Paleontologists Redescribe Enigmatic Carboniferous-Period Fish
A team of U.S. paleontologists has redescribed the morphology of a long-snouted ray-finned fish called Tanyrhinichthys mcallisteri and created a more complete and accurate reconstruction of the fish as a living animal.
Tanyrhinichthys mcallisteri lived around 300 million years ago (Carboniferous period) in an estuary environment in what is today New Mexico, the United States.
The fish was between 15 and 20 cm long and had sharp, recurved teeth.
“The specimen looks like someone found a fish and just pulled on the front of its skull,” said Jack Stack, a researcher in the Department of Earth and Environmental Sciences at Michigan State University and the Department of Earth and Environmental Sciences at the University of Pennsylvania.
“Many modern fish species, from the swordfish to the sailfish, have protuberant snouts that extend out in front of them, often aiding in their ability to lunge at prey. But this characteristic is much rarer in ancient fishes.”
In the 2010s, several more specimens of Tanyrhinichthys mcallisteri were found in the same quarry.
In a new study, Stack and colleagues from the University of Pennsylvania, Maryland-National Capital Parks and Planning Commission, and New Mexico Museum of Natural History and Science examined these specimens in detail and studied other species that dated to the same period.
“This sounds really simple, but it’s obviously difficult in execution as fossils are compressed flat when they are preserved,” Stack said.
What the scientists noticed cast doubt on the conception of Tanyrhinichthys mcallisteri as resembling a pike.
While a pike has an elongated snout with its jaws at the end of it, allowing it to rush its prey head-on, Tanyrhinichthys mcallisteri has an elongated snout with its jaws at the bottom.
“The whole form of this fish is similar to other bottom dwellers,” Stack said.
The team also noticed canal-like structures on Tanyrhinichthys mcallisteri’s snout concentrated in the top of its head, suggestive of the locations where sensory organs would attach.
“These would have detected vibrations to allow the fish to consume its prey,” said Dr. Lauren Sallan, a paleobiologist in the Department of Earth and Environmental Sciences and the Department of Biology at the University of Pennsylvania.
The scientists noted that many of the species that dwelled in similar environments possessed longer snouts.
“This also makes sense because it was an estuary environment with large rivers feeding into it, churning up the water, and making it murky,” Dr. Sallan said.
“Rather than using your eyesight, you have to use these other sensory organs to detect prey.”
Despite this, other features of the different ancient fishes’ morphology were so different from Tanyrhinichthys mcallisteri that they do not appear to have shared a lineage with one another, nor do modern sturgeon descend from Tanyrhinichthys mcallisteri.
Instead the long snouts appear to be an example of convergent evolution, or many different lineages all arriving at the same innovation to adapt well to their environment.
“Our work, and paleontology in general, shows that the diversity of life forms that are apparent today has roots that extend back into the past,” Stack said.
The team’s results were published in the Zoological Journal of the Linnean Society.
Jack Stack et al. Tanyrhinichthys mcallisteri, a long-rostrumed Pennsylvanian ray-finned fish (Actinopterygii) and the simultaneous appearance of novel ecomorphologies in Late Palaeozoic fishes. Zoological Journal of the Linnean Society, published online June 22, 2020; doi: 10.1093/zoolinnean/zlaa044