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Dental microwear reveals mammal-like chewing in the neoceratopsian dinosaur Leptoceratops gracilis - PubMed

  • ️Fri Jan 01 2016

Dental microwear reveals mammal-like chewing in the neoceratopsian dinosaur Leptoceratops gracilis

Frank J Varriale. PeerJ. 2016.

Abstract

Extensive oral processing of food through dental occlusion and orbital mandibular movement is often cited as a uniquely mammalian trait that contributed to their evolutionary success. Save for mandibular translation, these adaptations are not seen in extant archosaurs or lepidosaurs. In contrast, some ornithischian dinosaurs show evidence of precise dental occlusion, habitual intraoral trituration and complex jaw motion. To date, however, a robust understanding of the diversity of jaw mechanics within non-avian dinosaurs, and its comparison with other vertebrates, remains unrealized. Large dental batteries, well-developed dental wear facets, and robust jaws suggests that neoceratopsian (horned) dinosaurs were capable chewers. But, biomechanical analyses have assumed a relatively simple, scissor-like (orthal) jaw mechanism for these animals. New analyses of dental microwear, presented here, show curvilinear striations on the teeth of Leptoceratops. These features indicate a rostral to caudal orbital motion of the mandible during chewing. A rostrocaudal mandibular orbit is seen in multituberculates, haramiyid allotherians, and some rodents, and its identification in Leptoceratops gracilis is the first evidence of complex, mammal-like chewing in a ceratopsian dinosaur. The term circumpalinal is here proposed to distinguish this new style of chewing from other models of ceratopsian mastication that also involve a palinal component. This previously unrecognized complexity in dinosaurian jaw mechanics indicates that some neoceratopsian dinosaurs achieved a mammalian level of masticatory efficiency through novel adaptive solutions.

Keywords: Ceratopsia; Chewing; Dental microwear; Dinosauria; Jaw action; Jaw mechanics; Mastication; Ornithischia.

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Conflict of interest statement

The author declares there are no competing interests.

Figures

Figure 1
Figure 1. Histogram of striation orientations pooled from all teeth examined in Leptoceratops gracilis (CMN 8889).

Classes are in 10° increments. A single mode is present, with the greatest frequency of scratches in an arc from 30° to 60° . Line intersecting largest bar is the pooled sample mean angle (44.1°). N = 1,504.

Figure 2
Figure 2. Dental microwear on representative teeth of Leptoceratops gracilis (CMN 8889).

(A) Light microscope images of the seventh left maxillary, and (B) sixth left dentary teeth showing curvilinear microwear traversing the entire occlusal surface. Arrows indicate the direction of initiation and exit of the power stroke. Microwear near the site of initiation is oriented caudodorsally whereas the same wear near the end of the stroke is oriented rostrocaudally, a ≈50° shift in orientation. Dotted lines in (A) and (C) demarcate approximate junction of hard mantle dentine (HMD) with orthodentine (O). (C, D) SEM micrographs imaged from rectangles indicated in (A) and (B) respectively. At this magnification (100×) wear appears uniform with a striation dominated texture. (E, F) Rose diagrams of angular data from striations in (C) and (D). Rose diagrams are on a unit circle and summarize angular orientation relative to wear on a left dentary tooth. 90° = caudal and 180° = ventral directions. Values in lower right quadrant of rose diagrams are the mean angle and the length of the mean vector (r). Arrows in rose diagrams indicates direction and magnitude of r.

Figure 3
Figure 3. Light microscope images of additional teeth from Leptoceratops gracilis (CMN 8889) showing semicircular dental microwear.

(A) Eighth right dentary tooth. (B) 12th right dentary tooth. (C) 10th right maxillary tooth. (D) 11th right maxillary tooth. Colored arrows correspond to those in Fig. 6 and indicate (where visible) the overall orientation of microwear at the initiation (blue) and ending (red) of the power stroke. Arrow (black) in (D) indicates step between two facets (f1, f2) formed via differential wear. These facets were likely caused by the occlusion of this tooth against two opposing right dentary teeth at different stages of eruption. Images not to scale.

Figure 4
Figure 4. Micrographs of selected teeth from each of the four dental quadrants in Leptoceratops gracilis (CMN 8889).

(A) 13th left dentary tooth, (B) sixth left maxillary tooth, (C) eighth right dentary tooth, and (D) fourth right maxillary tooth. Orientations of micrographs are as follows; apical at left and basal at right. In left dentary and left maxillary micrographs distal is at top and mesial is at bottom. In right dentary and right maxillary images distal at bottom and mesial is at top. Rose diagrams summarize angular orientation relative to wear on a left dentary tooth. Description of rose diagram orientation and data are the same as in Fig. 2.

Figure 5
Figure 5. Light microscope images of dental microwear in additional specimens of Leptoceratops.

(A) Isolated right maxillary tooth assigned to Leptoceratops (YPM VPPU 018133) showing semicircular dental microwear. (B) Fifth right dentary tooth from Leptoceratops gracilis (AMNH FR 5205) showing a predominance of mesiodistally oriented wear near the base of the facet, and in a similar orientation as the labial shelf (LS). Colored arrows correspond to those in Fig. 6 and indicate the overall orientation of microwear at the initiation (blue) and ending (red) of the power stroke. Images not to scale.

Figure 6
Figure 6. Model of circumpalinal mastication in Leptoceratops.

(A) Adduction of the lower jaw dominated by action (blue arrow) of the m. adductor mandibulae externus group (mAME) and beginning of the power stroke of mastication. (B) Progression of the power stroke into a palinal (retraction) phase dominated by transition from the mAME to action (red arrow) of the m. addcutor mandibulae posterior (mAMP). Below each skull is a left dentary tooth with colored arrows demarcating the segment and direction of microwear striations that result from the aforementioned muscular actions.

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Grants and funding

Funding for this research was provided by grants from the Jurassic Foundation, Sigma Xi: Grants in Aid, The Geological Society of America, and a Stephen J. Gould Award from the Paleontological Society. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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