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Paleontological Society Fabricational Noise in Elephant Dentitions Author(s): V. Louise Roth Source: Paleobiology, Vol. 15, No. 2 (Spring, 1989), pp. 165-179 Published by: Paleontological Society Stable URL: http://www.jstor.org/stable/2400850 . Accessed: 24/09/2013 17:41 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Paleontological Society is collaborating with JSTOR to digitize, preserve and extend access to Paleobiology. http://www.jstor.org This content downloaded from 129.93.16.3 on Tue, 24 Sep 2013 17:41:31 PM All use subject to JSTOR Terms and Conditions

Fabricational Noise in Elephant Dentitions

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Fabricational Noise in Elephant DentitionsAuthor(s): V. Louise RothSource: Paleobiology, Vol. 15, No. 2 (Spring, 1989), pp. 165-179Published by: Paleontological SocietyStable URL: http://www.jstor.org/stable/2400850 .

Accessed: 24/09/2013 17:41

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Paleobiology, 15(2), 1989, pp. 165-179

Fabricational noise in elephant dentitions

V. Louise Roth

Abstract.-A marked retardation of dental ontogeny characterizes the family Elephantidae. As a consequence of this retardation, elephant teeth are subject to the forces of mastication, eruption, and progression while still in a developing and pliant stage. As specimens described here illustrate, the mechanical forces are often sufficient to deform the gross morphology of dentitions. Morpho- logical variation in elephant teeth can be regarded as "fabricational noise"-revealing information about the dynamic context in which the teeth develop. Accordingly, dental variation is less species- specific in elephants than in other mammals. The fossil record may comprise fewer species of elephants than is generally believed, and trends inferred to reflect rapid evolution within this family may in fact reflect phenotypic plasticity.

V. Louise Roth. Department of Zoology, Duke University, Durham, North Carolina 27706

Accepted: January 19, 1989

Introduction

The extensive and well-known fossil re- cord of the Elephantidae has provided evo- lutionary theory with widely cited examples of a variety of phenomena, ranging from or- thogenesis and rapid phyletic evolution, to explosive speciation, adaptive radiation, and punctuational change (Osborn 1942; Davis 1949; Watson 1949; Simpson 1953; Stanley 1979; Minkoff 1983; Raff and Kaufman 1983; Vrba 1987). Yet many inferences about the evolutionary history of the elephant family have been made without a full appreciation of variation within modern species and with- out regard for the sources of that variability (Roth 1982; Roth and Shoshani 1988). Much of the taxonomy of fossil elephants is based upon teeth (Osborn 1942; Cooke 1947; Maglio 1973; Madden 1981). This paper and its sequel (Roth in prep.) are analyses of variability in elephant dentitions.

The variation examined for these studies is of two types: (1) anomalies, or gross morpho- logical variants that are most clearly de- scribed verbally or with an illustration, and (2) intraspecific variation of an ordinary sort, which is reflected in dental measurements and can be compared quantitatively. Types (1) and (2) are in fact ends of a continuum. The cur- rent paper will focus on anomalies. Anoma- lies in dental morphology, if viewed simply as extremes in the spectrum of variability, can ?) 1989 The Paleontological Society. All rights reserved.

provide clues to the processes that normally shape teeth during their formation and attri- tion. In a second paper (Roth in prep.), vari- ability in one extant species, Elephas maximus, will be documented quantitatively. The quantitative variability that is described in the second paper suggests that the develop- mental plasticity exhibited in anomalies is manifest to some degree in all elephant teeth, and indicates that, if extinct species varied as much as E. maximus, the fossil record may comprise fewer elephant species than is com- monly assumed.

Fabricational Noise. -In this paper, variation is treated as fabricational noise. The term "fabricational noise" has been applied to fea- tures of morphology that contain information about the mechanism by which a structure is manufactured (Seilacher 1970, 1973). The pneu-like shapes of sand dollars, for example, suggest that their skeletons are formed by hardening around fluid-filled elastic mem- branes in tension (Seilacher 1979). The details of pattern in other structures, such as human fingerprints, mammalian nerve networks, or the surface marks on sharks' teeth (Rachootin and Thomson 1981; Hutchinson 1981; Reif 1973) appear not to develop according to strict genetic specifications; and yet variations in these patterns can reveal much about how the patterns are generated. For this reason, the term "fabricational artifact" may more closely

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166 V. LOUISE ROTH

reflect the meaning that has in fact been in- tended by the term "fabricational noise." (I will, however, retain the original terminol- ogy.)

In different individuals, morphogenetic processes operate within different genetic, epigenetic, and environmental contexts, and morphological variation is the result. The dif- ferent contexts in which morphogenesis oc- curs can be viewed as experiments; morpho- logical variation among specimens can thus reveal how details of a pattern are controlled and the nature of mechanical forces that op- erate during its development.

Fabricational noise in skeletal structures of invertebrates has proved to be very infor- mative (Seilacher 1970, 1973, 1979; Hickman 1980), but fabricational noise is rarely the fo- cus of research on vertebrates (the shark ex- ample above [Reif 1973] is an exception). The phenomenon has never, to my knowledge, been examined in mammals. One reason fab- ricational noise has seldom been sought in vertebrate skeletons may be that they char- acteristically include bone. Haversian bone, for example, is reworked and remodelled. Early states in ontogeny become obscured with time, and fabricational noise, though continually generated, is also continually obliterated. The structure of even Haversian bone could, however, be examined for re- cently generated fabricational noise. The ori- entation of trabeculae appears to follow the distribution of mechanical stresses in the bone (Thompson 1942, quoting mid-19th-century observers) but the mechanism for this adap- tive remodelling is still not well understood (Wainwright et al. 1982; Currey 1984). The growth of bone contrasts with the accretion- ary growth of molluscan shells: the septa of an ammonite, for example, preserve a record of ontogeny back to the earliest larval stages (e.g., Seilacher 1973; Landman et al. 1983).

Teeth are another important hard part in vertebrates. Variability in mammalian teeth is highly species-specific, and the formation of teeth is generally assumed to be tightly canalized (e.g., Butler 1982). Mammalian teeth usually form as discrete units early in ontog- eny, and their fabrication can therefore be disrupted only during a very early and brief

segment of the animal's lifetime. The dental cap and papilla appear, moreover, to exist in a kind of equilibrium, which allows the in- ternal dental epithelium to develop relatively free of external mechanical constraints (Gaunt and Miles 1967). The characteristically species- specific nature of mammalian dental mor- phology, and the relatively low variability among individuals within a species, is one reason teeth play such a prominent role in mammalian systematics.

Elephants, however, are exceptional mam- mals. In the remainder of this paper I will demonstrate that, however small its role in the dentitions of other mammals, variation in the form of fabricational noise is an important feature of elephant dentitions. I will first dis- cuss the mechanistic basis for this observation by describing some unique features of dental ontogeny in elephants and listing the major forces an elephant tooth experiences during its formation. Second, I will report some ex- treme examples of fabricational noise (anom- alies) in elephant teeth, which indicate that the teeth are subject to mechanical constraints of great magnitude and illustrate the extent to which elephant dentitions developing within different contexts can vary.

Elephant Dental Ontogeny.-I will concen- trate on the cheek teeth, the premolars and molars. Tusks are also teeth-in elephantids, tusks are permanent evergrowing incisors that are preceded in young juveniles by tiny milk tusks-but they differ from cheek teeth in morphology, development, and function (they are used for manipulating objects out- side of the mouth, and in social display and combat, rather than mastication: Sikes 1971; McKay 1973).

The toothrow of an elephant does not erupt in typical mammalian fashion. In each jaw quadrant, there are, in series, a total of six cheek teeth (representing six "serial cate- gories"). Each tooth is large, and the jaw can physically accommodate only one or two per quadrant at a time. Development and erup- tion are retarded. Teeth appear sequentially, and dental ontogeny continues through much of the life of an animal (Laws 1966; Sikes 1971; Roth and Shoshani 1988). New teeth arise within an alveolus in the posterior part of the

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DENTAL ANOMALIES IN ELEPHANTS 167

jaw; they erupt and gradually move forward, progressing as though on a conveyor belt. Wear occurs on the occlusal surface, and den- tal material at the anterior end of the tooth- row fragments and is shed.

The dental epithelium of an elephant cheek tooth exhibits a series of deep folds, or (to describe it more accurately) pockets. "Fold" would imply that, for most of its height, the enamel on either side of a fully developed tooth forms a free edge; instead, the enamel completely encloses a core of dentine (Figs. 1). The enamel-and-dentine units, called lamel- lae, are connected in series to one another at their bases. The entire tooth is encased in cementum, which also fills the troughs be- tween adjacent lamellae.

A single tooth forms as a conglomerate of separate lamellae, and the sequence of de- position of the hard tissues in one tooth can be deduced from a posterior-to-anterior gra- dient of morphology in the entire dentition of a newborn animal. For example, (see Ma- terials and Methods section for translation of acronyms) in LACM 172 and SBMNH 245 (Mammuthus exilis), UNSM 2090 and an un- numbered TMM specimen (M. columbi) and AMNH 54345 and 39030 (Elephas maximus), the first tooth is complete and beginning to erupt, but further to the posterior in the jaw, the formation of teeth is less complete. Den- tine and enamel of the lamellae of the second tooth are formed and the lamellae are united at their bases, but not yet cemented together, and a few loose, very thin rudiments of the anteriormost lamellae of a third tooth exist in the alveolus to the rear. In this posterior por- tion of each jaw, a series of three additional teeth ultimately forms on each side, and thus dental development continues. Lamallae at first solidify as individual, unconnected, hol- low units composed of enamel surrounding dentine and a pulp cavity. The lamellae are hollow because dentine has not yet complete- ly encroached on the extensions of the pulp cavity projecting into them. They are thickest at the crown, thinning out toward their bases. Gradually, the lamellae thicken and fill with dentine, unite at their bases, and become sealed together with cementum to form a sin- gle solid unit, the tooth. Despite the unusual

TROUGH

I BETWEEN LAMELLAE

LAMELLAE

PULP CAVITY

Eft D CE

FIGURE 1. (a) Schematic diagram showing the topology of the enamel of three adjacent lamellae of a single tooth. The enamel of the lamellae is formed into a series of pockets (pulp cavities), which open toward the base (root) of the tooth. At their bases, adjacent lamellae are joined; at the crown, the lamellae appear as distinct folds sep- arated by troughs. The pulp cavities ultimately fill with dentine. The sides and crown of the tooth, and the troughs between adjacent lamellae, eventually become encased in cementum, forming a solid unit as shown in (b). (b) Partially formed second mandibular cheek tooth (usually homologized with dP3) of an elephant, viewed from lin- gual and ventral aspect. (The tooth has the same orien- tation as the diagram in la; anterior is toward the left.) Grooves on the cementum surface of the tooth demarcate individual lamellae; three of the total of seven lamellae are indicated with arrows. The darkest regions at the base of the tooth are the subdivisions of the pulp cavity that extend up into a lamella (each cavity of which ul- timately fills with dentine). A skirt of cementum projects beyond and ensheaths the base of the tooth. Wear on the tooth from mastication produces an occlusal surface that is a horizontal section, as shown in (c). EN = enamel; DE = dentine; CE = cementum.

dental morphology of elephants, the pattern of development resembles that in other mam- mals, in which the deposition of enamel spreads from the peaks of cusps to their bases, and individual cusps unite when enamel from adjacent cusps spreads into the contiguous valleys (Gaunt and Miles 1967). In elephants, the process continues for decades, through each of six successive serial categories, until the last tooth in the series is fully formed and erupted.

This retardation of dental ontogeny in el-

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168 V. LOUISE ROTH

:-' Eruption, Progression, Addition of Lamellae,

Grovith a Eruption of Posterior Teeth

Resistai; - \\ i i JGravity4 Bone,Soft Tissues,\ 1 More AnterorTeeth Mastication

<ANT RIOR POSTERIOR>

Resitne fVMastication Bone, Sofi Tissues, Eruption, B More AnteriorTeett Progression,

Addition of Lamelage,

Growth 8 Eruptionof Posterior Teeth

FIGURE 2. Forces experienced by the teeth of an ele- phant as they develop, erupt, progress, and wear. This diagram would correspond to a lateral view of the left side of the head (both upper and lower jaws), with the teeth in place and all bone removed. The occlusal surfaces are toward the center of the diagram, and roots of the teeth point toward the top in the maxilla and bottom in the mandible; anterior is toward the left. Two teeth are in wear in each of the two jaw quadrants shown. The anterior tooth in both jaws is completely erupted and in wear, its roots are eroded, and some of the lamellae have been shed. The occlusal surfaces of the more posterior teeth extend approximately one-third of their lengths, and the remaining portions of these teeth would be en- cased in bone. Arrows indicate only the directions of the forces; their lengths are not intended to reflect their rel- ative magnitudes.

ephants not only prolongs the period of time during which fabricational noise may arise; it also exposes the dentition to a wide range of forces during its development.

Mechanical Forces Affecting Dental Ontogeny in Elephants.-In the course of their forma- tion, elephant teeth grow (increase in size), erupt, progress, are subject to gravity, and interact with food, other teeth, or other tis- sues in the jaw. Figure 2 illustrates diagram- matically the forces that act upon elephant teeth. In the following list, the forces are grouped according to their source. The list, although it is by no means exhaustive, prob- ably incorporates all major sources. (The list is heuristic, and the categories are not mu- tually exclusive: gravity, mastication, and eruption, for example, may all be involved in the mechanism of dental progression.)

(1) Mastication-During mastication, a tooth receives vertical crushing or grinding force (downward for mandibular teeth; up- ward for maxillary teeth) from its occlusal partner. The lower jaw is brought forward on the power stroke of the masticatory cycle (Ma- glio 1972), causing, in addition, a shearing force that is directed anteriorly on the max- illary teeth and posteriorly on the mandib- ular teeth. A newly erupted tooth in the rear of the jaw often comes into wear at its anterior end before its posterior end is completely formed. Thus the forces of chewing can affect morphogenesis, and fabricational noise aris- ing as a direct result of mastication should be most marked toward this posterior end.

(2) Growth-Development of an elephant tooth proceeds in an antero-posterior direc- tion: within the alveolus, plates form and are added to the posterior face of a molar. The crown of each lamella forms first. As a tooth erupts, its root elongates through the exten- sion of the cementum sheath that encases the lamellae. Thus growth should create pres- sures that are directed anteriorly and toward the crown of the tooth.

(3) Eruption and progression-These forces, like the forces contributed directly by the growth of the tooth, act vertically, toward the crown of each tooth (upward in the mandible; downward in the maxilla), and anteriorly (in the direction that the teeth move during pro- gression).

(4) Gravity-The weight of a tooth is di- rected toward its roots in the mandible, and toward its crown in the maxilla. Elephant teeth are massive. In the Asian elephant (Elephas maximus), which is only a moderate-sized member of the elephant family (Maglio 1973; Roth 1984), a single sixth mandibular tooth may weigh over 8 kg; a maxillary cheek tooth can weigh nearly 7 kg (Roth and Shoshani 1988).

(5) Interaction with other tissues-Each tooth in the jaw is buffetted by its own array of forces and experiences pressures in turn from other teeth with which it is in contact. Mastication has already been mentioned. The forces a tooth receives from the growth, erup- tion and progression of more posterior teeth within the same jaw quadrant approximately parallel the direct forces of the tooth's own

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DENTAL ANOMALIES IN ELEPHANTS 169

growth, eruption, and progression, and therefore augment them. Bone, teeth, and soft tissues anterior to a tooth resist the motion of progression, providing a force that is di- rected posteriorly. Resistance to eruption is offered by the bone and soft tissues of the jaw and is oriented more or less vertically, toward the root of each tooth. The jaw itself grows, and the space available to a growing tooth may affect the tooth's morphology.

Materials and Methods Osteological collections of the following

institutions and departments were surveyed for anomalous dental specimens of North American mammoths and Asian elephants: British Museum (Natural History) (Osteolo- gy) (BMNH); American Museum of Natural History (Mammalogy) (AMNH); U.S. Nation- al Museum of Natural History (Mammals) (USNM); Yale Peabody Museum (Osteology) (YPM); Philadelphia Academy of Natural Sci- ences (Mammalogy) (ANSP); Field Museum of Natural History (Mammals) (FMNH); Mu- seum of Comparative Zoology (Mammalogy) (MCZ); University of California Museum of Paleontology (Vertebrate Paleontology) (UCMP); University of Florida (Mammalogy and Vertebrate Paleontology) (UF); Univer- sity of Michigan Museum of Zoology (Mam- malogy) (UMMZ); Duke University (Verte- brate Zoology) (DU); Wayne State University (Osteology) (WSUMNH); Los Angeles Coun- ty Museum (Vertebrate Paleontology) (LACM); Santa Barbara Museum of Natural History (Vertebrate Zoology) (SBMNH); Ne- braska State Museum (Vertebrate Paleontol- ogy) (UNSM); and the University of Texas at Austin/Texas Memorial Museum (Vertebrate Paleontology) (TMM).

The mammoth specimens examined had chiefly been referred to the species Mammu- thus imperator, M. columbi, and M. exilis (the Santa Rosa Island mammoth). In the descrip- tions below, taxonomic distinctions will not in general be made within the sample of mammoths. All mammoth specimens are re- ferable to the genus Mammuthus, but species- level taxonomy is variable and terminology remains inconsistent (Kurten and Anderson 1980; Madden 1981). Specimens from Santa Rosa Island will be indicated, however. The

small size of the Santa Rosa mammoths (Stock 1935; Orr 1968; Roth in press) appears to have had important consequences for their dental morphology (Roth 1982), and their circum- scribed geographical provenance makes them a well-defined group.

Samples were as follows: Elephas maximus: 559 teeth, 69 of which were isolated, and the remainder of which were situated within the bone of 103 individual skulls; mammoths: 205 isolated teeth, plus the mandibles and/or maxillae of 84 additional individuals. Al- though the frequencies of severe anomalies in museum collections can be approximated by dividing the number of specimens de- scribed below by these total sample sizes, ex- cept where noted (for partial lamellae; see below), such frequency estimates must be considered minimal. The most severe exam- ples of distorted morphology were recorded preferentially, and anomalies in unerupted portions of teeth could not be observed. The frequency of anomalies in museum collec- tions may be an inflated estimate of the fre- quency within natural populations, however, if collection has been biassed in favor of odd- ities.

For most teeth, morphology could be de- scribed directly and recorded in photographs and sketches. In some, the relationships be- tween lamellae and between bone and tooth were clarified using conventional film-radio- graphic methods. The arrangements of la- mellae in two specimens (UNSM 39309 and TMM 108; see below) were too complex to resolve with either superficial examination or with standard X-rays, but they could be de- termined with computed tomography (CT scans). Contiguous 1-cm thick CT slices were obtained in coronal and parasagittal planes, using 120-140 kV, and 120-170 mA, with a GE 9800 CT scanner at the Radiology De- partment of the Duke University Medical Center.

Dental Anomalies: The Evidence of Fabricational Noise

Rare though they may be, anomalies un- ambiguously indicate that forces within the jaw can be of sufficient magnitude to deform developing teeth. These forces may displace entire teeth from their normal progression

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170 V. LOUISE ROTH

FIGURE 3. Mandibles of E. maximus with poorly aligned teeth in one of the two quadrants. Views are of the occlusal surface, with anterior to the left. In both jaws the teeth erupted and progressed normally on one side. A, In YPM 1428, the right side (uppermost, in the figure) is maloccluded, and the teeth have strayed lingually across the mandibular symphysis. Note that on the right side, the tooth in the posterior part of the mandible is more fully erupted than the corresponding tooth on the left. B, The fourth (IV) and fifth (V) cheek teeth on the left side (lowermost, in the figure) of ANSP 2404 are poorly aligned, apparently (i) allowing tooth V to advance lateral to tooth IV, and (ii) causing a bend in the fifth tooth where it extends anteriorly beyond the posterior end of tooth IV. Because of its atypical position, tooth V exhibits wear on its lingual surface.

through the jaw, or change the structure of the tooth itself, by either rearranging or de- forming its lamellar subunits. (Depending upon the angle at which occlusion sections the tooth, the configuration of enamel on the wear surface can also vary greatly, but vari- ation in patterns of attrition, per se, will not be reported here.)

The gradual forward migration of teeth through the jaw in elephants (dental pro- gression) is usually orderly, but it can become derailed (Fig. 3A). YPM 1428, a circus animal shot while on a rampage through the streets of Hamden, Connecticut (YPM osteology cat- alogue notes), is maloccluded on its right side. The teeth are also poorly aligned, and instead of proceeding longitudinally through the mandibular ramus, have pushed through the

bone toward the tongue on a course across the mandibular symphysis. Whereas on the left side tooth VI and the last remnants of tooth V are positioned and worn normally, on the right there are, in series, four separate tooth-units-either broken sections or dis- tinct teeth (it is not clear which). The normal synchrony between eruption and progres- sion was disturbed: the last tooth in the rear of the right mandible has erupted prema- turely with respect to its position in the jaw, its advancement apparently blocked by teeth anterior to it that had strayed off course.

The shape of the tooth itself may be molded by the jaw in which it is housed. All elephant molars curve to some degree (their curvature laterally or medially and toward crown or root renders maxillary, mandibular, right and

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DENTAL ANOMALIES IN ELEPHANTS 171

FIGURE 4. Mandible of a Santa Rosa Island dwarf mammoth (Mammuthus exilis), LACM (CIT) 106/67792, in antero- lateral view, with anterior to the left. Bone of the vertical rami of the jaw has eroded, leaving the posterior alveolus exposed. Note the curvature of the teeth and the manner in which lamellae are staggered and sheared with respect to one another.

left teeth distinguishable, for example [Ma- glio 1973]). The teeth of dwarf mammoths (Mammuthus exilis, from Santa Rosa Island, California) are large in relation to the bony portions of the jaws they occupy (Roth 1982), and their curvature is thereby accentuated. Specimen LACM (CIT) 106/67792 (Fig. 4) demonstrates this curvature especially clear- ly. The teeth of this mandible extend from the anterior edge of the toothrow back, through the entire occlusal region of the jaw, into the posterior alveolus (which, in life, was surrounded by bone). The longitudinal axis of each tooth follows a curve in three dimen- sions, tracing the main axis of the jaw: toward its posterior end each tooth curves laterally, following the divergence of the mandibular rami, and dorsally, following the curve of the ventral and posterior angle of the jaw, where the horizontal ramus grades into the vertical ramus and mandibular condyle. The curva- ture of each tooth arises through shifts in the

positions of the constituent lamellae. Succes- sive lamellae are aligned on a longitudinal axis that pierces them obliquely, rather than perpendicularly (as though the stack of la- mellae, arranged from anterior to posterior like a row of dominoes, had undergone shear). Each tooth also exhibits torsion about its lon- gitudinal axis: successive lamellae are slightly rotated in position, such that the roots of the most posterior lamellae point laterally and posteriorly, instead of in a strictly ventral di- rection.

In addition to shifting and rotating, la- mellae may also be displaced, giving the tooth a folded appearance. In the right mandible of E. maximus ANSP 2404 (Fig. 3B), tooth V is properly aligned for its entire length and is not bent. On the left side, however, the an- terior end of tooth V lies lateral to tooth IV. Posteriorly, beyond their points of contact, tooth V bends into a more natural alignment with the preceding tooth. Laws (1966) re-

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172 V. LOUISE ROTH

A~~~

FIGURE 5. Maxillary teeth of E. maximus, lingual view, anterior to the left. A, The posterior end of BMNH 39.4.18.6 curves buccally; B, a more typical morphology (BMNH 707x) is shown beneath. Scale divisions are one and five millimeters. Photo courtesy of the British Mu- seum (Natural History).

ported a similar distortion of a mandibular tooth of Loxodonta africana. The maxillary dentition of Jumbo (AMNH 3283), a famous captive individual of this species, provides another example of a longitudinally bent tooth (with displaced lamellae) that is poorly aligned with its successor (see Colyer 1936: fig. 589).

The bend in a tooth may be quite sharp. What ordinarily would be the posterior end of the maxillary tooth of E. maximus shown in Fig. 5A (BMNH 38.4.18.6) executes a hairpin turn; the final plates in the series lie anterior and lateral to their predecessors. A British

Royal College of Surgeons specimen dis- cussed by Colyer (1936: fig. 591) shows the same feature, as do (to a less extreme degree) BMNH 84.12.4.2 and an unnumbered UF specimen (both E. maximus). In each of these teeth, some time before the last plates became solidly fused to the rest of the tooth, they were displaced laterally.

Mandibular molars with a similar config- uration are illustrated by mammoth specimen UNSM 39309 (Fig. 6A). In this mandible, the posterior end of the left tooth doubles back upon itself laterally; the right molar is also somewhat hook-shaped, but less so. The jaw itself is misshapen, displaying anomalous swellings in the horizontal ramus (visible in Fig. 6B as a bulge in the lateral contour of the mandible) of both sides, but most markedly on the left. CT scans of the specimen (e.g., Fig. 7) reveal an abscess beneath each tooth. The normal, intimate relationship between dental root and mandibular bone was dis- rupted, and the abscesses may have interfered with normal dental progression: ordinarily, this longitudinal shift in the position of a tooth makes room for the new lamellae that are added sequentially at the posterior end. The anterior ends of the teeth are worn al- most flush with the bone of this mandible. This heavy wear is consistent with the inter- pretation that progression was obstructed. Laws and Parker (1968: p. 343) described ab- scesses and bony swellings "up to the size of a grapefruit" in jaws of East African L. afri- cana. The incidence of abscesses (up to 10%) in a population correlated with other mea- sures of population stress (Laws and Parker 1968).

The anomalous shapes of the teeth de- scribed above were the result of the rear- rangement of lamellae, but lamellae them- selves may be deformed. For example, a tooth will occasionally comprise one or more par- tial lamellae, visible on the occlusal surface as an enamel loop that does not traverse the entire breadth of the tooth. Such partial la- mellae intrude upon and disrupt the orderly parallel arrangement of successive lamellae, causing the lamellae immediately anterior and posterior to become offset or to bend in con- formation.

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DENTAL ANOMALIES IN ELEPHANTS 173

Af

I,Gh~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~'

FIGuRE 6. Mammoth mandible UNSM 39309, bearing abscesses and an unusual lateral curvature of the posterior ends of the teeth. A, Occlusal surface, anterior pointing downward; B, anterior view.

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174 V. LOUISE ROTH

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FIGuRE 7. CT scan slice of mandible shown in Fig. 6, UNSM 39309. This image of the left mandible was taken from close to the longitudinal midline of the tooth. The anteriormost edge of bone is to the right; scale at left is marked in 1-cm intervals. Beneath the anterior and pos- terior part of the tooth in this section, the roots retain a normal, intimate relationship with the surrounding bone of the jaw. The low-density (white) areas beneath the center of the tooth appear to be the result of an abscess that has eroded the base of the tooth and adjacent bone and caused the anomalous bulges clearly visible in Fig. 6B.

Partial lamellae appear to be dispropor- tionately common among the mammoths of Santa Rosa Island in comparison to the other mammoths or to Asian elephants, and this suggests poor canalization of tooth morphol- ogy in the dwarfed forms. Five specimens (a specimen can be an isolated tooth, a single tooth-bearing jaw or jaw fragment, or asso- ciated upper and lower jaws) out of 127 Santa Rosa Island mammoths, two out of 162 spec- imens of larger, mainland mammoths, and only a single isolated tooth out of the 172 specimens of E. maximus, bore partial lamel- lae. By a G-test of association (with Williams's correction; Sokal and Rohlf 1981), the differ- ence in frequency of partial lamellae was in- significant for E. maximus and non-island mammoths (P > 0.5), did not reach signifi- cance for island and non-island mammoths (0.25 > (P > 0.1), but for island mammoths and E. maximus it was highly significant (P < 0.001).

What I have called "partial lamellae" and what Laws (1966) described as "lateral acces- sory cusps" in Loxodonta africana appear to be the same phenomenon. The frequencies Laws reported (five of 235 jaws from Murchison Falls Park; zero out of 150 from Queen Eliz- abeth Park) differ significantly (P < 0.05, by

the tests used above). The higher incidence at Murchison Falls reinforces the interpre- tation that partial lamellae are indicators of disrupted canalization. All of Laws's speci- mens with partial lamellae occurred in this population. Population densities, calving in- tervals, and ages at first reproduction suggest that the Murchison Falls population was also more stressed and overcrowded (Laws 1966). The incidence at Murchison Falls, while ap- parently less than, is insignificantly (0.5 > P > 0.25) different from that observed in the collections of M. exilis.

It is possible that partial lamellae do not reflect poor canalization, but that they are genetically determined and their incidence in some areas is high because of founder ef- fects. If that were so, it would be a remarkable coincidence that they are most common in the two populations (Murchison Falls and Santa Rosa Island) that one has other reasons to believe were stressed (Laws 1966; Roth 1982, in press). It is worth noting that partial la- mellae do not necessarily occur on both sides of a given jaw; when they do, they do not necessarily occur between precisely corre- sponding complete lamellae. Partial lamellae occur on either lingual or buccal aspects of the tooth, irrespective of the jaw (upper or lower).

A particularly extreme case of deformed la- mellae is illustrated in Fig. 8. TMM 108, an isolated maxillary mammoth tooth, displays a normal wear surface, indicating that it had progressed into occlusion in the jaw and that at the level of the occlusal surface the lamel- lae are properly oriented. The most anterior lamellae are worn to within 2 cm of their bases, and the enamel of these bases joins consecutive lamellae in the usual way (Fig. 9A). At the posterior end, however, just dorsal to the wear surface, lamellae bend sharply a full 90 degrees to run horizontally. Subse- quent (more posterior) lamellae are stacked vertically, in horizontal layers that at the pos- terior tip of the tooth slump slightly toward its medial face. A CT scan (Fig. 9A) reveals a constriction of the lamellae midway between the occlusal surface and the apex (root tip) of the tooth. Such severe distortion must have occurred while the lamellae were still in a pliant stage, before they were fully miner-

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DENTAL ANOMALIES IN ELEPHANTS 175

~~~~~JI I I . I _. .

FIGURE 8. Maxillary cheek tooth of a mammoth, TMM 108, with deformed lamellae. A, Lingual surface, anterior to the right, occlusal surface facing downward; B, occlu- sal surface, anterior to the right.

alized. Areas of low density (white in Fig. 9) are regions of the intralamellar pulp cavities that were cut off by the constriction and therefore could not fill with dentine. In CT sections other than the one shown here, sim- ilar white areas are apparent within these and other lamellae; in each case, the areas of low density are within the dentine and bear the same relationship to the constricted parts of the lamella. Apical to the constriction, denser tissue within the same lamellae indicates that dentine deposition continued beyond (and after) the deformation of the tooth. More api- cally still, the lamellae bend again and be- come vertical.

This tooth appears to have been com- pressed from above and behind during its formation. To judge from the position I infer it originally occupied within the skull, the source of the pressure could have been a tumor in either the bone of the maxilla or

FIGURE 9. A, A 1-cm thick CT slice of TMM 108 (see Fig. 8A), selected from a series of contiguous 1.0-cm longi- tudinal slices using 120 kV and 120 mA; this one is from near the midline of the tooth. The horizontal edge at the bottom is the occlusal surface, and anterior is to the right. In these photographs, contact-printed from the digitized CT image, the darkest (and densest) tissue is enamel; lighter grey is alternately dentine and cementum. Note the areas vacant of dentine, appearing white, posterior and ventral to the constriction. B, One-cm thick CT slice (using 140 kV and 170 mA) of a more typical maxillary tooth, for comparison. The specimen (of E. maximus) has the same orientation as Fig. 6A, its occlusal surface form- ing the lower horizontal edge of the picture. (All scales are marked at 1-cm intervals).

possibly the optic nerve, or else another, unerupted, tooth. To have disturbed its predecessor in this way, an unerupted tooth would have to have been unusually well de- veloped and massive at a time this tooth, an- terior to it, was not yet fully mineralized.

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176 V. LOUISE ROTH

FIGURE 10. Fused maxillary cheek teeth of E. maximus (BMNH 1847.3.3.4), buccal view. Occlusal surface points upward and to the right. Between the teeth on the occlusal surface is an enamel loop (arrow) apparently derived from two fused lamellae. A cementum bridge a few mm thick links the two teeth, but more dorsally the teeth are distinct and separated by maxillary bone. Photo courtesy of the British Museum (Natural History).

TMM 108 has 25 lamellae (plus an additional unknown number that were lost to wear and progression), which makes it likely that it was the sixth cheek tooth in the series (Madden 1981; Roth 1982; Roth and Shoshani 1988). Any subsequent tooth would have been su- pernumerary.

Seventh cheek teeth occurred with a fre- quency averaging 9% (among animals with a fully erupted sixth tooth) in Laws's (1966) population samples of Loxodonta africana. For mammalian species, this frequency of super- numerary teeth is high. In only four species out of the approximately 130 mammalian genera Colyer (1936) surveyed do frequencies equal or exceed this level. Elephants lack pre- cise occlusion, so supernumerary teeth may cause little functional impairment, especially if they occur late in the series or late in life. In the course of dental progression, elephant teeth continually shift and adjust their posi- tions within the jaws. Elephant dentitions may consequently have an enhanced ability to ac- commodate an extra tooth, if the tooth be- comes well integrated into the normal pro- gression and eruption of the dentition.

Even the individuality of single teeth may be compromised. A cementum bridge con- taining an enamel loop apparently derived from two fused lamellae links the two teeth shown in Fig. 10, a right maxillary III and IV

from an E. maximus (BMNH 1847.3.3.4). The enamel loop visible on the occlusal surface between the teeth contains a dentine core, within which a smaller loop of enamel sur- rounds a tiny island of cementum. It appears that as the teeth developed, the dental lamina linking them persisted, and the dental sacs of the buds of the two teeth remained con- nected (see, e.g., Sharawy and Bhussry 1986). Colyer (1936) also described an elephant skull with fusion both in maxillary and in man- dibular teeth. Within the jaw of an elephant, the buds of adjacent teeth are often in suffi- ciently close association that they interact in compression or shear. Evidence of this is com- monly seen on the posterior and anterior faces of pairs of adjacent teeth, which often con- form to one another where they articulate in sequence. While the lamellae are still mallea- ble and moveable, pressure between succes- sive tooth buds may deform and rearrange them.

Discussion Alveolar bone will yield to forces exerted

by (or through) fully formed teeth. This fact is the basis for human orthodonture, and den- tal progression in elephants also depends upon it. The growth of a tooth can itself be restricted, however, and the tooth molded by

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DENTAL ANOMALIES IN ELEPHANTS 177

extrinsic forces, while it is developing and not yet fully calcified.

In humans, growth and calcification of den- tal roots continues long after the crown is fully formed-in some cases for one or two decades after birth (Schour and Massler 1940). Roots appear to be especially vulnerable to deflections imposed by sinuses, foramina, and neural canals (Taylor 1978). Morphological variability of the crowns, in contrast, is more difficult to relate to their physical environ- ment. It is noteworthy that in Taylor's (1978) thoroughgoing catalogue of morphological variants among human teeth, physical con- straints were discussed for roots, but were not discussed in the context of crown formation; attrition and wear were the only mechanical influences invoked to explain variation in the shape of the crown.

Dental ontogeny in elephants is prolonged, and elephant teeth develop within a partic- ularly dynamic environment. They are sub- ject to an array of forces, which were enu- merated above. The morphological variants described here indicate that elephant teeth can be deformed during their development and that the physical distortion may involve either the arrangement or the shapes of the lamellae. Although the shape of a developing tooth ultimately becomes fixed as the lamel- lae harden and unite at their bases, disruption of just one lamella (as in the case of partial lamellae) provides a template on which later- developing lamellae are distinctively mold- ed. Moreover, a variety of tooth shapes can be produced by slight shifts in the relative positions of the lamellae before they become fixed.

Ordinarily this transient flexibility is a con- venient feature for elephant teeth, which are large in relation to the jaws they occupy and which are continually in formation and mo- tion through the jaws as the animal grows. However, this flexibility also renders the teeth more susceptible to morphological distortion by obstructions. Although not all anomalies can clearly be related to mechanical forces or physical constraints within the jaws, several of the anomalies described here were found to be associated with physical obstructions: abscesses, supernumerary teeth, malocclu-

sions, and other impediments to normal pro- gression or eruption. In other mammals, by contrast, morphological variants arising post- natally are usually the result of infection or traumatic injury (Colyer 1936).

The variation illustrated here (Figs. 3-10) is obvious and extreme, but the study of fab- ricational noise in elephant dentitions could also be applied to subtle features of mor- phology.

For example, at its crown, a lamella may bear indentations which divide it into indi- vidual cuspules. Herpin (1933) observed that this subdivision tends to be most pronounced in the more posterior lamellae of a tooth, and he proposed a simple mechanical cause for the pattern. His analysis is flawed (mastica- tion and progression in fact generate a dif- ferent pattern of forces from the one his mod- el requires), but his conceptual approach, which foreshadowed that of fabricational noise, is promising.

In its subtlest manifestations, fabricational noise appears as quantitative variation. For example, a portion of the quantitative vari- ability in lengths, widths, and numbers of lamellae recently revealed in E. maximus by Roth and Shoshani (1988) is in all likelihood fabricational artifact.

At one extreme, the number of lamellae and the linear dimensions of an elephant tooth could be predetermined; alternatively, they could reflect the amount of space available for the tooth in the growing jaw. The true situation is likely to be intermediate. The elongation of the tooth, the generation of new lamellae, the growth of the jaw, and dental progression are all interrelated processes whose rates, because of limitations in space, must be mutually accommodating.

TMM 108 and UNSM 39309 (Figs. 6-9) were not discouraged from producing new lamel- lae in abnormally cramped situations, which suggests that the rate of generation of new lamellae is to some extent intrinsic and pre- determined. If (within limits) that is the case, and if, on the other hand, the developing teeth respond in size and shape to pressure from the surrounding tissues (as many of the examples described above suggest), they may also respond to quick expansion of the alveo-

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178 V. LOUISE ROTH

lar cavity or enlargement of the jaw during relatively rapid growth. A well-fed, rapidly growing animal should therefore have rela- tively broadly spaced, uncompressed lamel- lae. Genetically controlled studies of tooth size and lamellar number in elephants of var- ious sizes would be highly desirable, if they could reveal the nature of the developmental interdependence of these variables. In the meantime, however, because elephants are difficult experimental subjects at best, a more detailed study of fabricational noise in ele- phant dentitions could prove fruitful.

The likelihood that growth rates influence tooth morphology has profound implications for systematics. Lamellar frequency, which reflects the thickness and spacing of lamellae in the teeth, has been one of the most com- monly used criteria for identifying and dis- tinguishing species of fossil elephants (Os- born 1942; Cooke 1947; Maglio 1973; Madden 1981). Yet, as Kurten and Anderson remarked (1980: p. 351), "species delineation in mam- moths is not sharp." For example, the number of species of North American mammoths rec- ognized has ranged from Maglio's (1973) pro- visional value of four, up to as many as 16 (Osborn 1942). Even when the same specific epithets are accepted, divisions and synony- mies between taxa vary from author to author (c.f., Osborn 1942; Maglio 1973; Kurten and Anderson 1980; Madden 1981). This confu- sion will not be resolved until we are able to discriminate between species-specific char- acters and fabricational noise.

As enhanced appreciation for fabricational noise in elephant dentitions could markedly reduce the number of species recognized within the family. If variability in elephant dentitions has a simple mechanical basis, the key to the dramatic morphological variation and change in the fossil record of the Ele- phantidae may lie not in a rapid evolutionary diversification and speciation (e.g., Stanley 1979; Minkoff 1983), but rather in a consid- erable phenotypic plasticity.

Acknowledgments I am grateful to the staff and curators of the

institutions enumerated in the Materials and

Methods section for access to specimens in their care. I especially thank my colleagues at the University of Texas at Austin/Texas Memorial Museum and the Nebraska State Museum for arranging the loan of mammoth specimens and K. Bryan (BMNH) for arrang- ing for the photography in Figs. 5 and 10. E. Effman, assisted by F. Avery, generously shared his time (in predawn hours) and ex- pertise with the CT scanner at the Duke Uni- versity Medical Center. For their suggestions and comments on the manuscript, I thank R. F. Kay, J. G. Lundberg, V. J. Maglio, S. Vogel, and S. Wainwright. This work was supported in part by NSF Grants DEB 79-18482 and BSR 85-16818 and grants from the Duke Univer- sity Research Council.

Literature Cited BUTLER, P. M. 1982. Some problems of the ontogeny of tooth

patterns. Pp. 44-51. In Kurten, B. (ed.), Teeth-Form, Function, and Evolution. Columbia University Press; New York.

COLYER, F. 1936. Variations and Diseases of the Teeth of Ani- mals. John Bale, Sons and Danielsson, Limited; London.

COOKE, H. B. S. 1947. Variation in the molars of the living African elephant and a critical revision of the fossil Probos- cidea of Southern Africa. American Journal of Science 245:434- 457, 492-517.

CURREY, J. D. 1984. The Mechanical Adaptations of Bones. Princeton University Press; Princeton, New Jersey.

DAVIS, D. D. 1949. Comparative anatomy and the evolution of vertebrates. Pp. 64-89. In Jepsen, G. L., E. Mayr, and G. G. Simpson, (eds.), Genetics, Paleontology and Evolution. Prince- ton University Press; Princeton, New Jersey.

GAUNT, W. A., AND A. E. W. MILES. 1967. Fundamental aspects of tooth morphogenesis. Pp. 151-197. In Miles, A. E. W. (ed.), Structural and Chemical Organization of Teeth. Academic Press; New York.

HERPIN, A. 1933. Un point particulier de la morphologie des molaires des Elephants explique par l'action des causes me- caniques. Archives du Museum National d'Histoire Naturelle (Paris) 6:125-128.

HICKMAN, C. S. 1980. Gastropod radulae and the assessment of form in evolutionary paleontology. Paleobiology 6:276-294.

HUTCHINSON, G. E. 1981. Random adaptation and imitation in human evolution. American Scientist 69:161-165.

KURTEN, B., AND E. ANDERSON. 1980. Pleistocene Mammals of North America. Columbia University Press; New York.

LANDMAN, N. H., D. M. RYE, AND K. L. SHELTON. 1983. Early ontogeny of Eutrephoceras compared to Recent Nautilus and Mesozoic ammonites: evidence from shell morphology and light stable isotopes. Paleobiology 9:269-279.

LAWS, R. M. 1966. Age criteria for the African elephant Loxo- donta a. africana. East African Wildlife Journal 4:1-37.

LAWS, R. M., AND I. S. C. PARKER. 1968. Recent studies on ele- phant populations in East Africa. Pp. 319-359. In Crawford, M. A. (ed.), Comparative Nutrition of Wild Animals. Symposium of the Zoological Society of London 21.

MADDEN, C. T. 1981. Mammoths of North America. Unpub- lished Ph.D. dissertation, University of Colorado. Boulder, Col- orado.

This content downloaded from 129.93.16.3 on Tue, 24 Sep 2013 17:41:31 PMAll use subject to JSTOR Terms and Conditions

DENTAL ANOMALIES IN ELEPHANTS 179

MAGLIO, V. J. 1972. Evolution of mastication in the Elephan- tidae. Evolution 26:638-658.

MAGLIO, V. J. 1973. Origin and evolution of the Elephantidae. Transactions of the American Philosophical Society 63:1-149.

McKAY, G. M. 1973. Behavior and ecology of the Asiatic ele- phant in southeastern Ceylon. Smithsonian Contributions to Zoology 125:1-113.

MINKOFF, E. C. 1983. Evolutionary Biology. Addison-Wesley Publishing Company; Reading, Massachusetts.

ORR, P. C. 1968. Prehistory of Santa Rosa Island. Santa Barbara Museum of Natural History; Santa Barbara, California.

OSBORN, H. F. 1942. Proboscidea. American Museum Press; New York.

RACHOOTIN, S. P., AND K. S. THOMSON. 1981. Epigenetics, pa- leontology, and evolution. Pp. 181-193. In Scudder, G. G. E., and J. L. Reveal (eds.), Evolution Today. Proceedings of the Second International Congress of Systematic and Evolutionary Biology. Hart Institute for Botanical Documentation. Carnegie- Mellon University; Pittsburgh.

RAFF, R. A., AND T. C. KAUFMAN. 1983. Embryos, Genes, and Evolution. Macmillan Publishing Company; New York.

REIF, W. E. 1973. Morphologie und Skulptur der Haifisch- Zahnkronen. Neues Jahrbuch fur Geologie und Palaontologie Abhandlung B 143:39-55.

ROTH, V. L. 1982. Dwarf mammoths from the Santa Barbara, California Channel Islands: Size, Shape, Development and Evolution. Unpublished Ph.D. dissertation, Yale University. New Haven, Connecticut.

ROTH, V. L. 1984. How elephants grow: heterochrony and the calibration of developmental stages in some living and fossil species. Journal of Vertebrate Paleontology 4:126-145.

ROTH, V. L. In press. Dwarfism and variability in the Santa Rosa Island mammoth: an interspecific comparison of limb-bone sizes and shapes in elephants. In Hochberg, F. G., Recent Ad- vances in Channel Islands Research: Proceedings of the Third California Islands Symposium. Santa Barbara Museum of Nat- ural History; Santa Barbara.

ROTH, V. L. In preparation. Species diversity and variability in the Elephantidae: are elephants overly split?

ROTH, V. L., AND J. SHOSHANI. 1988. Dental identification and

age determination in Elephas maximus. Journal of Zoology 214: 567-588.

SCHOUR, I., AND M. MASSLER. 1940. Studies in tooth develop- ment: the growth pattern of human teeth, part II. Journal of the American Dental Association 27:1918-1931.

SEILACHER, A. 1970. Arbeitskonzept zur Konstruktions-Mor- phologie. Lethaia 3:393-396.

SEILACHER, A. 1973. Fabricational noise in adaptive morphol- ogy. Systematic Zoology 22:451-465.

SEILACHER, A. 1979. Constructional morphology of sand dollars. Paleobiology 5:191-221.

SHARAWY, M., AND B. R. BHUSSRY. 1986. Development and growth of teeth. Pp. 24-44. In Bhaskar, S. N. (ed.), Orban's Oral His- tology and Embryology, Tenth Edition. C. V. Mosby Company; St. Louis.

SIKES, S. K. 1971. The Natural History of the African Elephant. Weidenfeld and Nicolson; London.

SIMPSON, G. G. 1953. The Major Features of Evolution. Columbia University Press; New York.

SOKAL, R. G., AND F. J. ROHLF. 1981. Biometry. Second Edition. W. H. Freeman and Company; New York.

STANLEY, S. M. 1979. Macroevolution: Pattern and Process. W. H. Freeman and Company; New York.

STOCK, C. 1935. Exiled elephants of the Channel Islands, Cal- ifornia. Scientific Monthly 41:205-214.

TAYLOR, R. M. S. 1978. Variation in Morphology of Teeth. Charles C. Thomas; Springfield, Illinois.

THOMPSON, D. 1942. On Growth and Form. Cambridge Uni- versity Press; Cambridge.

VRBA, E. S. 1987. Ecology in relation to speciation rates: some case histories of Miocene-Recent mammal clades. Evolutionary Ecology 1:283-300.

WAINWRIGHT, S. A., W. D. BIGGS, J. D. CURREY, AND J. M. GOSLINE. 1982. Mechanical Design in Organisms. Princeton University Press; Princeton, New Jersey.

WATSON, D. M. S. 1949. The evidence afforded by fossil ver- tebrates on the nature of evolution. Pp. 45-63. In Jepsen, G. L., E. Mayr, and G. G. Simpson, (eds.), Genetics, Paleontology and Evolution. Princeton University Press; Princeton, New Jersey.

This content downloaded from 129.93.16.3 on Tue, 24 Sep 2013 17:41:31 PMAll use subject to JSTOR Terms and Conditions