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Evolutionary Aspects of Reptilian and Mammalian Enamel Structure

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Evolutionary Aspects of Reptilian and Mammalian Enamel Structure Scanning Microscopy Volume 1 Number 4 Article 38 8-16-1987 Evolutionary Aspects of Reptilian and Mammalian Enamel Structure A. Sahni Panjab University Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Biology Commons Recommended Citation Sahni, A. (1987) "Evolutionary Aspects of Reptilian and Mammalian Enamel Structure," Scanning Microscopy: Vol. 1 : No. 4 , Article 38. Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss4/38 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 1, No. 4, 1987 (Pages 1903-1912) 0891-7035/87$3.00+.00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA EVOLUTIONARYASPECTS Of REPTILIANAND MAMMALIAN ENAMEL STRUCTURE A Sahni Centre of Advanced Study in Geology, Panjab University CHANDIGARH-160014India (Received for publication December 08, 1986, and in revised form August 16, 1987) Abstract Introduction The evolution of enamel structure is dealt The present paper focusses attention on the with here on the basis of fossil reptiles and evolution of mammalian enamels based principally mammals ranging from the Triassic to the present. on the evidence of fossil vertebrates. This The evidence suggests that prismatic enamel had aspect involves not only the documentation of the developed in some therapsid reptiles and the stages leading to the development of prismatic mammal, Eozostrodon about 180 million years ago. enamels, but also the study of the structural For the next 100 million years, mammalian diversity of recent reptiles and mammals. While evolutionary history is sparingly documented and the prism packing patterns of extant reptiles and this is reflected in the poor record of enamel mammals has become better understood through the evolution during this period. The few Jurassic works of Cooper and Poole (1973) and Boyde (1964), reptiles and mammals studied suggest a pre­ there is still a lacuna with regard to the fossil prismatic structure. In the Late Cretaceous (BO record. During the last decade, considerable to 65 million years ago) when the fossil record attention has been paid to the enamel structure of improves, mammalian enamel investigated from North fossil-based evolutionary lineages, particularly American localities, are found to be prismatic; those concerning the Rodentia, Cetacea and allotherian (multituberculate) and metatherian Primates. This has largely become possible (marsupial) enamels are usually tubular, while because of a multidisciplinary approach involving eutherian (placental) ones are not. Prism palaeontologists and dental histologists, and the structure in Tertiary mammals in general, conforms growing awareness that enamels of fossilized to that of their present day descendants, but mammals have an excellent potential for there are discernible exceptions. The record of documenting structural diversity on a scale much evolutionary change in Tertiary mammals is larger than that known for living mammals. obscured by functional modifications related to Furthermore, all previous studies have shown that biomechanical stresses. Enamel structure may be fossilized mineralized tissue undergoes negligible secondarily modified; similar in phylogenetically diagenetic alteration through time and faithfully unrelated groups (eg., pauciserial enamel of early records the prism structure, even in animals that rodents) or dissimilar at the intra-familial level are more than 200 million years old. (eg., rodent families Ctenodactylidae and The pioneering efforts of Poole (1956, 1957) Ischryomyicae). Prismatic enamel is recorded from and Moss (1969) in studying fossil material by the tooth of a hatchling of the gavial, Gavialis light microscopy have recently been taken up gangeticus. world-wide by several workers using scanning electron microscopy (Fosse et al. 1973, 1978, 1985; Carl son and Krause 1985; Koenigswald 1980; Boyde and Martin 1984; Kozawa 1984; Sahni 1979). While this has led to a better documentation of the enamel structure of diverse fossil organism, these studies have mainly been descriptive accounts of prism patterns and have lacked a theoretical model to interpret evolutionary mech­ anisms. There are still several unsolved issues that preclude a better understanding of the evo­ KEHIORDS: Fossil reptile, mammal, Enamel lutionary processes in various recent and fossil evolution, geochronological distribution, prism organisms. Some of these points are discussed patterns and structure. below. Correspondence of phylogenies based on enamel structure and gross dental morphology Many workers have emphasized the value of 1903 A. Sahni enamel structure in establishing phylogenetic Structure in relation to function relationships (Fosse et al. 1978; Carlson and Krause 1985; Flynn 1982; Koenigswald 1980; Boyde The adaptive response of enamel structure at and Martin 1984). In general, the prism packing the crystallite and prism pattern level to such patterns follow taxonomic relationships estab­ biomechanical stresses generated by crown size, lished by evolutionary biologists. Primate, pro­ hypsodonty, rootless conditions, leading and boscidean, carnivore and taenioloabidoid multi­ trailing wear margins, and lophodonty is only now tuberculate enamel are characterized by a pre­ becoming better understood (Koenigswald 1980; dominance of Pattern 3 prisms, but their enamel Fortelius 1984; Rensberger and Koenigswald 1980). ultrastructure is clearly distinguishable even in This aspect is important because functionally small samples. The ungulates (both artiodactyls related changes in the enamel structure may be and perissodactyls) are characterized by Pat tern lost or reintroduced several times in phylogenetic 2 prisms along with the marsupials. Amongst all lineages and tend to obscure the main mammalian orders, the rodents represent the most evolutionary trends of a particular lineage. Such unified group on the basis of enamel ultrastruc­ modifications are not only significant at gross ture even though there are sharp and distinct levels, but also at levels of resolution by differences in enamel at the subordinal level. electron microscopy. The great variety of prism arrangements at various With the exception of some hypsodont arvi­ taxonomic levels hinders, in many ways, the colid rodents, in which the molar teeth are also delineation and record of phylogenetic lineages. rootless, the structure of the incisor is usually Though precise correspondence at lower taxo­ different from that of the molars. This diff­ nomic levels is usually lacking, there is a erentiation can first be observed in the incisors general congruity between evolutionary lineages of the ptilodontid multi tuberculate Mesodma based on enamel ultrastructure and those based on (Sahni 1979). The structure of the rodent incisor dental or cranial characteristics. This aspect is has been studied in better detail as it has been particularly true of the condition prevailing in considered to be of great taxonomic utility the multituberculates at the subordinal level (Korvenkontio 1934). According to Koenigswald (Carlson and Krause 1985) and agrees well with the (1980), the acquisition of mul tiserial Hunter­ situation prevailing in the Rodentia and the Schreger bands in the incisors of various phylo­ Primates, the only two other mammalian orders that genetical ly unrelated forms; Vombatus have been studied in any detail in this regard. (Marsupialia); Daubentonia (Primates); Myotragus Rodents are at present one of the best and Hippotragus (Artiodacty~a); and in_the studied examples of such a conformity: the two hystricomorph rodents is a functional adaptation. extant rodent enamel patterns, the multiserial (most hystricognaths) and the uniserial (myomorphs Materials and Methods and sciurornorphs) are clearly derivable from the pauciserial condition of the Early Tertiary The material on which the present study is rodents (Sahni 1980, 1984, 1985). When examined based was obtained from several sources:­ in greater detail, however, obvious discrepancies Professor RE Sloan, University of Minnesota at are seen to arise: the pauciserial condition is Minneapolis, provided a generous sample of the Bug found in al 1 Early Tertiary rodents even within Creek Ant hi 11 micrommal s of Late Cretaceous age. those families, for instance the Ctenodactylidae These include the multituberculates, Mesodma, and the Ischyromyidae which in the latter part of Cimexomys, Meniscoessus, Stygimys and Catopsalis; the Upper Tertiary acquired a multiserial and a the marsupial, Alphadon; the insectivores, uniserial enamel structure, respectively. The Cimolestes and Gypsonictop and the condylarthran, abruptness of this change from the pauciserial to Protungulatum. Professor TK Roy Chowdhary of the the extant rodent enamel conditions may either be Geological Studies Unit, Indian Statistical a signature of inadequate palaeontological samp- Institute, Calcutta, loaned samples of Triassic 1 ing during the Oligo-Miocene, or may reflect thecodonts and traversodontid cynodonts from the modifications of the muscle stresses generated in Gondwana Pranhita-Godavari Basin of peninsular the cranial and mandibular structure of hystrico­ India for enamel ultrastructure work. The other morphous and sciurognathous rodents. Wi1 son material investigated here is from
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