Evolution: horse shoe crab brain unchanged from Cambrian (Evolution)

by David Turell , Thursday, July 29, 2021, 15:20 (995 days ago) @ David Turell

Ancient fossil brain and present brain look the same:

https://theconversation.com/ancient-brains-a-look-inside-the-extraordinary-preservation...

"Charles Darwin famously discussed the “imperfections” of the geological record in his book On The Origin of Species. He correctly pointed out that unless conditions are just right, it’s unlikely for organisms to be preserved as fossils, even those with bones and shells.

"He also said “no organism wholly soft can be preserved”.

"However, after more than a century of fossil hunting since his book was published, we now know the preservation of soft creatures is indeed possible — including some of the most fragile animals, such as jellyfish.

"Burgess Shale-type deposits are much older, being Cambrian in age (typically 500 to 520 million years old). They contain an abundance of exceptionally preserved marine arthropods.

"These fossils are very important as they represent what are unmistakably some of the oldest animals, and can therefore inform us on their origins and earliest evolutionary history. Their remains are primarily preserved as carbon films in mudstone.

***

"Many Burgess Shale-type arthropod specimens preserve internal organs, especially the gut. But fewer show parts of the central nervous system, such as the optic nerves, ventral nerve cord or the brain.

"Our new fossil demonstrates arthropod brains can be preserved in an entirely different way. The specimen of the horseshoe crab, Euproops danae, comes from the world-famous Mazon Creek deposit of Illinois, in the US. Fossils from this deposit are preserved within concretions made of an iron carbonate mineral called siderite.

***

"One of the challenges of interpreting ancient arthropod anatomy is the lack of close modern relatives available for comparison. But luckily for us, Euproops can be compared to the four species of living horseshoe crabs.

"Even to the untrained eye, a comparison of the fossil’s nervous system with that of a modern horseshoe crab (below) leaves little question that the same structures are found in both species, despite them being separated by 310 million years.

"The fossil and living nervous systems match up in their arrangements of nerves to the eyes and appendages, and show the same central opening for the oesophagus to pass through.

"Uncovering these exceptional specimens gives palaeontologists a rare glimpse into the deep past, enhancing our understanding of the biology and evolution of long-extinct animals. It seems Charles Darwin need not have been so pessimistic about the fossil record after all."

Comment: see the pictures of comparison. Stasis, not an advance, can happen also. Modern fossil analysis techniques are far in advanced of what Darwin thought could be.