While hard appendages like bone and shell can be preserved as fossils for millions of years, palaeontology’s enduring problem has been a dearth of soft tissues in the fossil record. As a result, there are huge knowledge gaps about the evolution of soft tissues – including internal organs like the brain – among early life.
A new study, published today in Geology, has filled one such knowledge gap, describing the preserved brain and central nervous system (CNS) of a 310-million-year-old horseshoe crab (Euproops danae) that lived during the Carboniferous Period.
Australian lead author Russell Bicknell, of the University of New England (UNE), says that the discovery is unique because brain tissues are generally only preserved in amber deposits or Cambrian Burgess-Shale deposits, both of which are limited to certain windows of prehistory.
“Amber, or fossilised tree resin, often contains a variety of trapped organisms such as insects, preserving the most intricate details. Using sophisticated imaging technology, scientists are able to study these entombed creatures, including their tiny brains.
“However, we are somewhat limited when studying these particular fossils, as the oldest arthropods in amber only date back to the Triassic Period, around 230 million years ago.”
Burgess Shale-type deposits from the Cambrian Period – typically around 500 to 520 million years in age – are much older than amber, and also preserve brain structures as carbon films in mudstone.
“These Burgess Shale-type fossils are very important as they represent some of the oldest animals on Earth, and can inform us on their origins and earliest evolutionary history,” Bicknell says. But the window between the youngest Burgess Shale deposits and the oldest amber deposits has – until now – lacked any well-preserved brains for palaeontological research.
“So, even though we can see arthropod brains from a wide timespan, there’s actually really big gaps in between,” says co-author John Paterson, a palaeontologist also at the UNE. “What we’ve done with our new fossil discovery is fill one of those gaps.”
The research specimen comes from the famous Mazon Creek deposit in Illinois, US. Fossils from this deposit are preserved in concretions made from an iron carbonite mineral called siderite.
The central nervous system can provide a unique insight into the behaviour of extinct animals, but neurological tissues decay rapidly, often well before they can be preserved – which makes this find so serendipitous.
“We have shown, for the first time, that the Mazon Creek animals were not only moulded by the rapid formation of siderite that entombed their entire bodies, but also that the siderite quickly encased their internal soft tissues before they could decompose,” Paterson says.
One of the striking things about the fossil is that its CNS has been preserved in a stand-out white colour, starkly visible against the rest of the specimen.
“In our fossil, the brain of Euproops is replicated by a white-coloured clay mineral called kaolinite,” Paterson says. “This mineral cast would have formed later within the void left by the brain, long after it had decayed. Without this conspicuous white mineral, we may have never spotted the brain.”
The discovery marks the first time this siderite deposit has been conclusively shown to preserve a brain. One other specimen from the deposit, a bizarre creature known as the Tully Monster (Tullimonstrum), was found to have something resembling a central nervous system, but Paterson says the evidence was inconclusive. This specimen, on the other hand, has compelling evidence of a brain.
Paterson says this unique fossil shows how little the horseshoe crab brain has changed in the 310 million years since the Mazon Creek specimen died.
“Even though horseshoe crabs tend to be labelled living fossils, they have changed quite a bit in their external anatomy, as well as the environments they live in,” Paterson says. “But we can look at modern horseshoe crabs that are living in places like Asia or the United States and the brain looks very, very similar, which is pretty amazing.”
This extraordinary conservatism suggests that horseshoe crabs had already hit the evolutionary jackpot way back in the Carboniferous: “With that type of brain architecture, they must have hit on a winning formula and just kept with it.”