Ancient Fossil Brain Gives Clues on How Arthropods of Today Got Heads

When Javier Ortega-Hernández realised he was looking at a well-preserved fossilised brain over 500 million year old at the Smithsonian Institute, he knew he'd made a remarkable discovery.

"Preserved nervous systems allow us to understand the origin of the head in arthropods, the animal group that includes insects, crustaceans, millipedes and spiders," said Ortega-Hernández, who is a research fellow in Paleobiology in the Department of Earth Sciences at the University of Cambridge, over email. As arthropods are all over the world these days, Ortega-Hernández noted that figuring out how they originated would lead to greater understanding on the origin of modern ecosystems and complex animal life.

In a study published today, Ortega-Hernández reports on some of the oldest brains preserved in the fossil record, which originated from a middle Cambrian locality known as the Burgess Shale in British Columbia, Canada. The Cambrian period, more commonly known as the Cambrian Explosion, is an era dating back 500 million years when major groups of animals first appear in the fossil record—primarily when arthropods with hard exoskeletons and jointed limbs emerged. Prior to this period, organisms on earth were mostly made up of algae and soft-bodied jellyfish-like creatures.

For his study, Ortega-Hernández evaluated how heads likely evolved in early animals primarily by looking at material that was deposited and collected at the Smithsonian Institution in Washington DC and the Royal Ontario Museum in Toronto.

So what prompted his quest for ancient fossilised brains? In 2014, reports circulated that a 520 million-year-old fossil brain had been unearthed in China, so Ortega-Hernández decided to "examine some good candidates" that might have some preserved brain traces in the large collections available today on the Cambrian period.

"Fortunately, I had a lucky guess or two, and it turned out that some of the fossil arthropods did indeed have preserved brains," said Ortega-Hernández, who told me that they'd been overlooked as most people believe that nervous systems are too fragile to be preserved.

Although arthropods are ubiquitous in modern ecosystems, the origins of the arthropod head has been a fairly controversial topic. Known as the "arthropod head problem" in biology, Ortega-Hernández explained that arthropods are segmented animals with heads composed of several fused segments. "The problem is that arthropods are so varied in their shapes that is is often difficult to recognize which segments are equivalent in different groups," he explained.

"Finding the brain in the Burgess Shale arthropods allowed me to recognize that, even though they are very different in their appearance, various Cambrian species have a similar head organization," said Ortega-Hernández.

For his study, Ortega-Hernández compared two types of arthropod ancestors—the Helmetia expansa (a soft-bodied trilobite) and the Odaraia alata (a genus of submarine crustacean arthropod). He explained that both possessed a "visor-like plate on their heads called an "anterior sclerite"" on which were a pair of dot-like eyes used for sensing light. These eye-like features had nerve traces connecting them to the front part of the brain, and corresponded to how vision is controlled in modern arthropods.

Helmetia_complete_Smithsonian: Helmetia expansa, a soft-bodied trilobite-like arthropod from the middle Cambrian Burgess Shale. Photograph courtesy of Javier Ortega-Hernández (University of Cambridge).

"Fortunately, some specimens of Odaraia (the submarine arthropod) have a beautifully preserved brain that shows nerve connections to these eyelets, which means that we can make very precise comparisons with the brains of living arthropods, and make sense of how the heads of extinct species were organized," said Ortega-Hernández.

"The anterior sclerite has been lost in modern arthropods, as it most likely fused with other parts of the head during the evolutionary history of the group," said Ortega-Hernández in a press statement. "What we're seeing in these fossils is one of the major transitional steps between soft-bodied worm-like creatures and arthropods with hard exoskeletons and jointed limbs - this is a period of crucial transformation," he continued.

In the study, Ortega-Hernández also made comparisons with an even more distant shrimplike arthropod ancestor known as the Anomalocaris, which he dubbed the "great swimming predator of the Cambrian." Hernandez-Ortega found that this and several other related species also had a visor-like plate on their heads. When he compared preserved Anomalocaris brains with those of Odaraia and Helmetia brains, Ortega-Hernández discovered they all had the same visor plate (anterior sclerite). "Putting it all together, this means that the anterior sclerite originated in soft-bodied animals like Anomalocaris, and was inherited by arthropods with jointed limbs," he said.

With heads becoming more complex over time, Ortega-Hernández findings have shed light on how arthropods metamorphosized from soft-bodied to hard-bodied creatures, and how they got they heads.