How fossilization preserved a 310-million-year-old horseshoe crab’s brain

Researchers have uncovered a never-before-seen fossilized brain from a 310 million-year-old horseshoe crab, revealing some surprises about the evolution of these wannabe crustaceans, according to a new study.

The fossilized brain, which belongs to the extinct species Euproops Danae, was discovered at Mazon Creek in Illinois, where the conditions were just right to perfectly preserve the animal’s delicate soft tissue. 

There are four species of horseshoe crabs alive today — all of which sport hard exoskeletons, 10 legs and a U-shaped head. Despite their name, these “crabs” are actually arachnids that are closely related to scorpions and spiders, according to The National Wildlife Federation.

Although horseshoe crab fossils are relatively common, nothing was previously known about their ancient brains, the researchers said. 

“This is the first and only evidence for a brain in a fossil horseshoe crab,” lead author Russell Bicknell, a palaeontologist at the University of New England in Maine, told Live Science. The chances of finding a fossilized brain are “one in a million,” he added. “Although, even then, chances are they are even rarer.” 

Soft tissues that makeup brains are very prone to rapid decay, Bicknell said. “In order for them to be preserved, either very special geological conditions, or amber, are needed.”

In this case, geology helped to keep the soft tissue in tip-top condition over the years and preserve the brain — or at least a copy of the brain. “We have a mould of the brain, not the brain itself, so to speak,” Bicknell said. 

The deposits at Mazon Creek are made of an iron carbonate mineral called siderite, which forms concretions — mineral precipitations — that can quickly encase a dead body and fossilize it.

Although such concretions preserved the horseshoe crab’s body, the brain tissue still decomposed and eventually disappeared. However, as the brain rotted away it was replaced by a clay mineral called kaolinite, which created a cast of the brain.

Kaolite is white in colour, whereas siderite is dark grey. This colour contrast meant the brain fossil “stood out more than it would have normally” from the rest of the fossil, Bicknell said.

The hunt is now on for more ancient brains that might have been fossilized in the unique geological conditions that preserved this horseshoe crab.

“The Mazon Creek deposit is exceptional,” Bicknell said. “If we started looking, we may be lucky enough to find more [brain fossils].”

The discovery provided researchers with the unique opportunity to study how the arachnids’ brains evolved over time. But to the researchers’ surprise, they found that the ancient brain, which dates to the Carboniferous period (359 million to 299 million years ago), was remarkably similar to that of a modern horseshoe crab.

“Despite 300 million years of evolution, the fossil horseshoe crab brain is pretty much the same as modern forms,” Bicknell said.

The 70 Million-Year-Old History of the Mississippi River

In 1758, the French ethnographer Antoine-Simon Le Page du Pratz published The History of Louisiana, in which he wrote that the Mississippi River’s name meant “the ancient father of rivers.” Though his etymology was off—the Ojibwe words that gave us Mississippi (Misi-ziibi) actually mean “long river”—the idea has proven durable. “Ol’ Man River” buoyed Show Boat, the 1927 musical by Jerome Kern and Oscar Hammerstein II.

During the 1937 flood, Raymond Daniell wrote in the New York Times about frantic efforts to raise barriers “faster than old man river could rise.”

Now it appears that the Mississippi is far older than Le Page thought, and it used to be far bigger than the Ojibwe could have imagined. And it might even become that big again in the future.

These are the extraordinary new findings unearthed by geologists including Sally Potter-McIntyre at Southern Illinois University, Michael Blum at the University of Kansas and Randel Cox at the University of Memphis, whose work is helping us better understand the monumental events, beginning in late Cretaceous North America, that gave rise to the Mississippi, swelling it to gargantuan proportions.

In the late Cretaceous, around 80 million years ago, a mountain chain spanned the southern portion of the continent, blocking southbound water flows, so most North American rivers flowed to the Western Interior Sea or north to Canada’s Hudson Bay.

Eventually, a gap in those mountains formed, opening a path for the river we now know as the Mississippi to flow to the Gulf of Mexico. Scientists call that gap the Mississippi Embayment, but the rest of us know it as the Mississippi Delta, the vast flood plain that stretches from southern Missouri to northern Louisiana.

As recently as 2014, geological consensus held that the Mississippi began flowing through the embayment around 20 million years ago. But in 2018, Potter-McIntyre and her team concluded, based on the age of zircon fragments they excavated from sandstone in southern Illinois, that the river began flowing much earlier—some 70 million years ago.

The Mississippi was thus born when dinosaurs still roamed the planet; one can almost picture an alamosaurus bending its prodigious neck to drink from its waters. By contrast, the Missouri River, in its current form, dates back a mere two million years. Old Man River, indeed.

Still, 70 million years ago the Mississippi was nowhere near as large as it would become. Blum has detailed how the waterway grew as it added tributaries: the Platte, Arkansas and Tennessee rivers by the late Paleocene, then the Red River by the Oligocene.

Around 60 million years ago, the Mississippi was collecting water from the Rockies to the Appalachians; by four million years ago, its watershed had extended into Canada, and the Mississippi had grown to an enormous size, carrying four to eight times as much water as it does today, Cox and colleagues have found. “This was a giant river, on the order of the Amazon,” said Cox.

So the river’s larger-than-life role in culture was perhaps inevitable. Until the early 19th century, the Mississippi marked the western border between Spanish and American territory, and it continues to give life to the cities that sprang up along its route.

After Union forces captured Vicksburg, Mississippi, in 1863, President Abraham Lincoln saw the emancipated river as a symbol of a nation unified: “The Father of Waters again goes unvexed to the sea,” he wrote.

Mark Twain, the best publicist a river ever had, inspired 150 years’ worth of dreams about floating away from our troubles. And among members of the Ojibwe, Dakota and Chitimacha tribes, who still live on portions of ancestral lands in the Mississippi Valley, a spiritual connection to the river remains strong.

In 2013, Nibi Walk, a group of Indigenous women walked 1,500 miles along the Mississippi to advocate for clean water—an issue of vital importance to the 18 million Americans who get their drinking water from the river.

The river’s famed fluctuations have shaped American urbanization, too. The Great Flood of 1927 accelerated the Great Migration, as African Americans, disproportionately displaced, sought economic opportunity in cities such as Chicago and Detroit.

“Old Mississippi River, what a fix you left me in,” Bessie Smith sings in “Homeless Blues,” one of many songs about the 1927 flood. That disaster also ushered in an era of unprecedented public works, as the federal government sought to remake the river into a predictable route for moving bulk necessities like corn and coal.

The mighty river has inspired more than a thousand songs since 1900, including “Big River” by Johnny Cash and “Proud Mary,” in which John Fogerty (echoed later by Tina Turner) observes that “people on the river are happy to give.” That truism is confirmed every year when people who live along the Mississippi offer a meal and a shower to the dozens of strangers who test themselves against Old Man River by paddling small boats from Minnesota to the Gulf of Mexico.