In today's ocean, "giants" such as blue whales and humpback whales often migrate to the other side of the ocean in large numbers to breed and give birth in waters with few natural enemies. They gather on the same coastline year after year. Recently, a new study by scientists shows that nearly 200 million years before the evolution of giant whales, a school bus-sized marine reptile, ichthyosaurs, may have begun similar migration behaviors, giving birth and reproducing together in relatively safe seas.
Ichthyosaurs breeding in safe seas (pictures from the Internet)
There is a famous fossil site in Nevada, the Berlin Ichthyosaur State Park. It is located near an abandoned mining town. Many ichthyosaurs about 15 meters long were found in the site - the common Shawnee ichthyosaur, a predator similar to the giant dolphin, and has now become the state fossil of Nevada.
Berlin Ichthyosaur State Park contains at least 37 ichthyosaur fossils dating back approximately 230 million years. Why so many ichthyosaurs gathered here has long puzzled paleontologists. Some paleontologists have proposed that these ichthyosaurs died in mass stranding events. For example, similar stranding events may occur in modern whales due to pollution of the marine environment, but this hypothesis lacks strong scientific evidence to support it.
Recovery picture of an ichthyosaur stranded due to low tide (picture from the Internet)
A research team led by Vanderbilt University and Neil Kelley of the Smithsonian Institution's National Museum of Natural History conducted research on the above issues.
The most attractive thing for scientific researchers in Berlin Ichthyosaur State Park is a barn-like building, which retains a fossil quarry in situ. The quarry contains partial skeletons from seven different ichthyosaurs, which seemed to have died at the same time.
Researchers measured the ichthyosaur fossils in the stope (picture from the Internet)
The research team carefully studied all the fossils found in the stope and surrounding areas, and found that the limestone in the park produced many large specimens of adult Shawnee ichthyosaurs, but very few other marine vertebrates. There were almost no remains of creatures such as fish or other marine reptiles for these ichthyosaurs to eat, and no bones of juvenile Shawnee ichthyosaurs were found. Other fossils are mostly small invertebrates such as bivalves and ammonites.
3D scans of 7 different ichthyosaur bones (different colors) (picture from the Internet)
The research team discovered tiny ichthyosaur remains among new fossils collected in the park. After careful comparison of the bones and teeth using micro-CT X-ray scanning, it was discovered that these small bones are actually embryos and newborns of Shawnee ichthyosaurs.
Further analysis of the strata where different ichthyosaur bones were found shows that many of the ichthyosaurs in the park were found in strata that were at least hundreds of thousands of years apart.
Considering that there is no sufficient food source in the gathering place of ichthyosaurs, and only large adult Shawnee ichthyosaurs, embryos and newborns are found, and not even juvenile ichthyosaurs have been found, the research team speculates that this place is the place where Shawnee ichthyosaurs once gave birth. These large pelagic predators have returned here for generations to breed, and have reproduced for many generations over hundreds of thousands of years. This behavior is very similar to today's whales and existed 230 million years ago.
There have been two hypotheses for the cause of the mass die-off of ichthyosaurs: harmful algal blooms or large-scale volcanic activity, but new petrochemical analysis rules out these two factors. Different types of carbon test results show that there is no evidence that organic matter in ocean sediments suddenly increased, resulting in anoxia in the surrounding water so that ichthyosaurs suffocated to death; mercury is usually accompanied by large-scale volcanic activity, and mercury test results showed that no significant increase in mercury levels was found. These geochemical analyzes suggest that algal blooms or volcanic activity were not to blame for the mass die-off of ichthyosaurs. Exactly what catastrophic event caused the mass die-off of these marine reptiles is still unknown. The
research results were recently published in the journal Current Biology.
Paper information: Neil P. Kelley, Randall B. Irmis, Paige E. dePolo, et al.. 2022. Grouping behavior in a Triassic marine apex predator. Current Biology, 24(32): 5398-5405. DOI: 10.1016/j.cub.2022.11.005
Source: eurekalert.org, cbsnews.com, yubanet.com
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