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Stomping Around the Sump: Miocene Pygmy Gomphothere from Esmeralda County, Nevada

Fabian C. Hardy* and Joshua BondeUniversity of Nevada Las Vegas

ABSTRACT

Here we describe an individual gomphothere specimen recovered from Esmer-alda County, Nevada. It was recovered from an area known as “The Sump,” just 18 miles north of Dyer, NV, currently dominated by badland topography. Remnants of a paleo-forest are visible in the highlands of the site, while the central basin exposes mudrocks and sandstones, interfingered in places by several chalky, volcaniclastic layers. The sediments denote a past marsh environment, an ideal feeding ground for browsers of C3 plant material, such as trees and shrubs. A slot canyon feeds the in-active channel where the discovery was made. Sedimentary units of the Esmeralda Formation in the vicinity of the north end of Fish Lake Valley have previously been interpreted as Miocene (Claredonian North American Land Mammal Age).

The Gomphotheriidae were a family of proboscideans, widespread throughout the late Cenozoic era. Gomphotheres radiated throughout North America during the Miocene epoch, and diversified throughout the continent until their extinction in the late Pleistocene. Several species were participants in the Great American Biotic Inter-change, but only three have been recorded from South America.

Gomphotheres likely filled the niche of a dominant megafaunal browser, as their dentition and body morphology suggest that they were opportunistic feeders with a preference for C3 browse. The recovered specimen represents the maxillae, portions of the premaxillae, as well as the palatine, all in articulation. After removal of sediment with an air scribe, anterior and posterior sinus cavities were revealed. Two molars are present, and a portion of both tusks have been preserved within their cranial sockets.

We attribute this specimen to Gomphotherium minor based upon the combina-tion of small potential skull size with the extremely worn occlusal surface of the pre-served molars of the individual. Modern elephants exhibit similar tooth wear towards the end of their lives, eventually succumbing to starvation. Mawby (1968) described Megabelodon minor (later synonymized with Gomphotherium by Tobien 1973) based on a tuskless mandible recovered from the same formation. Diagnostic measurements of the teeth, maxillae, and palate were taken and compared to the type specimen and other longirostrine proboscideans. We believe that the individual collected from the Sump is of the same species, but this is difficult to confirm due to our lack of a dentary present.

Key Words: Paleontology, taxonomy, taphonomy, gomphothere

*E-mail: hardyf@unlv.nevada.edu

INTRODUCTION

The Gomphotheriidae are an extinct family of probos-cideans that were widespread during the late Cenozoic era (Sanchez, 2004). Members of this group are found throughout North America, ranging temporally from the Miocene to Plio-

cene epochs (roughly 23–2.6 Ma), with several South American species persisting through the Pleistocene (Dantas, 2013). The various South American gomphothere taxa are thought to have been Pleistocene immigrants from North America during the Great American Biotic Interchange. As a Neogene example of megafauna, gomphotheres provide insight into the biogeogra-phy of the Americas, and particularly strategies of dispersal and adaptation, which encouraged migration from North America.

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Gomphotheres are characterized by a brachycephalic and brevirostrine skull, with a curved mandibular symphysis (Mothé, 2013). Three independent lineages of gomphotheres fit with the temporal and morphological characteristics seen in an individual recovered from Nevada. There is currently a great deal of debate regarding the diversity of Gomphotheriidae, of-ten with the result of species being synonymized and reclassi-fied, especially with regards to ancestral relationships across North and South America (Mothé, 2013).

Phil Compton first discovered the specimen described here in June 2013 while hiking through exposures of the Miocene Esmeralda Formation exposed at the north end of Fish Lake Valley in a location referred to as the Sump. Mr. Compton re-ported his find to UNLV in March 2014, after which permis-sion was gained from the Bureau of Land Management Battle Mountain Field Office for collection of the specimen.

The Sump is found at the north end of Fish Lake Valley in the shadow of Boundary Peak to the west. This site is found due east of the fork of Highways 264 and 773, approximately 18 miles north of Dyer, Nevada. Deposits of The Sump are the late Miocene Esmeralda Formation. The unit is exposed in the vicinity of the Sump as well as across the Esmeralda County line in southern Mineral County. These deposits represent a syntectonic sedimentary package associated with the develop-ment of the Silver Peak detachment to the east. Nice fanning of dips of sedimentary units is exposed in the cliffs of the Sump, indicative of syntectonic sedimentation. The stratigraphy of the Sump is interbedded (vertically and horizontally) volcaniclas-tic units, tuffaceous coarse sandstones, and greenish siltstones. Beds show no obvious signs of well-developed pedogenesis. Most sandstones and siltstones are green to grey and show signs of poor drainage, suggestive of a swampy environment. Bed-ding is roughly decimeter to meter in scale. Modern topography of the area is typical badland topography, resultant from intense weathering.

A taxonomic analysis was performed on the maxillae, and we have assigned the individual to Gomphotherium minor, a species first described by Mawby (1968) as Megabelodon minor. The genus Megabelodon was later synonymized with Gomphotherium by Tobien (1973). This represents the third such individual recorded in Nevada, and reinforces the diver-sity of proboscideans in the Great Basin. The presence of Gom-photherium indicates that the region was able to support mega-fauna, with abundant plant material and readily available water.

Gomphothere Paleoecology and Habitat

The Miocene Great Basin was home to a diverse fauna, which includes proboscideans such as mastodons, mammoths, and gomphotheres. While mammoths and mastodons are char-ismatic taxa, gomphotheres are hardly media darlings. These shovel-tusked elephantiformes were the most widespread and diverse family of proboscideans in North America, yet there is a paucity of research surrounding them. The aim of this study

was to collect and verify the presence of another Gomphoth-erium minor in the Great Basin, and describe the paleoenviron-ment of the area.

There are 182 living and extinct species and subspecies of Proboscidea currently recognized within 44 genera (Shoshani, 2006). The period of greatest diversity within the order ranged temporally from the Miocene to Pliocene epochs (Dantas, 2013). North America was the source of diversification and the origin for the second radiation of Proboscidea during the early Miocene, which led to their nearly global distribution (Scher-ler, 2013). Three families diverged during the Oligocene and Miocene: the Mammutidae (mastodons), Elephantidae (mam-moths), and the Gomphotheriidae, which enjoyed a cosmo-politan distribution (Shoshani, 2006). The Gomphotheriidae consisted of 18 genera and 6 subfamilies, and the genus Gom-photherium contains 14 species (Figure 1).

Three independent lineages of Neogene gomphotheres contain species that display a long symphysis with a lack of lower tusks, and are found within the genera Gomphotherium, Gnathabelodon, and Eubelodon (Mawby, 1968). The genus Gomphotherium possesses upper tusks with enamel bands, and last molars with 3–5 lophids (Tobien, 1973). Our individual lacks the dentary, and we have identified it based on dental characteristics, tusk alveoli orientation, and maxillary charac-ters.

Most species had specialized dentition, including a pair of upper tusks and six pairs of upper and lower molars (Mothé, 2013). The dentition of genus Gomphotherium includes four lophids on the socketed M3 in both the maxillae and dentary (Mawby, 1968, Tassy, 2013). Their cheek teeth display hori-zontal displacement and diphyodonty, rather than the polyphy-odonty of mastodons and modern elephants (Alberdi, 2011). Gomphotherium minor has been described as the smallest of any previously studied post-Oligocene proboscideans (Mawby, 1968). This status has been based on the previously described type specimen for M. minor, and various fragmentary tooth, tusk, and jaw material (Mawby, 1968). The maxillae in Gom-photherium contain a pair of tusks which, because they are not replaced during the animal’s lifetime, record changes in diet and/or habitat (Fox, 2000). The bulk of the tusks are composed primarily of dentin, as in modern elephants, but Gomphotheri-um maxillary tusks possess a characteristic band of enamel that grows laterally along the structure (Fox, 2000; Prado, 2008; Steenkamp et al., 2008). The presence of this enamel band may have added structural stability and resistance to abrasion.

The Gomphotheriidae were thought to have been mega-herbivore generalist feeders, which were able to follow several biogeographic corridors through the Isthmus of Panama during the Great American Biotic Interchange (GABI) about 3.1 to 2.8 Ma (Sanchez et al, 2004). These corridors represented habitats that were conducive to gomphotheres, yet prevented emigra-tion by specialist feeders, such as Mammuthus (mammoth) and Mammut (Mastodon), across the isthmus (Prado, 2001, San-chez, 2004). The Sump Gomphotherium likely thrived in the

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Figure 1. Proboscidean phylogeny, displaying high degree of diversity within the Gomphotheriidae. Modified from Fossilworks (fossilworks.org), Alberdi (2004), Shoshani (2006), Prado (2008), and Todd (2010).

marshy to sylvan C3 rich environment present at the Miocene locality. The site reflected much wetter conditions as expressed by buff- to gley- colored fine sandstones and mudrocks, which are indicative of reducing wet environments. Also exposed within the same beds are the preserved remains of a petrified forest buried in volcaniclastic sediments.

A diet of primarily C3 woody plant material has been pro-posed by previous researchers based on tooth microwear and isotopic analysis on sister species within the genus (Domingo, 2012; Fox and Fisher, 2004; Asevedo et al., 2012). Their teeth were likely brachyodont, with relatively simple molar crowns (Fox and Fisher, 2004). Gomphotherium probably utilized its upper tusks in a similar manner to modern elephants, which use them to debark trees, root for food, and in social interactions (Steenkamp et al., 2008). There have been numerous isotopic studies conducted on gomphothere tooth enamel which suggest a mixed diet of C3 and C4 plant material, with a preference for C3 browse (Fox and Fisher, 2004; Asevedo, 2012; Domingo, 2012).

A contemporaneous, tuskless gomphothere skull from the Sump was described by Mawby (1986), and assigned to Mega-belodon cruziensis. The author hypothesized that the tuskless character of the individual was a functional adaptation rather than sexual dimorphism. The skull in question has since been misplaced, but the possibility for a genus or species level match with the Sump gomphothere in our study would be possible with its recovery. A further examination of the diet of Gom-photherium would be helpful, as a group may experience a large degree of change in its feeding habits not accompanied by a morphological adaptation (Fox and Fisher, 2004). When a significant change in diet is seen within a lineage or community, it is a possible indicator of ecological or evolutionary change, and a dietary reconstruction has the potential to characterize the trigger for its extinction (Fox and Fisher, 2004).

METHODS

The partially exposed maxillae were excavated in one day,

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Figure 2. Preparation of the maxillae in situ (Photo by Aaron Mayes).

Figure 3. Anterior view, displaying nasal cavity and tusk alveoli.

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as the unconsolidated stream sediments were easily brushed aside with standard paintbrushes. Chisels and rock hammers were used to move greater volume of sediment surrounding the bone. Polyvinyl acetate consolidant was applied in the field to prevent deterioration and damage during excavation (Figure 2). The bone was encased in burlap, aluminum foil, and a plaster jacket for safety of transport, and removed from the site.

In addition to the excavation of the gomphothere specimen, we briefly prospected exposures of the badlands in the vicinity of the dig site. A number of fossil bearing sites were discovered, including at least two types of camels, beaver, and a potential horse. All fossils collected will be reposited in Nevada State Museum, Carson City (NSM).

At the Las Vegas Natural History Museum, matrix was re-moved using pneumatic and hand tools. The absence of a ma-jority of the cranium provided the opportunity to expose a large portion of the anterior and posterior nasal cavity (Fig 3). Poly-vinyl acetate was applied throughout the process, to preserve the integrity of the bone and prevent fractures. The palatine, tusks, and tooth alveoli were exposed through this process (Fig-ure 4). After we had exposed the usable surfaces, the bone was measured according to von den Driesch (1976) where possible. Modifications were made where the original dimensions were absent, in an effort to provide specifics regarding present and

exposed surfaces. The individual will be assigned a specimen number and reposited in the NSM.

Our projected reconstruction of the complete skull is based primarily upon preexisting maxillary data from Tassy (2013) and Gilette (1993). A complete skull of G. minor is not avail-able, but the combination of the dentary dimensions from Maw-by (1968) and the maxillae from the Sump gomphothere allows for a confident restoration (Figure 5).

RESULTS

The specimen was found in a planar to cross-bedded, me-dium- to coarse-grained, tuffaceous sandstone. The isolated na-ture of the specimen implies that postmortem the specimen had enough time to disarticulate. Weathering on the surface of the specimen is no greater than Stage 1 (after Behrensmeyer, 1978) suggesting that the specimen did not spend any great amount of time exposed to subaerial weathering. No spiral fractures, score marks, or signs of readily observable abrasion are pres-ent. Thus, the specimen must have been preserved relatively closely to where the animal died, and was not exposed on the surface long enough to be weathered by ultraviolet exposure from the sun.

Table 1 contains measurements of various features of the

Figure 4. Ventral posterior view, displaying palatine and fractured right M3.

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Figure 5. Reconstruction of Gomphotherium minor skull based on dimensions from Tassy (2013), Gillette (1996), Mawby (1968), and Mawby (1986). Matrix remains on top of skull, to prevent further damage during preparation. Dotted lines represent inferred dimensions.

fossil. Measurements (2) and (3) measure the anterior and pos-terior palatial breadth, which fit well with the holotype dentary, described by Mawby (1968). No other known gomphotheres have displayed dimensions this small, except for juveniles (Fox, 2000; Alberdi, 2004; Prado, 2008; Tassy, 2013). Dimensions (16) and (17) are the result of the partial destruction of the molars due to exposure, which provided the opportunity to measure the thickness of enamel without further cutting into the tooth.

Dimensions (11) through (13) were key to diagnosing the individual, as molars are a diagnostic feature of the order. The tetralophodont M3 limits the individual to the genus Gomphoth-erium (Mawby, 1968; Fox, 2000). The relatively small size of the M3 combined with the extreme wear reaching Laws Cate-gory XXX confirms this as a very small and old individual. The Sump gomphothere’s tusks are incomplete, and did not retain the labial portion, making it impossible to determine the pres-ence of a lateral enamel band. Roughly half of the base of the right tusk is present, and we estimate its diameter to be 15cm. The tusk alveoli are relatively straight, lacking divergence seen in modern elephants.

Based on our projected reconstruction, the Sump gom-phothere’s skull would have measured roughly 40 cm in height, and 81 cm in length. This low profile would place it morpho-logically similar to the common gomphothere, Gomphotherium

productum, but its extremely small size makes this a tenuous association at best (Tassy, 2013).

DISCUSSION

The Sump is located in a region of the Great Basin known for Miocene swamps, and most likely contains a Clarendon-ian faunal assemblage similar to other exposures of the Esmer-alda Formation found in Mineral and Esmeralda Counties. The herbivores commonly found associated with Gomphotherium include Aepycamelus, Equidae, and Teleoceras, a pig-sized rhi-nocerotoid (MacDonald and Pelletier, 1958, Nelson and Mad-sen, 1987, Kita, 2014). Carnivores include Leptocyon and Epi-cyon, predecessors to modern forest-dwelling foxes and wolves (Macdonald and Pelletier, 1958; Nelson and Madsen, 1987). The Miocene Sump would have existed at an elevation about 1 to 1.5 km greater than it is today, based on botanical evidence, and as a result supported a typical Sierra Nevadan floral as-semblage (Wolfe, 1997). Grassland and woodland habitats dur-ing the Miocene supported a density of species-rich browser communities much higher than has been observed since (Janis, 2004).

Gomphotherium teeth were replaced horizontally through continuous reformation of the upper and lower jaws (Tassy,

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2013). This displacement generated an expansion of the pal-atine over time, and worked to maintain teeth in occlusion throughout their usable interval (Tassy, 2013). In modern el-ephants, the 3rd molar erupts once the animal nears 25-30 years of age (Laws, 1966; Lee, 2012; Tassy, 2013). The degree of wear on the maxillary M3 molars in the Sump gomphothere was extreme, eliminating the majority of easily measurable fea-tures, and causing the cusps to become confluent. This amount of wear is indicative of an older animal, similar to how modern elephant dentition reaches the end of usable teeth at Laws Cat-egory XXX (Laws, 1966; Lee, 2012).

The G. minor described by Mawby (1968) possessed a relatively unworn right mandibular m3 that had just come into occlusion at the time of death. Based on the height of this molar, we interpret the M3 of the Sump gomphothere to have a maxi-mum height of 50 mm at the first lophid. The delta crown height between maximum (adolescent) and measured (geriatric) is 36 mm, implying the teeth of the Sump gomphothere were nearly useless for mastication (Figure 4). On average, a modern el-ephant tooth reaches Laws Category XXX when the animal has reached 65 years of age, and displays comparable wear (Laws, 1966; Lee, 2012; Tassy, 2013). An animal will typically be un-able to consume its preferred diet, and eventually starves when tooth wear reaches Category XXX (Laws, 1966; Lee, 2012). The expected age of the Sump gomphothere was approximately

60+ years, (Jachmann, 1988). Further study based on first and third order features in cross section of the present tusk material could provide age estimation to a precision of within several weeks (Fisher, 2003).

The sex of the individual cannot be determined by tooth characteristics, but the presence and size of tusks may distin-guish a male from female (Lee, 2012). The elusive gomphoth-ere skull described by Mawby (1986) from the Esmeralda For-mation lacked tusks, but it has not been formally described or had its dimensions published. Should this individual prove to be a member of the same species, sexual dimorphism is a po-tential explanation for the absence of tusks. Another possibility is that the tusk alveoli were destroyed post-burial in the Mawby (1986) specimen. The distal portion of the alveoli may have been broken off, allowing the tusks to be removed during trans-port, while weathering eliminated evidence of fracturing.

This skull offers a unique opportunity to characterize the rostrum, as no other G. minor maxillaries are available, but such an analysis is beyond the scope of this project.

CONCLUSIONS

The lack of lower tusks and scoop-like bill seen in the ho-lotype specimen, combined with the paleoenvironment of the Sump, imply a predisposition towards selective browsing be-

Table 1. Measurements of the Sump gomphothere, modified from Driesch (1976).

Dimension mm

1) Greatest breadth on curvature on maxillae at midpoint of M3 alveoli 2892) Greatest posterior palatal breadth: at posterior extremity of M3 alveoli, measured alveolus to alveolus 1083) Greatest anterior palatal breadth: at anterior extremity of M3 alveoli 884) Maximum width, entire structure (measured “cheek to cheek”) 3265) Minimum width, entire structure (measured from labial alveolus to labial alveolus) 2756) Total length of palate - posterior to anterior, along central axis 2867) Greatest width between midpoint of base of tusk alveoli, proximal to nasal (measured tusk alveolus to tusk alveolus) 588) Height: anterior base of palatine to base of braincase? (top of sinus present) 1569) Approx tusk diameter, base of tusk within alveolus (half of cone present) 157

10) Length of visible tusk 17411) Greatest length of M3 18212) Labial M3 length 16213) Lengual M3 length 16514) Root widths:

Right Anterior 1 Right Middle 2

Right Posterior 3

45Not fully visible due

to alveolus Not visible due to

alveolus15) Height from absolute minimum cap of M3 root to occlusal surface 14 (extremely worn)16) Maximum thickness of tusk enamel/dentin 3817) Minimum thickness of tusk enamel/dentin 3218) Maximum width anterior nasal cavity 9419) Minimum width anterior nasal cavity 6320) Height, anterior palatal surface to base of nasal cavity 62

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havior. This feeding mode would have taken advantage of the close proximity of C3 plants to the water source, and a geriat-ric individual such as the Sump gomphothere would have cer-tainly remained nearby. Gomphotherium molar wear typically does not display intensive scratching seen in specialized graz-ers, which matches well with isotopic studies (Fox and Fisher, 2004; Calandra, 2008).

Based on available cranial measurements, particularly the dimensions of the molars and palatine, we have assigned the Sump gomphothere to Gomphotherium minor. This is the sec-ond confirmed individual of the species, and the potential exists for a link to the M. cruziensis individual discovered by Mawby (1986). Previous authors have suggested a synonymization with G. productum, but such a connection is difficult with a dearth of specimens.

The Great Basin was a vastly different environment during the Miocene, and the presence of multiple gomphotheres is a direct piece of evidence. The marshy, inland environment of the Sump provided a water source for a dense community of mammals, and the site will likely yield more exciting discover-ies in the future.

ACKNOWLEDGEMENTS

Phil Compton with his keen eye for detail made the ini-tial discovery of this specimen. We thank our contributors from the UNLV Dead Lizard Society, Lauren Parry, Dawn Reynoso, Andrew Rigney, Margarita Rodriguez, Steve Rowland, Chelsy Salas, and Oscar Vazquez for giving up their Spring Break to assist in this endeavor. We thank Aaron Mayes of the UNLV Photo Services for documenting our journey. We also thank the Bureau of Land Management for permission to collect, in par-ticular John Kinsner and Bryan Hockett. We thank the Las Ve-gas Natural History Museum for their generosity in providing our team with a preparation space.

REFERENCES

Alberdi, M.T., 2004, The Pleistocene Gomphotheriidae (Proboscidea) from Peru. Neues Jahrbuch für Geologie und Paläontologie: Abhandlungen, v. 231, issue 3, p 423–452.

Alberdi, M.T., 2011, Paleobiogeography of trilophodont gomphotheres (Mam-malia: Proboscidea). A reconstruction applying DIVA (Dispersion-Vi-cariance Analysis): Revista Mexicana de Ciencias Geologicas, v. 28, no. 2, p. 235–244.

Asevedo, L., Winck, G.R., Mothé, D., and Avilla, L.S., 2012, Ancient diet of thePleistocene gomphothere Notiomastodon platensis (Mammalia, Proboscidea,Gomphotheriidae) from lowland mid-latitudes of South America: Stereomicrowear and tooth calculus analyses combined: Qua-ternary International, v. 255, p. 42–52.

Behrensmeyer, A.K., 1978, Taphonomic and ecologic information from bone weathering: Paleobiology, v. 4, no. 2, p. 150–162.

Dantas, M.A.T., et al., 2013, A review of the time scale and potential geo-graphic distribution of Notiomastodon platensis (Ameghino, 1888) in the late Pleistocene of South America: Quaternary International, v. 317, p. 73–79.

Domingo, L., Koch, P.L., Grimes, S.T., Morales, J., and López-Martinez, N., 2012, Isotopic paleoecology of mammals and the Middle Miocene Cooling event in the Madrid Basin (Spain): Palaeogeography, Palaeo-climatology, Palaeoecology, v. 339-341, p. 98–113.

Driesch, A. von den., 1976, A guide to the measurement of animal bones from archaeological sites: Peabody Museum Bulletin No. 1, 135p.

Fisher, D.C., 2003, Tusk growth rate and season of death of Mammuthus colum-bi from Hot Springs, South Dakota, USA: Advances in Mammoth Re-search in Deinsea, v. 9, p. 117–133.

Fox, D.L., 2000, Growth increments in Gomphotherium tusks and implications for late Miocene climate change in North America: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 156, p. 327–348.

Fox, D.L., and Fisher, D.C., 2001, Stable isotope ecology of a late Miocene population of Gomphotherium productus (Mammalia, Proboscidea) from Porty of Entry Pit, Oklahoma, USA: Palaios, v. 16, p. 279–293.

Fox, D.L., and Fisher, D.C., 2004, Dietary reconstruction of Miocene Gom-photherium (Mammalia, Proboscidea) from the Great Plains region, USA, based on the carbon isotope composition of tusk and molar enamel: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 206, p. 311–335.

Jachmann, H., 1988, Estimating age in African elephants: a revision of Laws’ molar evaluation technique: African Journal of Ecology, v. 26, p. 51–56.

Janis, C.M., Damuth, J., and Theodor, J.M., 2004, The species richness of Mio-cene browsers, and implications for habitat type and primary productiv-ity in the North American grassland biome: Palaeogeography, Palaeo-climatology, Palaeoecology, v. 207, p. 371–398.

Kita, Z.A., Secord, R., and Boardman, G.S., 2014, A new stable isotope record of Neogene paleoenvironmentas and mammalian paleoecologies in the western Great Plains during the expansion of C4 grasslands: Palaeo-geography, Palaeoclimatology, Palaeoecology, v. 399, p. 160–172.

Laws, M., 1966, Age criteria for the African elephant: African Journal of Ecol-ogy, v. 4, p. 1–37.

Lee, P.C., Sayialel, S., Lindsay, W.K., and Moss, C.J., 2012, African elephant age determination from teeth: validation from known individuals: Afri-can Journal of Ecology, v. 50, p. 9–20.

Macdonald, J.R., and Pelletier, W.J., 1958, The Pliocene mammalian faunas of Nevada, U.S.A. Paleontologia, taxonomia y evolucion. Ciudad de Mexico: International Geological Congress, 1956: Report of the 7th Session v. 7, p. 365–388.

Mawby, J., 1968, Megabelodon minor (Mammalia, Proboscidea), a new species of mastodont from the Esmeralda Formation of Nevada: PaleoBios, v. 4, 11 p.

Mothé, D., Avilla, L.S., and Cozzuol, M.A., 2013, The South American Gom-photheres (Mammalia, Proboscidea, Gomphotheriidae): Taxonomy, Phylogeny, and Biogeography: Journal of Mammalian Evolution, v. 20, p. 23–32.

Nelson, M.E., and Madsen, J.H., 1987, Contributions to Geology. University of Wyoming v. 25.

Prado, J.L., Alberdi, M.T., Azanza, B., Sanchez, B., and Frassinetti, D., 2001, The Pleistocene Gomphotheres (Proboscidea) from South America: di-versity, habitats, and feeding ecology: The World of Elephants—Inter-national Congress, Rome.

Prado, J.L., and Alberdi, M.T., 2008, A cladistics analysis among trilophodont gomphotheres (Mammalia, Proboscidea) with special attention to the South American genera: Palaeontology, v. 51, part 4, p. 903–915.

Sanchez, B., Prado, J.L., and Alberdi, M.T., 2004, Feeding ecology, dispersal, and extinction of South American Pleistocene gomphotheres (Gom-photheriidae, Proboscidea): Paleobiology, v. 30, p. 146–161.

Scherler, L., Mennecart, B., Hiard, F., and Becker, D., 2013, Evolutionary his-tory of hoofed mammals during the Oligocene-Miocene transition in Western Europe: Swiss Journal of Geosciences, v. 106 (2), p. 349–369.

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Steenkamp, G., Ferguson, W.H., Boy, S.C., Ferreira, S.M., and Bester, M.N., 2008, Estimating exposed pulp lengths of tusks in the African elephant (Loxodonta africana africana): Journal of the South African Veterinary Association, v. 79, p. 25–30.

Tassy, P., 2013, L’anatomie cranio-mandibulaire de Gomphotherium angus-tidens (Cuvier, 1817) (Proboscidea, Mammalia): données issues du gisement d’En Péjouan (Miocène moyen du Gers, France): Geodiversi-tas v. 35, p. 377–445.

Tobien, H., 1973, On the evolution of Mastodonts (Proboscidea, Mammalia). Part 1: the bunodont trilophodont groups: Notizblatt des Hessischen Landesamtes für Bodenforschung zu Weisbaden, 101, 202–276.

Todd, N.E., 2010, New phylogenetic analysis of the family elephantidae based on cranial-dental morphology: The Anatomical Record, v. 293, p. 74–90.

Wolfe, J.A., Schorn, H.E., Forest, C.E., and Molnar, P., 1997, Paleobotanical evidence for high altitudes in Nevada during the Miocene: Science, v. 276, p. 1672.