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
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

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