A Stunningly Preserved Half-Billion Year-Old Fossil of a Strange Cambrian creature could change our understanding of how a sister group of vertebrates evolved, a new study suggests.
The fossil, described on July 6 in the journal Nature Communicationis the oldest of the genus and belongs to an ancient species of tunicate, Megasiphon thylakos. The discovery answers a fundamental question about what early tunicates looked like, filling an important gap in the animal tree of life.
The newly discovered fossil offers the “best possible window into what early tunicates looked like, what they lived, their ecology, their way of life, and this in turn allows us to make inferences about what early tunicates and early vertebrates were able to share a common,” study co-author Karma Nanglupostdoctoral researcher at Harvard University’s Museum of Comparative Zoology, told Live Science.
Tunicates are the sister group vertebrates, meaning they shared a common ancestor hundreds of millions of years ago. They are a diverse subphylum of invertebrate marine animals that encompass 3,000 modern species. They are divided into two distinct groups: ascidia and appendicular. Often referred to as “sea squirts”, ascidiacea begin life in a tadpole-like form, permanently attaching themselves to the sea floor before metamorphosing into barrel-like adults. Appendicularians retain their tadpole morphology until adulthood and continue to swim in the water column. Until now, it was unclear whether early tunicates were more like sea squirts or appendicularia – an equally important question for understanding the evolutionary origins of vertebrates, due to their close relationship to tunicates.
Related: Did the Cambrian Explosion Really Happen?
We know very little about ancient tunicates because we only have one fossil from the earliest period of their history: a strange creature that doesn’t look much like modern tunicates called Shankouclava, which was discovered two decades ago in China. The fossils could be rare because tunicates were relatively rare, or it could just be that their soft bodies don’t keep well, Nanglu said.
Evolutionary indices
Study co-authors Javier Ortega-Hernandez And Rudy Lerosey-Aubril, assistant professor of organismal and evolutionary biology and research associate, both at Harvard University, came across the newly described fossil in a collection housed at the Utah Museum of Natural History. Nanglu recognized the tell-tale characteristics of tunicates: a barrel-shaped body and siphoe-like growths, and – most notably – the dark stripes running across its body.
The team inspected the fossil using microscopy and high-resolution photography, and compared dissections of the fossil with a modern species of sea squirt. The dissections confirmed that the structural similarities between the two did not simply exist at the surface level. In fact, the dark bands represented similar muscle arrangements between M. thylakos and its modern counterpart.
“What this tunicate tells us is that tunicates were ancestrally more likely to be animals that had a lava-like shape like a tadpole and then metamorphosed into a barrel-shaped animal that had these two siphons pointing the water column,” Nanglu said.
In other words, this two-phase lifestyle probably dates back to the origins of tunicates. The findings also place the origin of tunicates around 500 million years ago, not so far off from estimates made using “molecular clocks” or mutation rates in DNA, the authors wrote. study in the article.
The identification of the fossil, which was originally found in the Middle Cambrian Marjum Formation in Utah, weighs in on the question of whether ascidiacea or appendiculariae were the first to branch off the tree. life of tunicates. The resemblance of M. thylakos to ascidiacea suggests that the body form of the ascidiacea is ancestral, a hypothesis supported by recent search.
In follow-up work, Nanglu wants to find fossils that may reveal the point where deuterostomes, which encompass chordates (including tunicates and vertebrates), hemichordates (including worm-like sea creatures) and echinoderms (like starfish and sea cucumbers), to reveal the point that connects them all in an evolutionary sense.
“We are always looking for other animals that fit into this tree of life,” Nanglu said.
Editor’s Note: An earlier version of this article included a title suggesting the fossil was 500,000 years old. It is 500 million years old.
