Authors
Parés-Casanova, Pere M.
Abstract
Form can be viewed as the geometrical combination of size and shape. It can be quantified by geometric morphometrics. Allometry is the statistical association between size and shape. Form of occlusal surfaces of both M1 from twenty-five clean skulls from horses belonging to the ‘Catalan Pyrenean Horse’ (Cavall Pirinenc Català’) breed were evaluated by means of geometric morphometric methods. No allometric changes (i.e., shape change according to size) was observed. These results suggest that the occlusal contour form does not change during wearing, a process that will be continuous during all horse’s life, thus molars with bigger occlusal did not have significantly different shape so jugal tooth occlusal contour (but which includes no information on occlusal relief) seems to be unsuitable as an indicator of age. Although it has been indicated that multiple anatomical regions on the occlusal surface of the jugal tooth can be used as an indicator of age, form is not suitable for this purpose. These results add to the corpus of information on how molar teeth change with wear among equids, and how similarities in form are maintained at comparable points throughout the wear sequence.
Introduction
Although they are the hardest of tissues, teeth are worn away by a constant use (Sisson and Grossman 1985) (Dyce, Sack, and Wensig 1999). The horse, like other domestic mammals, has a heterogeneous dentition that consists of incisors, canines, premolars, and molars. These latter are characterized by being hypsodont, e.g. possessing long crowns that are completely covered by coronal cement at eruption and continuing to erupt in length after apparition (Barone 1999) (Dyce, Sack, and Wensig 1999). Such teeth last until death (Barone 1999).
The equine masticatory apparatus is one of the best studied dental systems in domestic mammals. Among other, it can express feeding behaviour, physiology, and morphology. The crowns of equid jugal teeth (premolars and molars, “cheek teeth”) are made up of dentine, enamel and cementum (Dixon 2002). Cementum surrounds the crown and acts as an attachment for the periodontal ligament, securing the tooth to the alveolar bone, while it also contributes to the bulk of the crown. Pattern of tooth occlusal surfaces is response to normal growth processes, environmental influences, dental treatment, pathology and ageing wearing -the relative proportion of tooth-to-tooth contact (attrition), food-to-tooth contact (abrasion) (Taylor et al. 2013), erosion (dissolution by acidic substances), and abfraction (tissue loss that occurs along the gingival margin)-. Thus, the occlusal surface does not remain static throughout life but continually change. This wearing depends on the type of diet, individual and breed variation in the hardness of dental tissue, the ingestion of hard phytoliths (grits) in the diet, etc (Dixon 2002) (Ungar 2015). Jugal teeth slowly erupt to compensate for the constant grinding, which wears away 2-3mm per year. Zooarchaeologists have extensively recorded occlusal traits in the dentition in studies of equids’ remnants. Changes of this nature have particular relevance for this kind of researches.
Compared to traditional morphometrics, Geometric Morphometrics (GM) capture the shape of an object using coordinates retrieved from landmarks established on homologous points, maximizing the amount of biological information captured Cartesian coordinates instead of linear distances (Bookstein 1991). Shape is the geometric feature of an object except for its size, position and orientation.
Some researchers have considered a great variation in dental structures according to wear. The opinions regarding conservation of occlusal surface during wearing are not either uniform in literature. For instance, Carmalt finds a significant positive relationship between occlusal outer surface area (area of external cementum) and horse age for this tooth (Carmalt 2009). Inconsistency of researchers´ views led to the realization of this study.
So, we used GM techniques to analyse the relationship between occlusal molar surface size and shape in in the “Cavall Pirinenc Català” breed (CPC, ‘Catalan Pyrenean Horse’), a local breed from Catalonia (NE Spain). We investigate specifically the effects of allometry of occlusal size against occlusal shape in first maxillary (upper) molars (M1). The results of this study can contribute to more morphological information about ‘natural’ tooth surface form (size+shape) among permanent grazing horses. Moreover, dental form analysis may be used to compare dental morphology between horse population independently from occlusal topographic traits (enamel foldings), which can vary with wear. To the author’s knowledge, no similar work has been performed on any other horse breed to date. Ideally, this type of study should be performed directly on 3D, but there are many GM dental studies that have been successfully performed on 2D images of the teeth.
Material and methods
Sampling
Twenty-five dentate skull specimens were obtained from horses belonging to the CPC breed. CPC represents a small local population (<4 600 individuals) bred mainly for meat production in the NE part of the Pyrenees, being compact and broad-built (Infante González 2011). It is reared outdoors throughout the year (Infante Gil 2011), grazing fresh high mountain grasslands, near 3,000 m above sea level during spring and summer (Infante González 2011). Animals do not receive any systematic clinical care and live under extensive conditions (Infante González 2011).
Maxillary first molars (M1) were selected for this study as it offers a larger coronal cementum surface than mandibular counterpart. M1 erupts between 9-12 months of age. To maximize sample sizes, pieces from both sides were sampled. No cases of tooth diseases or abnormal occlusion were detected but teeth with severe attritional wear were included. Specimens are currently held at the private collection of the author. The cohort consisted predominantly of females, but no distinction between sexes was made for this study. None of the horses in this study had ever had their teeth floated (rasped) although it is uncertain what effect, if any, this may have on the anatomical regions measured as the occlusal surface of the teeth should not be floated during routine treatment.
Geometric morphometrics
M1 were photographed into their alveoli in their occlusal view [Facies occlusalis] with a Nikon® D70 digital camera (image resolution of 2 240 x 1 488 pixels) equipped with a Nikon AF Nikkor® 28-200 mm telephoto lens, on a grid baseboard, column, and adjustable camera arm, and ensuring that the lens was parallel to the baseboard. One picture was taken for each dental piece. A total of 6 homologous landmarks distributed on the right and left M1 was assessed (figure 1). Enamel pattern (protocone, hypocone, etc.) was excluded from the analysis because of their high wearing on some pieces. The software TpsUtil v. 1.50 (Rohlf 2015) was used to prepare and organize the files, which were digitized twice using TpsDig v. 2.16 (Rohlf 2015).
Allometry
To understand the change of occlusal shape, we tested the correlation between the external configurations (determined by the landmarks) and occlusal size, e.g., the influence that the occlusal size and location may have on the outline occlusal shape. Size was obtained from Centroid Size (CS), which is a measure of scale and can be defined as the square root of the summed squared deviations of the coordinates from their centroid (centroid being the "gravity centre" of the structure) (Adams, Rohlf, and Slice 2013). CS can be assimilated to “occlusal surface” in this research. Normality of CS was assessed by a Shapiro-Wilk W test. Covariation size -expressed as CS- and shape were investigated with multivariate (Procrustes coordinates) regression. The amount of shape variation explained by the proportion of variation for which the regression accounts as a percentage of the total variation was assessed with permutation tests (10,000 permutations). Permutation tests do not require normally distributed variables and can be applied to multivariate datasets that are not of full rank, such as Procrustes shape coordinates. Natural log transformation was applied to scale CS relative to the mean.
Morphometric and statistical analyses were conducted using the MorphoJ v. 1.06c (Klingenberg 2011) and PAST v. 2.17c (Hammer, Harper, and Ryan 2001) packages, with a 95% confidence level.
Results
Size
The distribution of CS did not differ significantly from normal (W=0.978; p=0.107).
Relationship between size and shape
Our analysis finds that there is a small but significant allometric variation in M1 shape. Shape was not related to size (Wilk’s λ=0.981; F12, 87=0.14; p=0.236) with a mere 1.4% of shape change explained by size change (Figure 2). The null hypothesis of isometry was not rejected (allometry was not present) so size represents no confounding factor for shape change.
Discussion
Our geometric morphometric investigation confirmed the absence of allometries in the occlusal contour of upper first molar (M1) of “Cavall Pirinenc Català” breed (CPC), e.g. shape (occlusal contour) remains similar independently of size (occlusal area), so mesial and/or distal borders of the tooth is not affected by wearing. As the allometric effect is very small, so it cannot be considered responsible for the morphological occlusal shape variation. These results can be deduced as ‘natural’ grazing wear as CPC is managed under extensive conditions (grazing) all year and receives no dental care.
It has been stated that the continued occlusal and distal wear has a strong influence on size and shape variation. The individual’s behavioural history influences tooth wear throughout the lifetime (Taylor et al. 2013). Mesowear reflects the amounts of attritive (tooth-on-tooth) wear and abrasive (food-on-tooth) wear. Equine jugal teeth taper towards their apices (Carmalt 2009) but it has been demonstrated that age and wearing have no significant effects on the overall occlusal enamel shape pattern of jugal teeth. Our data confirms this fact, as occlusal surface will not reduce its area over time although enamel and dentin ridges, contained within the confines of the outer cementum, can change. Indeed, the occlusal patterns of enamel foldings have been said to vary with wear, but according to our conclusions, occlusal shape does vary in a uniform wear pattern, so, in any case, shape contour must not be considered to predict age in horses. So, although it has been indicated that there are multiple anatomical areas on the occlusal surface of the jugal series can be used as an indicator of horse age (Carmalt 2009), form is not suitable for this purpose.
In cases in which the age of horses cannot be accurately determined from incisor examination, jugal teeth may be used (Carmalt 2009), but not on their shape. As this study has been performed on specimens belonging to the same breed, which is managed under quite similar conditions, the results obtained in this research provide meaningful insights into natural occlusal M1 form (size+shape) in a semi-extensive managed breed.
This wearing could be tested for other equine populations with different diets. Further studies would also be required for the rest of both upper and lower molars to explore whether similar results appear.
Further details
Supporting information
The contents of all supporting data are the sole responsibility of the author. Queries or any other issues regarding errors are requested to be addressed to him.
Data availability
Data are available upon reasonable request to author.
Conflicts of interest
The author confirms to have no conflict of interest in connexion to the study.
References
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