The oldest fossilised brain ever discovered was found in a deep sea worm that was 525 million years old.

 One of the oldest preserved brains ever discovered was located in a prehistoric worm discovered in China. The structure of the brain may also assist resolve a long-standing controversy over the origins of arthropods.

The earliest known example of a brain is probably found in a fossilised worm that dates back 525 million years and was discovered in China. The unexpected structure of the brain provides information on the development of arthropods, which include insects, arachnids, and crustaceans. It also offers a potential solution to a mystery that has baffled scientists for more than a century.

The fossilised creature, known as Cardiodictyon catenulum, was found in 1984 at a location in China's Yunnan province together with a large number of other fossils, collectively known as Chengjiang fauna.

An additional team of scientists reexamined the petrified creature and discovered that it had been concealing an astounding secret – an intact neurological system, including a brain — in a new research, which was published Nov. 24 in the journal Arthropod Evolution(opens in new tab).

Nicholas Strausfeld, a neurobiologist at The University of Arizona in Tucson and the study's principal author, said in a statement, "To our knowledge, this is the oldest fossilised brain we know of, so far" (opens in new tab).

Related: 310-million-year-old petrified brain discovered in perfect condition

Because scientists had previously thought that all soft tissue in C. catenulum had been destroyed over time, it took over 40 years for them to find the animal's brain.

Frank Hirth, an evolutionary neurologist at King's College London in the U.K. and research co-author, said in a statement that "until very recently, the prevailing assumption was [that] brains don't fossilise." He stated that previous researchers "would not even dare to look at it in hopes of discovering a brain" because of the fossil's modest size and age.

Recent studies of fossils that are identical and date to the same period, however, have challenged this assumption. To date, rudimentary fossilised brains have also been discovered in a 500 million year old relative of the penile worm, a 500 million year old bug-like creature, a 520 million year old "sea monster," and many 506 million year old three-eyed marine monsters.

Arthropod evolution is proceeding.

Even though the discovery of the ancient brain surprised the experts, they were more shocked by the critter's cranium's form and shape. Both the skull and the brain are non-segmented, which means they are not divided into many components. The rest of the fossil's body, however, is broken up into sections.

This anatomy was entirely unexpected, according to Strausfeld. For more than a century, scientists believed that long extinct arthropods had segmented heads and brains, much like current arthropods do. Most fossils of other early arthropod predecessors likewise have segmented heads and brains, he continued.

The "fundamental brain plan" may not have evolved significantly over the past half-billion years, according to the study's authors, who highlighted that the fossilised brain of C. catenulum still shares several essential traits with those of contemporary arthropods.

To learn more about how various brains have evolved throughout time, the researchers will next compare the preserved brain with the brains of other animal species.