Among vertebrates, placental mammals are particularly variable in the covariance between cranial shape and body size (allometry), with rodents a major exception. Australian murid rodents allow an assessment of the cause of this anomaly because they radiated on an ecologically diverse continent notably lacking other terrestrial placentals. Here we use 3D geometric morphometrics to quantify species-level and evolutionary allometries in 38 species (317 crania) from all Australian murid genera. We ask if ecological opportunity resulted in greater allometric diversity compared to other rodents, or if conserved allometry suggests intrinsic constraints and/or stabilizing selection. We also assess whether cranial shape variation follows the proposed “rule of craniofacial evolutionary allometry” (CREA), whereby larger species have relatively longer snouts and smaller braincases. To ensure we could differentiate parallel versus non-parallel species-level allometric slopes, we compared the slopes of rarefied samples across all clades. We found exceedingly conserved allometry and CREA-like patterns across the 10 million year split between Mus and Australian murids. This could support both intrinsic constraints and stabilizing selection hypotheses for conserved allometry. Large-bodied frugivores evolved faster than other species along the allometric trajectory, which could suggest stabilizing selection on the shape of the masticatory apparatus as body size changes.

3D skulls (stored in Morphosource - see associated link) from four Australian museums were landmarked in Viewbox 4.0. The 3D coordinates are stored here along with the molecular phylogeny modified from Smissen and Rowe 2018 and ecological trait information from Breed and Ford 2007. These are the raw data used for all analyses. Full methods for data collection provided in the associated American Naturalist article (or the preprint DOI provided below). Instructions for reproducing all analyses, figures, and tables are provided in the readme file as well as in the RMarkdown files provided in the zip file. Alternatively, the same data and code can be viewed at the GitHub link below. 

All scripts are in RMarkdown format (.Rmd) and can be opened in RStudio. There, you can edit and run code chunks as normal or use the Knit button to create HTML versions with both code and output. Remember to either set your working directory to the allometry-rodents folder on your computer or open an RStudio project from that folder.