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article imageScientists analyze 500-million-year-old brains of 'sea monsters'

By Megan Hamilton     May 10, 2015 in Science
Scuttling around in their Cambrian world over 500 million years ago, two species of hard-shelled, primitive arthropods could have no idea that they would help scientists to better understand how modern arthropods like lobsters and scorpions evolved.
However, that's exactly what they did.
Recently, scientists have been examining some of the oldest known fossil brains to be discovered, and this has given them a better idea of how primitive animals developed heads, IFLScience reports. These prehistoric brains belonged to early ancestors of our modern-day arthropods, an amazing group that includes insects, spiders, and crustaceans.
Their findings, published this week in Current Biology, uncover a key moment in the change-over from soft-bodied worm-like arthropod ancestors to those with hard exteriors like the creatures we are more familiar with today.
During a wondrous period known as the Cambrian Explosion, all sorts of evolutionary innovation was occurring. This is when most major animal groups began showing up in the fossil record, and this includes arthropods with hard exoskeletons and jointed limbs. It was definitely a good time to be an arthropod. Prior to this, most animals were soft-bodied, like worms and jellyfish are, and their heads weren't well defined.
To begin with, the researchers focused on an oval structure known as the anterior sclerite, which is found in the heads of prehistoric arthropods, LiveScience reports. It's a structure that has baffled scientists for some time, largely because some prehistoric arthropods have it, but others don't, and depending upon the quality of the fossil, its location in the head changes.
Which is where Odaraia alata and Helmetia expansa come in. Discovered in British Columbia's famed Burgess Shale during the early part of the 20th century, they are currently housed in a collection at the Smithsonian Museum.
Close-up of the fossilized brain in Odaraia alata  an arthropod resembling a submarine from the Midd...
Close-up of the fossilized brain in Odaraia alata, an arthropod resembling a submarine from the Middle Cambrian Burgess Shale.
Photo courtesy of Jean Bernard Caron (Royal Ontario Museum)
An analysis of these fossils indicates that the anterior sclerites were associated with the creatures' bulbous eyes. This provides evidence that, in these creatures, the oval structures were associated with nerves originating from the brain's anterior region, the study reported, per LiveScience.
Odaraia alata  an arthropod resembling a submarine from the Middle Cambrian Burgess Shale.
Odaraia alata, an arthropod resembling a submarine from the Middle Cambrian Burgess Shale.
Photo courtesy of Jean Bernard Caron (Royal Ontario Museum)
"We can say, 'Ah-ha, where does anterior sclerite come from? It comes from the anterior most part of the brain — the forebrain," said researcher Javier Ortega-Hernández, a research fellow in paleobiology at the University of Cambridge in the United Kingdom.
The eyes of these marvelous creatures were simple photoreceptors and were embedded in the hard plate of the anterior sclerite, IFLScience reports, and indeed, the fossils show the nerve traces originating from the front of the fossilized brain — the forebrain. In modern arthropods, the forebrain is associated with vision control. It's likely these prehistoric brains processed information in a fashion similar to modern cockroaches and crabs, and Odaraia and Helmetia needed these brains for sensing the environment, finding food, and escaping from predators.
"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," Ortega-Hernández says in a 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."
Ortega-Hernández can see similarities between Odoraia and Helmetia and anomalocaridids, an oddball group of swimming predators that lived during the Middle Cambrian. It's thought that these are early arthropod ancestors, even though they don't really look much like any living arthropod today. What he discovered is that the plate atop the anomalocaridid head shared similarities with the anterior sclerite, thus suggesting a common origin, IFLScience reports. It's the anterior sclerite that may connect the head of anomalocaridids with those of arthropods that are more recognizable.
"Heads have become more complex over time," Ortega-Hernández notes. "Here is an answer to the question of how arthropods changed their bodies from soft to hard. It gives us an improved understanding of the origins and complex evolutionary history of this highly successful group."
The Burgess Shale has offered up thousands of spectacular fossils since being discovered in 1909 by Charles D. Walcott. Now, some 65,000 specimens are housed in the Smithsonian's National Museum of Natural History, Paleobiology notes. Since Walcott's discovery, fossil deposits like the Burgess have been found widely dispersed in China, Greenland, Siberia, Australia, Europe, and the U.S.
The creatures were buried in an underwater avalanche of fine mud that exquisitely preserved fine details of the structures that comprised their soft parts. In most Cambrian deposits, only the hard parts are preserved, and because of this, information in the geologic record is limited.
Another interesting aspect of the Burgess Shale fossils is that these creatures demonstrably grew into a variety of shapes and sizes from more primitive Precambrian life forms. Many of the fossils appear to be ancestors of higher forms, and this includes everything from algae to the chordates (a major group of animals that include human primates). On the other hand, there are also some forms that don't seem to be related to any living forms and their disappearance presents a mystery.
Note: This digital journalist searched several sites to find out the sizes of Odaraia and Helmetia but was unsuccessful.
More about 500 million year old brains, "sea monsters", burgess shale, Cambrian explosion, precambrian
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