Diversity and Evolution of Hunter-Schreger Band Configuration in Tooth Enamel of Perissodactyl Mammals (original) (raw)
Related papers
Annals of the Carnegie Museum
In the Chalicotheriidae the main shearing facets of upper and lower molars possess a very significant modification of the direction of Hunter-Schreger bands (HSB). In contrast to the primitive horizontal HSB orientation, chalicotheres show distinctly U-shaped HSB that intersect the main cutting edges at almost right angles. This relationship is maintained throughout the various stages of wear. This specific characteristic is shared with Brontotheriidae. Certain other Perissodactyla, such as Tapiroidea and Rhinocerotidae, but definitely not the Equoidea, follow a similar tendency. Functionally, the reorientation of the HSB is interpreted as an adaptation to reduce abrasion. The reorientation of the HSB seems to be related to interprismatic matrix (IPM) parallel to the prisms. ZUSAMMENFASSUNG Bei den Chalicotheriidae zeigen die Hauptschneidekanten der oberen und unteren Molaren eine aulfallende Abweichung von der normalerweise horizontalen Orientierung der Hunter-Schreger-Bander (HSB). Die HSB sind U-formig gebogenen und stehen annahemd senkrecht auf den Hauptschneidekanten. Dieser Winkel bleibt durch diese spezielle Anordnung auch wahrend der fortschreitenden Abkauung erhalten. Dieses abgeleitete Merkmal teilen die Chalicotheriidae mit den Brontotheriidae. Andere Perissodactyla, wie die Tapiroidea und Rhinocerotidae, nicht aber die Equoidea, verfolgen eine ahnliche Strategic. Funktionell wird diese Reorientierung der HSB als Anpassung gedeutet, durch die der Abrieb der Schneidekante reduziert wird. Die Reorientierung der HSB scheint an eine Interprismatische Matrix (IPM) gebunden sein, die parallel zu den Prismen liegt. to some other Perissodactyla." Annals of the Carnegie Museum 63(1), 49-65.
2020
Both tooth shape and mode of mastication gained an enormous variability during the evolution of large herbivorous mammals, which allowed them to exploit diverse ecological niches. The variability in mastication includes specific adaptations to diet and is linked to the evolutionary history of a group. In addition, teeth are composed of the hardest tissue of the vertebrate body and thus are the most abundant vertebrate remains in the fossil record. Because of that and the resistance of the enamel to the wear process, teeth are preferred objects in paleontology. Teeth are worn and partly abraded by the continuous chewing processes and need to serve the entire lifespan of an animal. Wear creates facets typically appearing on the crown of molars forming light reflecting polished surfaces (Mills 1955, Butler 1973). These wear facets occur as matching pairs on upper and lower teeth; several numbering systems and nomenclatures were erected for descriptive and comparative purposes (see summ...
Two different crests have been identified in the upper cheek teeth of perissodactyls as the postprotocrista. Both occur in the upper premolars of some early perissodactyls, demonstrating that they are not homologous. The true postprotocrista extends distolabially from the protocone, often connecting the protocone and the metaconule, at least primitively. The other crest, here termed the endoprotocrista, extends less labially and more distally than the postprotocrista and does not connect to the metaconule. In some taxa, the hypocone arises from the distal end of the endoprotocrista. Thus, the endoprotocrista plays an important role in molarization of some perissodactyl upper premolars. Molarization is completed by separation of the hypocone from the protocone and the metaconule connecting to the hypocone, involving either migration or loss of the postprotocrista. The endoprotocrista is similar to the “Nannopithex fold” of North American adapid primates, but it is otherwise currently known only in some early perissodactyls. Perissodactyl upper premolars become molarized by at least two modes. One mode involves the development of the hypocone from the endoprotocrista, as described here. A second involves the enlargement and lingual migration of the paraconule, as inferred for the third upper premolar of early equids. In the upper molars, the hypocone appears to be derived from the cingulum. The present study highlights the difficulties in making inferences regarding the mode of premolar molarization from static morphology. This further reflects the uncertainty inherent in using modes of premolar molarization in phylogenetic analysis and concomitant problems in determining primary homology.
Journal of Mammalian Evolution, 2016
Among medium-to large-sized terrestrial 'ungulates,' there is often a relationship between increasing body size, correlated changes in diet, and increased complexity of the enamel microstructures [notably the development of Hunter-Schreger bands (HSB)]. An exhaustive survey of the enamel microstructures of living and extinct Hyracoidea demonstrates, however, that the Schmelzmuster within this order of mammals is generally one-layered and formed by radial enamel despite a large range of body sizes and dietary adaptations; HSB are remarkably absent. Radial enamel is characteristic of early diverging hyracoids, as well as more derived members of the extinct families Geniohyidae and Pliohyracidae, and the extant Procaviidae. Only some large 'Saghatheriidae,' and all members of the family Titanohyracidae, developed a more complex enamel microstructure (i.e., with prisms decussating), a unique condition among Mammalia that we name 'bundled enamel' (BE). This structure is reminiscent to some degree of both the 'Pyrotherium enamel' and the '3D enamel' of proboscideans. Hyracoids with BE represented a major component of the diversity of mid-to large-sized herbivores during the Paleogene in Africa. Like HSB, which are developed by most other 'ungulates,' the BE is regarded as a device for resisting propagation of cracks during mastication. Hyracoids never developed however the 'modified radial enamel' that is characteristic of most large and hypsodont perissodactyls and artiodactyls that entered Africa during the Miocene.
Anatomy and Embryology, 1992
Distribution of peritubular dentine was studied in cheek teeth of fallow deer (Dama dama), roe deer (Capreolus capreolus) and wild boar (Sus scrofa). In the two cervid species, especially intense peritubular dentine formation was found in the outer regions of mid and cuspal coronal dentine. In these areas a marked asymmetry occurred, peritubular dentine being predominantly secreted onto the side of the dentinal tubule walls nearest to the dentinoenamel junction. Intensity and asymmetry of peritubular dentine formation decreased cervically. In root dentine, the walls of the dentinal tubules were covered with only a thin peritubular dentine layer of even thickness. Here, in contrast to peripheral coronal dentine, the volume of intertubular dentine far exceeded that of peritubular dentine. In porcine coronal dentine, PTD asymmetry, being of lesser extent than in cervids, was observed only in peripheral areas of cuspal and flank regions of the cheek teeth. Because peritubular is more highly mineralized than intertubular dentine, the relative volume of dentine made up from the two components has an important influence on dentinal wear resistance. The significance of variations in volume and distribution of peritubular dentine between different dentinal regions for achieving and maintaining a functional occlusal surface is shown for cervid cheek teeth. Our results suggest that dentinal structure (in addition to enamel structure) should be taken more into consideration when discussing occlusal surface morphology in herbi- and omnivores from a functional point of view.
Four groups of equids, ''Anchitheriinae,'' Merychippine-grade Equinae, Hipparionini, and Equini, coexisted in the middle Miocene, but only the Equini remains after 16 Myr of evolution and extinction. Each group is distinct in its occlusal enamel pattern. These patterns have been compared qualitatively but rarely quantitatively. The processes influencing the evolution of these occlusal patterns have not been thoroughly investigated with respect to phylogeny, tooth position, and climate through geologic time. We investigated Occlusal Enamel Index, a quantitative method for the analysis of the complexity of occlusal patterns. We used analyses of variance and an analysis of co-variance to test whether equid teeth increase resistive cutting area for food processing during mastication, as expressed in occlusal enamel complexity, in response to increased abrasion in their diet. Results suggest that occlusal enamel complexity was influenced by climate, phylogeny, and tooth position through time. Occlusal enamel complexity in middle Miocene to Modern horses increased as the animals experienced increased tooth abrasion and a cooling climate.
PalZ, 2018
The dentition of Cambaytherium was investigated in terms of dental wear, tooth replacement and enamel microstructure. The postcanine tooth row shows a significant wear gradient, with flattened premolars and anterior molars at a time when the last molars are only little worn. This wear gradient, which is more intensive in Cambaytherium thewissi than in Cambaytherium gracilis, and the resulting flattened occlusal surfaces, may indicate a preference for a durophagous diet. The tooth replacement (known only in C. thewissi) shows an early eruption of the permanent premolars. They are in function before the third molars are fully erupted. During the dominant phase I of the chewing cycle the jaw movement is very steep, almost orthal, with a slight mesiolingual direction and changes into a horizontal movement during phase II. The enamel microstructure shows Hunter-Schreger-bands (HSB) in the inner zone of the enamel. In some teeth the transverse orientation of the HSB is modified into a zigzag pattern, possibly an additional indicator of a durophagous diet.
Ungulate cheek teeth: developmental, functional, and evolutionary interrelations
1985
Developmental and functional factors interact in complex ways reflected in evolutionary patterns. The paper addresses problems of coronal morphogenesis, amelogenesis, food comminution and digestion, mastication, tooth eruption and wear, in order to identify functional interrelations and developmental constraints in the evolution of cheek tooth morphology. A general mechanism explaining differen tial (levelling) wear of teeth in a row is proposed. The paradox of predicted positive allometry but observed isometry of tooth size to body size is resolved by inclusion of the time dimension in the equation. The reasons for the commonness of isometric scal ing in animals are discussed. Relative width of antagonists is found to discriminate be tween functional categories in terms of the dynamics of the power stroke in mastica tion, and can be used to interpret the functional meaning of major patterns of dental morphological radiation. Enamel structure at the level of arrangement of Hunter Schreger bands is interpreted in terms of different mechanical requirements (resis tance to crack propagation) in major morphological and functional categories. It is concluded that many aptive features probably or certainly did not arise for their cur rent functions, but are one-time constraints which have become incorporated into functional systems (exaptations rather than adaptations).
Journal of Mammalian Evolution, 2005
The previously unknown enamel microstructure of a variety of Mesozoic and Paleogene mammals ranging from monotremes and docodonts to therians is described and characterized here. The novel information is used to explore the structural diversity of enamel in early mammals and to explore the impact of the new information for systematics. It is presently unclear whether enamel prisms arose several times during mammalian evolution or arose only once with several reversals to prismless structure. At least two undisputed reversions or simplifications are known-in the monotreme clade from Obdurodon to Ornithorhynchus (via Monotrematum?), and (perhaps more than once) within the clade from archaeocete to a variety of odontocete whales. Similarly, both prismatic and nonprismatic enamel is present among docodonts. Seven discrete characters showing enough morphological diversity to be of potential importance in phylogenetic reconstructions may be identified as a more appropriate summary of enamel microstructural diversity among mammaliaforms than the single character "prismatic enamel-present/absent" employed in recent matrices. Inclusion of five of these characters in the matrix of Luo et al. (2002) modifies the original topology by collapsing several nodes involving triconodonts and other nontribosphenic taxa. There is considerable support for prismatic enamel as a synapomorphy of trithelodonts plus Mammaliamorpha, and multituberculates appear to have small or "normal" sized prisms as the ancestral condition, with some (as yet) enigmatic changes to nonprismatic structure in some basal members of the group and the appearance of "gigantoprismatic" structure as an autapomorphic state of less inclusive clades. Other potential qualitative characters and the need for attaining appropriate methods to incorporate quantitative features may be important for future analyses.