Some 310 million years ago, long-snouted Bandringa sharks are believed to have traveled downstream to a tropical coastline to spawn. Researchers from the University of Michigan and the University of Chicago have acquired and interpreted the fossilized remains of Bandringa sharks, derived from one of the earliest known shark nurseries, situated in Illinois.
The findings were published in the Jan. 7 issue of the Journal of Vertebrate Paleontology, and appears to highlight the earliest recorded example of shark migration. Such behavior patterns persist in the modern shark species of today. Some shark species, such as the tiger shark (Galeocerdo cuvier), range over huge distances to give birth to their young, with some females having been known to cross the Atlantic to do so.
Bandringa sharks are bottom-feeding predators that possessed prominent, elongated snouts that spanned lengths of around half their body. Bandringa sharks inhabited an ancient river delta system that covered the length of – what is today – the Upper Midwestern United States.
Bandringa Migrated From Freshwater Sites to the Open Ocean
The latest research was carried out by Lauren Sallan, a paleontologist working at the University of Michigan, alongside organismal biologist Michael Coates, who works at the University of Chicago.
The pair collected and re-assimilated all known samples of Bandringa shark fossils. Following their analysis, Sallan and Coates noted a striking difference between behavior patterns observed in Bandringa sharks, relative to that seen in the sharks of today; the fossils show that Bandringa sharks migrated from freshwater to saltwater regions. Sallan, who serves as assistant professor in the University of Michigan’s Department of Ecology and Evolutionary Biology, recently reflected over this unprecedented finding:
“This pushes migratory behavior in sharks way back… These sharks bred in the open ocean and spent the rest of their lives in fresh water. No shark alive today is known to do that.”
The genus formerly comprised two species, including Bandringa rayi and Bandringa herdinae. Following further investigation of all the catalogued fossils from the Mazon Creek in northern Illinois, it is believed that the characteristics used to delineate the two species could be attributable to differential taphonomy – the study of decaying organisms, and the ensuing process of fossilization – in adjacent marine and non-marine specimens.
The authors point out a lack of evidence to suggest the existence of morphologically distinct populations, before going on to describe the likelihood of a “monospecific Bandringa,” following evaluation of fossilized remains from 24 individuals:
“A monospecific Bandringa exhibiting complementary data sets from localities with different modes of preservation provides a more complete picture of hard- and soft-tissue anatomy than resident taxa from a single deposit.”
The physical differences observed between the two species – B. rayi and B. herdinae – are conjectured to be attributed to the different preservation processes that take place in marine and freshwater regions, according to Sallan and Coates. Whereas freshwater sites seem to increase the chance of preserving bone and cartilage structures, marine sites are more likely to preserve soft tissue remains.
Adults of the species roamed freshwater swamps and rivers. However, the female sharks migrated downstream to deposit their eggs in the shallow marine waters of a tropical coastline. All of the Bandringa collected from the marine sites were juvenile sharks, discovered next to egg cases. Meanwhile, adult specimens have only been identified around freshwater sites, including a number examined in Ohio and Pennsylvania.
Improved Insight Into Species Anatomy
After reclassifying Bandringa as a single species, and uniting data sets from each fossil site, the researchers believe they have gained a far better insight into the long-extinct shark’s anatomical features, including a number of findings that have not been previously reported. These features include ventrally-directed jaws, spiny protrusions emanating from the head and cheeks and an intricate set of sensory organs; these mechanoreceptors and electroreceptors are said to have been responsible for helping the sharks to identify potential prey within the waters. Meanwhile, it has been hypothesized that the jaws facilitated suction-feeding, while the needle-like spines may have played a role in self-defense.
Ultimately, Sallan and Coates believe that the sharks hatched from the egg cases found in the Mazon Creek of Illinois. The site was used as a shark nursery, where adult females would deposit their eggs, before migrating back upstream to their freshwater habitats.
By James Fenner
Top image credit: painting by John Megahan, University of Michigan