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Molar Diversity and Functional Adaptations in Mesozoic Mammals

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Molar Diversity and Functional Adaptations in Mesozoic Mammals 187 Molar diversity and functional adaptations in Mesozoic mammals Thomas Martin, Kai R. K. Jäger, Thorsten Plogschties, Achim H. Schwermann, Janka J. Brinkkötter, and Julia A. Schultz Introduction The vast diversity of molars seen in therian mammals is to add a crushing function to the plesiomorphic piercing derived from the apomorphic tribosphenic respectively and cutting action of the molariforms among early mam- pretribosphenic molar pattern of the Mesozoic mammalian maliaform lineages, independently from the evolution of precursors (Fig. 10.1). The tribosphenic pattern appeared the tribosphenic molar. Vivid examples are the Jurassic in the fossil record in two different lineages of mammals and Early Cretaceous docodontans that evolved complex (Luo et al. 2001), the Middle Jurassic australosphenidans molars with crushing functions (e. g., Kermack et al. 1987, (e. g., Ambondro, Flynn et al. 1999; Asfaltomylos, Martin & Butler 1988, Pfretzschner et al. 2005), suitable for process- Rauhut 2005) and Late Jurassic boreosphenidans (Jura- ing diverse diet, and also with disparate molar shapes and maia, Luo et al. 2011). The Southern Hemisphere austra- very divergent morphologies among docodontan taxa, such losphenidans are considered stem monotremes that have as some specialized teeth for piscivory of Castorocauda completely reduced their permanent teeth in extant taxa, (Ji et al. 2006). Another example is the Late Cretaceous whereas boreosphenidans comprise all other mammals Malagasy gondwanatherian Vintana with the first known (metatherians and eutherians) with a striking diversity of euhypsodont molars in mammalian evolutionary history dentitions (Luo 2007). Mammals with tribosphenic molars with complex enamel folding and enamel islets, suitable (boreosphenidans or tribosphenidans) are characterized by for processing tough plant materials (Krause et al. 2014, a two-phased masticatory power stroke with piercing and Schultz et al. 2014). cutting function during phase I and subsequent grinding Within the Deutsche Forschungsgemeinschaft (DFG) and crushing function during newly developed phase II, in Research Unit FOR 771 “Function and performance en- contrast to the single-phased power stroke in pretribosphe- hancement in the mammalian dentition – phylogenetic and nidans (only phase I present) (Hiiemäe & Kay 1972, 1973, ontogenetic impact on the masticatory apparatus” several Kay & Hiiemäe 1974, Hiiemäe 1976, 1978). The two-phased projects were devoted to the functional analysis of Meso- power stroke with crushing and grinding function allowed zoic mammaliaform molariforms and molars for a better the exploitation of more diverse food sources and likely understanding of the evolutionary history of mammalian fostered the great evolutionary success of therian mam- mastication. 3D investigation techniques such as µCT mals in the Cenozoic. But even before the appearance of combined with the Occlusal Fingerprint Analyser (OFA) tribosphenic molars in mammal evolution, other Mesozoic software (Kullmer et al. 2009, 2020, this volume) that was mammal lineages had already developed remarkably di- developed within the FOR 771 made a virtual reconstruc- verse dentitions and dietary specializations, and a second tion of the masticatory cycle and a quantitative analysis chewing phase seems to have evolved independently in of dental function and occlusal relationships possible. The several groups, including docodontans, multituberculates, following chapter provides a review of the morphological and gondwanatherians. The selective pressure to enhance diversity of Mesozoic mammalian molariforms and molars the ingestion and mastication of new food sources triggered and their various functional and dietary adaptations. a number of attempts in mammalian evolutionary history Triconodont molariforms – three cusps in a row for piercing and cutting The most plesiomorphic mammaliaform molariform teeth codontans and “amphilestids” (a paraphyletic assemblage are of the triconodont type with three primary cusps aligned of dentally plesiomorphic eutriconodontans) among Eu- in a mesio-distal row. This molariform type characterizes triconodonta. In more derived Eutriconodonta, the height stem mammalian Morganucodonta and has been retained difference between the central and mesial respectively as a symplesiomorphic character by crown mammalian distal cusps is much smaller, or they are of equal height, Eutriconodonta. as in the eutriconodontan family Triconodontidae. In de- Plesiomorphically, central cusp A/a is the highest, rived Eutriconodonta, an elongation of the cutting edges whereas mesial cusp B/b and distal cusp C/c are lower occurs compared to the plesiomorphic condition in Mor- (Fig. 10.2). This molariform type characterizes morganu- ganucodonta with relatively shorter cutting edges. The T. Martin & W. v. Koenigswald (eds.): Mammalian Teeth – Form and Function. Pp. 187-214, 22 figs. © 2020 by Verlag Dr. Friedrich Pfeil, München, Germany – ISBN 978-3-89937-266-3 DOI: http://doi.org/10.23788/mammteeth.10 Published 22 December 2020 188 Pappotherium Didelphodus tribosphenic Aegialodon III Boreosphenida Peramus pretribosphenic Amphitherium Dryolestes symmentrodont II Cladotheria Kuehneotherium 2 1 3 triconodont 4 Morganucodon 6 5 trigonid talonid I Mammaliaformes (conservative (modified) A B structure) Fig. 10.1. Evolution of the tribosphenic lower molar. A, schematic diagram with functional transformation. Yellow, cutting edges of trigonid; green, hypoflexid groove (mainly shearing); red, crushing and grinding area within the (incipient) talonid basin. B, lower tribosphenic molar with wear facets (hatched areas); numbers of wear facets after Crompton (1971). dental function of the triconodont molariform dentition is diversity of the cusps and the tooth size disparity of the mainly piercing and cutting with a predominantly orthal jaw triconodont molars suggest that, even with the same tri- movement. Smaller taxa such as the morganucodontans conodont pattern, there occurred functional differences, Morganucodon and Megazostrodon were insectivorous. leading to a dietary differentiation. Based on a microtextural analysis of molariforms, Gill et The chewing stroke of Morganucodon had been recon- al. (2014) concluded that Morganucodon mainly fed on structed as predominantly orthal with a slight movement in hard-shelled insects such as coleopterans. Larger taxa anterior or posterior direction (Crompton & Jenkins 1968). such as the eutriconodontan Gobiconodon were faunivo- Jäger et al. (2019), however, found evidence for a relatively rous (Jenkins & Schaff 1988), and Repenomamus, the high degree of freedom during the masticatory cycle ac- largest known Mesozoic mammal, was carnivorous, either cording to the varying orientations of striations on molars scavenging or predatory, or both. Hu et al. (2005) reported of Morganucodon. Due to the lack of guiding structures, the on a Repenomamus specimen with remains of a juvenile molars of Morganucodon have no auto-occlusion (sensu psittacosaur in the stomach region. The morphological Mellet 1985, Evans & Sanson 2006, based on the teeth of T. Martin & W. v. Koenigswald: Mammalian Teeth – Form and Function. – München (Pfeil) 2020 – ISBN 978-3-89937-266-3 189 lingual maxillary pits mesial A M4 M3 M2 M1 P4 P2P3 P1 i3 i2 i4 i1 B mesial m4 m3 m2 m1 p4 p3 p2 c lingual C mesial 1 mm D Fig. 10.2. Dentition of Morganucodon. A, left upper and B, left lower tooth row in occlusal view; C, left upper tooth row in lingual view and D, left lower tooth row in buccal view (mirrored). From Jäger et al. (2019). Carnivora). Further, it is now demonstrated by OFA analysis byproduct of attrition of tooth-to-tooth contact. The tooth that the occlusal movements in Morganucodon can have action related to these extensive facets represents only chewing strokes in orthal, distal and mesial orientations, a part of total masticatory functions. Further, the extent and are more versatile than only the orthal movement as of wear facets is dependent on ageing. It takes time for previously emphasized (Jäger et al. 2019). Most individuals individuals of Morganucodon to develop extensive and fully apparently developed a preferred chewing direction, which matched wear facets (Crompton & Jenkins 1968, Butler & suggests precise muscular control of the jaw movements Clemens 2001), but other dental occlusal functions may which had been inferred previously based on the match- start earlier than the formation of extensive wear facets. ing wear facets of Morganucodon (Crompton & Parker Morganucodon and Megazostrodon differ in pattern 1978). According to the crown morphology, distribution of cusps opposition between the upper and lower molars. of wear facets, and the OFA analysis (Jäger et al. 2019), Megazostrodon is characterized by an embrasure occlu- Morganucodon and Megazostrodon had a single-phased sion with lower molar cusp a entering between two upper chewing stroke. This study supported the notion that pierc- molars, and upper molar cusp A between two lower molars ing and shear-cutting were the main functions during the (Crompton & Jenkins 1968). For Morganucodon, a differing masticatory cycle of Morganucodon and Megazostrodon. occlusal mode was described, with lower molar cusp a According to Jäger et al. (2019), the teeth of Morganucodon occluding between upper molar cusps B and A, and up- were fully functional for piercing, shear-cutting, and (to a per molar cusp A occluding between lower molar cusps a lesser extent) shearing upon tooth eruption. The
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