Unexpected morphological diversity in ancient dogs compared to modern relatives

Abstract

Dogs are among the most variable species today, but little is known about the morphological variability in the early phases of their history. The Neolithic transition to farming may have resulted in an early morphological diversification as a result of changes in the anthropic environment or intentional selection on specific morphologies. Here, we describe the variability and modularity in mandible form by comparing 525 dog mandibles from European archaeological sites ranging from 8100 to 3000 cal. BC to a reference sample of modern dogs, wolves, and dingoes. We use three-dimensional geometric morphometrics to quantify the form of complete and fragmented mandibles. We demonstrate that an important morphological variability already existed before the Bronze Age in Europe, yet the largest, smallest, most brachycephalic or dolichocephalic extant dogs have no equivalent in the archaeological sample, resulting in a lower variation compared to modern relatives. The covariation between the anterior and posterior parts of the mandible is lower in archaeological dogs, suggesting a low degree of intentional human selection in early periods. The mandible of modern and ancient dogs differs in functionally important areas, possibly reflecting differences in diet, competition, or the implication of ancient dogs in hunting or defence.

Keywords: chalcolithic; dog; mandible; neolithic; shape.

Figure 1.

Origin and fragmentation patterns of the archaeological mandibles. (a) Map showing selected sites dating from the Mesolithic to the end of the Romanian Chalcolithic in Western Europe and Romania. Site key, chrono-cultural attribution and dating are presented in the electronic supplementary material, dataset S1. (b,c) Three-dimensional landmarks considered in the geometric morphometric analyses for the most complete (b) or fragmented (c) mandibles, matching the 10 subsets of fragments (letters A to J), represented on lateral (top image) or medial (bottom image) views of the mandible. Anatomical landmarks are in red, sliding semi-landmarks of curves are in blue, sliding semi-landmarks of surfaces are in green. ‘Module 1’ and ‘module 2’ represent the two modules used in the modularity tests performed on the complete mandibles. Definitions of the anatomical landmarks are provided in the electronic supplementary material, table S2. (Online version in colour.)

Figure 2.

Visualization of the variability in mandible shape and size in ancient dogs with complete mandibles (fragment A), with comparison with ancient wolves and modern Canis. (a) Principal component (PC) analyses on modern and ancient specimens with fragment A (66 modern dogs, eight modern dingoes, eight modern wolves, 127 ancient dogs and four ancient wolves). Icon size is proportional to the log₁₀ of the centroid size. Deformations at the minimum and maximum of the PC axes are represented in blue and pink, respectively. (b) Tree depicting the morphological similarity between mandibles based on pairwise Euclidean distances. Icon size is proportional to the log₁₀ of the centroid size. (c) Boxplot of the centroid sizes of modern and ancient canids. Ancient canids are orange, modern canids are in black. The labels of modern dogs indicate the breed (key is presented in the electronic supplementary material, dataset S1). Groups (a), (b), (c), (d), (e), (f) and (g) are mentioned in the text. (Online version in colour.)

Figure 3.

Visualization of the differences between modern (black/grey) and ancient (orange) dogs, based on the results of the CVA performed on mandible shapes (a) or allometry-free shapes (b) for fragments A and I. The sample size and percentage of cross-validation are indicated. On the upper line, shapes at the minimum and maximum of CV scores are superimposed to the mean shape of the CVA and vectors of deformations between the two shapes are represented. On the lower line, deformations from the mean shape to the minimum or maximum of the CV scores are magnified by 3. See the electronic supplementary material, figure S13 for visualizations with other fragments. (Online version in colour.)