Evolution: can work in reverse (Introduction)

by David Turell , Saturday, August 21, 2021, 19:22 (1188 days ago) @ David Turell

Newly found Cambrian fossils make the point:

https://phys.org/news/2021-08-rare-cambrian-fossils-utah-reveal.html

"Ctenophores, also known as comb jellies, are a group of over 200 living species of invertebrate animals with a transparent gelatinous body superficially resembling that of a jellyfish. There is much interest in ctenophore evolution in recent years as their controversial phylogenetic position in the animal tree of life has prompted conflicting hypotheses. While some studies suggest they might represent the earliest branching animals, others suggest a more traditional position as close relatives of jellyfish.

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"Despite their importance for understanding animal evolution, most information about ctenophores comes from living species alone as fossil comb jellies are extremely rare due to their gelatinous bodies. However, some fossil ctenophores have been discovered in early and middle Cambrian sites (about 520-500 million years ago) with exceptional preservation. These fossilized specimens, found around the world in sites including Burgess Shale in Canada and Chengjiang in South China, show that Cambrian ctenophores are a bit different from living representatives. The fossils include features such as a skeleton that supported the ctenes, or comb rows, as well as up to 24 comb rows—many more than the eight comb rows possessed by living species.

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"The first species, Ctenorhabdotus campanelliformis, has a small bell-shaped body with up to 24 comb rows and a wavy mouth opening. Intriguingly, this species shows two important features. First, there is a rigid capsule that protects the sensory apical organ, which represents the remains of the skeleton found in older ctenophores from the early Cambrian. Secondly, this species also shows a preserved nervous system. The nerves are long, and connect with a ring around the mouth. "This was quite an unexpected finding, as only one species (Euplokamis) of comb jellies today has comparable long nerves. Most modern comb jellies have a diffuse nervous net, and not well-defined long nerves," said senior author Professor Javier Ortega-Hernández, the Department of Organismic and Evolutionary Biology, Harvard University.

"The second species, Thalassostaphylos elegans, has a rounder appearance, approximately 16 comb rows, and a wavy mouth opening. Although this species does not show fossilized nerves, it has an important feature known as the "polar fields", which can be seen as two small dots on top of the apical organ. "These are also important for sensing the environment in living comb jellies, and finding evidence for them in the Cambrian is significant for understanding their evolution," said Ortega-Hernández. "Interestingly, Thalassostaphylos elegans does not have a rigid capsule, indicating that the skeleton found in early Cambrian ctenophores was already lost in some representatives by the mid-Cambrian."

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"The researchers conclude that Cambrian ctenophores had more complex nervous systems compared to those observed today. Living species of comb jellies have a diffuse nervous system similar to the structure of chicken wire, but very thin and transparent. Cambrian ctenophores' nervous systems were condensed with specific nerve tracks that basically ran along the length of the body and then as a ring around the mouth. This complex system is only seen in one living species, the Euplokamis, which is regarded as potentially being an early branching ctenophore living today. However, while Euplokamis has this elongated nerve structure that runs the length of the body, it does not have the ring around the mouth, so it too is simpler compared to Cambrian ctenophores.

"To better understand the evolution of this group, the team performed phylogenetic analysis which suggests the condensed nervous system is actually the ancestral condition and that only modern ctenophores have lost this complex nervous system and instead favored a more diffuse nerve net.

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"'This discovery means that there has a been a secondary simplification of comb jellies during their evolution, first losing the rigid skeleton, and then the discrete nerves observed in the fossils," said Dr. Luke A. Parry, Department of Earth Sciences, University of Oxford, UK. "These are insights that would be impossible to obtain from only studying living comb jellies, so the fossil record is providing a valuable glimpse into the evolution of these enigmatic animals." (my bold)

"Ortega-Hernández agreed, "In this context, Euplokamis would be showing a sort of vestigial organization of the nervous system, which are not seen in other living ctenophores. Ctenophores have a more complex evolutionary history than what can be reconstructed from their living representatives alone. Fossils allow us to understand the morphology that developed first and how it has changed through time.'"

Comment: these simple forms are nowhere like the complex animals that make the Cambrian Explosion such a huge gap in evolution. As simple forms they actually lost complexity over time. Evolution can run in reverse on very limited occasions. In Darwinist terms, it is obvious advanced complexification through natural selection isn't required. Animals can get by with less complexity than previously present.