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Ichthyosaur graveyard may be an ancient birthing ground

The Berlin-Ichthyosaur State Park (BISP) is well known for its remarkable fossil beds. Located in Nevada’s Humboldt-Toiyabe National Forest, the Park is famous for the many ichthyosaurs that have been unearthed there. In fact, the Park contains the largest number of fossil ichthyosaurs in the world, including the longest one ever discovered, at around 50 feet in length. Paleontologists have always pondered why so many (37) of the giant marine reptiles all ended up being fossilized in the same area – a new study has now proposed a possible reason. 

“We present evidence that these ichthyosaurs died here in large numbers because they were migrating to this area to give birth for many generations across hundreds of thousands of years,” said co-author and Smithsonian National Museum of Natural History curator Nicholas Pyenson. “That means this type of behavior we observe today in whales has been around for more than 200 million years.”

Ichthyosaurs are dolphin-shaped, predatory reptiles that inhabited the world’s oceans for a large part of the Mesozoic Era, between 250 million and 90 million years ago. They ate fish and other marine prey, and had a long snout with many small, conical teeth for catching their food. Ichthyosaurs had unusually large eyes that enabled them to see in the dark depths of the ocean, and their limbs were modified to form flippers. They varied in size from a diminutive 3 feet to an impressive 50 feeet, breathed air, were warm blooded and gave birth to live young. 

Over the years, some paleontologists have proposed that BISP’s ichthyosaurs died in a mass stranding event such as those that sometimes afflict modern whales, or that the creatures were poisoned by toxins such as from a nearby harmful algal bloom. The problem is that these hypotheses lack strong lines of scientific evidence to support them. 

The new research, from a team of scientists – including researchers from the University of Utah, the Smithsonian Institution, the Vanderbilt University, the University of Nevada, Reno, the University of Edinburgh, the University of Texas at Austin, the Vrije Universiteit Brussels, and Oxford University, involved using newer technology to analyze the ancient fossils of an ichthyosaur called Shonisaurus popularis

The researchers combined 3D scanning and analytical geochemistry with traditional paleontological methods, such as poring over archival materials, photographs, maps, field notes and drawer after drawer of museum specimens for shreds of evidence that could be reanalyzed. They focused first on a particular site in the Park where a large barn-like building houses what is known as Quarry 2. This is one of the main attractions for visitors because it displays the partial skeletons of seven Shonisaurus ichthyosaurs that all appear to have died around the same time. 

Unlike most other fossil displays, these individuals have not been fully excavated from the rock but remain embedded, with only enough of each fossil exposed to enable paleontologists to identify the type of bone. So visitors can see a part of a skull in one location, some flipper bones in another, and a tail in a third. The individuals lie precisely as they did, millions and millions of years ago, when they died, became covered in sediment, and slowly turned into rock. 

In order to try and understand why the ichthyosaurs had all died together, the research team collaborated with Jon Blundell, a member of the Smithsonian Digitization Program Office’s 3D Program team, and Holly Little, informatics manager in the museum’s Department of Paleobiology. While the paleontologists were physically measuring bones and studying the site using traditional paleontological techniques, Little and Blundell used digital cameras and a spherical laser scanner to take hundreds of photographs and millions of point measurements that were then stitched together using specialized software to create a 3D model of the fossil bed.

“When I first visited the site in 2014, my first thought was that the best way to study it would be to create a full-color, high-resolution 3D model,” said lead author Neil Kelley, an Assistant Professor at Vanderbilt University. “A 3D model would allow us to study the way these large fossils were arranged in relation to one another without losing the ability to go bone by bone.”

In addition, the researchers collected tiny samples of the rock surrounding the fossils both at the Quarry 2 site, and in nearby areas, and performed a series of geochemical tests on them. One test measured mercury levels, which often indicate that large-scale volcanic activity has taken place, and found no significantly increased levels. Other tests examined different types of carbon and determined that there was no evidence of sudden increases in organic matter in the marine sediments that would result in a shortage of oxygen in the surrounding waters (though, like whales, the ichthyosaurs breathed air).

The results of these geochemical tests, published in the journal Current Biology, indicated that the ichthyosaurs had not all perished because of some apocalyptic event that had caused havoc with their environment. Furthermore, they had not died in shallow waters as would have happened if they had stranded as a group. When the researchers analyzed the geology surrounding the other fossil remains found beyond Quarry 2, they identified an unusual pattern. 

Not only did the geologic evidence indicate that the bones of the dead ichthyosaurs had sunk down into the deep ocean at the time, but also it revealed that there were very few other marine vertebrates, such as fish, that had been fossilized in the same location. It appeared that there was not much food for the ichthyosaurs in this location. Although there are a lot of 50-foot adult Shonisaurus specimens present, the only other fossils at the BISP are those of small invertebrates, such as clams and ammonites, which would not have been food items for the ichthyosaurs. 

“There are so many large, adult skeletons from this one species at this site and almost nothing else,” said Pyenson. “There are virtually no remains of things like fish or other marine reptiles for these ichthyosaurs to feed on, and there are also no juvenile Shonisaurus skeletons.”

One final piece of evidence led the researchers to propose their new hypothesis. The team eventually found tiny ichthyosaur remains among the new fossils collected at the BISP, as well as hiding within the older museum collections. Careful comparison of the tiny bones and teeth, using micro-CT X-ray scans at Vanderbilt University, revealed that these small bones were in fact from embryonic and newborn Shonisaurus individuals.

“Once it became clear that there was nothing for them to eat here, and there were large adult Shonisaurus along with embryos and newborns, but no juveniles, we started to seriously consider whether this might have been a birthing ground,” said Kelley.

Today’s marine giants – such as blue and humpback whales – also group together for various purposes, including breeding, birthing and socialization. They routinely travel across oceans on regular migratory paths, feeding in certain oceanic regions and giving birth and mating in others. They often give birth when aggregated together, so that there is some safety in numbers, and also select waters in which predators are less common. Large groups of individuals migrate and congregate, year after year, in the same locations. This is what the researchers think may have been the story behind the large collection of ichthyosaur fossils at the BISP.

The detailed analysis of the various strata in BISP where clusters of ichthyosaur bones occur, showed that the fossils were not actually all deposited at the same time. Instead, the ages of the many fossil beds in the park differed by hundreds of thousands of years. This supports the idea that ichthyosaurs returned again and again to the same area and, each year some of them died and sank to the ocean floor. 

“Finding these different spots with the same species spread across geologic time with the same demographic pattern tells us that this was a preferred habitat that these large oceangoing predators returned to for generations,” said Pyenson. “This is a clear ecological signal, we argue, that this was a place that Shonisaurus used to give birth, very similar to today’s whales. Now we have evidence that this sort of behavior is 230 million years old.”

“One of the exciting things about this new work is that we discovered new specimens of Shonisaurus popularis that have really well-preserved skull material,” said Randall Irmis, NHMU chief curator and curator of paleontology, associate professor and co-author on the study. “Combined with some of the skeletons that were collected back in the 1950s and 1960s and that are at the Nevada State Museum in Las Vegas, it’s likely we’ll eventually have enough fossil material to finally accurately reconstruct what a Shonisaurus skeleton looked like.”

Image Credit: Gabriel Ugueto

By Alison Bosman, Staff Writer

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