Stepwise shifts underlie evolutionary trends in morphological complexity of the mammalian vertebral column

A fundamental concept in evolutionary biology is that life tends to become more complex through geologic time, but empirical examples of this phenomenon are controversial. One debate is whether increasing complexity is the result of random variations, or if there are evolutionary processes which actively drive its acquisition, and if these processes act uniformly across clades. The mammalian vertebral column provides an opportunity to test these hypotheses because it is composed of serially-repeating vertebrae for which complexity and organization can be readily measured. Here we test seven competing hypotheses for the evolution of vertebral complexity by incorporating fossil data from the mammal stem lineage into evolutionary models. Based on these data, we reject Brownian motion (a random walk) and uniform increasing trends in favor of stepwise shifts for explaining increasing complexity. We hypothesize that clade-specific adaptations associated with increased aerobic capacity in non-mammalian cynodonts may have provided impetus for increasing vertebral complexity in mammals.

Methods
Vertebral complexity and organization across synapsids. Dataset includes:

1. Complexity and organization metrics calculated from raw vertebral measures available from https://doi.org/10.5061/dryad.jm820mg. Range, polarization, irregularity, concentration and smoothness calculated following McShea (1991) on logged data. Clustering calculated as 1-the Hopkins Statistic.

2. Age ranges of non-mammalian synapsid species gathered from the literature, used for callibrating phylogeny

3. Phylogenetic trees as .Rdata files saved in the multiPhylo format. 60 phylogenetic trees generated using 'bin_timePaleophy' function with minimum branch length method and occurence dates randomly sampled from withing their time bin. The master tree reflects the most complete sampling (444 species), while the subsampled tree reflects taxa included in the analysis.