Rare Cambrian fossils from Utah reveal unexpected anatomical complexity in early comb jellies

Ctenorhabdotus Campanelliformis (top), and Thalassostaphylos Elegans (bottom): Artistic Reconstruction Holly Sullivan
Ctenophores are also known as comb jelly, and they consist of more than 200 species of living invertebrate animal species. Their transparent gelatinous bodies resemble that of jellyfish. Ctenophore evolution has been a topic of intense interest since their controversial phylogenetic location in the animal tree is causing conflicting hypotheses. Some studies suggest that they may be the oldest branching animals. Others suggest that they are closer relatives to jellyfish.

These hypotheses have important implications for understanding how animals came to be. It is possible that muscles or the nervous system could have come from multiple sources, which is a big deal since these features are so distinctive in animals today.

A team of international researchers published a study in iScience that described two new species fossil ctenophores, one of which had a preserved nervous system. This helps to illuminate the origins of sensory and nervous features in ctenophores.

Although they are important for understanding animal evolution, the majority of information about ctenophores is derived from living species. Fossil comb jellies with their gelatinous bodies are very rare and hard to find. Some fossil ctenophores were found in the middle and early Cambrian (about 520 to 500 million years ago) and have shown remarkable preservation. These fossilized specimens were found in a variety of locations around the globe, including in Canada's Burgess Shale and South China's Chengjiang. They show that Cambrian Ctenophores are quite different from their living counterparts. These fossils have features such as a support skeleton for the ctenes or comb row, and up to 24 rows. This is a lot more than the eight comb rows that are possessed by living species.

Holotype fossil specimen from Thalassostaphylos Elegans, mid-Cambrian Marjum formation in Utah. Light photograph (left), interpretative draw (center), magnification of the polar fields (right). Credit: Rudy Lerosey - Aubril and Luke Parry

Researchers describe the first known ctenophore fossils in America. They also add two species to the fragmented fossil record. These fossils, which are 500 million years old, were discovered in the Marjum Formation of the House Range in Utah. These marine deposits are remarkable for preserving delicate organisms that would otherwise not be found in the shelly fossil record. The fossils are flattened and preserved as films of organic carbon. This aids in the preservation of internal organs. The researchers used electronic microscopy to detect carbon film signals, which allowed them to identify original parts of the internal anatomy.

Ctenorhabdotus Campanelliformis is the first species. It has a small bell-shaped structure with up to 24 rows of combs and a wide mouth opening. This species has two interesting features. The first is the rigid capsule that protects and preserves the sensory apical, which contains the remains of the skeleton from older ctenophores dating back to the early Cambrian. This species also has a preserved nervous systems. The nerves connect to the mouth with a long, ring-like ring. This was a surprising finding as only one species of comb jellies (Euplokamis), has similar long nerves. The majority of modern comb jellies are composed of a diffuse nervous network and not well-defined long neurals," stated Professor Javier Ortega Hernndez from Harvard University's Department of Organismic and Evolutionary Biology.

Thalassostaphylos, the second species, has a more round appearance with approximately 16 comb rows and a wavy opening for its mouth. This species doesn't have fossilized nerves but it does have an important feature called the "polar fields". These are two dots that can be seen on the top of the apical orifice. Ortega-Hernndez said that these are important for sensing the environment of living comb jellies and that evidence from them in the Cambrian is crucial for understanding their evolution. "Interestingly, Thalassostaphylos Elegans doesn't have a rigid capsule. This indicates that some of the skeletons found in the early Cambrian Cambrian ctenophores had been lost by the mid-Cambrian.

Fossil specimen from Ctenorhabdotus Campanelliformis, mid-Cambrian Marjum Formation, Utah. Light photograph (left), elemental diagram showing nervous system and carbon films (center), with interpretative drawing (right). Credit: Luke Parry and Rudy Lerosey -Aubril.

The two new species of Utah are a great example of how the nervous system evolved, sensory structures developed, and the diversity of Cambrian Ctenophores. Researchers conclude that Cambrian Ctenophores had more complex nervous system than those currently observed. The nervous system of living comb jellies is very similar to that of chicken wire but much thinner and more transparent. Cambrian Ctenophores' nervous system was condensed using specific nerve tracks that ran along the length and then wrapped around the mouth. This complex system can only be seen in Euplokamis (a living species that is considered to have been an early branching Ctenophore). Although Euplokamis does have an elongated nerve structure running the length of its body, it doesn't have the ring around its mouth. This makes it simpler than Cambrian Ctenophores.

The team did a phylogenetic analysis to better understand the evolution and history of this group. It revealed that the ancestral condensed nervous system was actually the ancestral condition. Only modern ctenophores had this complex nervous system, and they favored a diffuse nerve net.

Dr. Luke A. Parry from the Department of Earth Sciences at the University of Oxford, UK, stated that this discovery indicates that there was a secondary simplification of the comb jellies' evolution. They lost their rigid skeleton and then developed the discrete nerves found in fossils. These insights are not possible to get from studying only living comb jellies. The fossil record provides valuable insight into the evolution of these mysterious animals.

Ortega-Hernndez agreed that Euplokamis could be showing a vestigial organization in the nervous system. This is not evident in any other living ctenophores. Ctenophores have a much more complicated evolutionary history than can be determined from living individuals. Fossils are a way to learn about the origins of morphology and how it changed over time.

The specimens of fossil material that has been studied are from the Bureau of Land Management and are permanent curated at Salt Lake City's Natural History Museum of Utah. Philip Gensler (acting regional paleontologist at the BLM) said that the Bureau of Land Management is proud of its efforts to preserve and protect paleontological resources located on public lands. This includes the Marjum Formation in west central Utah. This site is home to more than 100 species Cambrian-age fossils of invertebrates and offers opportunities for learning more about the evolution and environment of marine species 500 million years ago. The BLM supports research on public lands, and applauds Harvard University and the Natural History Museum of Utah for their outstanding preservation of specimens, scientific research, as well as revelations about the evolution of comb jelly.

To help with cataloguing and examining the large collection of Cambrian fossils found in the House Range of Utah, the Museum's curatorial staff reached out to Dr. Rudy Lerosey-Aubril (project coordinator and Research Associate, Department of Organismic and Evolutionary Biology at Harvard University) in 2019. The Cambrian deposits in western Utah are crucial to our understanding the Cambrian Explosion. Lerosey-Aubril said that our collaboration with the Natural History Museum of Utah has helped us to expand our research on these extraordinary fossil assemblages. This has led to exciting discoveries which we are eagerly to report.

Explore further Half-a-billion-year-old fossil reveals the origins of comb jellies

More information: Luke A. Parry and colleagues, Cambrian Comb Jellies from Utah Illuminate the Early Evolution of Nervous and Sensory Systems in Ctenophores, iScience (2021). Information for Journal: iScience Luke A. Parry et. al. Cambrian comb jelly from Utah illuminates the early evolution of nervous systems and sensory system in ctenophores (2021). DOI: 10.1016/j.isci.2021.102943

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