How the animals got heads

500-million-year-old brain fossils show decisive developmental stages

Fossil of Odaraia alata, an early Cambrian arthropod. Head and early eyes are easily recognizable. © Jean Bernard Caron, Royal Ontario Museum
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Decisive Transition: Two 500-million-year-old fossils show how shapeless molluscs made the first arthropods with heads. They are also among the oldest known fossils with recognizable brain, reports a British paleontologist. Thus, they show not only crucial transitions in the evolutionary history of modern insects and crabs, but also for the structure of all carcasses, writes the scientist in the magazine "Current Biology".

During the so-called Cambrian explosion about 500 million years ago, life on earth suddenly increased: fossils from this period show a huge amount of animal species that did not exist before and developed in a relatively short time. In this epoch, the first precursors of crustaceans appeared in the oceans - almost revolutionary creatures with a hard shell. In the pre-Cambrian era, most animals resembled soft jellyfish or microalgae.

From soft to hard

An important step on the way from these shapeless mollusks to the structured bodies of our time, especially the origin of the head, has now been clarified by Javier Ortega-Hern√°ndez of the University of Cambridge. He studied two fossils from the time of the Cambrian explosion: a soft trilobite and a bizarre creature called Odaraia alata. This animal already had the physique of an arthropod and is reminiscent of a lobster.

Despite their differences, the two animals had a lot in common, as the fossils prove: both had at the front end of their body a hardened plate, the so-called front sclerites. Embedded in it are eyes similar organs. Traces of nerves indicate a connection to the front of the brain. In this brain region, the visual center is located in modern arthropods such as crabs or insects.

More than head and brain

The two fossils from the Canadian Burgess Shale are a real stroke of luck: even in the excellent conditions of this site, well-preserved brains are not a common find. Such soft tissue is rarely preserved. However, the fossils described now not only show some of the oldest known brains, they are also among the best preserved. display

Ancestors of the Arthropods: Model of Laggania cambria, a representative of the Anomalocarididae. This species was about 60 inches long. Espen Horn / State Museum of Natural History Karlsruhe (CC BY-SA 3.0)

The fossils, however, not only show early head and brain structures. According to Ortega-Hern ndez, they also support another important developmental step: "What we see here is the answer to how the arthropods changed their bodies from soft to hard." The new findings also allowed a comparison with another group of animals, the Anomalocarididen. These are considered ancestors of the arthropods, so all modern insects, crustaceans and spiders. The Anomalocaridids resembled a few giant shrimps up to two meters long. In the Cambrian they were probably at the top of the food chain.

"A decisive epoch of change"

In fossils of these animals, the researchers found structures that contained remnants of the anterior sclerite - the early form of hard crust that characterized the first two fossils. This structure can no longer be found in the outer skeletons of modern animals: "The anterior sclerite has been lost in modern arthropods, as it is likely to have been lost during the evolution of this group with other parts of the family Head merged, "explains Ortega-Hern ndez.

"What we see in these fossils is one of the great transitions between worm-like animals with soft bodies and arthropods with hard outer skeleton and limbs with joints" "sums up the paliontologist. "This gives us a better understanding of the origins and complex evolutionary history of this highly successful group of organisms, " says Ortega-Hern ndez. "This was a crucial epoch of change." (Current Biology, 2015; doi: 10.1016 / j.cub.2015.04.034)

(University of Cambridge, May 11, 2015 - AKR)