New Paper: Evolution of multicuspid teeth in a Triassic fish from Utah

This is an old post I am archiving here on my website. It was originally posted on the KU Biodiversity Institute blog in February 2015.

Just yesterday, my newest paper was published online in the journal The Science of Nature: Naturwissenschaften about a rather unusual fish from the Upper Triassic Chinle Formation of southeastern Utah. The fish, Hemicalypterus weiri, was a deep-bodied, disc-shaped fish, with enameled ganoid scales covering the anterior portion of its flank, and a scaleless posterior half, which presumably aided in flexibility while swimming. Although Hemicalypterus was first described in the 1960s (Schaeffer, 1967), recent collecting trips recovered many new specimens of Hemicalypterus, and I decided to reinvestigate this enigmatic fish as part of my dissertation research.

While cleaning specimens of Hemicalypterus at the University of Kansas Vertebrate Paleontology prep lab, I noticed rather unusual teeth on the lower jaw that I had exposed from the rock matrix. These teeth look like a mouthful of little forks, and there were at least six individual teeth on the lower jaw. As I prepared other specimens, I found that these teeth were also on the premaxillae. Each tooth has a long cylindrical base and a flattened, spatulate edge with four delicate, individual cusps. I hadn’t seen anything like this before in other fossil fishes, and so I started searching the literature and talking to other ichthyologists.

Well, as it turns out, this tooth morphology has evolved multiple times in several independent lineages of teleost fishes, and quite often fishes with similar dentition scrape algae off of a hard substrate. These teeth indeed act like little forks (or “sporks” might be more appropriate) for these herbivorous/omnivorous fishes. Examples of extant fishes with similar teeth include freshwater forms such as the algae-scraping cichlids and characiforms, as well as many marine forms that are key in controlling algae growth in coral reef environments, such as acanthurids (surgeonfishes, tangs) and siganids (rabbitfishes). Of course, these modern-day fishes also feed on other things (e.g., phytoplankton), but algae is often the primary staple, and these fishes use this specialized dentition for a specific feeding behavior.

So while it is impossible to prove definitively what a species of fish that lived over 200 million years ago fed upon (without gut contents being preserved….or a time machine), it is still safe to infer that Hemicalypterus occupied an ecological niche space similar to algae-scraping cichlids or other modern-day herbivorous fishes and may have scraped algae off of a hard substrate, based on this unusual tooth morphology and its similarity to modern forms.

This discovery also extends evidence of herbivory in fishes clear back to the Early Mesozoic, whereas prior to this discovery it was assumed that herbivory evolved in the Middle Cenozoic in marine teleost fishes. Frankly, there was no evidence to say otherwise, as most Mesozoic fishes have general caniniform or styliform (peg-like) teeth, or they have heavy crushing or pavement-like teeth consistent with crushing hard-shelled organisms. The teeth of Hemicalypterus are very delicate, and wouldn’t really do well with durophagy. This is the first potential evidence of herbivory in the Mesozoic, and in a non-teleost, ray-finned fish.

Original Source: Gibson, S.Z. 2015. Evidence of a specialized feeding niche in a Late Triassic ray-finned fish: evolution of multidenticulate teeth and benthic scraping in †Hemicalypterus. The Science of Nature — Naturwissenschaften 102:10.

Also cited: Schaeffer, B. 1967. Late Triassic fishes from the western United States. Bulletin of the American Museum of Natural History 135: 289–342.