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Journal of Dental Research Journal of Dental Research http://jdr.sagepub.com/ The Origin and Evolution of the Human Dentition: A Palaeontological Review William K. Gregory J DENT RES 1920 2: 89 DOI: 10.1177/00220345200020011101 The online version of this article can be found at: http://jdr.sagepub.com/content/2/1/89.citation Published by: http://www.sagepublications.com On behalf of: International and American Associations for Dental Research Additional services and information for Journal of Dental Research can be found at: Email Alerts: http://jdr.sagepub.com/cgi/alerts Subscriptions: http://jdr.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav >> Version of Record - Mar 1, 1920 What is This? Downloaded from jdr.sagepub.com at COLUMBIA UNIV on September 26, 2014 For personal use only. No other uses without permission. THE ORIGIN AND EVOLUTION OF THE HUMAN DENTITION A PALAEONTOLOGICAL REVIEW WILLIAM K. GREGORY Columbia University and the American Museum of Natural History, New York City PART I. STAGES OF ASCENT FROM THE SILURIAN FISHES TO THE MAM- MALS OF THE AGE OF REPTILES CONTENTS I. Early stages of vertebrate evolution, seen in the fishes of the Paleozoic era.... 90 The ostracoderms of the Upper Silurian and Devonian ................... 90 The Devonian sharks ................................................ 91 The Devonian ganoids and lung-fishes .................................. 93 II. The emergence of four-footed, air-breathing vertebrates (primitive stegocephs) in the Upper Devonian and Lower Carboniferous ages ................. 97 III. The stem reptiles (Cotylosauria) and the mammal-like reptiles (Therapsida) of the late Paleozoic and early Mesozoic eras ............................ 100 IV. The Mesozoic mammals ................................................ 111 The Protodonta from the Upper Triassic of North Carolina ............... 111 Family Dromotheriidae ............. ................................ 111 Karoomys, a fore-runner of the Trituberculata from the Upper Triassic of South Africa.................................................... 114 Multituberculates from the Upper Triassic of England and Germany.. 115 Family Plagiaulacide ............ .................................. 115 Pachygeneleus, a possible ancestor of the multituberculates from the Storm- berg (Upper Triassic) of South Africa .............................. 117 The mammals of the Stonesfield Slate (Lower Jurassic)..................... 119 Order Triconodonta .............. ................................... 120 Family Triconodontide .......... .................................. 120 Order Trituberculata.... ...: .... 123 Family Amphitheriide ................. 123 Order Multituberculata? ................. 127 Stereognathus ................. 127 The South African mammal Tritylodon (Lower Jurassic) ..... .............. 128 Order Multituberculata? .............................................. 128 Family Tritylodontid.e............................................. 128 The mammals of the Purbeck and Morrison Beds (Upper Jurassic) .... .... .. 128 Order Multituberculata ................................................ 129 89 Downloaded from jdr.sagepub.com at COLUMBIA UNIV on September 26, 2014 For personal use only. No other uses without permission. 90 WILLIAM K. GREGORY Family Plagiaulacid .............................................. 129 Order Triconodonta ................................................. 130 Family Triconodontidze ............................................ 130 Order Triconodonta (?) .............................................. 133 Family Spalacotherildae ............................................ 133 Order Trituberculata ............................................... 136 Family Amphitherida ............................................ 136 Order Trituberculata ............................................... 139 Family Stylodontide ....................... ............... 139 Upper molars of Purbeck Trituberculata ................................ 141 Order Trituberculata ............................................... 147 Family Diplocynodontidw .......................................... 147 Summary and defence of the primitive characters of the Lower Jurassic Am- phitherium ...................................................... 149 V. The Late Mesozoic mammals (Upper Cretaceous) ....... .................. 151 Order Multituberculata............................................... 151 Family Polymastodontidae ................ .......................... 151 Order Marsupialia, Suborder Polyprotodontia ........................... 152 Family Cinolestida ........................................... 152 VI. The origin and evolution of the marsupials (Cretaceous to Recent) . 156 Suborder Polyprotodontia ....................... ...................... 157 Suborder Diprotodontia ............................................... 162 VII. Summary and conclusions for Part I ..................................... 166 I. EARLY STAGES OF VERTEBRATE EVOLUTION, SEEN IN THE FISHES OF THE PALEOZOIC ERA The later chapters in the evolution of the human dentition can only be understood in the light of the whole story, so it is advisable to begin at the beginning, or as near to the beginning as the palhonto- logical record extends. THE OSTRACODERMS OF THE UPPER SILURIAN AND DEVONIAN The problem of the origin of the vertebrates we must necessarily ignore, because the innumerable answers to it cannot be tested by an appeal to the paleontological record. We can, however, begin at a very early stage, represented by the ostracoderms of Silurian and Devonian times, which are on the whole the most primitive of all known chordates. It is true that the modern Amphioxus and the cyclostomes have acquired a world-wide reputation as primitive chordates but, in the absence of paleontological evidence, their real status is still unsettled; and, in their present conditions, they teem Downloaded from jdr.sagepub.com at COLUMBIA UNIV on September 26, 2014 For personal use only. No other uses without permission. EVOLUTION OF THE HUMAN DENTITION 91 with specializations which conceal their primitive characters. The Paleozoic ostracoderms, on the other hand, afford definite information of a pregnathostome stage of evolution which is lower and, as a whole, older, than any other group-of vertebrates. Although much excellent material is preserved in the museums of the world, none of these lowly fish-like chordates shows the least evidence of having attained "gill- arch jaws" of the normal vertebrate type. Some of them had a capacious mouth and, in the latest and more specialized ones (Bothrio- lepis, Pierichthys), the ossified dermal plates around the slit-like mouth acquired jagged edges and may have functioned in the prehension of food; but no known ostracoderm had either teeth or calcified oral and bronchial arches. For the most part they probably fed on small organisms found in the mud on the bottom of the streams in which they lived. Possibly they may have extracted the nutriment from FIG. 1. Birkenia elegans, A PRIM E FISH FROM TEE UPPER SILURIAN OF SCOTLAND Illustrating a stage of evolution before the formation of teeth and jaws. Natural size. After Traquair. the mud by ciliary tracts, as in Amphioxus and the larval lamprey. One of them (Birkenia) was shaped somewhat like Amphioxus and could perhaps dart about freely in the water, but most of them were depressed in form and probably lived on the bottom. None of them had attained the swift-swimming, predatory form, armed with strong teeth, which, there is much evidence to indicate, was the starting-point for the development of vertebrates higher in the scale. THE DEVONIAN SHARKS The numerous group of sharks, dating back to the Upper Silurian but well known only in the Devonian and later periods, are with good reason regarded as primitive in their class characters, since they all have the primitive vertebrate ground-plan of the endoskeleton. While most of the known ostracoderms had over-emphasized the exo- THE JOURNAL OF DENTAL RESEARCH, VOL. II, NO. 1 Downloaded from jdr.sagepub.com at COLUMBIA UNIV on September 26, 2014 For personal use only. No other uses without permission. 92 WIKLIAMK. GREGORY skeleton, the elasmobranchs very early built up an endoskeleton by precipitating calcium salts in the connective tissues and septa, which form a nucleus or core of the endoskeleton. They were active, preda- tory, fishes and developed further than did the ostracoderms the method of moving by rhythmic contraction of the muscle segments, FIG. 2. UNDER-SIDE OF THE HEAD OF Cladoselache fyleri, A. PRIMITIVE SHARK PROM THE UPPER DEVONIAN OF OHIO Showing the primary jaws, homologous with the bronchial arches, and bearing teeth on the skin inside the mouth. The eyes are surrounded by rows of sclerotic plates, which are homologous with the teeth and with the shagreen denticles of the skin. After Dean. or myomeres, arranged on either side of the mid-line. The develop- ment of the endoskeleton stiffened the median axis against the thrust of the myomeres and made possible the assumption of aggressive habits. In the earliest known sharks the mouth is supported by cartilagi- nous jaws of the "gill-arch" type. Even modern sharks retain much Downloaded from jdr.sagepub.com at COLUMBIA UNIV on September 26, 2014 For personal use only. No other uses without permission. EVOLUTION OF THE HUMAN DENTITION 93 evidence tending to show that the cartilage jaws are serially homolo- gous
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