Abstract

SOSA, Luis Marcelo  and  GARCIA LOPEZ, Daniel Alfredo. Structural variation of the masseter muscle in Typotheria (Mammalia, Notoungulata). Ser. correl. geol. [online]. 2018, vol.34, n.1, pp.1-6. ISSN 1666-9479.

The great morphologic diversity of the order Notoungulata is evident in remarkable traits, such as the body size, and this group includes from small, rodent size animals (e.g., Punapithecus) to forms exceeding 1000 kg in weight (e.g., Trigodon). Moreover, several cases of convergence with other placentals are known within the group, with rodent-like and lagomorphs-like forms in Typotheria and animals similar to some Perissodactyla or Artiodactyla within Toxodontia. This remarkable diversity is also expressed in specific traits developed on certain parts of the skeleton, such as the auditory region or the masticatory apparatus. In this contribution we present an analysis on the structural evolution of the area of origin of the masseter muscle focused on the suborder Typotheria. A sample of 12 representatives of that group was selected, plus two representatives of early diverging notoungulates (Henricosborniidae and Notostylopidae). This sample covers a span of approximately 43 Ma (Eocene-Pleistocene). On each case, the relative development and arrangement of the different parts of the masseter was deter mined evaluating the area of the muscular scars remaining on the zygomatic arch and its anterior root. T hree main conditions related to the origin of the masseter muscle were deduced, showing some temporal and phylogenetic arrangement. Condition 0 (e.g., Colbertia, Oldfeldthomasia) is characterized by a small origin of the superficial master and a small area for the deep master, is only present in Eocene representatives, and is comparable to the generalized plan present in some marsupials (e.g., Didelphis). Condition 1 (e.g., Notopithecus, Protypotherium) presents a moderately-developed deep master and an origin of the superficial masseter related to a conspicuous descendant process, being similar to some Lagomorpha (e.g., Or yctolagus). Finally, the condi-tion 2 (e.g., Griphotherion, Typotheriodon, Hegetotherium, Paedotherium) is the most complex, showing a spectrum of variations related to different stages of hypertrophy of the deep masseter which refex similarities to certain rodents (e.g., Sciuromorpha). This morphologic perspective shows the complexity achieved by the Notoun-gulata and brings a more complete frame on the evolution of this group, which represents one of the most characteristic elements of the South American fossil biota.

Keywords : Estructura. Músculo masetero. Typotheria. Notoungulata. Cenozoico. América del Sur..

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