Cambrian comb jellies had a more complex neuroanatomy than living species | Paleontology

Paleontologists have described two new species from the Cambrian period of Utah, which shed light on the early evolution of nerve and sensory characteristics of ctenophores (comb jellies).

Reconstruction of an artist from Ctenorhabdotus campanelliformis (top) and Thalassostaphylos elegans (low). Image credit: Holly Sullivan.

Ctenophores are a group of over 200 living species with a transparent gelatinous body superficially resembling that of a jellyfish.

While some studies suggest they might represent the earliest branched animals, others suggest a more traditional position as close relatives of jellyfish.

The two new species, named Ctenorhabdotus campanelliformis and Thalassostaphylos elegans, are a spectacular addition to the group’s rare fossils.

The specimens were found in the Marjum Formation in Utah’s House Range and represent the earliest ctenophoric fossils ever found in the United States.

Ctenorhabdotus campanelliformis had a small, bell-shaped body with up to 24 rows of combs and a wavy mouth opening.

Oddly enough, this species had two important characteristics: (i) a rigid capsule which protected the apical sensory organ, and (ii) a well-preserved nervous system; the nerves are long and connect with a ring around the mouth.

“It was a rather unexpected find, because only one species (Euplokamis) comb jellies today has comparable long nerves, ”said Professor Javier Ortega-Hernández, a paleontologist in the Department of Organic and Evolutionary Biology at Harvard University.

“Most modern comb jellies have a diffuse nerve network and long, undefined nerves.”

Thalassostaphylos elegans had a rounder appearance, about 16 rows of combs, and a wavy mouth opening.

It had an important feature known as “polar fields”, which can be thought of as two small dots above the apical organ.

“These are also important for detecting the environment in living comb jellies, and finding evidence for them in the Cambrian is important for understanding their evolution,” Professor Ortega-Hernández said.

“Interesting way, Thalassostaphylos elegans lacks a rigid capsule, indicating that the skeleton found in Early Cambrian ctenophores had already been lost in some representatives by the mid-Cambrian.

Professor Ortega-Hernández and his colleagues concluded that Cambrian ctenophores had more complex nervous systems than those seen today.

They also performed a phylogenetic analysis which suggests that the condensed nervous system is in fact the ancestral condition and that only modern ctenophores have lost this complex nervous system and instead favored a more diffuse nervous network.

“This finding means that there was a secondary simplification of the comb jellies during their evolution, losing first the rigid skeleton and then the discrete nerves seen in the fossils,” said Dr Luke Parry, paleontologist at the Department of Earth Sciences. at the University of Oxford.

“This is information that would be impossible to obtain by studying only living comb jellies, so the fossil record provides valuable insight into the evolution of these enigmatic animals.”

“In this context, Euplokamis would show a sort of residual organization of the nervous system, which is not observed in other living ctenophores, ”said Professor Ortega-Hernández.

“Ctenophores have a more complex evolutionary history than what can be reconstructed from their living representatives alone. “

“The fossils allow us to understand the morphology that first developed and how it changed over time.”

The discovery is reported in an article published in the journal iScience.

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Luke A. Parry et al. Utah’s Cambrian Comb Jellies shed light on the early development of the nervous and sensory systems in ctenophores. iScience, published online August 4, 2021; doi: 10.1016 / j.isci.2021.102943


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