New fossils show what the ancestral brains of arthropods looked like


Date:
August 20, 2021
Source:
University of Arizona
Summary:
Rare fossils preserving the brains of creatures living more
than half a billion years ago shed new light on the evolution of
arthropods such as insects and crustaceans.



FULL STORY ========================================================================== Exquisitely preserved fossils left behind by creatures living more than
half a billion years ago reveal in great detail identical structures
that researchers have long hypothesized must have contributed to the
archetypal brain that has been inherited by all arthropods. Arthropods
are the most diverse and species- rich taxonomic group of animals and
include insects, crustaceans, spiders and scorpions, as well as other,
less familiar lineages such as millipedes and centipedes.


==========================================================================
The fossils, belonging to an arthropod known as Leanchoilia, confirm the presence -- predicted by earlier studies in genetics and developmental
biology of insect and spider embryos -- of an extreme frontal domain
of the brain that is not segmented and is invisible in modern adult
arthropods. Despite being invisible, this frontal domain gives rise to
several crucial neural centers in the adult arthropod brain, including
stem cells that eventually provide centers involved in decision-making
and memory. This frontal domain was hypothesized to be distinct from
the forebrain, midbrain and hindbrain seen in living arthropods, and
it was given the name prosocerebrum, with "proso" meaning "front."
Described in a paper published today in the journal Current Biology,
the fossils provide the first evidence of the existence of this discrete prosocerebral brain region, which has a legacy that shows up during
the embryonic development of modern arthropods, according to paper lead
author Nicholas Strausfeld, a Regents Professor of Neuroscience at the University of Arizona.

"The extraordinary fossils we describe are unlike anything that has
been seen before," Strausfeld said. "Two nervous systems, already unique because they are identically preserved, show that half a billion years
ago this most anterior brain region was present and structurally distinct before the evolutionary appearance of the three segmental ganglia that
denote the fore-, mid- and hindbrain." The term ganglion refers to a
system of networks forming a nerve center that occurs in each segment
of the nervous system of an arthropod. In living arthropods, the three
ganglia that mark the three-part brain condensed together to form a
solid mass, obscuring their evolutionary origin as segmented structures.

Fossils of Brain Tissue are Extremely Rare Discovered in deposits of
the Kaili formation -- a geological formation in the Guizhou province of southwest China -- the fossilized remains of Leanchoilia date back to the Cambrian period, about 508 million years ago. The Kaili fossils occur
in sedimentary rock that has high concentrations of iron, the presence
of which probably helped preserve soft tissue, which subsequently was
replaced by carbon deposits.



==========================================================================
"The Kaili fossils open a window for us to glimpse the body plan
evolution of animals that lived more than half a billion years ago,"
said the paper's first author, Tian Lan of the Guizhou Research Center
for Palaeobiology at Guizhou University in China. "For the first time,
we now know that arthropod fossils of the Kaili formation have the
potential to preserve neural tissue that show us the primitive brain
of the early stem arthropod existing at the dawn of the animal world."
"Nervous systems, as other soft tissues, are difficult to fossilize,"
added co- author Pedro Martinez of the Universitat de Barcelona and
Institut Catala' in Barcelona, Spain. "This makes the study of the early evolution of neural systems a challenging task." The fossils also shed
new light on the evolutionary origin of two separate visual systems in arthropod evolution: pairs of front-facing eyes or sideward looking eyes,
the descendants of which are still present in species living today.

Many arthropods, including insects and crustaceans, have a distinct
bilateral pair of faceted compound eyes and another set of less obvious
eyes -- with more primitive architecture -- known as nauplius eyes, or
ocelli. These are structurally similar to the principal eyes of spiders
and scorpions. These simpler eyes correspond to the prosocerebrum's
forward eyes in Leanchoilia, in line with evidence obtained by previous
studies analyzing gene expression patterns during embryonic development
of living arthropods.

Leanchoilia's sideward eyes, on the other hand, relate to the
protocerebrum, which is the segmental ganglion defining the arthropod forebrain, lying just behind the prosocerebrum. In living arthropods, the protocerebrum provides the compound eyes of insects and crustaceans, or
the lateral single-lens eyes of arachnids, centipedes and millipedes. The visual centers serving those eyes also belong to the brain's protocerebral region.



========================================================================== Strausfeld explained that in living arthropods, the protocerebrum,
or forebrain, has incorporated -- in a way, swallowed up -- the ancient
centers provided by the prosocerebrum, so that it is no longer discernible
as a distinct anatomical entity.

The fossils are so well-preserved that they demonstrate that in addition
to frontward eyes, the prosocerebrum has also given rise to ganglia
associated with the labrum, or "upper lip," of modern arthropods. The
fossils also confirm an earlier hypothesis suggesting that the labrum
must have originally evolved from the grasping appendages of Radiodonta,
a group of stem-arthropods that were top predators during the Cambrian
period.

"When compared with other, similar fossil material belonging to more
advanced lineages, the organization of the Leanchoilia brain demonstrates
that the ganglionic arrangement of the early brain underwent condensation
and fusion of its components, which explains why in living species the prosocerebrum cannot be individually distinguished," Strausfeld said.

Implications for Brain Evolution in Vertebrates In addition to closing
a century-old gap in the understanding of arthropod brain evolution,
the findings have important implications for the early evolution of
vertebrate brains, Strausfeld said.

Although simple, fishlike animals existed at the same time as these
now- fossilized arthropods, there are no convincing fossils of their
brains and, thus, neither fossil evidence nor anatomical evidence for a prosocerebrum in vertebrates. Yet, modern studies show that genes defining
the fore- mid- and hindbrains of, for example, mice correspond to genes defining the three ganglionic divisions of the arthropod brain. And
in vertebrates, certain crucial centers involved in decision making
and in learning and memory have some genetic correspondences with the
higher centers in the arthropod brain, which originated in the ancient arthropod prosocerebrum.

Thus, it is plausible that even earlier than the Cambrian period,
possibly even before the evolution of segmentally organized body plans,
the common ancestor of both vertebrates and invertebrates possessed basic circuits for simple cognition and decision making. And while an ancient prosocerebral-like brain might have been present in the very early
ancestors of vertebrates, no such fossil has even suggested evidence
for a discrete, nonsegmental domain.

"Nevertheless, one can reasonably speculate that vertebrates have
embedded in their 'modern' brains parts of an ancient, non-segmented
brain that has so far only been demonstrable in an early arthropod,
such as Leanchoilia," Strausfeld said.

Additional co-authors on the study are Yuanlong Zhao of the Guizhou
Research Center for Palaeobiology at Guizhou University in Guiyang,
China; Fangchen Zhao of the State Key Laboratory of Palaeobiology and Stratigraphy of the Chinese Academy of Sciences in Nanjing, China;
and You He of Shanghai Synchrotron Radiation Facility.

========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Daniel Stolte.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Tian Lan, Yuanlong Zhao, Fangchen Zhao, You He, Pedro Martinez,
Nicholas
J. Strausfeld. Leanchoiliidae reveals the ancestral organization
of the stem euarthropod brain. Current Biology, 2021; DOI: 10.1016/
j.cub.2021.07.048 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210820111114.htm

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