Biodiversity and Evolutionary Relationships of Early Mesozoic Ray-Finned Fishes (Actinopterygii)

Abstract

Fishes have evolved to exploit multiple ecological niches. Extant fishes in both marine (e.g., rabbitfishes, surgeonfishes) and freshwater systems (e.g., haplochromine cichlids, characiforms) have evolved specialized, scoop-like, multidenticulate teeth for benthic scraping, feeding primarily on algae. The ray-finned fish †Hemicalypterus weiri Schaeffer, 1967, from the Upper Triassic Chinle Formation of southeastern Utah (~210–205 Ma), United States is the oldest example of a ray-finned fish with specialized multidenticulate dentition. †Hemicalypterus weiri was a lower actinopterygian species distinguished by its deep, disc-shaped compressed body; ganoid-scaled anterior half and scaleless posterior half; spinose, prominent dorsal and ventral ridge scales anterior to dorsal and anal fins; hem-like dorsal and anal fins with rounded distal margins; small mouth gape; and specialized, multicuspid dentition. †Hemicalypterus is phylogenetically remote from modern fishes, and its specialized dentition is a morphological convergence with those of several living teleost fishes with a likely function of these teeth being to scrape algae off a rock substrate. This finding contradicts previously held notions that fishes with multicuspid, scoop-like dentition were restricted to teleosts, and indicates that ray-finned fishes were diversifying into different trophic niches and exploring different modes of feeding earlier in their history than previously thought, fundamentally altering our perceptions of the ecological roles of fishes during the Mesozoic. †Hemicalypterus weiri is redescribed and rediagnosed based on new information collected from reexamination of museum specimens as well as examination of recently collected specimens. A phylogenetic analysis infers a placement of †Hemicalypterus within †Dapediiformes, with †Dapediiformes being recovered as sister to Ginglymodi within holostean actinopterygians. Fishes of the order †Redfieldiiformes (~22 genera, 26 spp.) are an extinct group of lower ray-finned fishes found globally in geologic deposits from the Early Mesozoic. Redfieldiiforms possessed a fusiform body shape, thick enameled (ganoid) scales covering the entire body, fixed maxilla, more upright jaw suspensorium, prominent and often tuberculated snout, and reduction in the dorsal lobe of the caudal fin. Redfieldiiforms represent one of the last remnants of a once- dominant lineage of fishes during the Paleozoic (†Palaeoniscimorpha), but the evolutionary relationships within the †Redfieldiiformes were last examined over thirty years ago and have never been assessed via a quantitative phylogenetic analysis. Redfieldiiform fishes are reexamined, with updates to the anatomical and morphological descriptions of certain taxa. Parsimony and maximum-likelihood analyses support a monophyletic †Redfieldiiformes with two families, †Brookvaliidae and †Redfieldiidae. The order †Redfieldiiformes is closely related to the †Scanilepiformes, †Perleidiformes, and †Mesopoma. A new classification of the †Redfieldiiformes is presented. A new species of †Lasalichthys is described based on new, three-dimensionally preserved specimens from the Upper Triassic Dockum Group of western Texas. The new species of †Lasalichthys is diagnosed by a combination of unique traits found in the skull, including shape of maxilla, shape of preoperculum and associated cheek bones, pattern and articulation of bones in the snout, and patterns of sensory line canals in the dermal skull bones. Specimens of this new species display novel patterns in the sensory line canals of the skull that have never been observed in any other redfieldiiform. The new species of †Lasalichthys represents one of the oldest redfieldiiform taxa in North America, and is recovered within the family †Redfieldiidae.