Occlusion and Function of Triconodont Dentitions

Abstract

Over the last decades, many studies have focused on the tribosphenic molar and its functional aspects because it is considered to be a key innovation in mammalian evolution. Early triconodont dentitions have been examined to a lesser degree, despite being the plesiomorphic mammalian tooth morphology and, therefore, closely linked to the evolution of precise occlusion and food processing, which contributed to the evolution of endothermy. The majority of these studies were limited to descriptions and two-dimensional modeling. In this thesis, micro-computed tomography (µ-Ct) and 3D models were used to test existing occlusal models and to analyze the dental function of the different triconodont molar morphologies. The triconodont molar is characterized by three linearly aligned main cusps a/A, b/B, and c/C. It is the characteristic molar pattern for the non-mammalian Mammaliaformes Morganucodonta and the early-diverging crown-group Mammalia Eutriconodonta, which were subjects of this study. <br /> The early-diverging mammaliaforms <i>Morganucodon watsoni</i>, <i>Megazostrodon rudnerae</i>, and <i>Erythrotherium parringtoni</i> had a primarily orthal occlusal path. Differences in the occlusion of the main cusps of <i>Morganucodon</i> compared to the embrasure occlusion of <i>Megazostrodon</i> and <i>Erythrotherium</i> were confirmed with the Occlusal Fingerprint Analyser (OFA). The occlusion of <i>Morganucodon</i> further showed variation in trajectory and cusp placement. The dentitions of Morganucodonta were well adapted to piercing and shear-cutting. ‘Shearing flanks’, which were the focus of previous studies, seemed to be rather a result of attrition, than functional areas in themselves. Positioning of the upper molars within the maxilla of <i>Morganucodon</i> suggests a predetermined tooth placement to allow space for the lower dentition. These results are in contrast to previous hypotheses that stated that <i>Morganucodon</i> relied on extensive wear in order to form a precise occlusion. <br /> While the molars of Morganucodonta emphasize the piercing capability with large, isolated cusps, the molars of Triconodontidae have a homogenous cusp-valley system that formed a continuous fore-aft crest that linked the entire molar series in a zig-zag pattern. It thus combines traits linked to both carnivorous diets (e.g. fore-aft cutting edges) and insectivorous diets (transverse crests and lobes). The molar series of triconodontids was highly uniform and adapted to a precise fit; lower molar cusps were self-sharpening within the valleys between upper molar cusps. While the high degree of precision ensured good cutting capabilities, its uniformity likely put the dentition under greater evolutionary constraints than other molar types with more heterogeneous cusp morphologies. This explains the stereotyped nature of the triconodontid molar, which underwent little change during the 60–85 Ma range of the family. Contrary to previous studies, embrasure occlusion was confirmed for Triconodontidae based on the OFA analysis. Embrasure occlusion can be therefore considered the universal occlusal mode for all taxa with triconodont molars with the exception of few Morganucodonta. Additionally, sequential tooth replacement was confirmed for Triconodontidae, which is in accordance with their phylogenetic position as early-diverging crown Mammalia. A unique pattern, on the other hand, is the development of m4 within the lingual side of the coronoid process, well above the tooth row of <i>Triconodon</i>. It was subsequently accommodated in the active tooth row via unusually prolonged and localized growth of the posterior part of the mandible (and, by implication, the base of the skull rostrum), with the m4 remaining in position and not erupting upward. This pattern is also seen among some later triconodontids and appears to be unique to the family. Over the course of this study, a new species of Triconodontidae <i>Triconodon averianovi</i> was described based on the partially reduced m4, its small p4, and gracile canine. <br /> Gobiconodontidae, a clade of early crown-group Mammalia, are unique among Mesozoic mammals due to their large size, the replacement of their molariforms, and carnivorous diet. A type of embrasure occlusion was present that caused extensive wear and resulted in deep grooves between the upper molars. The occlusion is centered around mesiodistally oriented crests. They form in the course of increased wear that results in the loss of the smaller b/B and c/C cusps and extend from the tip of the large a/A cusps to the base of the molariforms. These crests provide the main cutting capability during the single-phased power stroke. Gobiconodontidae and Triconodontidae both share mesiodistally oriented crests, a faunivorous diet, as well as lingually inclined upper molars. However, the occlusion of Gobiconodontidae differs from the precise uniform system of Triconodontidae. Its primary focus is on the formation of long crests at the costs of tooth material. Given the large size of Gobiconodontidae, it seems likely that bite force was emphasized over precision. This interpretation is in agreement with the previous hypothesis that the replacement of the molariforms might have been necessary to compensate for the loss of tooth material. <br /> The molars of all taxa, with a preserved maxilla, observed in this study were lingually inclined within the tooth row. This is interpreted as a mechanism to reduce the amount of roll required to keep the teeth in contact during occlusion.