High-precision body mass predictors for small mammals: A case study in the Mesozoic

Body mass is a pivotal quantity in palaeobiology but must be estimated
from an imperfect fossil record. We analyse the precision of skeletal
predictors of mammalian body mass as a mean to inform the Mesozoic mammal
record, including a new eutriconodont from North America. We focus on the
critical small end of the size spectrum – critical because the earliest
mammals were small, because small size persisted onto the stems of the
major extant radiations, and because small mammals compose a large
proportion of crown diversity. Linear regressions based on extant small
mammals indicate a universal correlation of body mass with observed
measurements, but with clear differences in precision. Postcranial
predictors outperform jaw and dental metrics, with certain femoral joint
dimensions providing surprisingly precise estimations. Overall, our data
indicate small-mammal evolution during the Mesozoic unfolded in patterns
of underappreciated complexity. Studying these dynamics is only possible
when estimating body mass within a strict, highly focused phylogenetic
context. The heuristic value of the estimators we provide here are not
limited to the Mesozoic but are phylogenetically justified for any
small-bodied mammal regardless of age.