Evolutionary Insights into Early Acanthomorphs: Fossils, Phylogenetics, and Morphological Patterns in Berycimorph Fishes (Acanthomorpha: Trachichthyiformes, Beryciformes, and Holocentriformes)

Abstract

“Berycimorphs,” a paraphyletic grouping of spine-rayed fishes that includes the orders Trachichthyiformes, Beryciformes, and Holocentriformes (Teleostei: Acanthomorpha), are examined in the context of their fossil record, phylogenetics, and phenotypic disparity to understand more wholly their evolutionary history. Together they form (in some sequential fashion) the outgroup to the most successful radiation of vertebrates, the percomorphs. Acanthomorphs account for approximately 20,000 species (57% of all ray-finned fishes), with 84% of these fishes being marine. Understanding their evolutionary dynamics across time and space are important in understanding how the ichthyofauna of our modern oceans were assembled, as well as in contextualizing the evolutionary success of Percomorpha. A new genus of Holocentriformes recovered from the Cretaceous–Paleogene (K–Pg) boundary is described and placed in a time-calibrated phylogenetic framework. Parsimony-based analysis is used to generate character state optimizations across Holocentriformes, resulting in several crown-group synapomorphies being reutilized as synapomorphies for the total-group. This study suggests that multiple lineages of total-group squirrel- and soldierfish may have survived the K–Pg extinction. Following this, Cretaceous, early Cenozoic, and recent acanthomorphs are placed into a phylogenetic framework primarily under parsimony and Bayesian inference. “Berycimorphs” account for approximately 50% of all Cretaceous acanthomorphs, though the relationships of these fossil forms to one another and to living “berycimorphs” remains poorly constrained. The results suggest hidden richness in forms and provides new insights into Cretaceous acanthomorph phylogenetics. Of note, several taxa previously placed on the stems of Trachichthyiformes, Beryciformes, or Holocentriformes are recovered within their own clade that has not been previously inferred. Importantly, the results alter the usage of node calibration taxa for several major acanthomorph clades, including Trachichthyiformes lacking Cretaceous node calibrations. Finally, the skulls of extant members of the three clades are phenotypically quantified using µCT and geometric morphometrics to examine how disparity and rates of phenotypic evolution vary across each “berycimorph” order and across shallow and deep water environmental contrasts. Results show that deep water “berycimorphs” are significantly more disparate than shallow water forms, and that rates of phenotypic evolution are highest in shallow water forms. These results are surprising, as recent studies of teleosts in general have shown both disparity and rate of phenotypic evolution to be highest in deep water settings. Notably, the living “berycimorphs” most often compared to the novel Cretaceous acanthopterygian clade fall within a restricted area of “berycimorph” morphospace. This may suggest that early acanthopterygians not only had hidden taxonomic richness, but were also phenotypically conversed and existed prior to the morphological expansion of the grouping. Future studies can build on this work by generated a fossilized birth-death phylogeny on Cretaceous, Paleogene, and Recent acanthomorphs, then incorporating the rich three-dimensional “berycimorph” fossil record into the extant sampling herein to elucidate these patterns quantitatively.