TABLE 1. List of specimens sampled for the morphometric component of this study. Acronyms are AMNH, American Museum of Natural History; BMNH, the Natural History Museum London; CMNH, Carnegie Museum of Natural History; MAE, Mongolian Academy of Sciences-American Museum Expedition; NML, World Museum Liverpool UK; UM, University of Montpellier; ROM, Royal Ontario Museum; TMM, Texas Memorial Museum; UMZC, University Museum of Zoology Cambridge; USNM, National Museum of Natural History Washington DC. Masseter types are given for extant rodents only, as defined by anatomical criteria discussed in the text and Maier and Schrenk (1987), Marivaux et al. (2004), Hautier et al. (2008), and Cox and Faulkes (2016).
| Species | Masseter type | Catalogue number |
| Acomys carhirinus | myomorphous | UMZC E2278 |
| Anomalurus beecrofti | hystricomorphous | UMZC E1403 |
| Aplodontia rufa | protrogomorphous | UMZC E1861 |
| Arvicola amphibius | myomorphous | UMZC E2805 |
| Bathyergus suillus | protrogomorphous | NML 19.8.75.14 |
| Capromys pilorides | hystricomorphous | BMNH 77-429 |
| Cricetomys gambianus | myomorphous | UMZC E2263 |
| Cuniculus paca | hystricomorphous | BMNH 52-149 |
| Cynomys lewisii | sciuromorphous | UMZC E1750 |
| Dipodomys agilis | sciuromorphous | UMZC E3022 |
| Dipodomys sp. | sciuromorphous | UMZC E3002 |
| Dipus sp. | hystricomorphous | UMZC E3165 |
| Eliomys quercinus | myomorphous | UM N354 |
| Geomys bursarius | sciuromorphous | BMNH 60-537 |
| Georhychus capensis | protrogomorphous | MNHN ZM-MO 1991-627 |
| Gerbillus watersi | myomorphous | UMZC E1971 |
| Glaucomys volans | sciuromorphous | TMM M-6332 |
| Gomphos elkema | MAE-BU 14467 | |
| Graphiurus hueti | hystricomorphous | UMZC E1909 |
| Heterocephalus glaber | protrogomorphous | UMZC 2019.30 |
| Heteromys gaumeri | sciuromorphous | UM 305 |
| Hydromys chrysogaster | myomorphous | UMZC E1921 |
| Hystrix indica | hystricomorphous | UMZC E3420 |
| Ischyromys douglassi | USNM 617532 | |
| Ischyromys typus | USNM 16828,AMNH 144628,ROM V1007, BMNH PVM7855,CMNH588 | |
| Jaculus sp. | hystricomorphous | UM N342 |
| Laonastes aenigmamus | hystricomorphous | UM TK2007 |
| Lepus californicus | UMZC E3941 | |
| Marmota bobak | sciuromorphous | UMZC E1762 |
| Napaeozapus insignis | hystricomorphous | UMZC E3161 |
| Neotoma fuscipes | myomorphous | UMZC E2675 |
| Paradelomys crusafonti | UM ACQ-6618 | |
| Pedetes capensis | hystricomorphous | UMZC E1459 |
| Perognathus lordi | sciuromorphous | UMZC E3061 |
| Platacanthomys lasiurus | myomorphous | UMZC E1917 |
| Rattus norvegicus | myomorphous | TMM M-2272 |
| Ratufa bicolor | sciuromorphous | UMZC E1570 |
| Spalax microphthalmus | myomorphous | UMZC E2867 |
| Tamias sp. | sciuromorphous | UM 284 |
| Tamiasciurus douglasii | sciuromorphous | UMZC E1655 |
| Zapus hudsonius | hystricomorphous | UMZC E3150 |
TABLE 2. List of cranial landmarks adapted from Morris et al. (2018) and shown graphically in Figure 4.
| Landmark | Description |
| 1 | Anteriormost point on internasal surface |
| 2 | Posteriormost midline point of nasal bone |
| 3 | Midpoint between medialmost points on orbital margins |
| 4 | Mid-sagittal point on fronto-parietal-suture |
| 5 | Anteriormost point on naso-premaxillary suture |
| 6 | Superiormost border of infraorbital foramen, from anterior aspect |
| 7 | Central tip of maxillary incisor, on enamel |
| 8 | Midpoint of lingual alveolar margin on upper central incisor |
| 9 | Midpoint between anterior margins of incisive foramina |
| 10 | Midpoint between anteriormost points of first cheek teeth |
| 11 | Posteriormost midline point on palatine |
| 12 | Anterior edge of the alveolus of the first cheek tooth |
| 13 | Posterior edge of the alveolus of the last cheek tooth |
| 14 | Dorsalmost point on the buccal alveolar margin of the upper incisor |
| 15 | Anterior point of frontal suture as it crosses the orbital margin |
| 16 | Posteriormost margin of the dorsal orbit on the frontal bone |
| 17 | Posteriormost point of the curve of the zygomatic arch on the anterior edge of the infra-temporal fossa |
| 18 | Anterior-dorsal point on margin of optic foramen, from anterior aspect |
| 19 | Anteriormost point of the curve of the zygomatic arch on the ventral anterior inferior edge of the infra-temporal fossa |
| 20 | Distal most point of infraorbital foramen, from anterior aspect |
| 21 | Inferiormost point of infraorbital foramen, from anterior aspect |
TABLE 3. Eigenvalues and percentage of shape variation explained by the first 14 principal components of the GMM analysis.
| PC | Eigenvalues | Variance% | Cumulative% | PC | Eigenvalues | Variance% | Cumulative% |
| 1 | 0.00944335 | 17.0 | 17.0 | 8 | 0.00248718 | 4.5 | 75.3 |
| 2 | 0.00754495 | 13.6 | 30.5 | 9 | 0.00182486 | 3.3 | 78.6 |
| 3 | 0.00645805 | 11.6 | 42.1 | 10 | 0.00160991 | 2.9 | 81.5 |
| 4 | 0.00539965 | 9.7 | 51.8 | 11 | 0.00157524 | 2.8 | 84.3 |
| 5 | 0.00471424 | 8.5 | 60.3 | 12 | 0.00129825 | 2.3 | 86.6 |
| 6 | 0.00334685 | 6.0 | 66.3 | 13 | 0.00108912 | 2.0 | 88.6 |
