Cambrian Explosion: still finding nervous systems (Introduction)

by David Turell , Thursday, January 27, 2022, 16:36 (821 days ago) @ David Turell

Reexamination finds nerves:

https://www.livescience.com/fossilized-arthropods-contain-preserved-nervous-tissue?utm_...

"Two tiny fossils, each smaller than an aspirin pill, contain fossilized nerve tissue from 508 million years ago. The bug-like Cambrian creatures could help scientists piece together the evolutionary history of modern-day spiders and scorpions.

"Still, it's not clear exactly where these fossils — both specimens of the species Mollisonia symmetrica — fit on the arthropod evolutionary tree, said Nicholas Strausfeld, a regents professor in the Department of Neuroscience at the University of Arizona, who was not involved in the study.

"That's because some features, like the animals' eyes and nerve cords, can be clearly identified in the fossils, but other parts of the nervous system cannot be so easily spotted. In particular, it's unclear whether or not the animals carry a brain-like bundle of nerves called a synganglion, and without this key piece of evidence, their relation to other animals remains fuzzy, Strausfeld said.

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"'It is … true that we do not have every single characteristic of the nervous system of this animal mapped out, because the fossils only tell us so much," Ortega-Hernández said. The researchers acknowledge this uncertainty in their new report, published Jan. 20 in the journal Nature Communications, and present a few different ideas as to how these fossils relate to ancient and modern-day critters. If more fossilized M. symmetrica are uncovered in the future, the species' place on the tree of life may eventually be resolved.

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"Scientists uncovered the first evidence of a fossilized arthropod brain from the Cambrian period about a decade ago, according to a 2012 report in the journal Nature Communications; arthropods are invertebrate animals in the phylum Arthropoda, a group that includes modern insects, crustaceans and arachnids, like spiders. Since that initial discovery 10 years ago, preserved nerve tissue has been found in more than a dozen Cambrian fossils, most of them arthropods, Ortega-Hernández said.

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"The researchers suspect that the arthropod also had seven pairs of tiny appendages, two fangs and six pairs of little limbs; that's based on a 2019 study, published in the journal Nature, that described a fossil from a different species in the Mollisonia genus that bore such appendages. However, it's highly unusual to find Mollisonia fossils with intact limbs, and both fossils used in the new study lack appendages, Ortega-Hernández noted.

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"In the Smithsonian fossil, a bulbous eye can be seen in the arthropod's head and a nerve cord can be clearly seen running down the length of its belly, with some nerves jutting out from its underside. In the Harvard specimen, one can see two huge, orb-like eyes on the head, and a bit of the nerve cord peeking out from beneath the animal's digestive tract, which obscures the rest of the cord.

"In both fossils, the study authors reported seeing optic nerves that run from the arthropods' eyes into the main body, but Strausfeld said the evidence for these nerves is "ambiguous," and ideally, these features would be clearer. And in both specimens, the authors noted that there's some sort of nerve tissue present in the head, but it's unclear whether this structure is a brain-like synganglion or something else entirely."

Comment: Another example of the complexity of Cambrian organisms with no precursors and the gap from the Edicaran layers is only 410,000 years, a blink of the eye in evolutionary terms.