Late Pleistocene toxodont remains of Tamaulipas, Mexico. Confirmation of the occurrence of Mixotoxodon larensis (Van Frank, 1957) and an analysis of sexual dimorphism

Highlights

• The toxodont discovered in Tampico confirms the presence of the Mixotoxodon in Tamaulipas.

• It is perhaps the best preserved toxodonts material in the México.

• The dental variations described by other authors, is only reflects of sexual dimorphism.

• The metric variations in molars are related to a difference in size between sexes.

• The lingual folds in molars are also dimorphic, but are influenced by other factors.

Abstract

The Mexican record of notoungulates is scarce. So far, the only described species in Mexico is represented by Mixotoxodon larensis, which has limited in Michoacán and Veracruz. This work describes the MG-ITCM-405P toxodont material discovered in Tampico (Late Pleistocene) that confirms the presence of the Mixotoxodon larensis in Tamaulipas (northeastern Mexico). The specimen consists of an almost complete jaw, which stands out for having the premolars (p2, p3 and p4) and molars (m1, m2 and m3) rights in good condition, being perhaps the best preserved toxodonts material in the country.

Statistical analyzes of sexual dimorphism (metric and morphological) of the Mixotoxodon larensis showed that the metric variations in molars are related to a difference in size between males and females, with m1 being the most dimorphic. The variations of the lingual folds of the molars also follow a similar pattern, but there is a strong influence of other variables such as age, diet and to a lesser extent, biogeographical variations.

Introduction

Toxodontidae is an extinct family of medium-to-large South American ungulates (Notoungulata), which were very diverse and abundant in the Neotropic during the Late Cenozoic (MacFadden, 2005; Nasif et al., 2000; Forasiepi et al., 2015; Bonini et al., 2017). Family members are distinguished by a specialized dentition, characterized by hipsodontic incisors, where the I2 and i3 are modified as "defenses", while the canines can be small, rudimentary or absent, while the jugal teeth can be protohipsodonte to hipselodonte(Madden, 1990; Bond et al., 2006; Bonini et al., 2017). In Pleistocene toxodonts it is common to consider them as ecological analogs of rhinos and hippos, not only because of the presence of an elongated skull, and a robust and heavy body, but also because of a mechanism in the extended knee (medial trochlear crest) that allowed them stand for long periods of time like horses and rhinos (Cope, 1897; Nasif et al., 2000; Shockey, 2001; Bonini et al., 2017).

The oldest known record of family Toxodontidae is known from the Oligocene of Patagonia, in southern Argentina (Bond et al., 2006; Bonini et al., 2017). They diversify throughout the Neogene in much of the subcontinent (Brazil, Peru, Ecuador, Bolivia, Uruguay, Venezuela, and Colombia) (Nasif et al., 2000). However, during the Pleistocene they arrive in North America in a limited way, represented only by a single confirmed genus Mixotoxodon (van Frank, 1957). Mixotoxodon is known from Central and North America, from Panama (Gazin, 1956, Lucas et al., 1997), Honduras (van Frank, 1957; Webb and Perrigo, 1984), Nicaragua (Leidy, 1886; Lucas et al., 1997), Costa Rica (Laurito, 1993; Lucas et al., 1997), El Salvador (van Frank, 1957; Laurito, 1993; Lucas et al., 1997; Cisneros, 2005); Guatemala (Woodburne, 1969); United States (Lundelius et al., 2013) and Mexico (Polaco et al., 2004; Rodríguez de la Rosa et al., 2011; Arroyo-Cabrales et al., 2010). Mexican record is known in Hihuitlán state of Michoacán (Polaco et al., 2004), and Pánuco and La Estribera, state of Veracruz (Pánuco and La Estribera) (Polaco et al., 2004; Rodríguez de la Rosa et al., 2011). However, Arroyo-Cabrales et al. (2010) mentioned that there is an unconfirmed record of Mixotoxodon in Tamaulipas (northeast of the country). So far, the location of the specimen remains unknown.

Van Frank describes Mixotoxodon larensis from the material found in San Miguel, Venezuela and mentions that there are some metric and morphological differences between the specimens he names as individual variations. Subsequently, other authors have recognized dental and cranial variations between Mixotoxodon material from South America, Central America, and Mexico. However, these differences have been interpreted differently, from individual variability (wear), intraspecific, ontogenetic variations (Rincón, 2011; Bond et al., 2006), post mortem distortions (Bond et al., 2006) to biogeographic (by speciation) (Porta, 1959, Polaco et al., 2004). Among the most frequent variations are the maximum extension of the symphysis, presence of enamel bands in folds or cones, size, and shape of dental pieces, among others. The objectives of this manuscript are: a) describe the toxodont material found in the city of Tampico, confirming the presence of Mixotoxodon larensis in Tamaulipas (northeastern Mexico); b) Analyze some cranial and dental variations to demonstrate whether or not there is sexual dimorphism in Mixotoxodon larensis.