Mugino O. Kubo

The Inter-Relationship between Dietary and Environmental Properties and Tooth Wear: Comparisons of Mesowear, Molar Wear Rate, and Hypsodonty Index of Extant Sika Deer Populations

In reference to the evolutionary trend of increasing cheek tooth height in herbivorous ungulates, the causes of dental abrasion have long been debated. Interspecific comparisons of extant ungulates have revealed that both phytoliths in grass and external abrasive matter may play important roles. Using analysis of extant sika deer living in various environments and showing continuous latitudinal variation in food habits from northern grazing to southern browsing, we quantitatively evaluated the influence of dietary and environmental properties on three dental variables: mesowear score (MS), molar wear rate, and M3 hypsodonty index. We used 547 skulls and 740 mandibles from 16 populations of sika deer to obtain the dental measurements. We found that only graminoid proportion in diet correlated with MS and the molar wear rate, implying that phytoliths in grass abrade dental tissues. In contrast, annual precipitation in habitat was not correlated with any of the dental variables. We also found a significant correlation between the molar wear rate (selective pressure for high-crowned molars) and the M3 hypsodonty index of extant sika deer, implying an evolutionary increment in molar height corresponding to the molar wear rate. Our intraspecific comparative analyses provide further support for use of mesowear analysis as a paleodiet estimation method; it not only reveals staple food types (graminoids or dicots) but also implies regional or seasonal variation in the diet of the species.

Associated evolution of bipedality and cursoriality among Triassic archosaurs: a phylogenetically controlled evaluation

Abstract Bipedalism evolved more than twice among archosaurs, and it is a characteristic of basal dinosaurs and a prerequisite for avian flight. Nevertheless, the reasons for the evolution of bipedalism among archosaurs have barely been investigated. Comparative analysis using phylogenetically independent contrasts showed a significant correlation between bipedality (relative length of forelimb) and cursoriality (relative length of metatarsal III) among Triassic archosaurs. This result indicates that, among Triassic archosaurs, bipeds could run faster than quadrupeds. Bipedalism is probably an adaptation for cursoriality among archosaurs, which may explain why bipedalism evolved convergently in the crocodilian and bird lineages. This result also indicates that the means of acquiring cursoriality may differ between archosaurs and mammals.

ANALYSIS OF TRIASSIC ARCHOSAURIFORM TRACKWAYS: DIFFERENCE IN STRIDE/FOOT RATIO BETWEEN DINOSAUROMORPHS AND OTHER ARCHOSAURIFORMS

ABSTRACT Fossilized trackways have rarely been analyzed quantitatively to examine major trends and patterns in evolution despite their potential utility, especially in understanding locomotory evolution. In the present study, trackways of Triassic archosauriforms were analyzed. The analyses showed foot and stride lengths of archosauriforms increased from the Early to Middle Triassic, especially those of dinosauromorphs, which tripled. Dinosauromorphs were much smaller in foot length and stride length compared to other archosauriforms during the Early Triassic. They reached similar stride length compared with other archosauriforms during the Middle Triassic and similar foot length in the Late Triassic. Stride/foot ratio is significantly higher in dinosauromorphs compared to other archosauriforms throughout the Triassic. This relatively long stride length of dinosauromorphs is attributed to either faster speed or higher relative hip height that was probably caused by their digitigrade foot posture. Analyses of trackway data sets, especially in combination with precise trackmaker assignment and age determination, would bring us more thorough knowledge about locomotory evolution of tetrapods that complements body fossil evidence.

Geographical Body Size Clines in Sika Deer: Path Analysis to Discern Amongst Environmental Influences

Using Japanese sika deer from boreal and subtropical forests, we investigated the effects of environmental factors on body size variation and tested the following ecological hypotheses pertaining to Bergmann’s rule in mammals: (1) heat conservation, (2) heat dissipation, (3) starvation resistance, (4) food availability and (5) insularity. Data on body sizes and habitats of sika deer from 31 populations on the Japanese archipelago were collected. Body mass and cranial greatest length (CGL; reflecting skeletal body size) were measured and analysed separately among gender groups. Path analyses were used to clarify inter-variable relationships and estimate direct and indirect effects of environmental variables on body size. Consistent with Bergmann’s rule, a clear latitudinal cline of body size was found for sika deer. Subsequent path analyses showed that the abiotic factors, specifically mean annual temperature and annual precipitation, had significant negative effects on body size, and annual temperature had the greatest effect among tested environmental variables. Winter severity and food availability during spring were significantly associated with body mass but not with CGL. Both heat conservation and dissipation hypotheses were accepted and food availability and starvation resistance hypotheses were applicable to variation of body mass but not to CGL, indicating that phenotypic changes in fat reserves strongly influence variation in body mass. Path diagram modelling of inter-variable relationships fit well for females but not for males, and unexplained variation of male body size suggested the presence of unidentified factors. Variation in mating systems among populations may effect body size variation of the male sika deer.

Dental microwear of a Late Triassic dinosauriform, Silesaurus opolensis

Silesaurus opolensis belongs to Silesauridae, the closest sister group to dinosaurs. The present study analyzed the dental microwear patterns of Silesaurus opolensis. Low pit-to-scratch ratios imply they did not feed on hard objects. Unimodal distributions of both wear-facet and non-facet scratch orientations indicate simple orthal jaw movement. Scratch orientation and density differ between microscopic regions in Silesaurus, and unlike hadrosaurid dinosaurs, the microwear patterns of small areas are not identical to those of whole teeth.

Compensatory response of molar eruption for environment-mediated tooth wear in sika deer

Abstract Teeth move continuously in the occlusal direction after they have approached the occlusal plane, which is considered a compensatory mechanism for loss of tooth crown by wear, although quantitative data have been inadequate. To clarify phenotypic correlation of tooth eruption and wear 10 populations of wild sika deer (Cervus nippon) in Japan were investigated. Mandibles, with ages assigned by cementum annuli of incisor roots, were used to estimate molar wear rate and eruption timing. Molar eruption status was assessed by observation of the buccal side of molars and coded as 3 ordinal levels according to the appearance of a cervical line (a boundary between molar crown and root). Logistic regression of the eruption status revealed that eruption timing of the lower 3rd molar (m3) was significantly different among populations (P < 0.0001) and correlated with m3 wear rate; faster molar wear resulted in faster molar eruption (P < 0.01). Eruption timing of lower 1st (m1) and 2nd (m2) molars did not show significant correlation with wear rate, possibly due to less variation in eruption timing in m1 and m2 than for m3. These findings indicate that the compensatory response of molar eruption occurs for m3 in accordance with wear mediated by environments.

Three-dimensional tooth surface texture analysis on stall-fed and wild boars (Sus scrofa)

Categorizing the archaeological remains of Sus scrofa as domesticated “pigs” or wild “boars” is often difficult because of their morphological and genetic similarities. For this purpose, we tested whether feeding ecological change of S. scrofa that accompanied their domestication can be detected based on the three-dimensional texture created on the tooth enamel surface by mastication. We scanned the lower tooth surface of one wild and one stall-fed populations of modern S. s. leucomystax and one wild population of S. s. riukiuanus by using a confocal laser microscope. The average body weight of S. s. leucomystax is twice as heavier as that of S. s. riukiuanus. The textures were quantified using the industrial “roughness” standard, ISO 25178, to prevent inter-observer errors and to distinguish small differences that were difficult to detect by two dimensional image observation. The values of parameters related to height and volume were significantly larger in the stall-fed population. Twenty parameters differed significantly between the stall-fed and wild population of S. s. leucomystax, which indicated that the feeding ecological difference affected the ISO parameters of the two boar populations. Six parameters also differed between the wild populations of S. s. leucomystax and S. s. riukiuanus. Surprisingly, no parameter differed between the populations of stall-fed S. s. leucomystax and wild S. s. riukiuanus. Consumption of hard nuts and/or agricultural fruits and crops by the wild population of S. s. riukiuanus may have produced a tooth surface texture similar to that of the stall-fed population of S. s. leucomystax. Further analysis of S. s. riukiuanus with a known diet is necessary to conclude whether ISO parameters reflect the dietary transition accompanying the domestication of Sus (e.g., wild, semi-domestic, and domestic). Until then, caution is needed in discriminating domesticated populations from wild populations that mainly feed on hard objects.

Masticatory jaw movement of Exaeretodon argentinus (Therapsida: Cynodontia) inferred from its dental microwear

Dental microwear of four postcanine teeth of Exaeretodon argentinus was analyzed using both two dimensional (2D) and three dimensional (3D) methods to infer their masticatory jaw movements. Results of both methods were congruent, showing that linear microwear features (scratches) were well aligned and mostly directed to the antero-posterior direction in all four teeth examined. These findings support the palinal masticatory jaw movement, which was inferred in previous studies based on the observation of gross morphology of wear facets. In contrast, the lack of detection of lateral scratches confirmed the absence of the lateral jaw movement that was also proposed by a previous study. Considering previous microwear studies on cynodonts, palinal jaw movements observed in Exaeretodon evolved within cynognathian cynodonts from the fully orthal jaw movement of its basal member. Although there are currently only three studies of dental microwear of non-mammalian cynodonts including the present study, microwear analysis is a useful tool for the reconstruction of masticatory jaw movement and its future application to various cynodonts will shed light on the evolutionary process of jaw movement towards the mammalian condition in more detail.

Nonplantigrade Foot Posture: A Constraint on Dinosaur Body Size

Dinosaurs had functionally digitigrade or sub-unguligrade foot postures. With their immediate ancestors, dinosaurs were the only terrestrial nonplantigrades during the Mesozoic. Extant terrestrial mammals have different optimal body sizes according to their foot posture (plantigrade, digitigrade, and unguligrade), yet the relationship of nonplantigrade foot posture with dinosaur body size has never been investigated, even though the body size of dinosaurs has been studied intensively. According to a large dataset presented in this study, the body sizes of all nonplantigrades (including nonvolant dinosaurs, nonvolant terrestrial birds, extant mammals, and extinct Nearctic mammals) are above 500 g, except for macroscelid mammals (i.e., elephant shrew), a few alvarezsauroid dinosaurs, and nondinosaur ornithodirans (i.e., the immediate ancestors of dinosaurs). When nonplantigrade tetrapods evolved from plantigrade ancestors, lineages with nonplantigrade foot posture exhibited a steady increase in body size following Cope’s rule. In contrast, contemporaneous plantigrade lineages exhibited no trend in body size evolution and were largely constrained to small body sizes. This evolutionary pattern of body size specific to foot posture occurred repeatedly during both the Mesozoic and the Cenozoic eras. Although disturbed by the end-Cretaceous extinction, species of mid to large body size have predominantly been nonplantigrade animals from the Jurassic until the present; conversely, species with small body size have been exclusively composed of plantigrades in the nonvolant terrestrial tetrapod fauna.