Fossil insects that are 500 million years old show remarkably well-preserved neural systems.

Scientists speculate that the organisms are connected to modern arachnids.

 

Fossils that date back 500 million years and are on display at the Smithsonian Institution (left panels) and the Harvard University Museum of Comparative Zoology (right panels) have retained nerve tissue. Picture courtesy of Ortega-Hernández et al. (2022) in Nature Communications.

The fossilized nerve tissue dates back 508 million years and can be found in two minuscule fossils, each smaller than an aspirin pill. The Cambrian bugs could aid in understanding how current spiders and scorpions evolved.

 

According to Nicholas Strausfeld, a regents professor in the Department of Neuroscience at the University of Arizona who was not involved in the work, it is still unclear where these fossils, both examples of the species Mollisonia symmetrical, belong on the arthropod evolutionary tree.

 

This is due to the fact that while some aspects of the nervous system can be plainly identified in the fossils—such as the eyes and nerve cords—other portions cannot. In particular, it is unknown whether or not the creatures possess a brain-like bundle of nerves termed a synganglion; without this proof, their relationship to other animals remains murky, as stated by Strausfeld.

 

According to the study's lead author, invertebrate paleobiologist and curator of the Harvard Museum of Comparative Zoology, Javier Ortega-Hernández, there is "this jumble in the middle of the head" in place of the synganglion. They know it's nerve tissue, but the researchers can't figure out how it's structured.

 

Because fossils can only reveal so much, "it is... true that we do not have every single trait of the neurological system of this animal laid out," Ortega-Hernández said. In their new article published on January 20 in the journal Nature Communications, the researchers acknowledge this ambiguity and give many hypotheses for how these fossils relate to extinct and extant creatures. Discoveries of other M. symmetrica fossils in the future could help determine the species' position on the evolutionary tree.

 

"Luck of the draw."

 

A "rarity," according to Ortega-Hernández, is the discovery of nerve tissue preserved in fossil form from the Cambrian period, which lasted from around 543 million to 490 million years ago. "I can only call it a fluke."

 

In 2012, researchers reported finding the fossilized remains of an arthropod brain from the Cambrian period. Arthropods are invertebrate organisms belonging to the phylum Arthropoda, which also includes contemporary insects, crustaceans, and arachnids like spiders. Since then, Ortega-Hernández added more than a dozen Cambrian fossils, mostly arthropods, have been recovered with preserved nerve tissue.

 

The fossils included in the current study were not unearthed from a remote location but rather were discovered deep within the collections of museums like the Smithsonian Institution in Washington, D.C., and the Harvard University Museum of Comparative Zoology in Cambridge, Massachusetts. Both pieces were unearthed in British Columbia's Burgess Shale layers, which date back to the middle Cambrian period.

 

The Harvard fossil is roughly 13 millimeters in length and 3.5 millimeters at its widest point, and it is angled such that you are gazing down on the arthropod. In contrast, the Smithsonian fossil provides a profile view of M. symmetrica; this specimen is only 0.3 inches (7.5 mm) in length and 0.06 inches (1.7 mm) in height.

 

According to Ortega-Hernández, neither fossil is particularly impressive at first glance. "Superficially, it is pretty uninteresting," he observed of the tiny Smithsonian fossil. The exoskeleton of M. symmetrica is composed of a basic head shield, a segmented trunk, and a posterior shield, similar to that of a pillbug but much longer and slimmer.

 

Based on a 2019 article describing a fossil from a different species in the Mollisonia genus that bore comparable appendages, researchers believe the arthropod had seven pairs of tiny appendages, including two fangs and six pairs of short limbs. Both of the fossils included in the new study have missing appendages, which is not surprising given how rare it is to discover Mollisonia fossils with whole limbs, as pointed out by Ortega-Hernández.

 

Ortega-Hernández remarked that, despite the lackluster aspect of the fossils, he noticed something unexpected when he put the Smithsonian M. symmetrica fossil under a microscope. When he examined the fossil, he commented, "Ooh, there's something strange inside of this animal." He discovered fully functional neural systems within the hard shells of each of these obscure arthropods. Because of organic carbon coatings, fossilization changed the nerve tissue into inky black splotches.

 

The fossil housed by the Smithsonian Institution depicts an arthropod with a bulbous eye on its head and a nerve cord extending the length of its abdomen, with some nerves protruding from its underbelly. The Harvard specimen has two enormous, orb-shaped eyes on its head, and a small portion of the nerve cord can be seen protruding from behind the animal's digestive tract, which covers the rest of the cord.

 

The researchers claimed to have seen optic nerves connecting the arthropods' eyes to the rest of their bodies in both specimens. Still, Strausfeld called the evidence for these nerves "ambiguous" and stated more clarity was preferable. The authors also detected the presence of nerve tissue in the heads of both species. However, they couldn't definitively say whether the nerve tissue was a synganglion similar to the brain or something else.

 

"We can tell there's something in there, but we don't have enough resolution to be able to say, 'Oh, it's clearly arranged in this way or that,'" According to Ortega-Hernández,

 

Data inaccuracies

 

The Harvard fossil depicts M. symmetrica from above. (Image: Ortega-Hernández et al., Nature Communications, 2022.)

Due to the lack of clarity in the fossil record, Ortega-Hernández claims that we still don't know how closely related M. symmetrica is to other species. The group, however, built two evolutionary trees based on the characteristics of the arthropods.

 

The two trees show that M. symmetrica and modern chelicerates are descended from the same ancestor, which suggests that the ancient animal's relatively simple nervous system gave rise to the highly condensed brain seen in modern members of this group which includes scorpions, spiders, horseshoe crabs, and ticks. Other major arthropod families from the Cambrian period, such as the megacheirans (which share a neurological system with contemporary chelicerates), are placed in different places across the trees.

 

Ortega-Hernández said that the relative positions of these groups on the evolutionary tree show either that chelicerate-like brains evolved in steps over time or that similar nervous systems evolved independently and at different times in some Cambrian arthropods and modern chelicerates, a process called "convergent evolution."

 

Given the available information, Strausfeld has stated that he would be "conservative" about attempting to locate M. symmetrica on an evolutionary tree. He said that he needs to see proof that nerves go all the way to the base of the arthropod's legs and that the optic nerves and synganglion (or lack thereof) are built in a certain way.

 

Strausfeld added, "I think one needs a better preparation, a better specimen" than what has been studied so far. Perhaps another specimen is stowed away in a museum.

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

Image source : https://pixabay.com/id/vectors/tanah-lapisan-terkena-menutupi-575641/

Bug-like fossils dating back 500 million years contain astonishingly well-preserved neural systems.

Bug-like fossils dating back 500 million years contain astonishingly maintained neural systems.