The Dance of Tusks: Rediscovery of Lower Incisors in the Pan-American Proboscidean Cuvieronius hyodon Revises Incisor Evolution in Elephantimorpha

Abstract

The incisors of proboscideans (tusks and tushes) are one of the most important feature in conservation, ecology and evolutionary history of these mammals. Although the absence of upper incisors is rare in proboscideans (occurring only in deinotheres), the independent losses of lower incisors are recognized for most of its lineages (dibelodont condition). The presence of lower incisors in the Pan-American gomphothere Cuvieronius hyodon was reported a few times in literature, but it was neglected in systematic studies. We analyzed several specimens of Cuvieronius hyodon from the Americas and recognized that immature individuals had lower incisors during very early post-natal developmental stages. Subsequently, these are lost and lower incisors alveoli close during later developmental stages, before maturity. Moreover, for the first time in a formal cladistic analysis of non-amebelodontine trilophodont gomphotheres, Rhynchotherium and Cuvieronius were recovered as sister-taxa. Among several non-ambiguous synapomorphies, the presence of lower incisors diagnoses this clade. We recognize that the presence of lower incisors in Cuvieronius and Rhynchotherium is an unique case of taxic atavism among the Elephantimorpha, since these structures are lost at the origin of the ingroup. The rediscovery of the lower incisors in Cuvieronius hyodon, their ontogenetic interpretation and the inclusion of this feature in a revised phylogenetic analysis of trilophodont gomphotheres brought a better understanding for the evolutionary history of these proboscideans.

First record of Notiomastodon platensis (Mammalia, Proboscidea) from Bolívia

“…Enamel may be absent or present on the upper tusks, shaped as a single band or covering the entire tusk in juvenile and some adult individuals. The upper tusks have a circular to oval cross-section, and the lower tusks are absent (Mothé et al, 2012(Mothé et al, , 2016. In accordance with the description of the specimen from Gran Chaco province, which is slightly upcurved, not twisted, with oval cross-section outline, it is here recognized as Notiomastodon platensis, because its diagnostic features match the characteristics of the V.M.001 tusk.…”

“…In accordance with the description of the specimen from Gran Chaco province, which is slightly upcurved, not twisted, with oval cross-section outline, it is here recognized as Notiomastodon platensis, because its diagnostic features match the characteristics of the V.M.001 tusk. Despite the poor preservation of the tusk, we are certain that it does not represent Cuvieronius because the arrangement of the hollow dentin cones is not twisted; and they follow the configuration of the Notiomastodon platensis tusk (untwisted and even, see fig 4 of Mothé et al, 2016).…”

“…The Tarija valley records of C. hyodon are considered one of the largest and most important occurrences of Quaternary proboscidean megafauna in South America (Prado et al, 2005;Ferretti, 2010;Mothé et al, in press) due the numerous complete individuals from all age classes (Boule & Thevenin, 1920;Mothé et al, 2016). However, although the fossil fauna from the Tarija valley has great diversity, including proboscideans, xenarthrans, carnivores, artiodactyls and perissodactyls (Tonni et al, 2009), no specimens with the diagnostic features of Notiomastodon platensis have been reported from this locality.…”

Sixty years after ‘The mastodonts of Brazil’: The state of the art of South American proboscideans (Proboscidea, Gomphotheriidae)

“…2). Some subsequent authors agreed with the taxonomy of Osborn (Madden, 1984;Ficcarelli et al, 1995;Ferretti, 2008;Moth e et al, , 2016Lucas, 2013; Moth e and Avilla, 2015), although others did not follow the taxonomic hypothesis of Osborn (1936), accepting the presence of the genus Stegomastodon in South America (Hoffstetter, 1950(Hoffstetter, , 1952Simpson and Paula Couto, 1957;Alberdi and Prado, 1995;Shoshani and Tassy, 1996;Alberdi et al, 2002Alberdi et al, , 2004Prado et al, 2003Prado et al, , 2005Prado and Alberdi, 2008). Hoffstetter (1950) reviewed proboscideans materials from Ecuador and coined the name Haplomastodon as a subgenus of Stegomastodon.…”

“…In his review of Argentinian proboscideans, Cabrera (1929) considered Cordillerion as a junior synonym of Cuvieronius and applied this name to the Andean proboscidean, using Mastotherium hyodon as its type species, a classification followed by subsequent authors (Hoffstetter, 1950(Hoffstetter, , 1952Simpson and Paula Couto, 1957;Tobien, 1973;Shoshani and Tassy, 1996;Alberdi et al, 2002Alberdi et al, , 2004Prado et al, 2002Prado et al, , 2003Prado et al, , 2005Prado and Alberdi, 2008;Moth e et al, 2016). Lucas (2008) indicated that Cabrera (1929) probably wanted to use the oldest valid species name for what he recognized as a single South American species of Cuvieronius, even though M. humboldtii was the originally designated type species (a violation of the ICZN rules, Article 68.2 as currently articulated in ICZN, 1999).…”