'Odd-looking' 500-million-year-old sea creature is connected to humans

Tunicates are peculiar sea creatures with diverse forms and lifestyles. Their adult form typically resembles a barrel with two protruding siphons. 

One siphon sucks in water and food particles, allowing the tunicate to feed using an internal filtering mechanism. The other siphon expels the water after feeding.

How this odd-looking creature could be related to vertebrates— which includes fish, mammals, and even humans—is hard to imagine were it not for that tadpole beginning. 

"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 tail to transform into this being with two siphons is just awe-inspiring," said Nanglu.

How did the 500 million-year-old tunicate live?

M. thylakos, the sole tunicate fossil with preserved soft tissue, is the oldest from Utah's middle Cambrian Marjum Formation. Molecular clock estimates indicate ascidiacean origin 450 million years ago. 

However, at 500 million years old, M. thylakos offers a clear glimpse into ancient tunicate anatomy and early evolution. Notably, it demonstrates the early establishment of the modern tunicate body plan after the Cambrian Explosion. This was a rapid evolutionary event around 541 million years ago, famed for the sudden emergence and diversification of many complex (or 'weird') animal life forms.

M. thylakos had all the essential hallmarks of an ascidiacean tunicate (one of two main tunicate lineages). But the feature that stood out to the team was the dark bands running up and down the fossil's body.

High-powered images of M. thylakos allowed the researchers to compare side-by-side to a modern ascidiacean. The researchers used dissected sections of the modern tunicate Ciona to identify the nature of Megasiphon's dark bands. 

The comparisons revealed remarkable similarities between Ciona's muscles, which allow the tunicate to open and close its siphons, and the dark bands observed in the 500-million-year-old fossil.

Nanglu explained Megasiphon's morphology implies that ancestral tunicates had a stationary adult form, utilizing its large siphons for filter feeding.

"This is an incredible find as we had virtually no conclusive evidence for the ancestral modes of life for this group before this," he highlighted. "It's so rare to find not just a tunicate fossil, but one that provides a unique and unparalleled view into the early evolutionary origins of this enigmatic group." 

"Given the exceptional quality of preservation and the age of the fossil, we can say quite a bit about the evolutionary history of the tunicates."

With the collection of hundreds of new fossils this spring, the researchers are confident that the Marjum Formation has just begun to unveil its secrets.

The complete study was published in Nature Communications on July 6 and can be found here.