The image above is an artistic rendition of Megasiphon thylakos, a bottom-dwelling creature that resided directly on the ocean floor. M. thylakos was sessile, meaning it did not move, and it primarily engaged in filter-feeding through its pronounced siphons. The depiction also includes other species often found at the Marjum Formation, the location where M. thylakos was unearthed. Various brachiopods and the spiny sponge Choia, prevalent in many Cambrian settings, are portrayed, along with the hemichordate Oesia, which existed in punctured tubes. Credit is given to Franz Anthony for the original artwork.
Karma Nanglu has expressed that his favored animal is consistently the one he is examining at the moment. Yet, his latest find may continue to fascinate him for an extended period: a 500-million-year-old fossil belonging to the unique and mystifying group of marine invertebrates known as tunicates.
Nanglu, a postdoctoral investigator in Harvard University’s Department of Organismic and Evolutionary Biology, enthused, “The excitement of discovering this creature rivals that of finding something while dangling from a mountain cliff or leaping from a helicopter. It’s just as thrilling.”
Published in Nature Communications, Nanglu and co-researchers detail the newly discovered fossil, named Megasiphon thylakos. They reveal that ancient tunicates existed as non-moving, filter-feeding adults and probably went through a transformation from a tadpole-like young stage.
Tunicates are remarkable organisms, exhibiting an array of shapes, sizes, and ways of life. Typically, an adult tunicate exhibits a barrel-like figure with two siphons jutting from its body. One siphon sucks in water containing food particles, allowing the creature to eat with an internal basket-like filtering mechanism. Once feeding is complete, the other siphon discharges the water.
Tunicates consist of two main lineages: ascidiaceans (commonly referred to as “sea squirts”) and appendicularians. While most ascidiaceans start as mobile tadpole-like beings and metamorphose into barrel-shaped adults with two siphons, living their mature life affixed to the seabed, appendicularians keep their tadpole appearance as they mature and freely swim in surface waters.
Images compare the newly discovered Cambrian tunicate Megasiphon thylakos with contemporary tunicates. Specifically, M. thylakos bears resemblance to the modern ascidiacean tunicates with its rounded, vase-like form and prominent siphons. Considering its 500-million-year age, this implies that historically, tunicates lived similarly to modern ascidiaceans: non-moving adults with siphons for filter feeding, a design reached after transforming from tadpole-like juveniles. Represented modern species include c: Ciona, d: Ascidiella, e: Molgula. Credit: Rudy Lerosey-Aubril (a,b) and Karma Nanglu (c,d,e).
Nanglu remarked on the tunicates’ life cycle, beginning as tadpole-looking larva, that eventually adheres to a rock and commences metamorphosis by absorbing its own tail to evolve into a being with two siphons, as “awe-inspiring.”
Interestingly, tunicates stand as the closest relatives to vertebrates, including fish, mammals, and humans. The connection between such a peculiar creature and vertebrates may seem hard to grasp were it not for the tadpole beginning. Studying tunicates is essential to comprehend our evolutionary origins, though they are nearly totally missing from the fossil record, with few convincingly identified as part of the group.
Due to the scarcity of fossils, scientists mainly depended on modern tunicate species to gather information. Without knowledge of the last common ancestor’s morphology and ecology, hypotheses suggested it was either a bottom-dwelling creature with two siphons, akin to the ascidiaceans, or a free-swimming organism resembling the appendicularians.
M. thylakos exhibited the fundamental characteristics of an ascidiacean tunicate, a barrel-like body, and two pronounced siphon-like protrusions. A distinguishing feature was the dark bands traversing the fossil’s body.
High-resolution images of M. thylakos enabled side-by-side comparisons to a current ascidiacean. Using dissected parts of the modern tunicate Ciona, the nature of Megasiphon’s dark bands was identified. The comparisons unveiled striking likenesses between Ciona’s muscles, responsible for opening and closing its siphons, and the dark bands found in the ancient fossil.
The morphology of Megasiphon, as stated by Nanglu, suggests “the ancestral way of life for tunicates involved a stationary adult that filter-fed with its extensive siphons.” Finding not just a tunicate fossil but one that offers a unique insight into this enigmatic group’s early evolutionary origins is rare indeed.
Anatomical insights into Megasiphon thylakos reveal two pronounced siphons and a barrel-like structure. Notable longitudinal muscles extended from the siphon tips to the body’s base, comparable to contemporary tunicates such as Ciona intestinalis. Even the individual muscle fibers, micrometers in size, can be paralleled between this ancient fossil and today’s tunicates. Credit: James Wheeler (a,d) and Karma Nanglu (b,c,e,f,g).
M. thylakos stands as the sole definitive tunicate fossil with soft tissue preservation known to date. Originating from Utah’s middle Cambrian Marjum Formation, it’s the most ancient of its kind. While visiting the Utah Museum of Natural History (UMNH) in 2019, the fossil was recognized as a tunicate by co-authors research associate Rudy Lerosey-Aubril and Professor Javier Ortega-Hernández (both in the Department of Organismic and Evolutionary Biology).
Ortega-Hernández said, “The fossil immediately seized our interest. Although our primary focus is Cambrian arthropods like trilobites, the striking resemblance of Megasiphon with contemporary tunicates was too compelling to ignore. We knew instantly that this fossil had a fascinating tale to tell.”
The Marjum Formation’s fossils date shortly after the Cambrian Explosion, a monumental evolutionary occurrence about 538 million years ago, transforming marine ecosystems. Notably, tunicates are almost entirely absent in Cambrian rocks, despite their abundance in modern oceans.
There are several U.S. Cambrian fossil sites with extraordinary preservation, often overshadowed by Canada’s Burgess Shale and China’s Chengjiang. “The discovery of Megasiphon underscores why we have been conducting fieldwork in Utah for the past decade,” noted Lerosey-Aubril. “The Marjum layers are of prime interest, as they preserve fossils of groups like tunicates or comb jellies, mostly missing from the Cambrian fossil record.”
Molecular clock assessments indicate that ascidiaceans emerged 450 million years ago. However, M. thylakos, at 500 million years old, provides the clearest glimpse into ancient tunicates’ anatomy and early evolutionary origins, with strong implications for understanding the evolution of this fascinating and vital group.
The research was carried out in collaboration with the UMNH, Harvard University’s Department of Organismic and Evolutionary Biology, and the Centre for Biodiversity Theory and Modelling at CNRS. It offers a profound insight into our evolutionary past, giving an intricate understanding of how invertebrates like tunicates have shaped and influenced the course of life on Earth. It underlines the importance of preserving and meticulously examining the world’s fossil record, both known and yet undiscovered, to comprehend the history of life on our planet fully.
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Table of Contents
Frequently Asked Questions (FAQs) about tunicate fossil
What is the significance of the 500-million-year-old fossil, Megasiphon thylakos?
The discovery of Megasiphon thylakos, a tunicate from the Cambrian era, provides an unparalleled view into the early evolutionary origins of marine invertebrates, particularly the tunicates. It shows that the ancient tunicates had a non-moving adult form with siphons for filter feeding, much like modern ascidiaceans. The exceptional preservation and age of the fossil allow a deep understanding of the tunicates’ evolutionary history.
What are tunicates, and why are they crucial to study?
Tunicates are marine invertebrates known for their variety in shapes and sizes and their unique lifestyles. They are the closest relatives of vertebrates, which include fish, mammals, and even humans. Studying them is critical for understanding our own evolutionary origins. They provide insights into ancestral forms and development, although they are rarely found in the fossil record.
How does the newly discovered fossil compare with modern tunicates?
The newly discovered fossil, Megasiphon thylakos, shares remarkable similarities with modern tunicates, such as a rounded barrel-like body and prominent siphons used for filter feeding. High-powered images allowed researchers to draw comparisons between the dark bands observed in the fossil and the muscles in modern tunicates, which enable the opening and closing of siphons.
Where was the fossil Megasiphon thylakos discovered?
The fossil was discovered in the middle Cambrian Marjum Formation in Utah and is recognized as the only definitive tunicate fossil with soft tissue preservation to date.
What insights does the discovery of Megasiphon thylakos provide about the Cambrian Explosion?
The discovery of Megasiphon thylakos illustrates significant evolutionary events shortly after the Cambrian Explosion, which occurred approximately 538 million years ago. It provides evidence that most of the modern body plan of tunicates was already established soon after this key event in Earth’s history, drastically altering marine ecosystems.
How did researchers identify the nature of Megasiphon’s dark bands?
Researchers used high-powered images of Megasiphon thylakos and conducted a side-by-side comparison to a modern ascidiacean. By comparing dissected sections of the modern tunicate Ciona, they were able to identify the nature of Megasiphon’s dark bands, revealing remarkable similarities between Ciona’s muscles and the fossil’s dark bands.
Who were the key contributors to the research and discovery of Megasiphon thylakos?
The research involved collaboration between Harvard University’s Department of Organismic and Evolutionary Biology, co-authors research associate Rudy Lerosey-Aubril, Professor Javier Ortega-Hernández, and postdoctoral researcher Karma Nanglu, along with the Natural History Museum of Utah (NHMU) and the Centre for Biodiversity Theory and Modelling at CNRS.
More about tunicate fossil
- Harvard University’s Department of Organismic and Evolutionary Biology
- Natural History Museum of Utah
- Centre for Biodiversity Theory and Modelling at CNRS
- Study on the Cambrian Explosion
- Overview of Tunicates
- Marjum Formation in Utah
5 comments
What a discovery! it’s interesting to think how this strange creature could be related to us humans. Keep up the good work scientists.
Wow! I never knew tunicates had such a rich history. This is incredible stuff. Does anyone know where I can find more about this?
Thats such a strange looking creature. Do modern tunicates look like that or have they changed a lot? Thanks for sharing this great article.
Is this the only fossil of its kind? I woud like to know more about how they discovered it. If you have any links please share.
I’ve been to Utah and seen some of the fossils there, but this is something else! i mean, a 500 million year old fossil? That’s amazing.