## 2 Materials and Methods
### Processing of the Primary Data: Geometric Morphometrics  
Photographs were taken as described in our previous article [12]. They were taken orthogonally to the medial aspect of each calcaneum at 30 cm of distance between the camera and the calcanea. We used a Nikon(r) D3400 (Nikon, Barcelona, Spain) digital camera in a fixed position equipped with an 18-55 mm objective set at a focal distance of 55 mm. To control the position and inclination of the camera, we used a spirit level, which ensured that the camera was situated identically in all photographs. A scale was included in each photograph to facilitate calibration by the software. We saved each image in JPG format.  
A set of 20 2D coordinates of homologous anatomical landmarks was collected on the fibular aspect (Figure 1) using the tpsDig v. 2.16 program [13]. Landmark definitions are: (1) Most concave point below the _tuber calcanei_; (2) Craniodorsal point of the _calcaneal tuber_; (3) Most dorsal point of the _calcaneal tuber_; (4) Caudoventral point of gastrocnemius enthesis; (5) Intersection point between the calcaneum neck and _sustentaculum tali_; (6) Uppermost dorsal point of _sustentaculum tali_; (7) Most caudal point of _sustentaculum tali_; (8) Most cranioventral point of _sustentaculum tali_; (9) Uppermost dorsal point of dorsal astragalus articular surface (_facies articulares talares_); (10) Most cranial point of dorsal face of _facies articulares talares_; (11) Most concave point between landmarks 10 and 12; (12) Most cranial point of malleolus articular surface; (13) Most concave point between landmarks 12 and 14; (14) Most ventral point of _facies articularis malleolaris_; (15) Most concave ventral point to _facies articularis malleolaris_; (16) Most cranioventral point of inferior _facies articulares talares_; (17) Most cranioventral point of cubonavicular articular surface (_facies articularis cuboidea_); (18) Most concave point of the caudal edge of the _facies articularis cuboidea_; (19) Most caudal point of _facies articularis cuboidea_; (20) Most concave point of the cranial edge of the _facies articularis cuboidea_. Nomenclature has been adjusted to _Nomina Anatomica Veterinaria_[14].  
Previously, to carry out any analysis, the consistence of the measurements (photographic and landmark positioning) were evaluated, and a repeatability experiment was conducted. Given that the last author of the present study is the person who developed the anatomical revision of the sheep calcanea based on geometric morphometrics [12], intra-observer error was carried out by him. This was done repeating photographs of two specimens 10 times during 10 different days. During this period, when each repeated photograph was taken, the tripod and the camera were mounted and dismounted each time. In addition, we also randomly selected 10 photographs of 10 different specimens from our initial sample (47 calcanea). Then, on the 30 photographs, the 20 landmarks were located twice during a 10-day period. After digitalisation, we used a Procrustes ANOVA test both among and within repeated calcanea photographs to statistically evaluate the measurements consistency [12].  
To test a hypothesis of bi-modularity of the development of the calcaneum, we divided the 20 2D landmarks into two different modules or blocks to describe the changes in size and shape of structure: one for the body and the process of the calcaneum (block 1 expressed by 4 landmarks: from landmark 1 to 4) and the other for the distal half of the bone (including talocalcaneal joint) (block 2 expressed by 16 landmarks: from landmark 5 to 20).  
Differences due to rotation, translation and scale were eliminated for each module through a Generalized Procrustes Analysis (GPA). As a size estimator, Centroid Size (CS) is used [15] ([PERSON] 1991). CS is obtained by calculating the square root of the sum of the distances, raised to the square, from each anatomical point to the centroid [1].