The life story of a gomphothere from east-central Mexico: A multidisciplinary approach

Highlights

• We report a multidisciplinary study of a Cuvieronius hyodon′s skull and tusks from the state of Tlaxcala, Mexico.

• Diet analyses showed that this animal was a mixed feeder and inhabited open zones with patches of wooden flora.

• The strontium isotopic ratios suggested that this gomphothere used to live in the Puebla-Tlaxcala Valley.

• We describe a detailed work of conservation and anatomical reconstruction of the gomphothere skull.

Abstract

The Gomphotheriidae family belongs to the Proboscidea order. Gomphotheres were elephant-like mammals whose representatives inhabited North America from the Middle Miocene to the late Pleistocene. In this work, a gomphothere remains from Santiago Tepeticpac (Tlaxcala, Mexico) are described. The comparative study with other Proboscidea, mainly from different localities of Mexico and South America, mostly Argentina and Brazil, allowed to identify them as Cuvieronius genus. The microwear signature together with carbon and oxygen stable isotopes showed that this animal was a mixer feeder and inhabited open zones with patches of wooded flora. These analyses also indicated the presence of high abrasiveness components and C₄ plants (grasslands) and medium to low wearing elements as well as C₃ vegetation (forest) in the site. The strontium isotopic signature implies that this gomphothere lived in the Puebla-Tlaxcala Valley. The paleoenvironmental inferences would show regional conditions.

Introduction

The Gomphotheriidae family is considered a long-living ancestral stock that gave origin to a succession of other groups. North America played an important role in the history of gomphothere biogeography and diversity (Lambert, 1996). From the late Barstovian to the Rancholabrean NALMAs (North America Land Mammal Age) many taxa immigrated to this continent from the Old World across Beringia. Gomphothere diversity also reached its maximum during this time span. From the late Clarendonian to the early Hemphillian, there were three known genera, Amebelodon, Gomphotherium and Serbelodon, followed by a reduction in the number of genera during the late Hemphillian (Lambert, 1996). During the Blancan (Pliocene-Pleistocene), three genera lived in North America: Rhynchotherium, Cuvieronius, and Stegomastodon, but only Cuvieronius survived until the Rancholabrean (Late Pleistocene) (Arroyo-Cabrales et al., 2007).

In Central America the fossil remains assigned to Gomphotheriidae that have been recorded from late Miocene to late Pleistocene with two genera: Cuvieronius and Gomphotherium (Lucas and Alvarado, 2010; Pasenko, 2012). Gomphotheres that crossed the Isthmus of Panama and arrived in South America in the Pleistocene are represented by two species which belong to Cuvieronius and Stegomastodon genera (Prado et al., 2005). However, Mothé et al. (2013, 2019) consider that the genus Stegomastodon was endemic of North America and never reached South America; instead, they proposed the name Notiomastodon for the South American gomphotheres abscript to Stegomastodon. Nevertheless, Prado et al. (2005) proposed that Notiomastodon should be a junior synonym for Stegomastodon.

Two genera lived in Mexico during the Middle to Late Pleistocene: Stegomastodon and Cuvieronius (Alberdi and Corona, 2005). The first genus has been reported only in two localities, Chapala and Valsequillo; in contrast, near 29 sites exist where remains of Cuvieronius have been described including one in Tlaxcala (Arroyo-Cabrales et al., 2007; Sá nchez Salinas et al., 2016). In 2014, during the conservation activities of the Archaeological Project Tepeticpac (TAP), fossil remains belonging to a gomphothere were accidently found and rescued in Santiago Tepeticpac, (Tlaxcala, Mexico) and later restored in the Coordinación Nacional de Conservación del Patrimonio Cultural (CNCPC), dependent on the Instituto Nacional de Antropología e Historia (INAH).

The main goals of this paper are to describe the fragmentary gomphothere skull found in Santiago Tepeticpec, making morphometric comparisons with existing data from other locations in South and North America, and integrate these data with previous records of this species in the region (Prado et al., 2002; Alberdi et al., 2008). Additionally, we provide dietary (microwear and stable isotopes C and O) and mobility (radiogenic Sr isotopes) data as well as palaeoenvironmental inferences linked to these findings and describe the restoration of the specimen.

Diet, habitat, and mobility characteristics of species found in a fossil locality are essential to understand the environmental evolution of a particular site through time. The principles and analyses used to infer biological aspects of fossil individuals and populations in proboscideans are biological actualism (Damuth and Janis, 2011), morphofunctional analysis (meso and microwear) (Rivals et al., 2012; Saarinen et al., 2015), and biogeochemical markers (δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr) (Koch et al., 1998; Hoppe and Koch, 2007).

Dental microwear (scars etched into enamel) results of the abrasion of a tooth's surface by food items such as plant phytoliths or due to exogenous grit or dust adhered on the surface of the vegetation consumed (Semprebon et al., 2016b). Microwear is, in general, a taxon-independent method that allows interpretation about the feeding behavior of the last days or weeks before an animal's death (Grine, 1986). Also, microwear provides a peek of the available vegetation and habitat as well as short-term dietary behavior and allows for inferences to be made regarding daily, seasonal, or regional variations in trophic habits (Semprebon et al., 2016b).

Nowadays, there are three methods for studying dental microwear: high magnification (Solounias et al., 2000; Solounias and Moelleken, 1992) in general directly on the original teeth, low magnification (Solounias and Semprebon, 2002), and texture analysis (Scott et al., 2005) both using principally the original teeth or high precision casts.

In the case of the proboscideans, the most used techniques are low magnification (Gutiérrez-Bedolla et al., 2016; Rivals et al., 2012, 2019, 2019; Semprebon et al, 2016a, 2016b, 2016a; Bravo-Cuevas et al., 2020) and texture analysis (Smith and DeSantis, 2018, 2020).

Carbon stable isotopes have been a vital tool to infer diets of Pleistocene herbivore and carnivore mammals because ¹³C isotopes recorded in the enamel reflect the photosynthetic signature of foods consumed (Pérez-Crespo et al., 2016a). Currently, in nature, the C₃ photosynthetic pathway occurs in trees and shrubs and some temperate grasses, with carbon isotopic values ranging between −36‰ and - 22‰. On the other hand, the C₄ photosynthetic pathway has δ¹³C values between −17‰ and −9‰ and is usually found in grasses as well as trees and shrubs from warm regions. The third photosynthetic pathway, CAM (Crassulacean Acid Metabolism) is found in succulent plants, like cacti, bromeliads or agaves, with δ¹³C values between −35‰ and −12‰ (Bender, 1971; Smith and Epstein, 1971; O'Leary, 1988; Farquhar et al., 1989; Ehleringer and Monson, 1993; Hayes, 2001; Andrade et al., 2007).

Herbivores eat plants, incorporating the carbon from those plants into their tissues and structures such as dental enamel. According to Cerling and Harris (1999), for large plant-eater mammals, the isotope enrichment factor (ε*) of ¹³C between the tooth enamel and their diet is 14.1 ± 0.5‰. Based on that enrichment, modern animals that eat C₄ plants will have δ¹³C values between −3‰ and +5‰. Carbon isotopic values between −8‰ and −22‰ will be found in herbivores eating C₃ plants, while those eating both types of plants will show δ¹³C values between −3‰ and −8‰ (Domingo et al., 2013).

However, based on what Tipple et al. (2010) found for atmospheric CO₂ values during the Pleistocene (δ¹³C_atmCO₂ = −6.5‰) with regard to the current values (δ¹³C_atmCO₂ = −8‰), it is inferred that Pleistocene mammals that feed on C₃ plants will have δ¹³C values in the enamel of −6.5‰ to −14.5‰ which can be divided into three groups: closed canopy forest (<-14.5‰), woodland to woodland-mesic or C₃ grassland (−14.5‰ to −9.5‰), open forest-xeric or C₃ grassland (−6.5‰ to −9.5‰), those with mixed diet: C₃/C₄ grassland (−6.5‰ to −1.5‰), and those feeding only on C₄ plants: C₄ grassland (˃-1.5) (Domingo et al., 2013; González-Guarda et al., 2018).

Oxygen is incorporated into animals by inhalation, from water in food, and mainly by ingested water. Such oxygen is in equilibrium with what is lost through CO₂ exhalation, feces, urine, and sweat. Other factors like physiology, climate, and habitat can modify such balance (Sánchez-Chillón, 2005). In mammals like gomphotheres, that are elephant relatives, it is assumed that oxygen in their enamel mostly comes from the ingested superficial water. The oxygen isotope ratio is affected by latitude, longitude, rain quantity, but mainly temperature (Dansgaard, 1964; Castillo et al., 1985; Levin et al., 2006; Yann et al., 2013). Oxygen isotopic composition is frequently used for paleoclimatic and paleoecological studies (Bocherens et al., 1996; Sponheimer and Lee-Thorp, 1999; Schoeninger et al., 2000). In addition, numerous studies have shown that δ¹⁸O values in mammals’ tooth enamel can be used in conjunction with ⁸⁷Sr/⁸⁶Sr in studies of geographical origin, seasonal and residential mobility, and migration (Balasse et al., 2002; Britton et al., 2009; Glassburn et al., 2018; Goepfert et al., 2013; Henton et al., 2009; Marín-Leyva et al., 2021).

Strontium is a trace element found in igneous, metamorphic, and sedimentary rocks, as well as in ground waters, soils, plants, and animals. This element has four natural occurring isotopes (⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr). A part of ⁸⁷Sr is radiogenic and formed by β⁻ decay of ⁸⁷Rb (Faure and Mensing, 2005; Slovak and Paytan, 2012).

Due to the high atomic mass of strontium, natural fractionation processes have negligible effect on its isotopic ratios. Strontium isotope ratios generally do not show significant changes during their passage from weathered rock to soil and finally into the food chain (Hurst and Davis, 1981; Beard and Johnson, 2000; Fietzke and Eisenhauer, 2006; De Souza et al., 2007; Wakabayashi et al., 2007; Halicz et al., 2008; Rüggeberg et al., 2008). Therefore, ⁸⁷Sr/⁸⁶Sr in soils, vegetation, and fauna generally reflect the original ⁸⁷Sr/⁸⁶Sr values of the bedrock (Capo et al., 1998). However, studies on biologically available ⁸⁷Sr/⁸⁶Sr indicate that heterogeneous ⁸⁷Sr/⁸⁶Sr in rock, sediment and soil samples of some regions can be due to additional strontium sediment sources, introduced by fluviatile transport or by fertilizers (Sillen et al., 1998; Price et al., 2002; Maurer et al., 2012).

Strontium can substitute calcium in the trophic web and is incorporated into hydroxyapatite crystals of dental enamel and bone (Comar et al., 1957). On the other hand, ⁸⁷Sr/⁸⁶Sr in tooth enamel is obtained during the animal's youth or teeth mineralization and generally remains as a closed system after its formation (Hillson, 1996). The ⁸⁷Sr/⁸⁶Sr ratio of dental enamel is compared with ⁸⁷Sr/⁸⁶Sr in sediments or rocks from the fossil site and its surroundings; if both ratios coincide, the animal can be classified as local. When both ratios are different, a look into the ⁸⁷Sr/⁸⁶Sr soil, rock, and plant database can provide evidence for the possible origin of the animal (Flockhart et al., 2015). However, it is known that the variation in bioavailable strontium in an area can be related to different environmental factors (geological and atmospheric) and that it can vary over time in a single region (Tucker et al., 2020). Also, in fossil mammal teeth, diagenetic alterations of Sr can occur mainly in the dentin, enamel-dentin junction, and the first 150–200 μm of enamel (Weber et al., 2021). Nevertheless, diagenetic strontium from teeth enamel can be removed by mechanical or chemical treatments (Tucker et al., 2020; Weber et al., 2021).

Taking these considerations into account, several studies have shown that ⁸⁷Sr/⁸⁶Sr analyses in animal tissue can be used to reconstruct geographic origin, home range, and mobility patterns including migration paths of extinct and extant species (Price et al., 1994; Chamberlain et al., 1997; Hoppe et al., 1999; Hoppe, 2004; Ábelová, 2006; Hoppe and Koch, 2007; Britton et al., 2009; Esker et al., 2019; Wallace et al., 2019; Pérez-Crespo et al., 2020; Marín-Leyva et al., 2021).