Cambrian Explosion: a whole nervous system (Introduction)

by David Turell , Thursday, March 17, 2016, 00:53 (3174 days ago) @ David Turell

Another review of Cambrian fossils and their central nervous system (CNS), Note they appear fully formed without precursors and really haven't changed much in today's world:-http://www.evolutionnews.org/2015/11/three_more_prob100791.html-"Another paper about Cambrian fossils appeared in Current Biology (see previous coverage here and here). This one is about "Preservational Pathways of Corresponding Brains of a Cambrian Euarthropod," suggesting that it's going to show brains of a really complex animal, a "true" or "good" arthropod -- you know, those complex animals with jointed appendages, brains, and a digestive tract? (insects, spiders, crabs, etc.). Sure enough, the color pictures jump out at you: brown stains at the heads of Chinese arthropod fossils are the actual remains of this animal's central nervous system (CNS), revealing "tripartite brain organization, cephalic nerves, and optic neuropils." Earlier finds showed some of these things, but the new fossils remove all doubt.-
"The record of arthropod body fossils is traceable back to the "Cambrian explosion," marked by the appearance of most major animal phyla. Exceptional preservation provides crucial evidence for panarthropod early radiation. However, due to limited representation in the fossil record of internal anatomy, particularly the CNS, studies usually rely on exoskeletal and appendicular morphology. Recent studies show that despite extreme morphological disparities, euarthropod CNS evolution appears to have been remarkably conservative.-
"'Let's parse this opener and translate the euphemisms. First, notice the quote marks around "Cambrian explosion," a subtle hint that the term is controversial. It's not. They state clearly that it is "marked by the appearance of most major animal phyla." Panarthropoda is a taxon that combines arthropods with tardigrades and onycophorans. The sentence means that yes, lots of different arthropods appear throughout the fossil record, revealing "extreme morphological disparities," i.e. outward differences. -***-"Most importantly, these fossils show that "a tripartite brain comprising three pre-stomodeal neuromeres had evolved by the early Cambrian" -- better, it appeared in the early Cambrian without any evidence of evolution before or after. They restate this important fact: "These features demonstrate that by Cambrian Stage 3 (circa 517 million years ago) arthropods had already acquired CNSs generally corresponding to those of extant taxa." -***-"So, we find complex animals with stains of their brains preserved in the rocks -- brains that look just like modern arthropod brains. The fossils were not disturbed by worms that existed alongside the arthropods, but apparently did not exist earlier in the Ediacaran. And then we see conserved genes that they say haven't evolved for 700 million years."-Comment: Looks just like a saltation to me