Rhynchocephalians were a successful, globally distributed group of diapsid reptiles that thrived in the Mesozoic. Multiple species of Clevosaurus existed worldwide in the Upper Triassic and Lower Jurassic, and they are characterised by shearing bladelike teeth perhaps functionally analogous to the carnassial teeth of mammals. Morphometric analysis shows that the dentary morphospace of clevosaurs differs significantly from that of other rhynchocephalians. Five Clevosaurus species occupied islands in the Bristol Channel archipelago of the UK, but generally not those occupied by mammaliaforms, suggesting dietary character displacement. Identifying the diet of such ancient, small tetrapods has been difficult. To identify the nature of their feeding, we apply finite element analysis to two near complete three-dimensional skulls of the species Clevosaurus hudsoni and Clevosaurus cambrica to estimate bite force, resistance to bending and torsion, and the distribution of stresses during biting. Both species had bite forces and tooth pressures sufficient to break apart chitin, indicating that like early Mesozoic mammaliaforms, clevosaurs could feed on tough-shelled beetles and possibly small vertebrates. In addition, the mechanical advantage of the jaws falls within the range of early mammaliaforms, so though we cannot demonstrate niche partitioning between members of both clades, it raises the prospect that they may have been functionally similar.

Supplementary information:

README: A PDF summarising and explaining all supplementary information and material included in this dataset, as well as including supplementary tables and figures.

1. CT-scan parameters
2. Homologous points on the mandibular ramus of Clevosaurus used for figure 6
3. Morphometrics data
4. Description of the adductor musculature of Clevosaurus
5. Finite element analyses output (.rpt)
6. Surface files for left mandibular ramus of Clevosaurus (.stl)
7. Supplementary tables

• Tables S1–S5

Figures S1–S5

Section C - Morphometrics data.

An excel file containing the list of landmarks used, specimen numbers, Procrustes distances results and groupings (e.g. cladistic, period, continent).

Section E - Finite element analyses output (.rpt)

Output data for Clevosaurus hudsoni (E1-E2) and Clevosaurus cambrica (E3-E6).

• Clevosaurus hudsoni: E1 with a bite position at the posterior-most tooth, E2 with a bite position at the anterior-most tooth.
• Clevosaurus cambrica (no surface-area to force scaling): E3 with a bite position at the posterior-most tooth, E4 with a bite position at the anterior-most tooth.
• Clevosaurus cambrica (with surface-area to force scaling): E5 with a bite position at the posterior-most tooth, E6 with a bite position at the anterior-most tooth.

Section F - Surface files for jaws of Clevosaurus (.stl):

Raw surface files for the lower left mandibular ramus of Clevosaurus hudsoni (F1) and Clevosaurus cambrica (F2) after reassembly. Note that since these jaws are surface files only, they do not retain separate label fields for the post-dentary bones and have not yet undergone the repairs that were required before pre-processing in FE was initiated.

Section G - Supplementary tables.

Excel spreadsheet summarising the following tables (tables S1-S5).