The impact of paleoclimatic changes on body size evolution in marine fishes

Body size is an important species trait, correlating with lifespan,
fecundity, and other ecological factors. Over Earth’s geological history,
climate shifts have occurred, potentially shaping body size evolution in
many clades. General rules attempting to summarize body size evolution
include Bergmann’s rule, which states that species grow to larger sizes in
cooler environments and smaller sizes in warmer environments; and Cope’s
rule, which poses that lineages tend to increase in size over evolutionary
time. Tetraodontiform fishes (including pufferfishes, boxfishes, and ocean
sunfishes) provide an extraordinary clade to test these rules in
ectotherms owing to their exemplary fossil record and the great disparity
in body size observed among extant and fossil species. We examined
Bergmann’s and Cope’s rules in this group by combining phylogenomic data
(1,103 exon loci from 185 extant species) with 210 anatomical characters
coded from both fossil and extant species. We aggregated data layers on
paleoclimate and body size from the species examined, then inferred a set
of time-calibrated phylogenies using tip-dating approaches for use in
downstream comparative analyses of body size evolution using models that
incorporate paleoclimatic information. We find strong support for a
temperature-driven model in which increasing body size over time is
correlated with decreasing oceanic temperatures. On average, extant
tetraodontiforms are 2–3 times larger than their fossil counterparts,
which otherwise evolved during periods of warmer ocean temperatures. These
results provide strong support for both Bergmann’s and Cope’s rules,
trends that are less studied in marine fishes compared to terrestrial
vertebrates and marine invertebrates.