Our ancient cousins: 500-million-year-old tunicate holds intriguing secrets

Karma Nanglu, a postdoctoral researcher in the Department of Organismic and Evolutionary Biology at Harvard University, says his favorite animal is whichever one he’s working on at the time. His most recent research, however, focuses on an animal that may keep its place as his favorite for a while: an extraordinary 500-million-year-old fossil from a fascinatingly bizarre group of marine invertebrates known as tunicates.

Nanglu’s research, published in the journal Nature Communications, describes this ancient specimen, named Megasiphon thylakos, and provides a surprising insight into the early evolution of tunicates. “This animal is as exciting a discovery as some of the stuff I found when hanging off a cliffside of a mountain, or jumping out of a helicopter. It’s just as cool,” said Nanglu.

Our closest invertebrate relatives

Tunicates, commonly known as sea squirts, are unusual creatures that come in a range of shapes and sizes and have diverse lifestyles. An adult tunicate typically has a barrel-like shape with two siphons, or tubes, projecting from its body. One of these siphons draws in water, complete with food particles, allowing the animal to feed through an internal filter. After feeding, the second siphon expels the water.

Although these tunicates are our closest invertebrate relatives, their life cycle is nothing short of astounding. Nanglu explained, “This idea that they begin as tadpole-looking larva that, when ready to develop, basically headbutts a rock, sticks to it, and begins to metamorphosis by reabsorbing its own tail to transform into this being with two siphons is just awe-inspiring.”

The tunicate family tree branches into two main lineages, the ascidiaceans, which are sessile (non-moving), and the mobile appendicularias. Most ascidiaceans begin life resembling tadpoles before metamorphosing into barrel-shaped adults, leading stationary lives attached to the seafloor. By contrast, appendicularians maintain their tadpole-like appearance into adulthood, freely swimming in upper waters.

Interestingly, tunicates are more closely related to vertebrates, which includes species ranging from fish to humans, than to any other invertebrate group. The tadpole stage of tunicates provides a glimpse into this unexpected connection, offering crucial insights into our own evolutionary origins. However, this has been challenging as tunicates have left only a scant presence in the fossil record.

Oldest known fossil of its kind

M. thylakos was unearthed from the middle Cambrian Marjum Formation in Utah, the oldest known fossil of its kind and the only definitive tunicate fossil with preserved soft tissue to date. It displays all the hallmarks of an ascidian tunicate – a barrel-shaped body and two prominent siphon-like growths. However, the most intriguing feature was the dark bands running along the fossil’s body.

Using high-resolution imaging, Nanglu and his colleagues compared M. thylakos to a modern ascidian species, Ciona. The similarities between the dark bands of the fossil and the muscles that allow modern tunicates to control their siphons were remarkable. 

“Megasiphon’s morphology suggests to us that the ancestral lifestyle of tunicates involved a non-moving adult that filter fed with its large siphons,” Nanglu explained.

The fossil was identified as a tunicate by co-authors Rudy Lerosey-Aubril and Professor Javier Ortega-Hernández, both from Harvard’s Department of Organismic and Evolutionary Biology. They discovered the fossil while visiting the Utah Museum of Natural History (UMNH) in 2019. 

“The fossil immediately caught our attention…the close morphological similarity of Megasiphon with modern tunicates was simply too striking to overlook, and we immediately knew that the fossil would have an interesting story to tell,” said Ortega-Hernández.

Evolutionary history of ancient tunicates

The fossil’s age, approximately 500 million years, places it in the era following the Cambrian Explosion, a pivotal moment in Earth’s history when most major animal groups appeared in the fossil record for the first time, radically transforming marine ecosystems. Yet, tunicates are conspicuously absent in Cambrian rocks, despite their diversity and abundance in modern oceans.

According to molecular clock estimates, ascidian tunicates originated about 450 million years ago. The older M. thylakos fossil, therefore, offers a unique glimpse into the anatomy and evolutionary history of ancient tunicates, suggesting that most features of modern tunicates were already established shortly after the Cambrian Explosion. 

“Given the exceptional quality of preservation and the age of the fossil, we can actually say quite a bit about the evolutionary history of the tunicates,” said Nanglu.

With hundreds of new fossils collected this spring, the researchers believe that the Marjum Formation is far from revealing all of its secrets, and they eagerly await the surprises that may be in store.

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