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- MATERIALS AND METHODS
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The serrated, or denticulated, ziphodont teeth of theropod dinosaurs display variability in their extent of denticulation. The functional model proposed here tests the hypothesis that denticles will not exist in areas that do not frequently contact the substrate. This area, defined as the “dead-space,” is determined by the direction the tooth moves through the fleshy substrate. The extent of denticulation, as well as the dead-space dimensions, is measured from photographs of 235 isolated and in situ theropod teeth, to determine a meaningful relationship between the two variables. Both Euclidean and geometric morphometric methods are employed, and the data are expressed in bivariate and ordination plots. The model predicts the direction of tooth movement through the curvature of the tip/apex. Tooth position and taxon are considered. The results show that the mesial margin is usually partially denticulated, while the distal margin is usually totally denticulated. Curved teeth have large dead-spaces, and tend to be less denticulated mesially. Straighter teeth are more extensively denticulated, to the point where they became symmetrical. The mesial denticulation is determined by the dead-space, and dictated by the substrate contact. The dead-space almost always predicted less extensive denticulation; a consequence of the model's limitations. Tooth curvature increases with a more distal position, due to rotation based on the proximity to the hinge. Denticulation indicates that theropods used a distally oriented puncture to modify the substrate, similar to modern analogues. Although there is little taxonomic variation, Troodontidae show unique and extreme degrees of mesial denticulation. Anat Rec, 292:1297–1314, 2009. © 2009 Wiley-Liss, Inc.
- Top of page
- MATERIALS AND METHODS
- Literature Cited
Theropod dinosaurs were the major terrestrial predators for most of the Mesozoic. Because of their large size and structural features, there is immense interest in the feeding dynamics of these animals. Therefore, theropod functional morphology in the context of feeding behavior has been investigated from many perspectives. Modeling cranial structuring, musculature, and kinesis has given insight into the skull's resistance to stress, potential bite force, and niche partitioning among species (Busbey, 1995; Henderson, 1998, 2002; Mazzetta et al., 1998; Molnar, 1998; Rayfield et al, 2001, 2007; Rayfield, 2004). Forelimb structure indicates a range of ability for securing prey with the manus (Holtz, 2002; Ostrom, 1969; Sereno, 1993; Tykoski and Rowe, 2004). Hind limb structure, as well as general size and body dimensions, is correlated with the theropod running speed (Farlow et al., 1995; Fastovsky and Smith, 2004; Larson, 1997; Seller and Manning, 2007), leading to several conclusions about the hunting and/or scavenging abilities (Horner and Lessem, 1993; Horner 1994). Although dentition is most definitely involved in food processing, and most likely acquisition, significantly less research has been done concerning the morphology of theropod teeth from a functional perspective.
This may be due to the unique dental morphotype of theropods. The majority of theropod dinosaurs possess ziphodont dentition, which is characterized by laterally compressed, curved, serrated teeth (Langston, 1975; Prasad and Lapparent de Broin, 2002). The degree of curvature and flattening can vary greatly (i.e., tyrannosaurids have lost much of their lateral compression), but all appear to possess serrations. These serrations, referred to as denticles, rest on carinae located along the margins of the tooth. The majority of carnivorous archosaurs throughout the Mesozoic era possessed ziphodonty (Benton, 2004). These include theropods, crurotarsians, and basal archosaurs (Benton, 2004; Farlow et al., 1991; Holtz, 2004; Senter, 2003; Smith et al., 2005). Within modern archosaurs though (i.e., crocodylians) it has been replaced by undenticulated, conical teeth. Ziphodonty has also convergently appeared in other tetrapod clades, including Permian pelycosaurs such as Dimetrodon (Farlow et al., 1991). Ziphodonty only occurs at present in certain members of the squamate family Varanidae (Auffenberg, 1981; Molnar, 2004).
Several comparative morphometric studies have been conducted on the diversity of theropod teeth, but most of these have focused primarily on the description and/or taxonomic identification of isolated teeth (Carr and Williamson, 2004; Currie et al., 1990; Molnar, 1998; Sadlier and Chapman, 1999; Sankey et al., 2002; Smith 2005, 2007; Smith et al., 2005; Samman et al., 2005). Consequently, there has been little analysis of dental function. Chandler (1990) suggested that theropod teeth are designed to cut by a combination of puncturing and drawing through flesh; the primary substrate modified by theropods. Farlow et al., (1991) depict these teeth as multipurpose implements, used for killing, cutting, and crushing bone. Because of increased tooth width and bending resistance, bone crushing may have been possible in larger taxa such as tyrannosaurids (Meers, 2003). Henderson (1998, 2002) used tooth lengths, heights, and orientations in addition to theropod skull measures to determine relative skull strengths and potential niche partitioning among theropods. Sadlier and Chapman (2002) used geometric morphometrics to determine that tooth shape reflects the variability in force applied to different tooth positions within individuals.
To gain better insight into the function of theropod teeth, researchers have also investigated the morphology and behavior of living ziphodont varanid analogues, especially the Komodo monitor, Varanus komodoensis. Studies of V. komodoensis indicate a unique feeding methodology associated with ziphodonty (Auffenberg, 1979, 1981; D'Amore and Blumenschine, in press), and cranio-dental similarities between theropods and V. komodoensis suggest similar food processing methods (Busbey, 1995; Carpenter, 1998; Molnar and Farlow, 1990; Paul, 1988; Rayfield et al., 2001).
Denticles are assumed to be a crucial component in theropod tooth function, yet few studies have described their functional value. In a study of the teeth of elasmobranch sharks, Frazzetta (1988) proposed that the serrated edges allow a blade to move through a substrate with less force than a smooth blade would require. As the serrations are drawn across this substrate they bind to a small portion of it and tear it, a mechanism described as “grip and rip” (Abler, 1992, p 178). Less force is needed to draw the serrated surface across the substrate because friction is reduced. Serrations are functionally analogous to denticles, so that a denticulated shark tooth would theoretically move into and through flesh more efficiently than one with smooth edges.
Abler (1992) simulated denticulated tooth performance in tyrannosaurids by using tooth casts and actual metal blades to determine the force needed to modify flesh. His results suggest that tyrannosaurids did not use their denticles to cut through meat by widening the size of a wound, as do sharks. Instead, tyrannosaurids used their teeth to “puncture and grip,” and denticles facilitated this (Abler, 1992, p 179).
Based on these results, it would seem to be advantageous to have denticulated carinae along the portions of the tooth margins that contact the substrate. Several authors have noted that the entire margin may not be denticulated. This is especially true for the mesial, or leading edge, of a tooth crown (Fig. 1). Chandler (1990, p 38) stated that mesial denticles near the base “become so small that they often appear to merge to form an unserrated keel.” Currie et al. (1990) report that mesial denticles in dromaeosaurids occur on less than half the length of the margin. Molnar and Carpenter (1989) note a similar condition in tyrannosaurids (see also Carr and Williamson, 2004). Abler (1992) states that in tyrannosaurids the mesial denticulation “tends to end where the tooth meets it maximum width.” Smith (2005) reported that the denticulated mesial carina terminates above the tooth base, but distal denticulation extends to the base in tyrannosaurids. Smith also noted a trend of decreasing mesial denticulation as each tooth moves closer to the hinge in position. The distance from the base of the mesial carina to the base of the tooth crown shows significant variance (Samman et al., 2005).
This study proposes a model that explains the variability in denticulation in functional terms. The model predicts that denticulated carinae will only exist in areas that make frequent contact with the substrate during typical feeding behaviors. These denticles will modify the substrate they contact, assisting the tooth in processing food. Conversely, areas with little contact with the substrate will lack denticles.
The amount of contact any portion of a tooth has with the substrate is based on the direction the tooth moves relative to the substrate. Rieppel (1979, p 812) describes this tooth movement or “line of action” for Varanus salvator. The orientation and curvature of the line of action of any tooth depends on its position relative to the hinge or hinges that the host bone rotates around, and the necessity that the apex of the tooth makes contact with the substrate. The apex has the smallest volume, focusing the force onto a smaller area and increasing the likelihood of puncturing the substrate (see also Chandler, 1990; Frazzetta, 1988; Lucifora et al., 2002). This results in axial loading of the tooth, which increases efficiency and reduces the chance of breakage.
As a tooth moves along its defined line of action, the degree of contact with the substrate will vary across the tooth's area. Opposite to the line of action is an area of the tooth that may not contact the substrate; this area is defined here as the tooth's “dead-space” (Fig. 2). This condition is considered analogous to the “trailing surface” of mammal molars (Evans and Sanson, 2003). This model predicts that because the dead-space does not make contact with unmodified substrate, it should have no denticles. The purpose of this study is to test this model of theropod tooth function by comparing the extent on denticulation with the dead-space. Theropod teeth are photographed and analyzed using both Euclidean and geometric morphometrics, focusing mainly on apex characters and denticulation. The line of action is determined based on the apex morphology, and a dead-space is proposed for each tooth. This dead-space is then compared to the degree of denticulation. If denticulation does reflect the extent of contact between a given tooth and the substrate, I hypothesize that the height of the dead space will correlate with the degree of denticulation. Conversely, the null hypothesis states that there is no meaningful relationship between the dead-space and the extent of denticulation. This is also examined in relationship to tooth position in the dental arcade, as well as the clade of the theropod it came from.
Figure 2. A diagrammatical representation of a theropod tooth and its interaction with the substrate. 1: The tooth moves toward the substrate in the direction of the line of action. 2: The tooth punctures the substrate apex first to allow for axial loading. 3: The tooth continues to move through the substrate along the line of action. Opposite this motion a dead-space forms (shaded). The arrow indicates the height of the dead space, which is the point where denticulation would terminate if it is dictated by frequent contact with the substrate.
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