A Companion to Dental Anthropology

By Joel D. Irish and G. Richard Scott

Companion to Dental Anthropology presents a collection of original readings addressing all aspects and sub-disciplines of the field of dental anthropology—from its origins and evolution through to the latest scientific research.

• Represents the most comprehensive coverage of all sub-disciplines of dental anthropology available today
• Features individual chapters written by  experts in their specific area of dental research
• Includes authors who also present results from their research through case studies or voiced opinions about their work
• Offers extensive coverage of topics relating to dental evolution, morphometric variation, and pathology

• Language: English
• Publisher: Wiley
• Release date: Oct 21, 2015
• ISBN: 9781118845370

Book preview

CHAPTER 1

Introduction to Dental Anthropology

Joel D. Irish and G. Richard Scott

The Foreword by Clark Larsen nicely addresses the content and purpose of this volume. Beyond that, the reader can look at the table of contents to ponder the topic of each chapter without us having to (re)state the obvious. So we will keep this chapter short and to the point. Think of it primarily as reading instructions, or at least suggested guidelines, to get the most out of this book.

What you hold before you, whether tactilely or virtually, is a text/reader/reference book or, if all else fails, paperweight, on the subject of all things teeth and tooth related. That is, we are talking teeth from the perspective of dental anthropologists. So what is the definition of dental anthropology? There is no need to reinvent the wheel when we have the internet. According to the Medical Dictionary for the Dental Professions (2012), it is [a] branch of physical anthropology concerned with the origin, evolution, and development of dentition of primates, especially humans, and to the relationship between primates’ dentition and their physical and social relationships. That definition works.

The reasons why teeth are studied are numerous. For one thing, they are made up of the two hardest tissues in the body, so are the most likely to be preserved in the fossil and archaeological records. Think about it. Many fossil primate species are defined based on teeth. Given that teeth are the only components of the skeleton to come into direct contact with the environment, we can learn about diet, health, and even certain cultural factors of individuals and groups. Throw in the facts that tooth size and shape have high genetic components in expression and that, unlike bone, this expression does not remodel itself during life (other than via crown wear and pathology), and we have an excellent source to estimate biological origins and relationships across time and space. There is much more, but you can read about that in the remaining 30 chapters of this book.

Who You Are, and Our Suggestions

At this point, we would like to know something about you so we can offer suggestions on getting the most out of this volume. Are you a dental anthropology beginner, such as an upper-division or graduate student taking an initial course in this branch of physical anthropology, or an interested layperson? If so, then you may be using this volume as a textbook or secondary class source. In that case, we suggest you read the rest of this chapter and then, importantly, skip directly to Chapter 7. The latter contains the fundamental terms and terminology needed to fully grasp the content of each remaining chapter. Pay special attention to the differences in tooth class designations; they vary among studies of primate, fossil hominin, and modern human dentitions (e.g., a lower first premolar may be labeled as P3, LP3, or LP1). After learning the basics, flip back to Chapter 2 and read the rest of the volume in order.

Are you an advanced graduate student, post doc, or a newly minted professional who wants to bone up on dental anthropology or focus on specific topics of interest to your own research or teaching? Then think of this book as a reader/reference, where you can reacquaint yourself with the basics, and get up to speed on new methods, areas of research, and references, and/or gain some personal insight from experts in the field. We suggest that you skim the rest of this chapter and start right in on Chapter 2. You should know enough to get by regarding dental terms, and besides, it will all come back to you as you peruse the material.

Or are you a dental professional, with strong academic or perhaps clinical experience, who is inclined to use this volume for reference purposes? If so, then you can probably skip the rest of this chapter, as well as Chapters 7–8, and perhaps any other chapter(s) covering material on which you are an expert. Then go ahead and cherry pick the sections and chapters that you find to be of interest.

Volume Organization and Content

This section heading may seem to imply that we are going to restate the obvious, but as I have already said, we are not. Rather, we are chiefly providing some rationale for the order and structure of chapters. This book is organized exactly in the way the first editor has taught dental anthropology as an upper-division/graduate-level university course for the past couple of decades. That is, it proceeds in a more or less sequential manner, both procedurally and temporally:

Part I: Context (see Chapter 2) provides a diachronic review of where this subdiscipline of physical (or biological) anthropology came from, and a list of some of the major players involved.

Part II: Dental Evolution (Chapters 3–6) covers the origins and variation of teeth in an evolutionary perspective—from their first appearance in non-primates (which, according to the above definition is actually outside the realm of dental anthropology), to variation among non-human primates and fossil hominins, including pre-modern Homo sapiens.

Part III: The Human Dentition (Chapters 7–9) introduces the terminology, anatomical structures, and functions of the teeth and supporting structures that are necessary for dental anthropological research.

Part IV: Dental Growth and Development (Chapters 10–13) provides key information on dental genetics, crown and root formation, eruption, final form, and variation therein.

Part V: Dental Histology from the Inside Out (Chapters 14–16) continues on from the previous part by providing detail on the structure and material properties of tissues that comprise the teeth.

Part VI: Dental Morphometric Variation in Populations (Chapters 17–20) shows how tooth shape and size can be quantified in samples to estimate intra- and inter-population variation and affinities, that is, involving the big picture in anthropology.

Part VII: Dental Morphometric Variation in Individuals (Chapters 21–24) follows up on the preceding part by focusing on individual characterization and differences; forensic applications are essential, but the ability to reconstruct life histories, as in an archaeological context, is necessary as well.

Part VIII: Dental Health and Disease (Chapters 25–28) covers the importance of dental pathology, like caries and enamel hypoplasia, for understanding life histories (including diet, indications of stress, etc.) at both the population and individual levels.

Part IX: The Future of Dental Anthropology (Chapters 29–31) shows a glimpse of where the subdiscipline is headed, by describing state-of-the-science approaches to assess the link among morphological variants, the use of chemical analyses, and an overview of non-destructive techniques to image the inside of teeth. All in all, the future looks bright.

Expertise and a Personal Touch

No matter whether you use this book as a text, reader, reference, or something else, it is important to know that each chapter was written by an expert (or experts) in that area of dental research. For example, do you want to know about the origins and early evolution of teeth? The author of Chapter 3, Peter Ungar (2010), literally wrote the book on the subject. The same goes for Peter Lucas (2004) concerning jaw function in Chapter 9, and the authors of Chapter 16 on enamel structure; in the latter case Daniel Antoine (2001) wrote his entire PhD thesis on enamel(!) and Simon Hillson (1996, 2000, 2014) has no fewer than three books in which this tissue is detailed. However, a quick perusal of the references in these chapters shows that the authors do much more than write books, as the many publications in peer-reviewed professional journals indicate. Along these lines, check out all the articles by the first author of Chapter 13 on tooth classes and the field concept, Grant Townsend (e.g., among others, 2005, 2009), and Chapter 27’s author, Debbie Guatelli-Steinberg (e.g., among others, 2004, 2012) on dental stress indicators (enamel hypoplasia). Each of the remaining authors could be singled out for kudos in this manner, but there are just too many. So please make sure to look through their short biographies at the beginning of this book to get an idea of who they are.

Now think about this. If you were tasked with creating a textbook (or reader/reference) on any topic of your choosing, would you not want it to be written by the best and the brightest? Ordinarily, textbooks are written by one or a few authors. Such an approach is great for chapter consistency in writing style, format, and so forth. Nevertheless, those few authors are likely not experts on every topic in the book. For example we—your kindly book editors—are dental morphologists, and we also know a little about other aspects of dental anthropology. However, we are not experts in primatology, paleoanthropology, genetics, forensics, or histology. So we got the experts to write about what they know best, while doing a light touch editing job to strive for consistency across chapters in writing style, format, and so forth. But wait, there is more. Beyond presenting material specific to each chapter topic, many authors provided findings from their own research, maybe a case study or two, and/or voiced opinions about their or other areas of dental anthropology. As such, they personalized their chapters so you can see that there is more to research than just the same old empirical or scientific approach.

Now on to Chapter 2 (or Chapter 7)

So, if you have read this far you must not be a dental expert, unless you simply wanted to see how everything turned out in the end. In any event, sit back, tactilely or virtually grab this volume firmly, and start reading (and above all, learning). The 40+ authors put a lot of work into their chapters. We hope you enjoy reading them as much as we did when putting this edited volume together.

REFERENCES

Antoine, D. (2001). Evaluating the Periodicity of Incremental Structures in Dental Enamel as a Means of Studying Growth in Children from Past Human Population. PhD thesis, University College London.

Guatelli-Steinberg, D., C.S. Larsen, and D.L. Hutchinson (2004). Prevalence and the Duration of Linear Enamel Hypoplasia: A Comparative Study of Neandertals and Inuit Foragers. Journal of Human Evolution, 47: 65–84.

Guatelli-Steinberg, D., R.J. Ferrell, and J. Spence (2012). Linear Enamel Hypoplasia as an Indicator of Physiological Stress in Great Apes: Reviewing the Evidence in Light of Enamel Growth Variation. American Journal of Physical Anthropology, 148: 191–204.

Hillson, S. (1996). Dental Anthropology. Cambridge: Cambridge University Press.

Hillson, S. (2000). Teeth, 2nd edition. Cambridge: Cambridge University Press.

Hillson, S. (2014). Tooth Development in Human Evolution and Bioarchaeology. Cambridge: Cambridge University Press.

Lucas, P.W. (2004). Dental Functional Morphology: How Teeth Work. Cambridge: Cambridge University Press.

Medical Dictionary for the Dental Professions. (2012). Dental Anthropology. Retrieved December 7, 2014 from http://medical-dictionary.thefreedictionary.com/dental+anthropology.

Townsend, G., Harris, E.F., Lesot, H., Clauss, F., and Brook, A.H. (2009). Morphogenetic Fields within the Human Dentition: A New, Clinically Relevant Synthesis of an Old Concept. Archives of Oral Biology, 54S: S34–S44.

Townsend, G., Richards, L., Hughes, T., Pinkerton, S., and Schwerdt, W. (2005). Epigenetic Influences May Explain Dental Differences in Monozygotic Twin Pairs. Australian Dental Journal, 50: 95–100.

Ungar, P.S. (2010). Mammal Teeth: Origin, Evolution, and Diversity. Baltimore, MD: Johns Hopkins University Press.

CHAPTER 2

A Brief History of Dental Anthropology

G. Richard Scott

Several papers have been written on the history of dental anthropology from both world (Dahlberg 1991; Scott 1997; Scott and Turner 2008) and regional (Brown 1992, 1998; Kosa 1993) perspectives. To avoid duplication, this brief history adopts a different approach. It brackets developments in three periods: foundations—the period preceding the classic edited volume Dental Anthropology (Brothwell 1963); development—from 1963 to Advances in Dental Anthropology (Kelley and Larsen 1991); and maturation—from 1991 to the present. The emphasis is on the various ways anthropologists have used teeth to further our understanding of primate and human evolution, variation, health, and behavior.

Foundations (Nineteenth Century to 1963)

The role of the physical or biological anthropologist is to describe biological variation and explain it in terms of adaptation, evolution, and history. In pre-Darwinian times, anthropologists focused largely on racial variation and classification. Teeth played no role in the early classifications of Blumenbach, Cuvier, and others, who focused on externally visible physical characteristics like skin, hair, and eye color and form, and skull types.

In the nineteenth century, a handful of anthropologists started exploring the utility of teeth and their role in understanding human health and behavior (e.g., P. Broca and crown wear, L.H. Mummery and oral pathology) and variation (W.H. Flower and tooth size). More substantive developments were taking place in paleontology, where Richard Owen, Edward Drinker Cope, and Henry Fairfield Osborn were laying the foundations of comparative odontology (Peyer 1968).

With the post-Darwinian acceptance that humans were primates and their closest relationship was to apes, more researchers started paying heed to tooth size and morphology in primates, the still limited array of fossil hominins, and recent human populations. For a broad-scale approach, a major contribution was The Origin and Evolution of the Human Dentition by William King Gregory (1922). Although Gregory discussed tooth morphology in humans, he minimized the significance of variation among the geographic races, mostly because he was familiar with a limited number of variables like upper and lower molar cusp number, Carabelli’s trait, and shovel-shaped incisors. Around this time, Hrdlička (1920) wrote the classic paper on shovel-shaped teeth, wherein he noted the close similarity between Native Americans and north Asians and their contrast to European and African populations.

Hrdlička was among the first researchers to appreciate the extent of dental variation among world populations because he had access to thousands of skeletal remains at the US National Museum of Natural History. Other significant contributions in this period were regional in scope. Classic examples include the characterization of the Australian Aboriginal dentition by T.D. Campbell (1925) and the Bantu by J.C. Middleton Shaw (1931). Slightly later, P.O. Pedersen (1949) and C.F.A. Moorrees (1957) produced classic monographs on the dentitions of East Greenland Eskimos and Aleuts, respectively. These various authors, as well as the many referenced below, will be cited throughout this volume.

The monographs already noted, plus significant journal articles on the Indians of Pecos Pueblo (Nelson 1938) and Texas Indians (Goldstein 1948), provided descriptions of tooth size, morphology, dental pathologies, and crown wear. Comparative data were still limited. During this period, a seminal paper by R.W. Leigh (1925) focused on the contrasts in dental health between tribes who lived in different environments and practiced diverse subsistence activities. Leigh was among the first to demonstrate clearly the impact of agriculture (and agricultural practices such as grinding grain) on the human dentition.

Some researchers, including A.A. Dahlberg (Figure 2.1a), focused primarily on tooth morphology. One of his most important early contributions was The Changing Dentition of Man (Dahlberg 1945), in which he laid out the fundamental principles of morphogenetic fields in the human dentition, as a follow-up to the field theory papers of P.M. Butler (1937, 1939). Following the modern evolutionary synthesis of the early 1940s, anthropologists started thinking more in terms of genes than types, stimulating papers such as Genetic Analysis of Racial Traits of the Teeth (Lasker 1950), and subsequent attempts to decipher the modes of inheritance of common morphological traits (cf. Kraus 1951, 1957; Kraus, Wise, and Frei 1959; Lasker 1957).

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Figure 2.1 (a) Albert A. Dahlberg (right) and the first editor (left) at Arizona State University. (b) Christy G. Turner II (left) and the second editor (right) at Scott’s home in Fairbanks, Alaska. Both photographs taken a long, long time ago.

By the mid-1950s, A.A. Dahlberg (1956) determined that the systematic study of human dental variation was impeded by a lack of standardization. Toward that end, he developed a series of plaster plaques that showed ranked expressions for key morphological traits of the human dentition, including shovel-shaped incisors, the hypocone, Carabelli’s trait, and the protostylid. These were widely adopted and observations based on these standards appeared in many articles for the next two decades.

Prior to 1963, studies of fossil hominin teeth were mostly descriptive in nature. Two key monographs did appear during this time: "The Dentition of Sinanthropus pekinensis" (Weidenreich 1937) and The Dentition of the Australopithecinae (Robinson 1956). Although observations had been made on Neanderthals, there was no attempt to systematically describe and compare their teeth, beyond taurodont molars, shoveling, and pronounced basal cingula. In general, fossil hominin and primate dentitions were not subject to the kinds of intensive studies that have characterized the past 50 years.

Development (1963–1991)

An important landmark in the history of the field was the publication of Dental Anthropology (Brothwell 1963). This edited work emanated from the Symposia of the Society for the Study of Human Biology. In the volume there are 15 papers: 3 dealt with primate teeth, 1 with fossil hominin teeth, and 11 with recent human populations. For the middle of the twentieth century, those contributions were proportional to research that fell under the heading of dental anthropology.

Following the publication of Dental Anthropology, the field showed significant growth. Between 1963 and 1991, numerous articles and dissertations dealt with various aspects of the human dentition. The period saw major efforts directed at developing standards for the study of tooth crown and root morphology (e.g., Turner 1970; Turner, Nichol, and Scott 1991), crown wear (Molnar 1971, 1972; Scott 1979; Smith 1984; Lovejoy 1985), and markers of developmental stress, especially linear enamel hypoplasia (Goodman and Armelagos 1985; Goodman and Rose 1990). Studies on the oral health of past and present populations received increasing attention, especially the negative impacts of agriculture on dental caries (Hillson 1979; Turner 1979; Lukacs 1981, 1982). The International Symposium of Dental Morphology, which first met in Copenhagen, Denmark in 1965, became a regular conference fixture and produced five edited volumes that highlighted research in dental ontogeny, genetics, and variation (Pedersen, Dahlberg, and Alexandersen 1967; Dahlberg 1971; Butler and Joysey 1978; Kurtén 1978; Russell, Santoro, and Sigogneau-Russell 1988).

Starting from the foundations laid by A.A. Dahlberg, the study of tooth crown and root morphology expanded exponentially during this period. With an ever-increasing list of nonmetric crown and root traits, C.G. Turner II (Figure 2.1b) and his students initiated studies in populations throughout the world. The three-wave model for the peopling of the Americas developed by Turner (1971, 1983, 1984, 1985, 1986; Greenberg, Turner, and Zegura 1986) brought dental morphology to the attention of the broader archaeological, biological, and linguistic communities. Regional studies demonstrated that tooth morphology was a sensitive indicator of population affinity below the level of continental populations (Scott 1973; Scott and Dahlberg 1982; Scott et al. 1983; Harris 1977). Focusing on Asia, Turner (1976) introduced the revolutionary notion that the prehistoric inhabitants of the Japanese archipelago, the Jomon, were ancestral to the Ainu of Hokkaido and Sakhalin and not the modern population of Japan. This idea was substantiated by analysis of odontometric data on Jomon, Ainu, Japanese, and other Asian populations (Brace and Nagai 1982). Turner (1987, 1990) demonstrated that there were two major dental patterns in Asia; that is, Sinodonty and Sundadonty. The Sinodont pattern characterized populations in northeast Asia and derived New World populations, while the Sundadont pattern, with its focus in southeast Asia, characterized derived populations in Polynesia and Micronesia. Australian Aborigines did not fall neatly within either pattern, although Turner (1990, 1992) suggested that they were proto-Sundadonts.

Early studies of hominin fossil teeth provided detailed descriptive data on the crowns and roots of individual fossils. The problem was a lack of comparative standards. In the 1980s, Bernard Wood and collaborators made the first systematic morphological observations on australopithecines and early Homo (Wood and Abbott 1983; Wood and Engleman 1988; Wood and Uytterschaut 1987; Wood, Abbott, and Graham 1983; Wood, Abbott, and Uytterschaut 1988). With larger fossil samples, it became possible to characterize taxa in terms of trait frequencies, a dramatic improvement over individual fossil descriptions. Bermúdez de Castro (1986, 1988) initiated comparable research on the dentition of Middle Pleistocene hominins from Spain.

Odontometric studies on fossil hominins saw significant advances during this period. Authors who made particular contributions include C.L. Brace (1967; Brace and Mahler, 1971), M. Wolpoff (1971), and D.W. Frayer (1978), who focused on metric trends in hominin dental evolution from the australopithecines to the modern Homo sapiens of the Mesolithic. The authors concentrated on buccolingual (BL) and mesiodistal (MD) diameters, but at least for these variables, comparative data were available to provide a perspective on differences and trends in tooth size and molar size sequence polymorphisms. To evaluate the evolution of sex dimorphism in hominin and primate evolution, Oxnard (1987) applied multivariate morphometric analysis to tooth size variables of apes, fossil primates, and australopithecines.

Daris Swindler (1976) provided an excellent monograph, The Dentition of Living Primates, on non-human primate teeth. This volume covered dozens of species, with illustrations, descriptions of dietary behavior, eruption sequences, crown morphology, and tables with summarized data on MD and BL dimensions, where descriptive statistics were based on small samples and not simply individual primates. Other researchers, including M.C. Dean, R. Kay, W.G. Kinzey, J. Sirianni, A. Rosenberger, B.H. Smith, and others, developed new insights into the variation and development of primate teeth. Topics explored included tooth combs in prosimians, developmental rates, growth disturbances, enamel thickness, canine honing, microwear analysis, and the interaction of crown morphology and dietary behavior. This foundation set the stage for problem-oriented research on tooth form, function, and evolution for diverse primate species.

In 1985, during the annual meeting of the American Association of Physical Anthropologists (AAPA) in Knoxville, TN, researchers who focused on teeth listed more than 150 anthropologists and dentists with teaching and research interests in the field. This resulted in the formation of a Dental Anthropology Group (DAG). At the 1986 AAPA meeting in Albuquerque, NM, the Dental Anthropology Association (DAA) was founded (İşcan, 1989). In that same year, the association started publishing the Dental Anthropology Newsletter. From these humble beginnings, the DAA would see extensive growth over the next 25 years. Along with other specialty organizations (e.g., Paleopathology, Human Biology), the DAA began holding formal meetings in conjunction with the annual meeting of the AAPA.

Maturation (1991–present)

Advances in Dental Anthropology (Kelley and Larsen 1991) marked a significant shift in approaches and practitioners. Many of the pioneers of dental anthropology were dentists and oral biologists. Although these researchers are still a significant component of the field, many more anthropologists (Figure 2.2) now have a primary focus on the teeth of primates, fossil hominins, and recent humans—both archaeological and living.

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Figure 2.2 Many of the dental anthropologists referenced in this chapter and elsewhere in the volume, at the Albert Dahlberg Memorial Symposium on Dental Morphology and Evolution, 1995 meeting of the American Association of Physical Anthropologists, Oakland, California. Front row (l–r): A.M. (Sue) Haeussler, Thelma Dahlberg, Patricia Smith. Middle row (l–r): Yaşar Işcan, Andrea Cucina, Lassi Alvesalo, Grant Townsend, John Mayhall, John Lukacs, Simon Hillson, Tasman Brown. Back row (l–r): Donald Morris, Diane Hawkey, Richard Scott, Phillip Walker, Edward Harris, Joel Irish, Yuji Mizoguchi.

In 2000, the Dental Anthropology Newsletter morphed into Dental Anthropology; articles submitted for publication now underwent peer review and the quality of the journal increased significantly. Several individuals, including S.R. Loth (Florida Atlantic University), A. (Sue) M. Haeussler (Arizona State University), Edward F. Harris (University of Tennessee), and, currently, Christopher Schmidt (University of Indianapolis) guided the evolution of this publication from a simple newsletter to a quality journal distributed in both paper and PDF forms, with high-quality color illustrations.

For the past two decades, Cambridge University Press has provided a high-profile venue for the publication of numerous books on teeth that can be used as textbooks and serve as scientific references in dental anthropology. These volumes include Human Adult Odontometrics (Kieser 1991), Dental Anthropology (Hillson 1996), The Anthropology of Modern Human Teeth: Dental Morphology and Its Variation in Recent Human Populations (Scott and Turner 1997), Development, Function and Evolution of Teeth (Teaford, Smith, and Ferguson 2000), Primate Dentition: An Introduction to the Teeth of Non-Human Primates (Swindler 2002), Dental Functional Morphology: How Teeth Work (Lucas 2004), Technique and Application in Dental Anthropology (Irish and Nelson 2008), Anthropological Perspectives on Tooth Morphology: Genetics, Evolution, Variation (Scott and Irish 2013), and Tooth Development in Human Evolution and Bioarchaeology (Hillson 2014). Other significant volumes appearing after 1991 include Dental Anthropology: Fundamentals, Limits, and Prospects (Alt, Rosing, and Teschler-Nicola 1997), Human Dental Development, Morphology, and Pathology: A Tribute to Albert A. Dahlberg (Lukacs 1998), and Dental Perspectives on Human Evolution: State of the Art Research in Dental Paleoanthropology (Bailey and Hublin 2007). In addition, more volumes emanated from the meetings of the International Symposium on Dental Morphology (Smith and Tchernov 1992; Radlanski and Renz 1995; Mayhall and Heikkinen 1999; Zadzinska 2005; Koppe et al. 2009).

The number of papers published in the past 20 years on topics directly or indirectly related to dental anthropology is staggering. To do this work justice would require a literature review comparable to the annual review article on dental anthropology written over 25 years ago (Scott and Turner 1988). To set the stage for developments in the field that post-date this volume, broad areas of research include the following:

Evo-Devo—experimental research on the role of genes and proteins in dental ontogeny, including the role specific proteins play in controlling tooth size and cusp number.

Genomics—integration of human genomics with visible dental phenotypes.

Geometric morphometrics—detailed quantitative analysis of key landmarks, moving odontometrics beyond MD and BL diameters.

Micro-CT—a non-destructive technique that allows a detailed three-dimensional image of the enamel–dentine junction, especially useful in studying rare fossil hominin teeth.

Microwear analysis—advanced methods for microscopic examination of crown wear.

Dental calculus—underappreciated yet ubiquitous material in the dentition that has recently yielded information on plant phytoliths, starches, microbial DNA, and stable carbon and nitrogen isotopes.

Future Directions

All of the areas just listed are applicable to the array of subjects on which dental anthropologists focus: fossil and living non-human primates, fossil hominins, archaeologically derived skeletal remains, and modern human populations. Increasingly sophisticated methods of quantitative analysis (e.g., GIS applied to tooth crown surfaces) and representation (e.g., 3D scans) allow researchers to pursue old lines of inquiry with far better tools and lead to new lines of inquiry not yet anticipated. To illustrate, while conducting research on the Greenlandic Norse at the Panum Institute in Copenhagen in 1986, P.O. Pedersen and I bemoaned the fact that dental calculus made observations on crown morphology and size difficult, but did not itself serve as useful research material. We failed to anticipate the many lines of research taken up in the last decade that employ our former nemesis—dental calculus—to address a range of anthropological problems. Time will tell what new materials and methods will move dental anthropology forward over the next decades. However, for now, the remaining chapters in this volume, beginning with the origins of teeth and ending with the current state of the science and future directions, provide a good indication of where we are today, and where we are going in the not-too-distant future.

REFERENCES

Alt, K.W., F.W. Rosing, and M. Teschler-Nicola (eds.) (1997). Dental Anthropology: Fundamentals, Limits, and Prospects. Vienna: Springer-Verlag.

Bailey, S.E., and J.-J. Hublin (eds.) (2007). Dental Perspectives on Human Evolution: State of the Art Research in Dental Paleoanthropology. Berlin: Springer.

Bermúdez de Castro, J.M. (1986). Dental Remains from Atapuerca (Spain). I. Metrics. Journal of Human Evolution, 15: 265–287.

Bermúdez de Castro, J.M. (1988). Dental Remains from Atapuerca/Ibeas (Spain). II. Morphology. Journal of Human Evolution, 17: 279–304.

Brace, C.L. (1967). Environment, Tooth Form, and Size in the Pleistocene. Journal of Dental Research, 46: 809–816.

Brace, C.L., and P.E. Mahler (1971). Post-Pleistocene Changes in the Human Dentition. American Journal of Physical Anthropology, 34: 191–203.

Brace, C.L., and M. Nagai (1982). Japanese Tooth Size, Past and Present. American Journal of Physical Anthropology, 59: 399–411.

Brothwell, D.R. (ed.) (1963). Dental Anthropology. New York: Pergamon Press.

Brown, T. (1992). Dental Anthropology in South Australia. Dental Anthropology Newsletter, 6: 1–3.

Brown, T. (1998). A Century of Dental Anthropology in South Australia. In J.R. Lukacs (ed.), Human Dental Development, Morphology, and Pathology, edited by J.R. Lukacs. Eugene, OR: University of Oregon Anthropological Papers, No. 54, pp. 421–441.

Butler, P.M. (1937). Studies of the Mammalian Dentition. I. The Teeth of Centetes ecaudatus and Its Allies. Proceedings of the Zoological Society of London, B107: 103–132.

Butler, P.M. (1939). Studies of the Mammalian Dentition: Differentiation of the Post-Canine Dentition. Proceedings of the Zoological Society of London, B109: 1–36.

Butler, P.M., and K.A. Joysey (eds.) (1978). Development, Function and Evolution of Teeth. New York: Academic Press.

Campbell, T.D. (1925). The Dentition and Palate of the Australian Aboriginal. Adelaide: Hassell Press.

Dahlberg, A.A. (1945). The Changing Dentition of Man. Journal of the American Dental Association, 32: 676–690.

Dahlberg, A.A. (1956). Materials for the Establishment of Standards for Classification of Tooth Characters, Attributes, and Techniques in Morphological Studies of the Dentition. Chicago, IL: Zollar Laboratory of Dental Anthropology, University of Chicago (mimeo).

Dahlberg, A.A. (ed.) (1971). Dental Morphology and Evolution. Chicago, IL: University of Chicago Press.

Dahlberg, A.A. (1991). Historical Perspective of Dental Anthropology. In M.A. Kelly and C.S. Larsen (eds.), Advances in Dental Anthropology. New York: Wiley-Liss, pp. 7–11.

Frayer, D.W. (1978). Evolution of the Dentition in Upper Paleolithic and Mesolithic Europe. Lawrence, KS: University of Kansas Publications in Anthropology, No. 10.

Goldstein, M.S. (1948). Dentition of Indian Crania from Texas. American Journal of Physical Anthropology, 6: 63–84.

Goodman, A.H., and G.J. Armelagos (1985). Factors Affecting the Distribution of Enamel Hypoplasias within the Permanent Dentition. American Journal of Physical Anthropology, 68: 479–493.

Goodman, A.H., and J.C. Rose (1990). Assessment of Systemic Physiological Perturbations from Dental Enamel Hypoplasias and Associated Histological Structures. Yearbook of Physical Anthropology, 33: 59–110.

Greenberg, J.H., C.G. Turner II, and S. Zegura (1986). The Settlement of the Americas: A Comparison of the Linguistic, Dental, and Genetic Evidence. Current Anthropology, 24: 477–497.

Gregory, W.K. 1922. The Origin and Evolution of the Human Dentition. Baltimore, MD: Williams and Wilkins.

Harris, E.F. (1977). Anthropologic and Genetic Aspects of the Dental Morphology of Solomon Islanders, Melanesia. PhD thesis, Arizona State University, Tempe.

Hillson, S.W. (1979). Diet and Dental Disease. World Archaeology, 11: 147–162.

Hillson, S.W. (1996). Dental Anthropology. Cambridge: University of Cambridge Press.

Hillson, S.W. (2014). Tooth Development in Human Evolution and Bioarchaeology. Cambridge: Cambridge University Press.

Hrdlička, A. (1920). Shovel-Shaped Teeth. American Journal of Physical Anthropology, 3: 429–465.

Irish, J.D., and G. Nelson (eds.) (2008). Technique and Application in Dental Anthropology. Cambridge: Cambridge University Press.

İşcan, M.Y. (1989). The Emergence of Dental Anthropology. American Journal of Physical Anthropology, 78: 1.

Kelley, M.A., and C.S. Larsen (eds.) (1991). Advances in Dental Anthropology. New York: Wiley-Liss.

Kieser, J.A. (1991). Human Adult Odontometrics. Cambridge: Cambridge University Press.

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PART II

Dental Evolution

CHAPTER 3

Origins and Functions of Teeth: From Toothed Worms to Mammals

Peter S. Ungar

Many dental anthropologists are fixated on a single species, Homo sapiens, or, at most, the few hundred that make up our biological order, Primates. And when we think about dental evolution, we usually consider only the hominins or, at most, the primate fossil record. However, there is so much more to the story of teeth. There are tens or even hundreds of thousands of species alive today with teeth, depending on how you define them. Teeth like ours first appeared about half a billion years ago, and we can trace their evolution from early vertebrates to early mammals and beyond. This chapter introduces some of the key players, milestones, and trends; for a more thorough discussion see Ungar (2010, 2014). The larger fossil record offers important context for dental anthropologists to better understand and appreciate the form, function, and evolution of human teeth.

The Earliest Teeth

If we define teeth as hardened structures in or near the mouth that serve in food acquisition and processing, hundreds of thousands of species have them. Some, like the chelicerae of spiders and radulae of mollusks, are chitinous ribbons. Others, like the curved, triangular parts of Aristotle’s Lantern in sea urchins, are calcium carbonate elements. Yet others, like the rasping hooks of hagfishes and the piercing spikes of lampreys, are keratinous structures. The variety of teeth not made from calcium phosphate (like humans’ are) is staggering. In fact, the number of species with hardened feeding structures completely unrelated to ours is an order of magnitude greater than the number with teeth like ours. These myriad forms are fascinating in their own right, and each is an important example of how natural selection has met the challenges of feeding in different species (Ungar 2010, 2014). If our goal is to understand evolution in the broader sense, Darwin’s endless forms most beautiful and most wonderful, dental anthropologists should not ignore the hundreds of thousands of species with hardened feeding structures in their mouths, let alone the tens of thousands with teeth like humans’.

If our goal is to understand the origin and evolution of our own dentition, we might define teeth in terms of homology, as structures derived from a common ancestor. By this definition, only vertebrates have real teeth. While spiders, mollusks, and sea urchins all have three primary layers of cells that form during embryonic development—that is, ectoderm, mesoderm, and endoderm (see Chapter 11)—only vertebrates have a fourth, the neural crest (Hall 2000). The odontoblasts (Chapter 15) that make the dentin cores of our teeth are derived from neural crest cells. Since it takes a neural crest to make our kind of teeth, only vertebrates can have them. So we must limit our search for the origin of teeth, as we know them, to early members of our own subphylum, the Vertebrata.

Models for the Earliest Teeth

Where to begin? To search for the earliest teeth, we need to understand how they first evolved. This subject has captured the imagination of dental researchers for a very long time (see Donoghue 2002). They are often linked with scales and the appearance of the jaw. Most current ideas trace to Ørvig (1967, 1977), who argued that teeth are differentiated from odontodes, dentin structures that enclosed internal pulp cavities housing blood vessels and attached to bases of bone or cartilage. The small, tooth-like placoid scales, or denticles, that give shark skin its sandpaper-like texture are a good model. The basic idea is that teeth were co-opted from placoid denticles around the margins of the mouth when the jaws first evolved. If so, teeth probably formed when embryonic ectoderm pushed inward to make the primitive mouth during the evolution of the jaw. Indeed, the dental lamina, which is responsible for patterning teeth, comes from ectodermal epithelial tissue (Sire and Huysseune 2003). This notion has developed into the outside-in hypothesis, which posits that denticles from the face moved inward toward the oral cavity to form an enlarged set of teeth at the margin of the evolving jaw (Reif 1982; see Gillis and Donoghue 2007).

Not everyone agrees that teeth first evolved from skin denticles. Moya Smith and her colleagues (e.g., Smith and Coates 2001; Smith 2003) argue that teeth were actually derived from the pharanygeal denticles that migrated out to the mouth rim. Indeed, many fish today have pharyngeal teeth—sometimes quite elaborate ones that are very effective in food processing. Zebra fish in fact have teeth in their pharynx but not mouth. If this inside-out hypothesis is correct, teeth came from the embryonic endoderm, and probably evolved independently of jaw evolution.

Fossil Evidence for the Earliest Teeth

The fossil record for vertebrate origins is obscured in the haze of deep time, but there are some contenders for the earliest tooth-bearing vertebrates. Consider the conodonts, a common but enigmatic group of jawless eel-like animals that lived from at least 510 to 220 million years ago (mega-annums or Ma). Soft-tissue impressions suggest they had a notochord and chevron-type myomeres, consistent with them being vertebrates (e.g., Donoghue, Sansom, and Downs 2006); they are a good place to start our search for teeth.

Conodonts have dental elements in the assumed region of the pharynx that look much like teeth in chemistry, form, and inferred function. They are made of calcium phosphate and divided into crowns and bases likened to enamel and dentin respectively (Sansom, Smith, and Smith 1994; Donoghue 2001). Their morphology and microwear also offer evidence of complex occlusion and chewing hundreds of millions of years before the first tetrapods brought upper and lower teeth together (Purnell 1995; Donoghue and Purnell 1999). Still, the microscopic structure of conodont elements differs fundamentally from that of our teeth, and offers little direct support for the inside-out hypothesis (Donoghue 2001; Kemp 2002).

The thelodont Loganellia scotica is perhaps a better candidate. These fish lived nearly 440 Ma and, while they lacked oral teeth and jaws, they did have oropharyngeal denticles joined into sets that look more like teeth than scales. These denticles line the branchial bars and are patterned in a way that some consider homologous with our teeth. They also appear to have evolved within the throat independently of both dermal denticles and the evolution of the jaw (Johanson and Smith 2005). While these had the same odontode-like structure as teeth today, they were laid out somewhat differently, suggesting to many researchers that they were not homologous to our teeth either, and therefore do not support the inside-out hypothesis.

Fossil evidence for the outside-in hypothesis seems more compelling. The ostracoderms, which appeared about half a billion years ago and dominated the seas for almost 100 million years, offer evidence for potential precursors in early jawless fishes. These had a scaly tail and head armor made from tiny hardened plates of calcium phosphate. Each had an outer surface of dentin, sometimes capped with a more mineralized enamel-like tissue, all covering a pulp chamber that housed blood vessels. While ostracoderms clearly did not have teeth per se, many did have odontode-like plates on the rim of their mouth, with small nubs or barbs that almost certainly functioned in feeding (Tarrant 1991; Purnell 2002; Elliot, Reed, and Loeffler 2004).

Early jawed vertebrates (gnathostomes) also offer evidence to support the outside-in hypothesis. Some acanthodians had tooth-lined jaws well over 400 Ma. Some formed whorls lining the jaw (spiral or arched cog-like conveyor belts with sharp, recurved cones or triangles rotating into place for use), and others had rows of individual teeth fused to the jaw bone, added one by one to the front when those behind became worn or broken. Yet others had both. Some, such as the ischnacanthids, had lip and cheek scales that look like tooth whorls, increasing in size with proximity to the mouth. This could be a smoking gun for the outside-in hypothesis (Blais, MacKenzie, and Wilson 2011). Further, like later tooth whorls (but not the pharyngeal denticles of Loganellia), their cusps got larger as new ones were added.

The Evolution of Teeth before the Mammals

Once teeth evolved, the pressure was on to make them work better. We can identify important milestones and trends through evolution by comparing living sharks to other fishes, fishes to amphibians, amphibians to reptiles, and reptiles to mammals. Hardened caps (i.e., enameloid and then true enamel) evolved, as did new and innovative ways of attaching tooth to jaw. There are tendencies toward reduced number, distribution, and replacements of teeth. Increasingly elaborate crown shapes appeared, as did variation in form and function of teeth along the tooth row. Precise occlusion and chewing were not far behind. All of this happened well before the first mammal connected squamosal to dentary bone (see later).

Hardened Tooth Caps

Known acanthodians did not have hardened caps covering their tooth crowns. Maybe, then, early gnathostomes figured out how to make teeth before they learned how to strengthen them. Vertebrates today commonly cover their crowns with a highly mineralized tissue, enameloid for most fishes, and enamel for most amphibians, reptiles, and mammals. Enameloid develops from both odontoblast (neural crest–derived) cells and ameloblast (ectoderm-derived) cells, whereas enamel forms from ameloblasts only (see Sander 2000; Line and Novaes 2005; Chapter 16). The two tissue types both harden the tooth crown, but they differ in microscopic structure and organic-matrix composition (Gillis and Donoghue 2007). While hardened tooth caps may have evolved separately many times (see earlier), genetic evidence suggests that our type of tooth enamel first evolved within the lobe-finned fishes more than 350 Ma (Kawasaki and Weiss 2006; Shintani et al. 2007).

Attaching and Anchoring Teeth

There are many ways to connect tooth to jaw. Some vertebrates attach their teeth directly to the tip or side of the jaw, whereas others embed them in sockets. Some anchor them with mineralized bony tissue and others with unmineralized fibers. Tendencies within higher-level groups give us a sense of how our tooth–jaw connection evolved. Sharks attach teeth by a common sheet of connective tissue, whereas bony fishes fasten them to the jaw individually (Berkovitz 2000). Bony fishes typically attach them to the tip of the jaw, whereas amphibians and most reptiles anchor them to the side. Today, only a few fish species, crocodilians, and mammals have tooth sockets, although many more, like dinosaurs and toothed birds, did in the past. However, mammals are different from the others (Gaengler 2000). Crocodiles, for example, have replacement teeth in the same sockets as their predecessors, whereas mammals replace the walls of milk tooth sockets when permanent teeth erupt. Crocodiles also have a partly mineralized periodontal ligament, essentially intermediate between a more primitive bony attachment and the mammalian fibrous one (McIntosh et al. 2002).

Number, Distribution, and Replacements of Teeth

There are differences among higher-level taxa in average tooth number, distribution of teeth in the oral cavity, and number of tooth generations/replacements. Fish can have thousands of teeth in the mouth at a time. While amphibians usually have fewer, they often have more than are typical for reptiles. Mammals tend to have fewer still. To be sure, there are many exceptions to these trends. Some species in each group have a reduced number of teeth, or have lost them entirely, and others have more. The spinner dolphin, for example, has up to 260 teeth in the mouth at once, a respectable number even by the standards of many non-mammalian vertebrates. Nevertheless, over deep time a trend toward decreasing numbers within our lineage seems clear.

Other differences among higher-level vertebrate taxa include tooth location and generation number (see Butler 1995). Fishes frequently have teeth widely distributed across the oral cavity and pharynx, whereas amphibians and reptiles have more limited attachments, albeit still often involving several bones of the skull. Mammals, in contrast, have their teeth embedded in no more than three bones, the premaxilla, maxilla, and dentary. The number of tooth generations also tends to decrease from fishes to amphibians, reptiles, and, ultimately, mammals. Sharks can replace their teeth 200 times, whereas crocodiles do so only 45–50 times (Reif 1984). Those that need precise occlusion, like agamid lizards, have fewer tooth replacements (Nydam, Gauthier, and Chiment 2000). Mammals have at most one.

Crown Differentiation

While one may envision fish, amphibian, and reptile teeth as simple pegs, many have evolved complex crowns with several cusps, serrations, or other features. Tooth types can also vary dramatically within a mouth, depending on function. Heterodonty (different tooth types or classes) is not a uniquely mammalian trait. Consider the sheepshead fish, with its eerily human-like incisiform front teeth for scraping barnacles from rocks and pilings, and its round, flattened back ones for crushing hard-shelled prey. Iguanas, in contrast, have conical, recurved front teeth for cropping vegetation, and laterally compressed, bladelike ones lined with tiny cusps for shredding plant parts. Indeed, non-mammalian tooth shapes can vary greatly with diet, even among closely related species. Compare the blunt, rounded crowns of Varanus olivaceous, the Gray’s monitor, used for crushing shells, with the piercing, finely serrated, steak knife–like marginal teeth of V. komodoensis, the Komodo dragon. There are countless other examples among both living and especially fossil vertebrates (see Ungar 2010). Think of the heterodontosaurid dinosaurs, with their small, peg-like front teeth, large, canine-like tusks, and complex back teeth, often chisel shaped with ridges along the biting edge. What mammalian teeth today can rival the elaborate dental battery of a ceratopsid or hadrosaur?

Early Occlusion and Chewing

Occlusion, precise alignment of opposing teeth, and chewing are often considered uniquely mammalian traits. They are not. Occlusion first appeared in land vertebrates some 300 Ma. The diadectids, a probable sister group to the amniotes, and the better-known edaphosaurids had bulbous cheek teeth with clear evidence of attritional facets (Laurin and Reisz 1995; Modesto 1995). We can look again to the dinosaurs. The ornithopods developed an ingenious way to grind their food, despite lower jaw movements constrained to vertical opening and closing (Norman 1984). Occlusal surfaces of the lowers were angled so the inner, lingual sides were taller than the outer ones—and the uppers were the opposite. As the lowers pressed into the uppers, they wedged upper left and right jaw bones outward, and their tooth rows apart. Muscles or ligaments connecting the bones would have presumably rotated the teeth back into place as the mouth began to open, so opposing occlusal surfaces slid across one another for grinding tough vegetation (Norman and Weishampel 1985). If we look beyond the mammals, we begin to appreciate the innovative ways in which evolution has adapted tooth and jaw to achieve efficient food processing.

Evolution of Mammalian Teeth and Mastication

Still, there is something special about the way mammals chew. We cannot understand our teeth without first appreciating this. Aristotle recognized as much millennia ago in De partibus animalium. He wrote:

Of the two separate portions which constitute the head, namely the upper part and the lower jaw, the latter in man and the viviparous quadrupeds [mammals] move not only upwards and downwards, but also from side to side; while in fishes and birds and oviparous quadrupeds, the only movement is up and down. The reason is that this latter movement is the one required in biting and dividing food, while the lateral movements serve to reduce substances to a pulp. To such animals, therefore as have grinder-teeth this later motion is of service; but to those animals that have no grinders, it would be quite useless; and they are therefore invariably without it.

The important points are that mammals differ from other vertebrates in how they chew, that horizontal movements (side to side or back to front) of the jaw are important to mammalian mastication, and that chewing and tooth shape are matched for efficient food breakdown (see Chapter 9). How did mammalian mastication evolve? What was the role of teeth in the process? The answers to these questions are written in stone, a 100-million-year fossil record of the mammal-like reptiles and earliest mammals (Kemp 2005). Mammals are the only surviving members of an ancient group of amniotes called the synapsids. Synapsids are distinguished by a number of traits, including their namesake single arch, or window, through the side of the skull. There were three major radiations of synapsids, beginning more than 300 Ma: the pelycosaurs, the therapsids, and, finally, the mammals. Their fossil record offers evidence for the differentiation of teeth, reorganization of the chewing muscles, replacement of the jaw joint, reduction to two tooth generations and cessation of indefinite growth of the jaw, and the appearance of prismatic tooth enamel.

Dental Division of Labor

While mammals are not the only vertebrates with a dental division of labor (see earlier), heterodonty is an important key to food acquisition and processing for our biological class. Many pelycosaurs had enlarged canine-like tusks distinct from their other teeth. Think of the well-known sail-backed Dimetrodon, with its big, thick front teeth, huge caniniforms, and recurved, laterally compressed postcaniniforms (Jenkins 2001). Therapsids had even more dental differentiation. Massive saber or tusk-like canines, and small, sharp postcanines, were common, and some had distinctive, interlocking incisor teeth (see Rubidge and Sidor 2001). Some dinocephalians even had inter-digitating postcanine teeth. However, mammal-like heterodonty began to emerge with the more derived therapsid cynodonts. Not only did many have elaborate crowns, but some showed increasing complexity from the front to the back of the postcanine tooth row, foreshadowing the division of premolar and molar teeth (Sidor and Hopson 1998). One later group of cynodonts, the trithelodontids, had back teeth that we would expect of the ancestral mammal: a single row of cusps running front to back, with crests and wear facets indicating use as shears for slicing tough foods (Gow 1980).

Reorganization of the Chewing Muscles

The muscles that move the mandible had to be reorganized for the jaw joint to transition from a simple hinge for vertical opening and closing to one with a significant horizontal component. Not only did opposing teeth need to be brought together in a precise manner for mastication, but chewing muscles had to produce bite forces in a line passing through the tooth row while minimizing stress on the jaw joint (Reisz and Müller 2004). The primitive condition involves two muscles that insert on the back of the lower jaw, an internal adductor that takes its origin from the palate, and an external one arising from the side of the skull. These form a sling of sorts for closing the mouth and bringing opposing teeth together.

The evolution from this sling to today’s mammalian chewing musculature can be traced through changes in sizes, shapes, and orientations of muscle attachment sites on fossil crania and mandibles (see Kemp 2005). The synapsid arch itself gave tendons an extended area for attachment, allowing for a larger external adductor to produce a strong, controlled bite (Tarsitano et al. 2001). Pelycosaurs and therapsids show increased attachment areas, with distances between attachment sites and the jaw joint adapted for a more powerful, efficient bite. Moreover, differentiation of the external adductor into temporalis and masseter, as well as changes to the internal adductor attachment in the cynodonts, meant more precise, controlled jaw movements (see Rubidge and Sidor 2001).

The Jaw Joint

Mammalian mastication requires a very special sort of jaw joint, one that is flexible enough for movements in the horizontal and vertical planes, and stable enough to withstand and dissipate the forces required for food fracture. In fact, the mammalian jaw joint is so unique and important that many consider it the defining attribute for our biological class. Pelycosaurs and primitive therapsids retained a simple hinge joint. A bone on the bottom of the cranium, the quadrate, projected down and fit into a trough, or recess, in the articular, a bone on the back of the mandible. The quadrate shrank in advanced therapsids, and a ligament eventually formed between an adjacent bone, the squamosal, and the lower jaw, to help stabilize and take pressure off the joint. A projection from the mandible, the condyle, then evolved to fit into a recess on the squamosal bone, in what was, by definition, the earliest mammal. This new temporomandibular joint became increasingly dominant over time, eventually replacing the articular–quadrate joint completely (Rubidge and Sidor 2001; Kielan-Jaworowska, Cifelli, and Luo 2004; Kemp 2005).

Diphyodonty and the Cessation of Growth of the Dentary

Diphyodonty, the reduction of tooth generations to no more than two, is another trait sometimes used to define mammals. It is associated with the cessation of jaw growth in adulthood (non-mammalian gnathostomes have indeterminate growth, and their jaws get progressively longer throughout life) and with the need for precise occlusion during mastication. In most vertebrates, smaller teeth are replaced by larger ones over and over again as the animal and its jaw grow indefinitely. Teeth are replaced every second or third position along the row, so there is not much of a gap at any one time (Berkovitz 2000). Mammals have a very different pattern of tooth replacement. Placentals usually have two sets of front teeth and premolars, with permanent ones pushing out their deciduous predecessors as they erupt. Molars have no deciduous counterparts, and are added one behind the other as space is made available, until the jaw stops growing.

There are, of course, exceptions. Toothed whales evidently never develop