A singular Hegetotheriinae (Notoungulata, Typotheria) from the late Oligocene-Early Miocene of the Subandean Region of Bolivia

Marcelo Alfredo Reguero David Eric Tineo Paula Bona Leandro Martín Pérez Gustavo Dardo Vergani Gloria González Ruiz Daniel Gustavo Poiré About the authors

Abstract

Geological studies in the northern sector of the Chaco foreland Basin, Bolivia, yielded new fossils coming from late Oligocene-late Miocene of the Petaca Formation. Few fossil mammals were known from the Subandean Region of Bolivia. We report a partially complete mandible of a hegetotheriid Hegetotheriinae (Notoungulata, Typotheria) from Abapó (Río Grande River). The specimen (YPFB-LIT-PAL-005) is very close in size and dental morphology to the late Oligocene (Deseadan South American Land Mammal Age, SALMA) — Santacrucian (early Miocene) Prohegetotherium schiaffinoi (Kraglievich 1932Kraglievich L. 1932. Nuevos apuntes para la geología y paleontología uruguayas. Anales del Museo de Historia Natural de Montevideo, 3:1-65.) from Fray Bentos (Uruguay and Argentina), Salla (Bolivia), Divisadero Largo-Potrerillos and Quebrada Fiera (Argentina). However, mandibular characteristics, as the shape with a marked change in height along its length, increasing towards the back, a prominent masseteric crest, a deep mandibular groove, and a remarkable thickening of the ventral rim of the mandible, indicate differences between this specimen and Prohegetotherium schiaffinoi and the other Hegetotheriinae. The affinity of YPFB-LIT-PAL-005 with P. schiaffinoi suggests a late Oligocene to early Miocene for the upper bearing horizon of the Petaca Formation (in Abapó), an older age than previously assigned to the top of this unit (late Miocene), and confirms the taxon distribution between ∼ 36 ° to ∼ 17 ° south latitude.

KEYWORDS
Notoungulata; Hegetotheriidae; Petaca Formation; late Oligocene-early Miocene; Bolivia

INTRODUCTION

The South American native ungulates, which include five extinct orders (Astrapotheria, Litopterna, Notoungulata, Pyrotheria and Xenungulata), underwent a remarkable diversification during the major part of the Cenozoic Era. Among them, notoungulates are by far the most diverse (Simpson 1980Simpson G.G. 1980. Splendid lsolation. The curious history of South American Mammals. New Haven: Yale University Press, 266 p., Billet 2011Billet G. 2011. Phylogeny of the Notoungulata (Mammalia) based on cranial and dental characters. Journal of Systematic Palaeontology, 9(4):481-497. https://doi.org/10.1080/14772019.2010.528456
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).

Typotheria notoungulates include small to medium-sized forms with the following apomorphic traits: a rodent-like anterior dentition with I1 somewhat enlarged and upper molars with obliterated occlusal surface (Cifelli 1993Cifelli R.L. 1993. The phylogeny of the native South American ungulates. In: Szalay F.S., Novacek M.J., McKenna M.C. (Eds.). Mammal Phylogeny (p. 195-216). Placentals. New York: Springer.). Within these small Typotheria, Hegetotheriidae and Archaeohyracidae are considered as forming the clade Hegetotheria (Cifelli 1993Cifelli R.L. 1993. The phylogeny of the native South American ungulates. In: Szalay F.S., Novacek M.J., McKenna M.C. (Eds.). Mammal Phylogeny (p. 195-216). Placentals. New York: Springer., Croft et al. 2003Croft D.A., Bond M., Flynn J.J., Reguero M.A., Wyss A.R. 2003. Large archaeohyracids (Notoungulata, Typotheria) from Central Chile and Patagonia, including a revision of Archaeotypotherium. Fieldiana, Geology (New Series), 49:1-38. https://doi.org/10.5962/bhl.title.5264
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), an entity previously regarded as a separate suborder (Simpson 1967Simpson G.G. 1967. The beginning of the Age of the Mammal in South America. Part Il. Bulletin of the American Museum of Natural History, 137:1-259.). Hegetotheriids are small typotherian notoungulates; some of the latest representatives were very similar in overall morphology to modern rabbits or certain caviomorph rodents (Elissamburu 2004Elissamburu A. 2004. Análisis morfométrico y morfofuncional del esqueleto apendicular de Paedotherium (Mammalia, Notoungulata). Ameghiniana, 41(3):363-380., Elissamburu and Vizcaíno 2004Elissamburu A., Vizcaíno S.F. 2004. Limb proportions and adaptations in caviomorph rodents (Rodentia, Caviomorpha). Journal of Zoology, 262(2):145-159. https://doi.org/10.1017/S0952836903004485
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, Reguero et al. 2007Reguero M.A., Dozo M.T., Cerdeño E. 2007. A poorly known rodent-like Mammal (Pachyrukhinae, Hegetotheriidae, Notoungulata) from the Deseadan (Late Oligocene) of Argentina. Paleoecology, biogeography, and radiation of the rodent-like ungulates in South America. Journal of Paleontology, 81(6):1301-1307. https://doi.org/10.1666/05-100.1
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). Many recent studies suggested that hegetotheriids arose from the latest archaeohyracids (Cifelli 1993Cifelli R.L. 1993. The phylogeny of the native South American ungulates. In: Szalay F.S., Novacek M.J., McKenna M.C. (Eds.). Mammal Phylogeny (p. 195-216). Placentals. New York: Springer., Croft et al. 2003Croft D.A., Bond M., Flynn J.J., Reguero M.A., Wyss A.R. 2003. Large archaeohyracids (Notoungulata, Typotheria) from Central Chile and Patagonia, including a revision of Archaeotypotherium. Fieldiana, Geology (New Series), 49:1-38. https://doi.org/10.5962/bhl.title.5264
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). Hegetotheriids have been traditionally divided into two subfamilies: Hegetotheriinae and Pachyrukhinae (Simpson 1945Simpson G.G. 1945. The principles of classification and a classification of mammals. Bulletin of the American Museum of Natural History, 85:1-350.). The paraphyletic Hegetotheriinae (Croft and Anaya 2006Croft D.A., Anaya F. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of the Hegetotheriidae. Journal of Vertebrate Paleontology, 26(2):387-399. https://doi.org/10.1671/0272-4634(2006)26[387:ANMMHN]2.0.CO;2
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, Reguero and Prevosti 2010Reguero M.A., Prevosti F.J. 2010. Rodent-like notoungulates (Typotheria) from Gran Barranca, Chubut Province, Argentina: phylogeny and systematics. In: Carlini A.A., Madden R.H., Vucetich M.G., Kay R.F. (Eds.). The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia (p. 148-165). Cambridge: Cambridge University Press., Cerdeño and Reguero 2015Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
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, Seoane et al. 2017Seoane F.D., Roig Juñent, S., Cerdeño E. 2017. Phylogeny and paleobiogeography of Hegetotheriidae (Mammalia, Notoungulata). Journal of Vertebrate Paleontology, 37(1):e1278547. https://doi.org/10.1080/02724634.2017.1278547
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) includes the following genera: Prohegetotherium Ameghino 1897Ameghino F. 1897. Mammiferes crétacés de l’Argentine—deuxieme contribution a la conaissance de la faune mammalogique des couches a Pyrotherium. Boletín del Instituto Geográfico Argentino, 18:406-521., from the Deseadan (late Oligocene) and Santacrucian (early Miocene, middle Member of the Mariño Formation) South American Land Mammal Ages (SALMAs) of Mendoza, Argentina, and the Deseadan (late Oligocene) of Bolivia and Uruguay; Sallatherium Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
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, from the Deseadan of Bolivia; Hegetotheriopsis Kramarz and Paz (2013)Kramarz A.G., Paz E.R. 2013. Un Hegetotheriidae (Mammalia, Notoungulata) basal del Mioceno temprano de Patagonia. Revista Mexicana de Ciencias Geológicas, 30(1):186-195. from the Deseadan and Colhuehuapian SALMAs (late Oligocene-early Miocene) of Argentina; Hegetotherium Ameghino 1887Ameghino F. 1887. Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo La Plata. Boletín del Museo La Plata, 1:1-26. from the Colhuehuapian, Santacrucian and Colloncuran SALMAs (early Miocene–early middle Miocene) of Argentina, as well as Chile (Croft et al. 2004Croft D.A., Flynn J.J., Wyss A.R. 2004. Notoungulata and litopterna of the early Miocene Chucal fauna, Northern Chile. Fieldiana, Geology (New Series), 50:1-49. https://doi.org/10.5962/bhl.title.5228
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, Flynn et al. 2005Flynn J.J., Croft D.A., Charrier R., Wyss A.R. Hérail F., García M. 2005. New Mesotheriidae (Mammalia, Notoungulata, Typotheria), geochronology and tectonics of the Caragua area, northernmost Chile. Journal of South American Earth Sciences, 19(1):55-74 https://doi.org/10.1016/j.jsames.2004.06.007
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, Flynn et al. 2008Flynn J.J., Charrier R., Croft D.A., Gans P., Herriott T., Wertheim J.A., Wyss A.R. 2008. Chronologic implications of new Miocene mammals from the Cura-Mallín and Trapa Trapa formations, Laguna del Laja area, south central Chile. Journal of South American Earth Sciences, 26(4):412-423. https://doi.org/10.1016/j.jsames.2008.05.006
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, Bostelmann et al. 2018Bostelmann J.E., Castro N., Moreno K., Garcia M., Fosdick J., Campos-Medina J., Croft D.A., Montoya-Sanhueza G. 2018. Stratigraphy and paleontology of Caragua, Arica and Parinacota regions, Chile, part 2: biostratigraphy and geochronology of the late Miocene sedimentary sequence. In: Congreso Geológico Chileno, 15., 2018, Concepción. Actas… Resumen 1321.), and Bolivia (Cerdas and Nazareno localities; Croft et al. 2009Croft D.A., Anaya F., Auerbach D., Garzione C., MacFadden B.J. 2009. New Data on Miocene Neotropical Provinciality from Cerdas, Bolivia. Journal of Mammalian Evolution, 16:175-198. https://doi.org/10.1007/s10914-009-9115-0
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, Croft et al. 2016Croft D.A., Carlini A.A., Ciancio M.R., Brandoni D., Drew N.E., Engelman R.K., Anaya F. 2016. New mammal faunal data from Cerdas, Bolivia, a middle-latitude Neotropical site that chronicles the end of the Middle Miocene Climatic Optimum in South America. Journal of Vertebrate Paleontology, 36(5):e1163574. https://doi.org/10.1080/02724634.2016.1163574
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); and Hemihegetotherium Rovereto (1914)Rovereto C. 1914. Los estratos Araucanos y sus fósiles. Anales del Museo Nacional de Historia Natural de Buenos Aires, 25:1-247. (including Pseudohegetotherium) from the Collon Curá Formation (middle-late Miocene), the Chasicoan (late Miocene) and Huayquerian (late Miocene) of Argentina (Croft and Anaya 2006Croft D.A., Anaya F. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of the Hegetotheriidae. Journal of Vertebrate Paleontology, 26(2):387-399. https://doi.org/10.1671/0272-4634(2006)26[387:ANMMHN]2.0.CO;2
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, Kramarz and Bond 2017Kramarz A.G., Bond M. 2017. Systematics and stratigraphical range of the hegetotheriids Hegetotheriopsis sulcatus and Prohegetotherium sculptum (Mammalia: Notoungulata). Journal of Systematic Palaeontology, 15(12):1027-1036. https://doi.org/10.1080/14772019.2016.1266047
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, Vera 2019Vera B. 2019. A new species and the record of Hemihegetotherium (Notoungulata, Hegetotheriidae) in the Middle to Late Miocene of Patagonia, Argentina. Journal of South American Earth Sciences, 93:23-35. https://doi.org/10.1016/j.jsames.2019.04.017
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), and Quebrada Honda (Croft and Anaya 2006Croft D.A., Anaya F. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of the Hegetotheriidae. Journal of Vertebrate Paleontology, 26(2):387-399. https://doi.org/10.1671/0272-4634(2006)26[387:ANMMHN]2.0.CO;2
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) and Muyu Huasi (Villarroel and Marshall 1989Villarroel C., Marshall L.G. 1989. A new fossil land mammal locality of late Miocene (Huayquerian) age from Muyu Huasi, southcentral Bolivia. Boletín del Servicio Geológico de Bolivia, La Paz, Serie A, 4:27-40.), Bolivia.

A few fossil mammals were so far known from the Subandean Region or adjoining areas of Bolivia (López-Murillo 1975López-Murillo L.O. 1975. Informe estratigráfico de los ríos Alto Moile, Alto Eterazama, Ichoa, Alto Beni y Tequeje. Informe interno YPFB, Santa Cruz, Bolivia, GXG-2600, 95 p.; Sanjinés and Jiménez 1976Sanjinés G., Jiménez F. 1976. Comunicación preliminar acerca de la presencia de fósiles vertebrados en la Formación Petaca del Área de Santa Cruz. Revista Técnica de YPFB, 4(3):147-156., Marshall and Sempere 1991Marshall L.G., Sempere T. 1991. The Eocene to Pleistocene vertebrates of Bolivia and their stratigraphic context, a review. Fósiles y Facies de Bolivia, 12(3-4):631-652., Poiré et al. 2013Poiré D.G, Tineo D.E, González-Rigas G., Bona P., Toledo N., Reguero M., Scarano A., Vergani G.D., Pérez L.M. 2013. Hallazgos de vertebrados continentales de la Formación Petaca (Oligoceno tardío), Cuenca del Chaco, Bolivia. Ameghiniana, 50(6):65R.). Concerning the notoungulates, Marshall and Sempere (1991)Marshall L.G., Sempere T. 1991. The Eocene to Pleistocene vertebrates of Bolivia and their stratigraphic context, a review. Fósiles y Facies de Bolivia, 12(3-4):631-652. reported that YPFB geologists collected in Quebrada Saguayo, 60 km WNW of Santa Cruz de la Sierra, a right maxilla with P2–M2 assigned to the notohippid ?Rhynchippus sp. (considered a typical Deseadan notoungulate). This material was recovered from the lower horizons of the Petaca Formation (i.e., a pink sandstone 2 m above the base of the unit).

This study describes a material of hegetotheriid Hegetotheriinae represented by a mandible (YPFB-LIT-PAL-005) from the upper section of the Petaca Formation, Río Grande Tatarenda section (near the town of Abapó), southwest of the department of Santa Cruz, Bolivia (Fig. 1). This specimen was previously reported by Reguero et al. (2018)Reguero M.A., Tineo D., Poiré D., Bona P., Pérez L.M. 2018. a new specimen of Prohegetotherium (Notoungulata, Hegetotheriidae) from the Paleogene of the Subandean (Central Andes), Bolivia. In: Congreso Latinoamericano de Paleontología de Vertebrados, 6., 2018, Villa Leyva, Boyacá, Colombia. Memorias… p. 51-52. as a hegetotheriine closely related to the Deseadan species, Prohegetotherium schiaffinoi (Kraglievich 1932Kraglievich L. 1932. Nuevos apuntes para la geología y paleontología uruguayas. Anales del Museo de Historia Natural de Montevideo, 3:1-65.), from Argentina, Bolivia, and Uruguay. Stratigraphically, this Bolivian hegetotheriid provides insight into the Hegetotheriinae diversity and allows valuable comparisons between late Paleogene — early Neogene South American ungulate faunas of low and high latitudes.

Figure 1
Location map of the study area in the Río Grande Tatarenda section of the Petaca Formation. Black star indicates locality where Prohegetotherium cf. P. schiaffinoi was collected.

GEOLOGICAL SETTING

The syn-orogenic sequences accumulated in the Chaco foreland basin during the Oligocene — Pleistocene are called the Chaco Group (Stebinger 1920Stebinger E. 1920. Report in oil geology of the región between Cochabamba and Samaipata, States Cochabamba and Santa Cruz, Bolivia. Informe Interno Standard Oil Company Bolivia, GXG-82., Harrington 1922Harrington G.L. 1922. North end Aguaragüe range from Camatindi to Cuevo Quebrada. Informe interno Standard Oil Company Bolivia, Santa Cruz, Bolivia, 15., White 1925White K.D. 1925. Bolivian stratigraphy south of the Río Grande. Informe interno Sandard Oil Company Bolivia, Santa Cruz, Bolivia.). In the study area (northern sector of the basin), the Chaco Group consists of the Petaca, Yecua, Tariquía, Guandacay and Emborozú Formations (Ayaviri 1967Ayaviri A. 1967. Estratigrafía del Subandino meridional. Informe Interno de YPFB, Santa Cruz, Bolivia, GXG-1215.) (Fig. 2A). Particularly, in the studied area, the Petaca Formation overlies discordantly (erosive surface) the Cajones sequences, and the contact with the overlapping Yecua Formation is sharp and unconformable (transgressive surface) (Fig. 2B). This is evidenced by the change from semi-arid fluvial (Petaca) to inland wetland/lacustrine (Yecua) depositional environments (Fig. 3A).

Figure 2
(A) Simplified stratigraphy of the Cenozoic Chaco foreland Basin. Summary of the Depositional systems (l, m, and e refer to late, middle, and early, respectively). (B) Lithologic log of the Petaca Formation in the study area, showing facies associations and their vertical relationships. Note the bone indicating the stratigraphic provenance of Prohegetotherium cf. P. schiaffinoi in the upper section (black arrow).
Figure 3
(A) Photograph of the outcrop showing the contact between the Petaca and Yecua formations (white line). Note the layer where the material was preserved (black rectangle, see detail in Figure 3B); (B) detail of the bed where Prohegetotherium cf. P. schiaffinoi was found; (C) upper Petaca Formation at overview of channel belts. Interpreted photomosaic showing architectural style of braided fluvial channels. Note how each sandstone-filled channel belt affects both old channel belts and floodplain deposits. Black rectangle on the right, corresponds to Figure 3A.

The Petaca Formation (Birkett 1922Birkett D.S. 1922. Preliminary reporto N Guariri and Saipuru domes, SE Bolivia. Informe interno Standard Oil Company Bolivia, Santa Cruz, Bolivia, 10 p.) consists of greenish-gray to reddish massive sandstones, cross-bedded sandstones and massive conglomerates, with notable presence of calcareous nodules and calcrete levels (Requena 1981Requena E. 1981. Secciones estratigráficas de las serranías de Charagua y Candado. Informe interno YPFB, Santa Cruz, Bolivia., Uba et al. 2006Uba C.E., Heubeck C., Hulka C. 2006. Evolution of the late Cenozoic Chaco Foreland Basin, southern Bolivia. Basin Research, 18(2):145-170. https://doi.org/10.1111/j.1365-2117.2006.00291.x
https://doi.org/10.1111/j.1365-2117.2006...
, Vergani et al. 2012Vergani G.D., González G., Poiré D.G., Tineo D.E. 2012. Análisis sedimentológico y estratigráfico a partir de la integración de datos de superficie y subsuelo en reservorios gasíferos de la Formación Petaca (Oligoceno-Mioceno), Campo Tajibo, Bolivia. In: XVI Congreso Peruano de Geología, 16., and SEG 2012 Conference, 2012, Lima, Perú. Actas…, Poiré et al. 2013Poiré D.G, Tineo D.E, González-Rigas G., Bona P., Toledo N., Reguero M., Scarano A., Vergani G.D., Pérez L.M. 2013. Hallazgos de vertebrados continentales de la Formación Petaca (Oligoceno tardío), Cuenca del Chaco, Bolivia. Ameghiniana, 50(6):65R.). The fossil remains were found within the sandy beds (Fig. 3B). The deposits of this unit consist of tabular to lenticular bodies with erosive bases, 0.5–7 m thick, and lateral extensions between 50–150 m (Fig. 3C). Their width/thickness ratio ranges between 50 and 100, classified as narrow to broad sheets (Gibling 2006Gibling M.R. 2006. Width and thickness of fluvial channel bodies and valley fills in the geological record: a literature compilation and classification. Journal of Sedimentary Research, 76(5):731-770. https://doi.org/10.2110/jsr.2006.060
https://doi.org/10.2110/jsr.2006.060...
). The bodies generally show a vertical and lateral stacking forming multiepisodic channel belts, up to 20 m thick, which are delimited by basal concave erosive surfaces (Fig. 3C). Paleocurrent data show a predominant direction to the west (Uba et al. 2005Uba C.E., Heubeck C., Hulka C. 2005. Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia. Sedimentary Geology, 180(3-4):91-123. https://doi.org/10.1016/j.sedgeo.2005.06.013
https://doi.org/10.1016/j.sedgeo.2005.06...
, 2006Uba C.E., Heubeck C., Hulka C. 2006. Evolution of the late Cenozoic Chaco Foreland Basin, southern Bolivia. Basin Research, 18(2):145-170. https://doi.org/10.1111/j.1365-2117.2006.00291.x
https://doi.org/10.1111/j.1365-2117.2006...
). Channel bodies are sporadically interbedded with massive to laminated mudstone beds with mudcracks. Both fine-grained deposits and sandy bodies present pedogenic features such as soil aggregates, rhizoconcretions and bioturbation.

The Petaca Formation is interpreted as interconnected fluvial system deposits with floodplain development (Marshall et al. 1993Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301., Uba et al. 2005Uba C.E., Heubeck C., Hulka C. 2005. Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia. Sedimentary Geology, 180(3-4):91-123. https://doi.org/10.1016/j.sedgeo.2005.06.013
https://doi.org/10.1016/j.sedgeo.2005.06...
, 2006Uba C.E., Heubeck C., Hulka C. 2006. Evolution of the late Cenozoic Chaco Foreland Basin, southern Bolivia. Basin Research, 18(2):145-170. https://doi.org/10.1111/j.1365-2117.2006.00291.x
https://doi.org/10.1111/j.1365-2117.2006...
, Vergani et al. 2012Vergani G.D., González G., Poiré D.G., Tineo D.E. 2012. Análisis sedimentológico y estratigráfico a partir de la integración de datos de superficie y subsuelo en reservorios gasíferos de la Formación Petaca (Oligoceno-Mioceno), Campo Tajibo, Bolivia. In: XVI Congreso Peruano de Geología, 16., and SEG 2012 Conference, 2012, Lima, Perú. Actas…). The channels are dominated by the sandy bed-load, accumulated from the construction of bars originated by the migration of 3D megaripples and current ripples. The bars are represented by lateral and frontal accretion components generating both simple and compound forms. The presence of amalgamated sandy bodies separated by erosive surfaces is interpreted as a design of active channels of moderate to low sinuosity that represent braided channel belts. The thin mudstone beds in the top-sets represent floodplain deposits accumulated during the migration of the channels (Page et al. 2003Page K.J., Nanson G.C., Frazier P.S. 2003. Floodplain formation and sediment stratigraphy resulting from oblique accretion on the Murrumbidgee River, Australia. Journal of Sedimentary Research, 73(1):5-14. https://doi.org/10.1306/070102730005
https://doi.org/10.1306/070102730005...
). Calcretes and paleosols indicate low sedimentation rates in a semi-arid climate (Cecil 1990Cecil C.B. 1990. Paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks. Geology, 18(6):533-536. https://doi.org/10.1130/0091-7613(1990)018%3C0533:PCOSRO%3E2.3.CO;2
https://doi.org/10.1130/0091-7613(1990)0...
, Uba et al. 2005Uba C.E., Heubeck C., Hulka C. 2005. Facies analysis and basin architecture of the Neogene Subandean synorogenic wedge, southern Bolivia. Sedimentary Geology, 180(3-4):91-123. https://doi.org/10.1016/j.sedgeo.2005.06.013
https://doi.org/10.1016/j.sedgeo.2005.06...
, 2006Uba C.E., Heubeck C., Hulka C. 2006. Evolution of the late Cenozoic Chaco Foreland Basin, southern Bolivia. Basin Research, 18(2):145-170. https://doi.org/10.1111/j.1365-2117.2006.00291.x
https://doi.org/10.1111/j.1365-2117.2006...
, Vergani et al. 2012Vergani G.D., González G., Poiré D.G., Tineo D.E. 2012. Análisis sedimentológico y estratigráfico a partir de la integración de datos de superficie y subsuelo en reservorios gasíferos de la Formación Petaca (Oligoceno-Mioceno), Campo Tajibo, Bolivia. In: XVI Congreso Peruano de Geología, 16., and SEG 2012 Conference, 2012, Lima, Perú. Actas…). Marshall et al. (1993)Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301. dated biostratigraphically the upper part of the Petaca Formation based on the Chasicoan-Montehermosan (late Miocene — early and middle Pliocene) cf. Vassallia minuta (Xenarthra, Pampatheriidae). However, there are some inconsistencies about the provenance and the systematic treatment of this material (see below).

MATERIALS AND METHODS

The YPFB-LIT-PAL-005 specimen is housed in the vertebrate paleontology collection of the Yacimientos Petrolíferos Fiscales de Bolivia (YPFB, Santa Cruz de la Sierra).

The anatomical study has been carried out through direct and bibliographic comparisons, using type specimens and reference material from collections in the institutions listed below (see “Institutional abbreviations”), as well as using several scientific contributions on Hegetotheriidae (e.g., Rovereto 1914Rovereto C. 1914. Los estratos Araucanos y sus fósiles. Anales del Museo Nacional de Historia Natural de Buenos Aires, 25:1-247., Reguero 1999Reguero M.A. 1999. El problema de las relaciones sistemáticas y filogenéticas de los Typotheria y Hegetotheria (Mammalia, Notoungulata): análisis de los taxones de Patagonia de la Edad-mamífero Deseadense (Oligoceno). Thesis, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, 350 p. Available from: <https://bibliotecadigital.exactas.uba.ar/download/tesis/tesis_n3136_Reguero.pdf>. Accessed on: Mar, 1999.
https://bibliotecadigital.exactas.uba.ar...
, Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
, Croft and Anaya 2006Croft D.A., Anaya F. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of the Hegetotheriidae. Journal of Vertebrate Paleontology, 26(2):387-399. https://doi.org/10.1671/0272-4634(2006)26[387:ANMMHN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2006)2...
, Kramarz and Paz 2013Kramarz A.G., Paz E.R. 2013. Un Hegetotheriidae (Mammalia, Notoungulata) basal del Mioceno temprano de Patagonia. Revista Mexicana de Ciencias Geológicas, 30(1):186-195., Cerdeño and Reguero 2015Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
; Kramarz and Bond 2017Kramarz A.G., Bond M. 2017. Systematics and stratigraphical range of the hegetotheriids Hegetotheriopsis sulcatus and Prohegetotherium sculptum (Mammalia: Notoungulata). Journal of Systematic Palaeontology, 15(12):1027-1036. https://doi.org/10.1080/14772019.2016.1266047
https://doi.org/10.1080/14772019.2016.12...
, Seoane et al. 2017Seoane F.D., Roig Juñent, S., Cerdeño E. 2017. Phylogeny and paleobiogeography of Hegetotheriidae (Mammalia, Notoungulata). Journal of Vertebrate Paleontology, 37(1):e1278547. https://doi.org/10.1080/02724634.2017.1278547
https://doi.org/10.1080/02724634.2017.12...
, Vera and Ercoli 2018Vera B., Ercoli M.D. 2018. Systematic and morphogeometric analyses of Pachyrukhinae (Mammalia, Hegetotheriidae) from the HuayquerÍas, Mendoza (Argentina): biostratigraphic and evolutionary implications. Journal of Vertebrate Paleontology, 38(3):e1473410. https://doi.org/10.1080/02724634.2018.1473410
https://doi.org/10.1080/02724634.2018.14...
, Ercoli et al. 2019Ercoli M.D., Álvarez A., Candela A.M. 2019. Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates. Communication Biology, 2:202. https://doi.org/10.1038/s42003-019-0423-5
https://doi.org/10.1038/s42003-019-0423-...
, Seoane and Cerdeño 2019Seoane F., Cerdeño E. 2019. Systematic revision of Hegetotherium and Pachyrukhos (Hegetotheriidae, Notoungulata) and a new phylogenetic analysis of Hegetotheriidae. Journal of Systematic Palaeontology, 17(19):1635-1663. https://doi.org/10.1080/14772019.2018.1545146
https://doi.org/10.1080/14772019.2018.15...
).

Hypsodonty index follows Reguero et al. (2010)Reguero M.A., Candela A.M., Cassini G.H. 2010. Hypsodonty and body size in rodent-like notoungulates. In: Carlini A.A., Madden R.H., Vucetich M.G., Kay R.F. (Eds.). The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia (p. 362-374). Cambridge: Cambridge University Press. and was calculated by dividing the m1 height by the m1 anterior/posterior length.

Teeth measurements were taken with a Mitutoyo digital caliper (0.01 mm accuracy). Two variables were measured for each tooth: length, corresponding to maximum mesiodistal diameter; and width, measured perpendicular to length.

Photographs were taken with a Nikon D3000 digital camera.

Institutional abbreviations

AMC, Amherst College Museum, Amherst, Massachusetts, USA; AMNH, American Museum of Natural History, New York, USA; MAMC, Museo de Arqueología de Canelones, Uruguay; MCNAM-PV, Museo de Ciencias Naturales y Antropológicas “J.C. Moyano”, colección Paleontología de Vertebrados, Mendoza, Argentina; MLP, Museo de La Plata, La Plata, Argentina; MNHN-DP, Museo Nacional de Historia Natural de Montevideo, Montevideo Uruguay; PZ-Ctes, PRINGEPA (Programa de Investigación Geológica y Paleontológica), Corrientes, Argentina; UATF-V, Vertebrate Paleontology Collections, Universidad Autónoma Tomás Frías, Potosí, Bolivia; UF, University of Florida, Gainsville, USA; YPFB-LIT-PAL, Yacimientos Petrolíferos Fiscales, colección paleontológica, Santa Cruz de la Sierra, Bolivia.

Abbreviations of teeth nomenclature

c, lower canine; i, lower incisor; m, lower molar; p, lower premolar.

RESULTS

Systematic paleontology

Order NOTOUNGULATA Roth (1903)Roth S. 1903. Los ungulados sudamericanos. Anales del Museo La Plata, 5:1-36.

Suborder TYPOTHERIA Zittel (1893)Zittel K.A. 1893. Handbuch der Palaeontologie, IV. Bd. Vertebrata (Mammalia). Munich: R. Oldenbourg, 590 p. (sensu Reguero and Castro 2004Reguero M.A., Castro P.V. 2004. Un nuevo Trachytheriinae (Mammalia, y Notoungulata) del Deseadense (Oligoceno tardío) de Patagonia, Argentina: implicancias en la filogenia, biogeografía y bioestratigrafía de los Mesotheriidae. Revista Geológica de Chile, 31(1):45-64. https://doi.org/10.4067/S0716-02082004000100003
https://doi.org/10.4067/S0716-0208200400...
)

Family HEGETOTHERIIDAE Ameghino (1894)Ameghino F. 1894. Enumération synoptique del espèces de mammifères fossiles des formations éocènes de Patagonie. Boletín de la Academia Nacional de Ciencias en Córdoba, 13:259-445. https://doi.org/10.5962/bhl.title.77348
https://doi.org/10.5962/bhl.title.77348...

Subfamily HEGETOTHERIINAE Ameghino (1894)Ameghino F. 1894. Enumération synoptique del espèces de mammifères fossiles des formations éocènes de Patagonie. Boletín de la Academia Nacional de Ciencias en Córdoba, 13:259-445. https://doi.org/10.5962/bhl.title.77348
https://doi.org/10.5962/bhl.title.77348...

Genus Prohegetotherium Ameghino (1897)Ameghino F. 1897. Mammiferes crétacés de l’Argentine—deuxieme contribution a la conaissance de la faune mammalogique des couches a Pyrotherium. Boletín del Instituto Geográfico Argentino, 18:406-521.

Type species

Prohegetotherium sculptumAmeghino (1897)Ameghino F. 1897. Mammiferes crétacés de l’Argentine—deuxieme contribution a la conaissance de la faune mammalogique des couches a Pyrotherium. Boletín del Instituto Geográfico Argentino, 18:406-521..

Included species

The type species Prohegetotherium shumwayi Loomis (1914)Loomis F.B. 1914. The Deseado Formation of Patagonia. Concord: Rumford Press, 232 p., P. schiaffinoi (Kraglievich 1932Kraglievich L. 1932. Nuevos apuntes para la geología y paleontología uruguayas. Anales del Museo de Historia Natural de Montevideo, 3:1-65.), and P. malalhuense Cerdeño and Reguero (2015)Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
.

Geographic and stratigraphic distribution

Prohegetotherium occurs in several Deseadan (late Oligocene) localities of Argentina (Patagonia, Mendoza and Corrientes) (Ameghino 1897Ameghino F. 1897. Mammiferes crétacés de l’Argentine—deuxieme contribution a la conaissance de la faune mammalogique des couches a Pyrotherium. Boletín del Instituto Geográfico Argentino, 18:406-521., Bond et al. 1998Bond M., López G., Reguero M.A., Scillato-Yané G.J., Vucetich M.G. 1998. Los mamíferos de la Formación Fray Bentos (Edad Deseadense, Oligoceno superior?) de las provincias de Corrientes y Entre Ríos, Argentina. Asociación Paleontológica Argentina, Publicación Especial, 5(1):41-50., Cerdeño and Reguero 2015Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
). It was also recorded in the Santacrucian (early Miocene) of the Mariño Formation (Cerdeño et al. 2008Cerdeño E., López G.M., Reguero M. 2008. Biostratigraphic considerations of the Divisaderan faunal assemblage. Journal of Vertebrate Paleontology, 28(2):574-577. https://doi.org/10.1671/0272-4634(2008)28[574:BCOTDF]2.0.CO;2
https://doi.org/10.1671/0272-4634(2008)2...
). Outside Argentina Prohegetotherium has been recorded in Cachapoal, Chile (early Oligocene, Croft et al. 2008aCroft D.A., Charrier R., Flynn J.J., Wyss A.R. 2008a. Recent additions to knowledge of Tertiary mammals from the Chilean Andes. In: Simpósio de Paleontología, 1., 2008, Chile. Actas… p. 1-7., Croft et al. 2008bCroft D.A., Flynn J.J., Wyss A.R. 2008b. The Tinguiririca fauna of Chile and the early stages of ‘modernization’ of South American faunas. Arquivos do Museu Nacional, 66(1):191-211.), Salla, Bolivia (late Oligocene, Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
), and Fray Bentos, Uruguay (late Oligocene, Fray Bentos Formation, Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
).

Remarks on the systematic status of Prohegetotherium

Florentino Ameghino (1897)Ameghino F. 1897. Mammiferes crétacés de l’Argentine—deuxieme contribution a la conaissance de la faune mammalogique des couches a Pyrotherium. Boletín del Instituto Geográfico Argentino, 18:406-521. erected this genus based on the two syntypes, probably from the Deseadan locality Cabeza Blanca in central Patagonia. Ameghino did not provide a detailed description of this taxon, but remarked that the upper premolars have an anterolabial sulcus (parastylar sulcus, in the current dental terminology) and a well-developed canine. Recently, Kramarz and Bond (2017)Kramarz A.G., Bond M. 2017. Systematics and stratigraphical range of the hegetotheriids Hegetotheriopsis sulcatus and Prohegetotherium sculptum (Mammalia: Notoungulata). Journal of Systematic Palaeontology, 15(12):1027-1036. https://doi.org/10.1080/14772019.2016.1266047
https://doi.org/10.1080/14772019.2016.12...
re-examinated the type specimens of Prohegetotherium sculptum (MACN A 52-443, left maxillary fragment with the alveolus for the canine and P1–P3; paralectotype: MACN A 52-444, left portion of maxillary with incomplete P3–M3, and part of nasals and frontals). They found that this material exhibits several dental and cranial characters not recognized in previous studies (Reguero 1999Reguero M.A. 1999. El problema de las relaciones sistemáticas y filogenéticas de los Typotheria y Hegetotheria (Mammalia, Notoungulata): análisis de los taxones de Patagonia de la Edad-mamífero Deseadense (Oligoceno). Thesis, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, 350 p. Available from: <https://bibliotecadigital.exactas.uba.ar/download/tesis/tesis_n3136_Reguero.pdf>. Accessed on: Mar, 1999.
https://bibliotecadigital.exactas.uba.ar...
, Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
), and concluded that no specimen other than the types can be assigned to P. sculptum. Seoane and Cerdeño (2019)Seoane F., Cerdeño E. 2019. Systematic revision of Hegetotherium and Pachyrukhos (Hegetotheriidae, Notoungulata) and a new phylogenetic analysis of Hegetotheriidae. Journal of Systematic Palaeontology, 17(19):1635-1663. https://doi.org/10.1080/14772019.2018.1545146
https://doi.org/10.1080/14772019.2018.15...
suggested the need for a deep revision of all the materials assigned to Prohegetotherium, particularly those excluded from P. sculptum by Kramarz and Bond (2017)Kramarz A.G., Bond M. 2017. Systematics and stratigraphical range of the hegetotheriids Hegetotheriopsis sulcatus and Prohegetotherium sculptum (Mammalia: Notoungulata). Journal of Systematic Palaeontology, 15(12):1027-1036. https://doi.org/10.1080/14772019.2016.1266047
https://doi.org/10.1080/14772019.2016.12...
.

Comparison with the other species of Prohegetotherium

YPFB-LIT-PAL-005 cannot be properly compared with Prohegetotherium sculptum, because the species is only known by the upper dentition (Kramarz and Bond 2017Kramarz A.G., Bond M. 2017. Systematics and stratigraphical range of the hegetotheriids Hegetotheriopsis sulcatus and Prohegetotherium sculptum (Mammalia: Notoungulata). Journal of Systematic Palaeontology, 15(12):1027-1036. https://doi.org/10.1080/14772019.2016.1266047
https://doi.org/10.1080/14772019.2016.12...
). Chaffee (1952Chaffee R.G. 1952. The Deseadan vertebrate fauna of the Scarritt Pocket, Patagonia. Bulletin of the American Museum of Natural History, 98:503-562., pl. 16, Figs. 2 and 3) described a mandible, AMNH 29605, from the Deseadan of Patagonia and referred to P. sculptum although there is no anatomical correspondence between the type material of P. sculptum and AMNH 29605. The similar size and matching occlusion allowed it to hypothesize that AMNH 29605 could represent the lower dentition of Prohegetotherium sculptum. The lower teeth of AMNH 29605 have the p4 in line with the tooth row and are also larger than those of the YPFB-LIT-PAL-005.

Prohegetotherium shumwayiLoomis (1914)Loomis F.B. 1914. The Deseado Formation of Patagonia. Concord: Rumford Press, 232 p. is based on only one specimen, a right maxillary fragment with upper premolars and molars (Loomis 1914Loomis F.B. 1914. The Deseado Formation of Patagonia. Concord: Rumford Press, 232 p., p. 64, Fig. 29). According to the figure and the dental measurements given by Loomis (1914)Loomis F.B. 1914. The Deseado Formation of Patagonia. Concord: Rumford Press, 232 p., the premolars are slightly shorter and narrower than in the lectotype of P. sculptum.

The cheek teeth morphology of YPFB-LIT-PAL-005 is very similar to that of the specimens assigned to Prohegetotherium schiaffinoi rather than to those of P. malalhuense, as Reguero and Cerdeño (2005)Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
suggest from comparison with the Bolivian material from Salla. Later, Cerdeño and Reguero (2015)Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
assigned to Prohegetotherium schiaffinoi several specimens from the Deseadan locality of Quebrada Fiera and Divisadero Largo (Mendoza Province, Argentina). Comparing directly with the latter sample, YPFB-LIT-PAL-005 shows a similar imbrication of the p4, with elongated trigonid and a posterolingual inflexion of m3, but a different elongation of p3 and a distinctive inflexion/groove in m3. These features can be present in some Miocene hegetotheriids (Cerdeño and Reguero 2015Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
, Seoane and Cerdeño 2019Seoane F., Cerdeño E. 2019. Systematic revision of Hegetotherium and Pachyrukhos (Hegetotheriidae, Notoungulata) and a new phylogenetic analysis of Hegetotheriidae. Journal of Systematic Palaeontology, 17(19):1635-1663. https://doi.org/10.1080/14772019.2018.1545146
https://doi.org/10.1080/14772019.2018.15...
, Vera 2019Vera B. 2019. A new species and the record of Hemihegetotherium (Notoungulata, Hegetotheriidae) in the Middle to Late Miocene of Patagonia, Argentina. Journal of South American Earth Sciences, 93:23-35. https://doi.org/10.1016/j.jsames.2019.04.017
https://doi.org/10.1016/j.jsames.2019.04...
). However, several morphological features seen on the mandibular ramus of YPFB-LIT-PAL-005, i.e., dentary exhibiting a marked change in height along its length, a prominent masseteric crest and a remarkably thick ventral margin of the dentary, make differences between this specimen and Prohegetotheium schiaffinoi.

Prohegetotherium schiaffinoi (Kraglievich 1932Kraglievich L. 1932. Nuevos apuntes para la geología y paleontología uruguayas. Anales del Museo de Historia Natural de Montevideo, 3:1-65.)

Holotype

MNHN-DP-186, partial maxilla with P2–M2. Fray Bentos Formation, Cañada de las Mulas, Santa Lucía River, Canelones Department, Uruguay. Deseadan SALMA.

Geographic and stratigraphic distribution

Uruguay: Fray Bentos Formation; Bolivia: Salla, “Upper Salla Beds”; and Argentina: Corrientes and Entre Ríos (Deseadan SALMA, Fray Bentos Formation).

Remarks

Kraglievich (1932)Kraglievich L. 1932. Nuevos apuntes para la geología y paleontología uruguayas. Anales del Museo de Historia Natural de Montevideo, 3:1-65. erected the species Propachyrucos? schiaffinoi (holotype MNHN-DP-186) on a maxillary fragment with P2–M2 from the Fray Bentos Formation, Uruguay. Reguero (1999)Reguero M.A. 1999. El problema de las relaciones sistemáticas y filogenéticas de los Typotheria y Hegetotheria (Mammalia, Notoungulata): análisis de los taxones de Patagonia de la Edad-mamífero Deseadense (Oligoceno). Thesis, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, 350 p. Available from: <https://bibliotecadigital.exactas.uba.ar/download/tesis/tesis_n3136_Reguero.pdf>. Accessed on: Mar, 1999.
https://bibliotecadigital.exactas.uba.ar...
and Reguero and Cerdeño (2005)Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
considered it as part of the genus Prohegetotherium, and these latter authors provided a more complete description of Prohegetotherium schiaffinoi based on the Bolivian material.

Remarks on the taxonomic status of “Ethegotherium carettei” from the Mariño Formation (Miocene) of Mendoza and its affinity with Prohegetotherium schiaffinoi

Ethegotherium carettei” was first described as a new species of Prohegetotherium (Minoprio 1947Minoprio J.L. 1947. Fósiles de la Formación del Divisadero Largo. Anales de la Sociedad Científica Argentina, 146:365-378.) and later assigned to a new genus Ethegotherium by Simpson et al. (1962)Simpson G.G., Minoprio J.L., Patterson B. 1962. The mammalian fauna of the Divisadero Largo Formation, Mendoza, Argentina. Bulletin of the Museum of Comparative Zoology at Harvard College, 127:139-293.. Later, López (2002López G.M. 2002. Redescription of Ethegotherium carettei (Notoungulata, Hegetotheriidae) from Divisadero Largo Formation, Mendoza Province, Argentina. Ameghiniana, 39(3):295-306., 2010López G.M. 2010. Divisaderan: Land Mammal Age or Local Fauna? In: Madden R., Carlini A.A., Vucetich M.G., Kay R. (Eds.). The Paleontology of Gran Barranca. Cambridge: Cambridge University Press, p. 410-417.) revalidated the species Ethegotherium carettei. López and Manassero (2008)López G.M., Manassero M. 2008. Revision of the stratigraphic provenance of Ethegotherium carettei (Notoungulata, Hegetotheriidae) by sedimentary petrography. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 248:1-9. https://doi.org/10.1127/0077-7749/2008/0248-0001
https://doi.org/10.1127/0077-7749/2008/0...
indicate that the sediment containing the holotype of E. carettei was found to share more similarities with the sediments of the Mariño Formation than the Divisadero Largo Formation. The stratigraphic provenance of this species therefore does not likely correspond to the Divisadero Largo Formation and should not pertain to the Divisaderan Fauna. Reguero and Cerdeño (2005)Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
and Cerdeño and Reguero (2015)Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
subsumed it within Prohegetotherium schiaffinoi and extended the chronological distribution of this species to the early Miocene (Cerdeño et al. 2008Cerdeño E., López G.M., Reguero M. 2008. Biostratigraphic considerations of the Divisaderan faunal assemblage. Journal of Vertebrate Paleontology, 28(2):574-577. https://doi.org/10.1671/0272-4634(2008)28[574:BCOTDF]2.0.CO;2
https://doi.org/10.1671/0272-4634(2008)2...
).

Prohegetotherium cf. P. schiaffinoi

(Figs. 4A4D)

Figure 4
Prohegetotherium cf. P. schiaffinoi, YPFB-LIT-PAL-005. (A) Pair of mandibles bearing left p4-m3 and right m2-3 in occlusal view; (B) original and schematic drawing of the occlusal view of the left horizontal ramus; (C) labial view of the right horizontal ramus; (D) ventral view of the right horizontal ramus. Scale bar equals 10 mm.

Material

YPFB-LIT-PAL-005, mandible with left p4-m3 and right m2-m3 (Fig. 4A).

Geographic occurrence

Vicinity of the town Abapó (19°1’17”S - 63°33’25”W), 160 km SW of Santa Cruz de la Sierra, Bolivia (Fig. 1).

Age and distribution

Petaca Formation, late Oligocene-early Miocene (Marshall and Sempere 1991Marshall L.G., Sempere T. 1991. The Eocene to Pleistocene vertebrates of Bolivia and their stratigraphic context, a review. Fósiles y Facies de Bolivia, 12(3-4):631-652.).

Description

In lateral view, the horizontal ramus of YPFB-LIT-PAL-005 is sturdy and low, being proportionally shorter and lower than in other hegetotheriids, i.e. Hegetotherium and Hemihegetotherium species. The horizontal mandibular ramus is very low at the level of p4 (9.6 mm), increasing in height backwards (15.3 mm at m1/m2 level); its ventral border is slightly convex and very thick, especially in the posterior sector; the posterior end of the symphysis extends up to the level of the alveolus of p3. In the masseteric area, there is a well-defined masseteric crest (best exhibited on the right ramus) and its anterior process has its maximal development at the level of the posterior end of m3 (Fig. 4B). A deep masseteric groove is present immediately anterior to the masseteric crest (Figs. 4C and 4D). The angular process has a rounded contour, and the ventral masseteric fossa, which is associated with the insertion of mm. masseter lateralis, is well defined as in Hegetotheriidae Pachyrukhinae (Ercoli et al. 2019Ercoli M.D., Álvarez A., Candela A.M. 2019. Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates. Communication Biology, 2:202. https://doi.org/10.1038/s42003-019-0423-5
https://doi.org/10.1038/s42003-019-0423-...
) (Fig. 4C). There is a large mental foramen between m2 and m3, close to the inferior border of the ramus.

The p4 (Fig. 4B) is not molariform and is the smallest tooth of the series. It has two labial lobes being the anterior (trigonid) slightly smaller than the posterior (talonid) and anterolingually rounded and posterolabially elongated. The anterior face of this tooth shows a thin dentine band, and the lingual face is rather straight. The labial groove is deeply marked. It is imbricated with the m1; the distolingual extremity of each tooth (p4 and m1) is just lingual to the mesial end of the next most distal tooth. The size and morphology of the posterior lobes of the p4 and molars are similar to those observed in Prohegetotherium schiaffinoi (Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
, Cerdeño and Reguero 2015Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
). On the contrary, in other hegetotheriid species, i.e., Hegetotherium mirabile, the p4 has a square trigonid, very narrow labial sulcus, and a linguodistal projection on the talonid.

The m1–m2 (Fig. 4B) are similar to each other. These two cheek teeth are characterized by a straight lingual face with a lingual projection near the distal end; a trigonid with a rounded to slightly pointed labial face; a talonid with a triangular lingual face; and absence of mesial and distal enamel where the tooth contacts the adjacent teeth, and without posterolingual projection unlike Hegetotherium. Their posterolingual corner projects lingually, unlike other species of Prohegetotherium, e.g. P. shumwayi.

The m3 displays a rounded trigonid similar to m1−m2, but smaller, a subtriangular talonid and a posterolingual groove (plg) like some specimens of the Santacrucian Hegetotherium mirabile Ameghino 1887Ameghino F. 1887. Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia austral y depositados en el Museo La Plata. Boletín del Museo La Plata, 1:1-26.. It is bilobate, but with the posterior lobe much longer than the anterior and its labial face convex, without talonid groove unlike Hegetotherium and Hemihegetotherium. The m3 of YPFB-LIT-PAL-005 differs from m1–m2 by several characteristics, including a narrower talonid; absence of enamel along the distal end of the lingual face; presence of a notch near the distal end of the lingual face (Figs. 4B and 4D); and similar in size to m2, a character not seen in other species of Prohegetotherium in which the m3 is always longer than m2 (Fig. 4B, Tab. 1). The first three features are also present in other hegetotheriid species, e.g., Hegetotherium mirabile and H. cerdasensis, although the lingual talonid notch tends to be smaller than in YPFB-LIT-PAL-005. In this regard, YPFB-LIT-PAL-005 resembles Prohegetotherium schiaffinoi (see Cerdeño and Reguero 2015Cerdeño E., Reguero M.A. 2015. The Hegetotheriidae (Mammalia, Notoungulata) assemblage from the late Oligocene of Mendoza, central-western Argentina. Journal of Vertebrate Paleontology, 35(2):e907173. https://doi.org/10.1080/02724634.2014.907173
https://doi.org/10.1080/02724634.2014.90...
: Fig. 5F). As in Prohegetotherium schiaffinoi, there is a posterolingual groove on m3 (best seen on right m3).

Table 1
Measurements (in mm) of the lower teeth and mandible bone of Prohegetotherium cf. P. schiaffinoi and compared specimens.

DISCUSSION

Mandibular morphology of YPFB-LIT-PAL-005

The dental morphology and dimensions of YPFB-LIT-PAL-005 (Tab. 1) show several similarities with the Deseadan species Prohegetotherium schiaffinoi from Salla (Bolivia), Fray Bentos (Uruguay), Corrientes and Mendoza (Argentina). However, several morphological distinctions of the mandibular ramus between this specimen and the holotype and referred material of P. schiaffinoi should be mentioned: i.e., dentary of YPFB-LIT-PAL-005 with a marked change in height along its length, and the ventral rim of the dentary remarkably thick. The particular mandibular anatomy of YPFB-LIT-PAL-005 is not frequent in Prohegetotherium. It is probably associated with a shortening of the mandible and narrowing of the symphysis (broken and badly preserved in YPFB-LIT-PAL-005). A similar mandibular characteristic is present in the holotype of Sallatherium altiplanensis (UF 91621) from the Deseadan (late Oligocene) of Salla (Bolivia), but in this species the mandible is higher and robust and has a marked symphyseal narrowing at the level of i1-c.

The mandible YPFB-LIT-PAL-005 exhibits a mosaic of features similar to the pachyrukhine species. Ercoli et al. (2019)Ercoli M.D., Álvarez A., Candela A.M. 2019. Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates. Communication Biology, 2:202. https://doi.org/10.1038/s42003-019-0423-5
https://doi.org/10.1038/s42003-019-0423-...
, based on the study and reconstruction of the masticatory muscles of some hegetotheriidae pachyrukhines, i.e., Paedotherium and Tremacyllus, recognize the presence of a true sciuromorph condition, defined by an anterior portion of the deep masseter muscle originating from a wide zygomatic plate that reaches the rostrum, a trait traceable since the Oligocene in Hegetotheriidae. In lateral view of YPFB-LIT-PAL-005, the horizontal rami are low, shorter than in other hegetotheriine species, and have a deep groove posteriorly immediately in front of the masseteric crest, and a prominent anterior process (Figs. 4C and 4D). This groove is linked to the passage of the masseter muscle superficialis pars reflexa (Ercoli et al. 2020Ercoli M.D., Álvarez A., Moyano S.R., Youlatos D., Candela A.M. 2020. Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus. Journal of Mammalian Evolution. https://doi.org/10.1007/s10914-020-09516-7
https://doi.org/10.1007/s10914-020-09516...
). The ventral and dorsal masseteric fossae are well defined and associated with the insertions of muscle masseter lateralis and medialis respectively (Fig. 4C). The singularity of the mosaic of masticatory features of the mandible YPFB-LIT-PAL-005 is interpreted as related to a large mediolateral component in chewing movements, more similar to Paedotherium (Ercoli et al. 2019Ercoli M.D., Álvarez A., Candela A.M. 2019. Sciuromorphy outside rodents reveals an ecomorphological convergence between squirrels and extinct South American ungulates. Communication Biology, 2:202. https://doi.org/10.1038/s42003-019-0423-5
https://doi.org/10.1038/s42003-019-0423-...
).

Paleobiology of the Hegetotheriidae from Abapó

Hegetotheriids are generally reconstructed as grassers and open habitat specialists that might have lived in burrows resembling rabbits (leporids) or various South American rodents (caviids, chinchillids) in lifestyle (Sinclair 1909Sinclair W.J. 1909. Mammalia of the Santa Cruz Beds. In: Scott W.B. (Ed.). Reports of the Princeton University Expeditions to Patagonia, 1896–1899 (p. 1-110). E. Schweizerbart’sche Verlagshandlung (E. Nägele). Stuttgart: Princeton University. v. 4., Genise 1989Genise J.F. 1989. Las cuevas con Actenomys (Rodentia, Octodontidae) de la Formación Chapadmalal (Plioceno superior) de Mar del Plata y Miramar (provincia de Buenos Aires). Ameghiniana, 26(1-2):33-42., Elissamburu 2004Elissamburu A. 2004. Análisis morfométrico y morfofuncional del esqueleto apendicular de Paedotherium (Mammalia, Notoungulata). Ameghiniana, 41(3):363-380., Croft 2016Croft D.A. 2016. Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America. Bloomington: Indiana University Press, 304 p., Ercoli et al. 2020Ercoli M.D., Álvarez A., Moyano S.R., Youlatos D., Candela A.M. 2020. Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus. Journal of Mammalian Evolution. https://doi.org/10.1007/s10914-020-09516-7
https://doi.org/10.1007/s10914-020-09516...
). The Petaca hegetotheriid has a moderate hypsodonty (HI ∼ 2.4). Janis (1988)Janis C.M. 1988. An Estimation of Tooth Volume and Hypsodonty indices in Ungulate Mammals, and the Correlation of these Factors with Dietary Preference. Mémoires du Muséum National d’Histoire Naturelle, 53:367-387. pointed out that hypsodonty occurs in herbivores feeding on any type of low vegetation that would be subjected to abrasive dust and grit coverage. In the context of hypsodont species, this ungulate was probably a wide-ranging species that lived in gallery forests, being able to eat close to water bodies and lagoons that occurred in the floodplains developed under humid and subtropical climate (Croft 2016Croft D.A. 2016. Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America. Bloomington: Indiana University Press, 304 p.).

Based on sedimentologic and paleontologic analyses, Poiré et al. (2013)Poiré D.G, Tineo D.E, González-Rigas G., Bona P., Toledo N., Reguero M., Scarano A., Vergani G.D., Pérez L.M. 2013. Hallazgos de vertebrados continentales de la Formación Petaca (Oligoceno tardío), Cuenca del Chaco, Bolivia. Ameghiniana, 50(6):65R. suggested a paleoenvironment for the Chaco Foreland (Petaca Formation) characterized by open habitats with grasslands under a semiarid climate during the late Oligocene. The hegetotheriid from Petaca probably was a mixed feeder, consuming a variable diet obtained close to the ground subjected to abrasive dust and grit coverage (Reguero et al. 2010Reguero M.A., Candela A.M., Cassini G.H. 2010. Hypsodonty and body size in rodent-like notoungulates. In: Carlini A.A., Madden R.H., Vucetich M.G., Kay R.F. (Eds.). The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia (p. 362-374). Cambridge: Cambridge University Press.). In other hegetotheriids, e.g., Hemihegetotherium trilobus, with similar morphology of the teeth to Prohegetotherium schiaffinoi, the type of mesowear is more similar to that of a browsing mammal (Croft 2016Croft D.A. 2016. Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America. Bloomington: Indiana University Press, 304 p.).

The mandibular features of YPFB-LIT-PAL-005 are more compatible with dietary habits that include hard and brittle or turgid fruits, emphasizing crushing, and secondarily grinding, instead of the extensive grinding of grazers (Ercoli et al. 2020Ercoli M.D., Álvarez A., Moyano S.R., Youlatos D., Candela A.M. 2020. Tracing the Paleobiology of Paedotherium and Tremacyllus (Pachyrukhinae, Notoungulata), the Latest Sciuromorph South American Native Ungulates – Part I: Snout and Masticatory Apparatus. Journal of Mammalian Evolution. https://doi.org/10.1007/s10914-020-09516-7
https://doi.org/10.1007/s10914-020-09516...
).

Diversity of Hegetotheriinae in the Paleogene-Neogene of Bolivia

Pro hegetotherium cf. P. schiaffinoi from the Petaca Formation increases the diversity of hegetotheriids during the Paleogene/Neogene in low and mid-latitudes of South America. The presence of Prohegetotherium cf. P. schiaffinoi in the top of the Petaca Formation in Abapó suggests an age consistent with other contemporaneous Deseadan faunas from northern and southern latitudes of South America. In close geographic proximity to the Abapó locality, the Deseadan Salla Beds have yielded extremely abundant remains of Prohegetotherium schiaffinoi (Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
). Kay et al. (1999)Kay R.F., Madden R., Vucetich M.G., Carlini A., Mazzoni M., Re G., Heizler M, Sandeman H. 1999. Revised geochronology of the Casamayoran South American land mammal age: climatic and biotic implications. Proceedings of National Academy of Science, 96(23):13235-13240. https://doi.org/10.1073/pnas.96.23.13235
https://doi.org/10.1073/pnas.96.23.13235...
assigned an age between 27 and 25.8 Ma for the richest fossiliferous levels.

Abapó is the sixth locality in Bolivia in which hegetotheriine have been found, and the eighth in South America located north of approximately 23° south latitude. Other records of Bolivian hegetotheriids are: two species, Prohegetotherium schiaffinoi and Sallatherium altiplanense from the Deseadan of Salla, western Bolivia (Reguero and Cerdeño 2005Reguero M.A., Cerdeño E. 2005. New Hegetotheriidae (Notoungulata) from the Deseadan (Late Oligocene) of Salla (Bolivia). Journal of Vertebrate Paleontology, 25(3):674-684. https://doi.org/10.1671/0272-4634(2005)025[0674:NLOHMN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2005)0...
); Hemihegetotherium trilobus from Quebrada Honda, unnamed formation (Honda Group), southern Bolivia, middle Miocene, Laventan SALMA (Croft and Anaya 2006Croft D.A., Anaya F. 2006. A new middle Miocene hegetotheriid (Notoungulata: Typotheria) and a phylogeny of the Hegetotheriidae. Journal of Vertebrate Paleontology, 26(2):387-399. https://doi.org/10.1671/0272-4634(2006)26[387:ANMMHN]2.0.CO;2
https://doi.org/10.1671/0272-4634(2006)2...
, McGrath et al. 2018McGrath A., Croft D.A., Anaya F. 2018. Two new macraucheniids (Mammalia: Litopterna) from the late middle Miocene (Laventan South American Land Mammal Age) of Quebrada Honda, Bolivia. Journal of Vertebrate Paleontology, 38(3):e1461632. https://doi.org/10.1080/02724634.2018.1461632
https://doi.org/10.1080/02724634.2018.14...
); Hegetotherium cerdasensis from the ?late Miocene fauna of Nazareno and Cerdas, Potosí, Bolivia (Croft et al. 2016Croft D.A., Carlini A.A., Ciancio M.R., Brandoni D., Drew N.E., Engelman R.K., Anaya F. 2016. New mammal faunal data from Cerdas, Bolivia, a middle-latitude Neotropical site that chronicles the end of the Middle Miocene Climatic Optimum in South America. Journal of Vertebrate Paleontology, 36(5):e1163574. https://doi.org/10.1080/02724634.2016.1163574
https://doi.org/10.1080/02724634.2016.11...
); and Hemihegetotherium from the ?late Miocene of Muyu Huasi, southern Bolivia (Villarroel and Marshall 1989Villarroel C., Marshall L.G. 1989. A new fossil land mammal locality of late Miocene (Huayquerian) age from Muyu Huasi, southcentral Bolivia. Boletín del Servicio Geológico de Bolivia, La Paz, Serie A, 4:27-40.). Hegetotheriids have also been reported from the late early Miocene (Santacrucian) Chucal fauna from northern Chile (Hegetotherium cf. H. mirabile; Croft et al. 2004Croft D.A., Flynn J.J., Wyss A.R. 2004. Notoungulata and litopterna of the early Miocene Chucal fauna, Northern Chile. Fieldiana, Geology (New Series), 50:1-49. https://doi.org/10.5962/bhl.title.5228
https://doi.org/10.5962/bhl.title.5228...
), and dubiously from the middle to late Miocene Urumaco Formation, Venezuela (Linares 2004Linares O.J. 2004. Bioestratigrafía de la fauna de mamíferos de las formaciones Socorro, Urumaco y Codore (Miocene medio—Plioceno temprano) de la región de Urumaco, Falcon, Venezuela. Paleobiología Neotropical, (1):1-26.).

Temporal implications of the hegetotheriid of Abapó section

Traditionally, the Petaca Formation was assigned to the late Oligocene-late Miocene (Sempere et al. 1990Sempere T. 1990. Cuadros estratigráficos de Bolivia: propuestas nuevas. Revista Técnica de YPFB, 11:215-227., Marshall and Sempere 1991Marshall L.G., Sempere T. 1991. The Eocene to Pleistocene vertebrates of Bolivia and their stratigraphic context, a review. Fósiles y Facies de Bolivia, 12(3-4):631-652., Marshall et al. 1993Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301.). No absolute dating (magnetostratigraphy or isotope analysis) has been performed for this unit; the age of the Petaca Formation is constrained by biostratigraphic data coming from different localities of the Subandean belt.

The base of this unit was assigned to the Deseadan (late Oligocene-earliest Miocene) based on the notohippid cf. Rhynchippus found in Quebrada Saguayo, about 60 km WNW of Santa Cruz (Sempere et al. 1990Sempere T. 1990. Cuadros estratigráficos de Bolivia: propuestas nuevas. Revista Técnica de YPFB, 11:215-227., Marshall and Sempere 1991Marshall L.G., Sempere T. 1991. The Eocene to Pleistocene vertebrates of Bolivia and their stratigraphic context, a review. Fósiles y Facies de Bolivia, 12(3-4):631-652., Marshall et al. 1993Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301.). Few data are available for this specimen. Marshall et al. (1993)Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301. did not illustrate this material or not provide it a catalogue number. The only reference provided by these authors is that YPFB’s geologists collected a right maxilla with P2-M2 of ?Rhynchippus which come from a pink sandstone 2 m above the base of the unit (p. 294).

Marshall et al. (1993)Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301. mentioned the presence of the pampathere cf. Vassallia minuta (Chasicoan-Montehermosan SALMA) in the top of the Petaca Formation. The identification of this material is doubtful. It is based on five right dentary teeth coming from the Río Yapacani (90 km WNW of Santa Cruz de la Sierra), not properly described or illustrated (Pascual et al. 1973Pascual R., Odreman-Rivas O.E., Zetti J. 1973. Determinación de un resto fósil procedente de Santa Cruz (Bolivia). Informe interno YPFB, Pal. 0858, n. RL-640., Villarroel 1974Villarroel C. 1974. Informe sobre dos mamíferos fósiles del Terciario subandino, ProvinciaIchilo, Santa Cruz. Informe interno YPFB, Santa Cruz, Bolivia, Pal. 0932, n. GS-1075, 5 p., Sanjinés and Jiménez 1976Sanjinés G., Jiménez F. 1976. Comunicación preliminar acerca de la presencia de fósiles vertebrados en la Formación Petaca del Área de Santa Cruz. Revista Técnica de YPFB, 4(3):147-156., Marshall et al. 1993Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301., p. 294). Even more, the stratigraphic position and collection number of this specimen are not available. About its geographical position, Marshall et al. (1993Marshall L.G., Sempere T., Gayet M. 1993. The Petaca (Late Oligocene-Middle Miocene) and Yecua (Late Miocene) formations and their tectonic significance of the Subandean-Chaco Basin, Bolivia. Documents des laboratoires de géologie de la Faculté des Sciences de Lyon, (125):291-301., p. 294) pointed out:

An attempt was made by LGM to relocate this site on November 14,1990, and it was discovered that the Petaca Formation occurs in fact about 20 km more to the southwest than originally illustrated. Despite the biostratigraphic importance for the late Miocene-early Pliocene of the pampathere Vassallia, there is no certainty about the provenance of this material.

We consider all this evidence unreliable to justify the age assigned to the unit. Instead, the only other hypothetical possibility is that the bearing horizon of cf. Vassallia minuta could be stratigraphically higher than that of cf. P. schiaffinoi. Our working hypothesis is that Vassallia minuta and cf. Prohegetotherium schiaffinoi come from different stratigraphic levels, and probably from different units. Moreover, Vassallia minuta could come from the Yecua Formation.

Here we describe in detail and accurately documents the presence of a hegetotheriid with close affinity to the Deseadan-Santacrucian Prohegetotherium schiaffinoi in the top of the Petaca Formation (Abapo section) based on more precise stratigraphic and biostratigraphic evidence. Thus, an older age, late Oligocene to early Miocene, is suggested for the top of this unit at Abapó section.

CONCLUSIONS

The study of the Hegetotheriinae from Abapó resulted in the record of the genus Prohegetotherium for the first time in the Petaca Formation.

The Abapó specimen is very close to the Deseadan-Santacrucian Prohegetotherium schiaffinoi. However, some features of the mandible, as the shape with a marked change in height along its length, increasing towards the back, a prominent masseteric crest and a deep mandibular groove, are different from that of this species.

The YPFB-LIT-PAL-005 broadens the knowledge of Hegetotheriinae for the late Oligocene – early Miocene of Bolivia and expands the geographic distribution of the genus Prohegetotherium to low latitudes of Chaco foreland in Bolivia.

The close affinity of the YPFB-LIT-PAL-005 to the Deseadan-Santacrucian species Prohegetotherium schiaffinoi suggests an older age, from late Oligocene to early Miocene, for the upper levels of Petaca Formation cropping out in Abapó.

Overall, new geochronological studies, regional stratigraphic correlations of the Petaca Formation, and more fossil evidence are needed to resolve the age of this unit in the Chaco foreland basin.

  • M.A.R.: Methodology, investigation, formal analysis, figure design, writing-original draft, writing-review. D.E.T.: Field work, geological description and discussion, figure design, writing-original draft, writing-review. P.B.: Comparison and paleontological discussion, investigation, formal analysis, figure design, writing-original draft. L.M.P.: Field work, fossil discoverer, writing-original draft, writing-review, editing draft, correspondence author. G.D.V.: Field work, geological description and discussion, writing-review. G.G.: Field work, geological discussion, English correction. D.G.P.: Field work, geological discussion, project administration, writing-original draft.

ACKNOWLEDGMENTS

We thank the editorial staff and the reviewers of BJG for comments that greatly improved the manuscript. To Dr. Cecilia Deschamps (MLP) for correcting the text in English. We also thank to the Bolivian GeoAmbiente Rangers Group for the support and hospitality provided; fieldwork without the assistance of Obando, Sellier and Osman would not have been possible. This research was financially supported by the National Scientific Council of Argentina (CONICET) and Pluspetrol S.A. Thanks are also due to Pluspetrol Bolivia.

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Publication Dates

  • Publication in this collection
    14 May 2021
  • Date of issue
    2021

History

  • Received
    09 July 2020
  • Accepted
    18 Feb 2021
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