightly worn dp4 and m1.The nMpi (Fig. 9D') is located on the labial wall of the dentary between dp4-m1.Both teeth are high-crowned, with open flexids, and continuous enamel around the entire crown, without cementum.The dp4 is pentalophodont and mesiodistally elongated, with a convex posterolophid.The mesial margin of the tooth is concave with a slight anterior extension.In this stage of wear, the metaconid is well-developed.The hypoflexid has concave margins and it extends transversely about halfway across the occlusal surface.The lingual flexids are open.The second crest (metalophulid II?) is long; the mesolophid and hypolophid are short.The mesial and distal mesoflexids (sensu Boivin and Marivaux, 2018) are short.The m1 has a convex metalophulid I and posterolophid.The m1 hypoflexid is wide and slightly posteriorly concave in its anterior margin.MLP 90-III-5-12 is very similar to MACN-A 10205, a right mandibular fragment with dp4-m1, collected by C. Ameghino in the SCF (without provenance), and assigned to Eocardia montana by Pérez (2010).The CBF specimen MACN-A 10205 is a little larger and is slightly more worn than MLP 90-III-5-12, which is evident in the dp4 because the antero-and mesial mesoflexid are closed and the metaconid is indistinct.Comments.Vucetich (1994) assigned to Eocardia?five specimens from Cerro Boleadoras, indicating that all probably correspond to a single species.The genus Eocardia has been reported for the early -middle Miocene of Patagonia Argentina and early Miocene of San Juan Province, Argentina.The revision performed here allows us to assign two of them (MLP 90-III-5-12 and MLP 91-I-10-1) to Eocardia montana, following Scott (1905) and Pérez (2010), being the first record of this species in CBF.This species is a very common element of the SCF in Patagonia (Pérez, 2010;Arnal et al., 2019).López et al. (2011) reported this species for the Chinches Formation (early Miocene, San Juan Province).Other CBF specimens not examined here-MLP 90-III-5-37 (90-390), MLP 90-III-5-15 (90-347), and MLP 90-III-5-10 (90-289)-were assigned as ?Eocardia by Vucetich (1994).
Rodentia/Hystricognathi/Caviomorpha/ Cavioidea Luantus Ameghino, 1899 Luantus propheticus Ameghino, 1899 (Fig. 9E-G) Referred material.See Appendix 1. Description.Two isolated teeth MLP 91-I-10-8c-d (Fig. 9E-F) are referred to Luantus propheticus because of the following characters: molars are protohypsodont (high-crowned but retaining roots, retain flexids/fossetids, have marginal enamel that is continuous around the crown except lingually at the base of the crown in moderate stages of wear, and have wide hypoflexids with straight, V-shaped borders.In addition, the size coincides with other specimens of this taxon (Kramarz, 2006b;Pérez, 2010).The m1 or m2 (MLP 91-I-10-8c-d) have straight lingual walls, and the mesial and distal margins are slightly convex.The protoconid and hypoconid areas lack the constriction commonly observed in euhypsodont taxa, like Eocardia.The lingual fossetids are elongate and narrow.The mesofossetids are the smallest of the fossetids, as in other related cavioids (e.g., Neoreomys, Phanomys, Eocardia, Schistomys).The anterofossetids are oriented slightly mesiolabially, whereas the meso-and metafossetids are more transverse.The smaller of the two molars (Fig. 9E) has discontinuous enamel on most of the lingual wall.On the other molar (Fig. 9F), the enamel is interrupted only at the mesiolingual corner of the crown base.MLP 91-I-10-8e (Fig. 9G) is a slightly-worn m3 in a small dentary fragment.The lingual flexids are still open.The enamel is continuous around the crown margins, except at the base of the mesiolingual corner.The molar is high-crowned, in lingual view, the base is wider than the apex, and the root is unformed, indicating a juvenile stage.In occlusal view, the mesial lobe is broader transversely than the distal one.The mesial margin of the hypoflexid is straight and the distal is concave mesially.The metalophulid I is elongate, the hypolophid is straight and relatively wide, and the posterolophid reaches the lingual border.The antero-and mesoflexid form a unique lingual flexid because the second crest in position is lateromedially short.The posteroflexid is wide and rounded, and the deepest of the flexids.Even though this m3 is similar to both Luantus propheticus (e.g., MACN SC 2457, MACN A 52-610?) and L. toldensis (e.g., MACN SC 4035), its size is more similar to the type species, which justifies assigning it to L. propheticus, Comments.MLP 91-I-10-8 consists of five isolated molars of different taxa.Two of them are referred here to Eocardia montana (MLP 91-I-10-8 a-b, see above) and the other three to Luantus propheticus (MLP 91-I-10-8 c-d-e).The genus has been reported for the early Miocene of Patagonia Argentina and BioBío region, Chile.Luantus propheticus is recorded from an unknown locality from the "Piso Astrapothericulense" (only the holotype MACN A 52-610), several localities of the Pinturas Formation (Santa Cruz Province; Kramarz, 2006b) and UFZ of the Sarmiento Formation (Chubut Province, Vucetich et al., 2010).(1994), we refer to P. pusillus two other specimens from CBF: MLP 90-III-5-14 and 91-I-10-2c.
Comments.Three specimens from CBF referred to Prolagostomus sp. by Vucetich (1994) are assigned to Pliolagostomus notatus in this revision.The genus was recorded in the early-middle Miocene of Patagonia, Argentina.
Pliolagostomus? (Fig. 9L) Referred material.See Appendix 1. Description.MPM-PV 19169 is a left dentary fragment with p4.The premolar is similar to that of Pliolagostomus notatus but at the dorso-ventral midpoint of the lateral surface of the dentary is a small mental foramen, contrasting with the condition of the type species in which the foramen is more dorsally positioned.On the other hand, Pliolagostomus friasensis, Prolagostomus, and Lagostomus lack the foramen (Rasia and Candela, 2019).(Arnal et al., 2017).Vucetich (1994) assigned to Acarechimys sp.only one specimen, MLP 90-III-5-4 (field number 90-335) and another, MLP 91-I-10-5 (91-530a), as "Gen.sp.indet.B".Both specimens are assigned here to species of Acarechimys (A. minutissimus and A. minutus respectively) because of their morphology and size.They are very small with brachydont cheek teeth (very low crowns with well-developed roots), present discrete cusps and have narrow crests separated by wide flexids.The lower incisor seems to be long.The masseteric crest of the dentary is well-developed.

Rodentia/Hystricognathi
Acarechimys minutissimus (Ameghino, 1887) (Fig. 9P) Referred material.See Appendix 1. Description.MLP 90-III-5-4 is assigned to Acarechimys minutissimus based on its morphology and size.The molars are very small and lack metalophulid II.The posterior arm of the metaconid and posterior extension of the metalophulid I are absent, contrasting with the condition of A. minutus.
The notch for the insertion of the tendon of nMpi is straight and located below dp4-m1, as in A. minutus.
Comments.As with the type species, A. minutissimus presents one of the broadest geographical and temporal distribution: early to middle Miocene from Sarmiento, Pinturas, Santa Cruz, and Collón Curá Formations of Patagonia Argentina; early Miocene of Chile; middle Miocene Villavieja Formation of Colombia (Arnal et al., 2017).

Acarechimys?
Referred material.See Appendix 1. Description.The specimen (MLP 90-III-5-1) shows the nMpi placed between the dp4?-m1 on the lateral wall of the dentary.The m2 has a similar size to that of Acarechimys minutus; it is lower-crowned and seems to be trilophodont; however, it is too damaged to assign it at species level.
Rodentia/Hystricognathi/Caviomorpha/ Octodontoidea Stichomys Ameghino, 1887 Stichomys sp. (Fig. 9Q-S) Referred material.See Appendix 1. Description.These specimens referred to Stichomys are larger than Prostichomys, and are characterized by having rooted but relatively high-crowned cheek teeth with flat occlusal surfaces and undifferentiated cusps.The deciduous premolars are retained throughout life, as in Adelphomys.The second crest of the m1 and/or m2 (mesolophid sensu Kramarz, 2001) is generally absent, unlike in Prostichomys; an indistinct second crest and a reduced anterofossetid in m1 and/or m2 occasionally are present (e.g., MLP 91-I-10-9, Fig. 9S).Stichomys differs from Adelphomys in having a convex anterior surface of the incisors (Ameghino, 1887).However, the incisors in the new specimens are not well enough preserved to distinguish this feature, so we agree with the assignment to Stichomys sp. by Vucetich (1994).Comments.The genus was reported from the early-middle Miocene of Patagonia, Argentina, and probably early Miocene of Chile.Vucetich (1994) reported six specimens of Stichomys sp., but only five were reviewed here.MLP 91-I-10-17 (91-565) was not available for revision.Stichomys is known from Pinturas Formation, SCF, CBF, and Río Frías Formation (Vucetich, 1984;Vucetich et al., 1993;Kramarz, 2005;Arnal et al., 2019) of Patagonian Argentina, and the early Miocene of Pampa del Castillo, Chile (Flynn et al., 2002).
Referred material.See Appendix 1. Description.This specimen is a fragment of a small left dentary retaining an m3 with a trilophodont pattern with the oblique crests, characteristic of Adelphomyinae.However, these features alone are insufficient to assign MPM-PV 19173 at the generic level.
Rodentia/Hystricognathi/Caviomorpha/ Erethizontoidea Steiromys Ameghino, 1887 Referred material.See Appendix 1. Comments., not reviewed here, was assigned to Steiromys sp. by Vucetich (1994: 368, fig.12).It consists of a left dentary fragment with m1-m2, with a conservative dental morphology and tetralophodont pattern.Steiromys is slightly higher-crowned than closely related Eosteiromys Ameghino 1902 (Colhuehuapian).Steiromys was recorded in the early to middle Miocene of Patagonia Argentina, early Miocene of Chile and Peru, and middle Miocene of Colombia.
Comments.MLP 90-III-5-19 (90-291), a right dentary fragment with m1-m2, was reported and figured by Vucetich (1994: 369) as Gen et sp indet.D of Dasyproctidae?The nMpi is placed below m1 and joined to the masseteric crest.The cheek teeth are extremely worn and broken; only the m2 is well enough preserved, is very high-crowned, and has thick peripheral enamel.Cementum in the hypoflexid and on the external walls of the crown is very unusual in caviomorphs of the early-middle Miocene.In the lateral view, the hypoflexus is closed; in the occlusal view it is narrow and transversely reaches halfway across the crown.The labial and posterior walls are convex.This combined morphology is different from any other known caviomorph.Another specimen, MPM-PV 19174, consists of a dentary fragment with a poorly preserved m1-m3.The thick enamel layer is very similar to that of MLP 90-III-5-19.

The fossil record
The number of specimens obtained from the CBF is rather limited when compared with those from the penecontemporaneous strata of the SCF (see below) at the Atlantic coast (Vizcaíno et al., 2012a;Kay et al., 2012), as well as along the Río Santa Cruz (Fernicola et al., 2019a, b;Kay et al., 2021) and the Río Chalía (Cuitiño et al., 2021).Also, the preservation of the remains is rather poor.We found only one specimen in situ, a partial articulated armadillo with its carapace.The cause of the poor fossil content relative to the SCF east of the Andes could be similar to that proposed for this unit at Lago Posadas, some 60 km south, that is, it may be related to the scarcity of fine-grained, especially pyroclastic, materials (see above) that could produce relatively higher preservation potential through fast episodic burial of dead animals on the floodplains of those areas, and to rocks more resistant to erosion with a consequently lower "release rate" of fossils (Cuitiño et al., 2019a).
Among mammals, in the new collection (MPM-PV) we have not recorded metatherians, vermilinguans, astrapotheres, toxodont notoungulates (toxodontids and homalodotheriids), and macraucheniid litopterns.Carlini et al. (1993) reported the presence of a vermilinguan Protamandua rothi, but the specimen has not been catalogued nor further studied.Although vermilinguans, homalodotheriids and macraucheniids are usually quite rare, metatherians, astrapotheres and, particularly, toxodontids are common in other penecontemporaneous outcrops.The apparent absence of astrapotheres, toxodonts, and macrucheniids at CBF is hard to interpret.On one hand, macraucheniids tend to be uncommon and their absence could be a consequence of sampling or taphonomic conditions.But the absence of toxodonts is quite surprising, given their ubiquity and abundance in the records elsewhere in the SCF.Nevertheless, we discern no obvious climatic or vegetational implications for the absence of toxodonts (see section 5.3.Paleoenvironments).
Unfortunately, the MLP-DU collection is not completely cataloged so far and, as mentioned above, not fully available currently.It includes phorusrhacid birds, tortoises, and other major groups of mammals.Metatherians (sparassodonts and paucituberculates) counts 21 specimens that were not reviewed here, but included in the list for the record.Although there are no certain records of astrapotheres, toxodonts and macraucheniid litopterns, field observations of poorly preserved slivers of enamel of large mammals most probably represent toxodonts or astrapotheres.We had access to only some rodents and tortoises.The integrated faunal list reported here includes at least 38 taxa: one anuran, one testudines, three birds and at least 33 mammals, including metatherians, xenarthrans, litopterns, notoungulate typotheres and rodents (Table 1).Almost half the specimens are rodents, represented by all four clades of caviomorphs: Caviodea, Chinchilloidea, Erethizontoidea, and Octodontoidea.Among them, cavioids and chinchilloids are the most abundant and characterized by proto-to euhypsodont taxa (e.g., Neoreomys, Eocardia, Luantus, Perimys, and Prolagostomus); erethizontoids are limited to one specimen of Steiromys, and octodontoids (including three new taxa; M. Arnal personal communication, 2021) seem to be the most diverse group.
A novel finding is the southernmost record of the family Testudinidae, in South America and the world, represented by Chelonoidis cf.Ch. gringorum.It confirms a more widespread and diverse distribution in Patagonia than previous mentions of testudinid specimens from Chubut and Santa Cruz provinces (see above).The species Chelonoidis gringorum is known from the early-middle Miocene of Chubut Province (Simpson, 1942;De la Fuente 1988, 1994;de la Fuente et al., 2018;Oriozabala et al., 2018), but has not been recorded in the penecontemporaneous faunas of the SCF in the Santa Cruz Province.Given the large size and common occurrence of tortoises in many other localities and the vast size of the SCF fossil assemblages (Vizcaíno et al., 2012a;Fernicola et al., 2019a), we interpret the absence of this clade in the SCF as a real absence, not an artifact of rarity or collection bias.The southernmost occurrence of tortoises on the continent is Chelonoidis from the late Oligocene (Deseadan) of Cabeza Blanca (Chubut, the oldest occurrences of the family in South America) and the supposedly early Miocene (Santacrucian) of Bajo de la Oriental (Santa Cruz Province;De Broin and De la Fuente, 1993;De la Fuente et al., 2018).

Correlations
Based on the available geochronological data, the CBF is correlated to the middle or upper sections of the SCF in several sites of the Austral-Magallanes Basin of southern Patagonia, including Lago Posadas, the Río Santa Cruz and Río Chalía valleys (Perkins et al., 2012;Cuitiño et al., 2016Cuitiño et al., , 2019aCuitiño et al., , 2021)), and north (e.g., Cañadón de las Vacas) and south of Río Coyle (Perkins et al., 2012;Trayler et al., 2020;Figs. 1 and 10).The CBF is younger than the Pinturas Formation east of the Meseta del Lago Buenos Aires (De Barrio et al., 1984;Bown and Larriestra, 1990;Escosteguy et al., 2003;Perkins et al., 2012;Aramendía et al., 2019).The lower beds of the SCF in Río Bote, near Lago Argentino, and in Sierra Baguales (Chile) are considerably older (~19-17 Ma; Cuitiño et al., 2016;Bostelmann et al., 2013) than the CBF, while the upper beds in those areas are of uncertain age.West of the study area in Meseta Guadal (Chile), the available ages for the lower part of the Santa Cruz (or Galeras) Formation (Encinas et al., 2019) indicate that it is older than the CBF, whereas the age for its upper part is unknown and could be, at least in part, contemporaneous with the CBF.
North of the Austral-Magallanes Basin, the CBF could be correlated to the Río Frías Formation at Río Cisnes (Chile) and Meseta Guenguel (Flynn and Swisher, 1995;De la Cruz and Cortés, 2011;Folguera et al., 2018).It also shows temporal overlap with the lower part of the Collón Curá Formation, particularly with the Pilcaniyeu Ignimbrite which has a reported U-Pb zircon age of ~15.5 Ma (Bucher et al., 2019;López, 2020).Finally, there is no chronostratigraphic overlap between the CBF and the older Colhué Huapi Member of the Sarmiento Formation in southern Chubut Province (Ré et al., 2010;Dunn et al., 2013).
As mentioned above, the fossil vertebrates have been recorded from the lower part of the stratigraphic section (representing a ~150 m-thick portion) of the CBF, between ~16 and ~15.1 Ma (late Burdigalianearly Langhian; Fig. 2).CBF vertebrates reported here mainly belong to taxa recorded in the SCF and can be considered Santacrucian sensu lato (Table 1).This is in accordance with the age based on the rodents proposed by Vucetich (1994), who assigned them an early Miocene age and established correlations with the Santacrucian (based on the presence of the genera Stichomys and Perimys), probably late Santacrucian (based on the presence of adult "eocardiid" rodents without fossets and fossetids).Up to now, as Vucetich (1994) mentioned for the rodents, we have not identified taxa typical of "Friasian" and Colloncuran faunas.For instance, we find no specimens of the rodents Alloiomys, Protacaremys denisae, Maruchito trilofodonte, Neosteiromys?tordillense (Vucetich et al., 1993), or the primate Proteropithecia (Kay et al., 1998).These hypotheses based on the record of rodents and primates deserve further consideration based on the new evidence available (see below).
Younger faunal correlations have been proposed for the CBF.Ugarte (1956) considered that the fauna from CBF could be correlated with the Friasian s.l.(1993).Up to now, these taxa have not been reported from other localities clearly assignable to an age younger than the Santacrucian.For this reason, we consider them to be geographic or ecological variants with typical Santacrucian localities from extra-Andean Patagonia, as already proposed for other localities close to the Andes (Fernicola et al., 2014;Cuitiño et al., 2016).
Most of the caviomorphs rodents from CBF present extensive biochrons and some taxa reviewed here have also been reported for older and younger rocks.Vucetich (1994) considered Stichomys to be exclusive to the Santacrucian.Now it has a broader temporal range from the older "Pinturan" (Pinturas Formation, Santa Cruz Province) and younger Colloncuran (Río Frías Formation, Chubut Province; Arnal et al., 2019).A more precise identification of the "eocardiids" studied by Vucetich allows the allocation of some of them to Eocardia montana, a taxon exclusive of Santacrucian age (Santa Cruz and Chinches formations, Argentina), and others to Luantus propheticus, previously recorded in the Pinturas Formation (Kramarz and Bellosi, 2005; Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation... Kramarz, 2006b) and the uppermost level (UFZ) of the Sarmiento Formation at Gran Barranca (Kramarz et al., 2010).This is the first report of the genus and species for the CBF.Luantus also has been reported also for the Andean Santacrucian of the Cura-Mallín Formation in Chile (Solórzano et al., 2020(Solórzano et al., , 2021)), but not from Santacrucian localities of the SCF in extra-Andean Patagonia, east of the Andes.
Steiromys, a porcupine reported by Vucetich (1994) is also known from "Pinturan'' and Colloncuran ages (Vucetich et al., 1993;Kramarz, 2004) 1) in agreement with its likely chronological age.The record of Luantus propheticus indicates the persistence in the west of a taxon that appears in older rocks but had become extinct in extra-Andean Patagonia, suggesting geographic or ecological differences with Santacrucian faunas from extra-Andean Patagonia.

Paleoenvironments
Following the observations of Aramendía et al. (2019), the sedimentary features of the CBF near Los Antiguos-Jeinemeni Rivers suggest sediment accumulation by a mixed-load fluvial paleoenvironment that grades upward transitionally to an alluvial fan.Cuitiño et al. (2019a) reconstructed depositional environments and the associated fossil vertebrate fauna of the thick section of the SCF at Lago Posadas.These authors indicated that the unit accumulated in a fluvial system with extensive floodplains, occasionally interrupted by low-sinuosity, sand-dominated channels.Fine-grained sediments with dominant reddish coloration and moderate to low paleosol development, abundant crevasse splay sandstones, and lack of plant remains, suggest deposition in a low-gradient, oxygenated setting under elevated sedimentation rates.Similar sedimentary features are observed for the CBF, albeit with a higher proportion of channelized, cross-bedded sandstone deposits and fewer paleosol horizons, resembling the upper part of the SCF at Lago Posadas.Neither plant remains nor trace fossils were observed in the CBF.North of the study area, Aramendía et al. (2022) recognized eolian dune deposits within the CBF that, although restricted, may suggest the occurrence of intervals of low-humidity conditions.
The records of anurans and testudines indicate the coexistence of different conditions in the landscape.The extant anuran Calyptocephalella gayi is an aquatic to semi-aquatic frog that dwells in lowland lakes, ponds, and slow moving streams (Cei, 1962) up to 500 m above sea level, in general associated with the Nothofagus forests (Muzzopappa and Báez, 2009).Thus, the record of Calyptocephalella indicates the presence of flooded areas and permanent body waters, possibly developed in a forested area (Fernicola and Albino, 2012;Muzzopappa, 2019).
By contrast, the extant tortoise Chelonoidis chilensis lives in a variety of arid environments and can tolerate seasonal temperature changes by basking or using burrows (Waller et al., 1989;Richard, 1999); however, high humidity is necessary for hatching its eggs, and the sex is determined in high temperatures (30±1 ºC) (see Romito et al., 2015 and references therein).The modern distribution of Ch. chilensis is controlled by warm temperatures and high precipitation, at least during the reproductive period (Ruete and Leynaud, 2015).Extant southernmost populations of Ch. chilensis are found in shrub-like and woody associations with diverse perennial plants, in plains and piedmont terrains (from "Chaco" plains to "Monte"; see Waller et al., 1989).Thus, the record of Chelonoidis cf.Ch. gringorum in CBF indicates higher temperatures than today and abundant rainfall, at least seasonally.Continued cooling and aridification of the Patagonian region during the last few millions of years may have caused the retraction of tortoises into more northern latitudes (Richard, 1999).
Among birds, Phorusrhacos, Patagornis and Psilopterus belong to two guilds of phorusrhacids: non-volant cursorial predators (Phorusrhacinae and Patagornithinae, together with Mesembriornithinae), and predators with some flying ability (Psilopterinae).According to Degrange et al. (2012) these groups suggest an open habitat dominated by shrubland or grassland vegetation.
On account of the limited sample sizes and incomplete taxonomic evaluation of all the mammalian taxa, we have not performed quantitative analyses as for Santacrucian mammal assemblages of the SCF eastward at the Atlantic Coast and the Río Santa Cruz (Kay et al., 2012(Kay et al., , 2021 respectively) respectively).However, the combined record of arboreal or semiarboreal, browsers and frugivores (e.g., megatherioid sloths and the porcupine Steiromys) on one hand, and grazing mammals (e.g., typotheres and eocardiid rodents) on the other hand, suggest the presence of both forested and parkland vegetation for the CBF, supporting the interpretation of Vucetich (1994).As is also the case for the SCF at Lago Posadas, the available evidence does not resolve whether these two environments coexisted or alternated during the deposition of the formation, and it does not tell us about whether one environment predominated over the other.The record of conifers and Nothofagaceae in the overlying Río Correntoso Formation in this region suggests that forests persisted in the area at least until the middle Miocene (Pujana et al., 2020), and that the two types of environments coexisted in the eastern areas of Patagonia, where semi-deciduous forests graded into savannas with gallery-forest components with canopy heights exceeding 20 m (Kay et al., 2012(Kay et al., , 2021)).According to Kay et al. (2021), this range of habitats occurs today where the mesic inland Atlantic forests of southern Brazil, northeastern Argentina and eastern Paraguay give way northwestward into the more xeric Paraguayan Gran Chaco.Kay et al. (2021) also reconstructed the climatic condition in a suite of SCF localities (that they identified as FL 1-7) in the lower member of the SCF along the Atlantic coast, and at Barrancas Blancas and Segundas Barrancas Blancas inland along the Río Santa Cruz, as closely resembling one another.The Mean Annual Precipitation (MAP) estimates were 1,635 mm for FL 1-7, 1,451 mm for Barrancas Blancas, and 1,504 mm for Segundas Barrancas Blancas.The Mean Annual Temperature (MAT) estimates were between ~21 and ~22 °C for FL 1-7 and Segundas Barrancas Blancas, possibly lower at 16 °C for Barrancas Blancas, with the confidence intervals for the estimates overlapping widely.Temperature seasonality was modest (3 to 4 °C) and similar for all localities.However, the presence of the caviomorph rodents Prolagostomus and Pliolagostomus and the typothere Pachyrukhos are also indicative of aridity.According to Kay et al. (2021) and Vizcaíno et al. (2021), these genera are ground-dwelling taxa with coarse herbivorous diets that appear in the upper parts of SCF and suggest a trend to aridification in the upper part of that formation along the Río Santa Cruz at the Segundas Barrancas Blancas locality (between ~16.5 and ~15.6 Ma) and South to the Río Coyle at Monte Tigre-Cañadón Las Totoras, and along the Río Gallegos (younger than ~17 Ma).Vucetich (1994) also suggested that the presence of several octodontoids rodents with reduced lophs in the CBF might indicate that more arid conditions prevailed in CBF compared with the SCF in the Atlantic coastal localities.The rodent assemblage of CBF reported here includes a large proportion of protohysodont (Neoreomys, Luantus) and euhypsodont (Perimys, Prolagostomus, Pliolagostomus, Eocardia) taxa, octodontoids with reduced occlusal cheek tooth surfaces, and very scarce erethizontids represented by Steiromys.This assemblage suggests habitats with open vegetation and arid environments.

Conclusions
This contribution provides the first comprehensive study of the paleontology and geology of the Cerro Boleadoras Formation (early-middle Miocene) in the Cerro Boleadoras and Cerro Plomo area (northwestern Santa Cruz Province).It constitutes the first effort to provide a faunal list of this area, with precise location within the stratigraphic section, and to correlate the CBF with the better known SCF in other localities of the Austral-Magallanes Basin.Together with the recent study of Lago Posadas (Cuitiño et al., 2019a), it represents one of the few reports for the early-middle Miocene from the modern-day eastern slope of the Southern Patagonian Andes.The main conclusions of this contribution are: • The sedimentary features of the CBF suggest mixed-load fluvial deposits that grade upward transitionally to an alluvial fan setting.Many specimens and several vertebrate groups previously recovered from the CBF remain to be studied.We expect that the updated and integrated information and conclusions provided here would serve as a framework and working hypotheses for further research on the geology and paleontology of the early to middle Miocene localities in western Patagonia.Much further field prospecting of the early to middle Miocene outcrops in and nearby the Andes is needed to envision the broad scenario of the environmental evolution of Austral South America during this crucial interval of time.

FIG. 3 .
FIG. 3. Exposures of the Cerro Boleadoras Formation at the Cerro Boleadoras and Cerro Plomo area. A. Panoramic view, from the west, of the collecting area.In the background the Cerro Zeballos and Meseta del Lago Buenos Aires volcanic rocks are visible.B. Views of the Cerro Plomo from the north, and C. The Cerro Boleadoras from the south.White arrows indicate the exposures where fossils were collected; yellow arrows in A and C indicate the same channelized sandstone deposits.
See Appendix 1. Description.MPM-PV 19187 is a damaged trochlea metatarsi II, lacking the posteromedial process, comparable in size to that of Patagornis marshi (partial measurements: length=32 mm; depth 26 mm; width 15 mm; Fig. 6C-D).Comments.Patagornis has been recorded in the early-middle Miocene of Patagonia, Argentina.
. 7F) Referred material.See Appendix 1. Description.Osteoderms of the movable bands (MPM-PV 17525 and 19126) are long and narrow, with three longitudinal ridges, the laterals slightly divided transversely into tubercules.The fixed osteoderms (MPM-PV 17520; Fig. 7F) are shorter and relatively wider posteriorly (anteroposterior length, 7 mm; transverse width, 5 mm); the median ridge is reduced anteriorly; the lateral ridges constitute rows of tubercles, which extend across the anterior border of the osteoderm.Comments.The genus Prozaedyus has been recorded in the late Oligocene-middle Miocene of Patagonia, and early Miocene of San Juan Province, Argentina; also from the early Miocene of Chile and the earlymiddle Miocene of Bolivia.In the Santacrucian fauna
See Appendix 1. Description.The genus Protypotherium has euhypsodont premolars and molars, and premolariform premolars.The upper molars have a lingual sulcus that
(middle Miocene) based on Rosendo Pascual's comments to him about some mammals recorded in Cerro Boleadoras area.Scillato-Yané et al. (1993) also suggested that the xenarthrans from CBF would indicate an age similar to those of the Río Frías Formation in alto Río Cisnes, Chile.Scillato-Yané and Carlini (1998) named a new armadillo, Stenotaus planus, and Carlini et al. (2013) named a new Planopinae sloth (Prepoplanos boleadorensis) from CBF, indicating a supposed younger age for the formation, based on allegedly advanced features of the scutes of the former, and in accordance with the speculations of Scillato-Yané et al.

FIG. 10 .
FIG. 10.Chronostratigraphic correlation scheme of Burdigalian-Langhian lithostratigraphic units rich in terrestrial fossil vertebrates from southern and central Patagonia.Orange bars correspond to the Cerro Boleadoras Formation; yellow bars correspond to several exposures of the Santa Cruz Formation; green bars correspond to other units.The source of the age for each locality is provided on the figure.
Fernicola and Vizcaíno (2019)nicola and Vizcaíno (2019)considered five species of Peltephilus to be valid for the SCF: Pe. strepens, Pe. pumilus, Pe. giganteus, Pe. nanus, and Pe.ferox.The limited number of osteoderms in our collections and the fact that they are all isolated does not permit a precise analysis of morphological variability, preventing us from assigning a specific identification.Xenarthra/Cingulata/material.See Appendix 1. Description.MPM-PV 17526 shows the moveable osteoderms with a wide longitudinal central figure separated by two shallow grooves from two similarly wide lateral figures (Fig.

1891a Stenotatus cf. S. planus Scillato-Yané and Carlini, 1998 (
for the two species are slight and within the range of variation of different species of extant cingulates such as Chaetophractus vellerosus and Zaedyus pichiy, concluded that there is just one valid species, Prozaedyus proximus. Fig. 7G-H) Referred material.See Appendix 1. Comments.The genus Stenotatus was recorded from the early Oligocene to middle Miocene of Patagonia Argentina, early Miocene of San Juan Province, Argentina, early Miocene of Chile, and early-middle Miocene of Bolivia.It has been Gonzaláz-Ruiz (2010) agreed that S. planus as the only species found in the CBF, but also reported S. planus in the localities La Olguita y La Porteña (Scillato-Yané y Carlini, 1998; González-Ruiz et al., 2005; González-Ruiz, 2010) which would also belong to the Río Zeballos Group based exclusively on faunal correlations.Croft et al. (2009) reported Stenotatus planus from early-middle Miocene outcrops from Bolivia.Xenarthra/Cingulata/Chlamyphoridae/ "Euphractinae"/"Eutatini" Proeutatus Ameghino,
compared with the movable osteoderms (18.0 mm anteroposterior length; 8.00 mm transverse width; 6.0 mm thickness; MPM-PV 19121; Fig.7I), with a sculptural pattern which is a modification of that seen in the movable osteoderms.Comments.Province; H. mirabile, occurs in the early-middle Miocene (SCF) in Santa Cruz Province, and in the middle Miocene (Collón Curá Formation) in Río Negro and Neuquén provinces, and H. cerdasensis

)
Referred material.See Appendix 1. Description.The specimens reported here are fragmentary dentaries with premolars and/or molars.The available material does not allow us to observe whether there is a conspicuous diastema between the cheek teeth and the anterior teeth, a distinctive feature of Pachyrukhos.However, the lower cheek teeth of the referred specimens have the straight lingual faces, conspicuous labial sulci and the very small size Seoane and Cerdeño (2019) (e.g., MPM-PV 19159; Fig.8B-D) compared with other typotheres, features that let us to refer these specimens provisionally to P. cf.P. moyani.Comments.The genus has been recorded in the early-middle Miocene of Patagonia, Argentina and the early Miocene of Chile.Seoane and Cerdeño (2019)recognized two valid species within Pachyrukhos: P. politus, early Miocene (Sarmiento Formation) Chubut Province, and P. moyani, early-middle Miocene (SCF) Santa Cruz Province, and middle Miocene (Collón Curá Formation) Río Negro and Neuquén provinces.Pachyrukhos moyani was also reported from the SCF at Lago Posadas(Cuitiño

.
Protypotherium is widely distributed both temporally and geographically: late Oligocene-middle Miocene of Patagonia Argentina, late Miocene of Buenos Aires, Entre Ríos and La Rioja provinces, late Oligocene of Uruguay, early-middle Miocene of Chile, and late Miocene of Bolivia and Venezuela.Fernández and Muñoz (2019) recognized Protypotherium australe, Protypotherium praerutilum, and Protypotherium attenuatum for the penecontemporaneus SCF at the localities of the Río Santa Cruz.After the manuscript of this article was accepted, Vera et al. (2021) described intheratheres from CBF, including a new genus and species.
The genus Prolagostomus has been reported for the early-middle Miocene of Argentina, Chile, and Bolivia.In addition to the specimens from CBF assigned to Prolagostomus sp. by Vucetich Description.The specimens assigned here to Prolagostomus (Appendix 1) have euhypsodont cheek teeth formed by two laminae (or lobes) connected by a narrow isthmus.The molariforms have rounded walls and the hypoflexus/id are sinuous, differing from those of Pliolagostomus and Lagostomus, which are straight.Comments.
Perimys because the cheek teeth are euhypsodont and bilophodont, with the hypoflexus/id conspicuously broader and filled with more cementum than in Prolagostomus and Pliolagostomus.As a result, the teeth of Perimys have a U-shape occlusal outline.The molars reported here are medium to large-sized for Perimys species, similar to P. onustus or P. intermedius; however, the specimens are badly damaged which prevents definite assignment to a species.Comments.The genus was recorded in the earlymiddle Miocene of Patagonia, Argentina, and earlymiddle Miocene of Chile.Vucetich (1994: 368-369)reported six specimens of Perimys sp. from CBF.She considered all as belonging to the same species due to the similarity in morphology and the large size.
Referred material.See Appendix 1. Description.These specimens(MPM-PV 19170,  MPM-PV 19172)are assigned to • The lower part of the CBF, with an estimated age between ~16.5 Ma and 15.1 Ma (late Burdigalian-early Langhian), temporally overlaps with the middle or upper sections of the SCF in many sites of the Austral-Magallanes Basin of southern Patagonia, the Río Frías Formations in Chile, and the lower Collón Curá Formation of northern Patagonia.• Faunal differences between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías, and Collón Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones.• The taxa list of the vertebrate fauna of CBF includes most of the groups recorded in the SCF: one anuran, one testudines, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents).Fragmentary remains of large mammals (e.g., toxodonts, astrapotheres) were present but, as well as metatherians (sparassodonts and paucituberculates), they have not yet been evaluated.• The report of Chelonoidis cf.Ch. gringorum constitutes the southernmost record of tortoises in South America and worldwide.• The faunal assemblage indicates a Santacrucian age sensu lato.• The presence of Luantus propheticus, a taxon recorded in older strata from other localities, suggests geographic or ecological differences with Santacrucian faunas from extra-Andean Patagonia.• The association of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both forested and more open environments.• The available evidence does not resolve whether these two environments coexisted or alternated or whether one environment predominated over the other.• The presence of key taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos supports the trend to aridification previously recorded for the upper part of the Santa Cruz Formation (between ~16.5 and ~15.6 Ma) along the Río Santa Cruz and South to the Río Coyle.