Discovery of 500-million-year-old fossil reveals astonishing secrets of tunicate origins

Date:
July 6, 2023
Source:
Harvard University, Department of Organismic and Evolutionary
Biology
Summary:
Researchers describe a 500 million-year-old tunicate fossil
species. The study suggests that the modern tunicate body plan
was already established soon after the Cambrian Explosion.


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FULL STORY ========================================================================== Karma Nanglu says his favorite animal is whichever one he's working
on. But his latest subject may hold first place status for a while:
a 500-million-year-old fossilfrom the wonderfully weird group of marine invertebrates, the 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, postdoctoral researcher
in the Department of Organismic and Evolutionary Biology at Harvard
University.

In a new study in Nature Communications, Nanglu and coauthors describe
the new fossil, named Megasiphon thylakos, revealing that ancestral
tunicates lived as stationary, filter-feeding adults and likely underwent metamorphosis from a tadpole-like larva.

Tunicates are truly strange creatures that come in all shapes and sizes
with a wide variety of lifestyles. An adult tunicate's basic shape is
typically barrel-like with two siphons projecting from its body. One of
the siphons draws in water with food particles through suction, allowing
the animal to feed using an internal basket-like filter device. After
the animal feeds, the other siphon expels the water.

There are two main tunicate lineages, ascidiaceans (often called "sea
squirts") and appendicularias. Most ascidiaceans begin their lives
looking like a tadpole and mobile, then metamorph into a barrel shaped
adult with two siphons. They live their adult life attached to the
seafloor. In contrast, appendicularians retain the look of a tadpole as
they grow to adults and swim freely in the upper waters.

"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," sais Nanglu.

Interestingly, tunicates are the closest relatives of vertebrates,
which includes fish, mammals, and even humans. How this odd-looking
creature could be related to vertebrates is hard to imagine were it not
for that tadpole beginning. Tunicate's close relationship to vertebrates
makes studying them critical for understanding our own evolutionary
origins. Unfortunately, it's not easy to do as tunicates are almost
completely absent from the entire fossil record, with only a handful of
fossils appearing convincingly as members of the group.

With so few fossils, scientists relied mainly on what could be learned
from modern tunicate species. Because no one knew the morphology
and ecology of the last common ancestor of the tunicates, scientists
could only hypothesize that it was either a benthic animal with two
siphons, like the ascidiaceans, or a free-swimming animal like the appendicularians.

M. thylakoshad all the basic hallmarks of an ascidiacean tunicate,
a barrel- shaped body and two prominent siphon-like growths. 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 conduct a
side- by-side comparison 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.

"Megasiphon's morphology suggests to us that the ancestral lifestyle
of tunicates involved a non-moving adult that filter fed with its large siphons," said Nanglu. "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." M. thylakos is the only definitive tunicate fossil with soft tissue preservation that has been discovered to date. It is the oldest of its kind originating from the
middle Cambrian Marjum Formation in Utah. The fossil was recognized as
a tunicate by co-authors research associate, Rudy Lerosey-Aubril, and
Professor Javier Ortega-Herna'ndez (both in the Department of Organismic
and Evolutionary Biology) while visiting the Utah Museum of Natural
History (UMNH) in 2019.

"The fossil immediately caught our attention," said Ortega-Herna'ndez, "although we mostly work on Cambrian arthropods, such as trilobites
and their soft bodied relatives, 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." Fossils from the Marjum Formation date from shortly after the Cambrian Explosion, one of the most significant evolutionary events in
Earth's history which occurred approximately 538 million years ago. During
this time the most major animal groups appeared in the fossil record
for the first time radically changing marine ecosystems. Tunicates,
however, are noticeably absent in Cambrian rocks even though they are
diverse and abundant in modern oceans.

There are many Cambrian fossil sites with exceptional preservation in
the United States, but these are often overlooked compared to those from
the Burgess Shale in Canada and Chengjiang in China. "The discovery of Megasiphon perfectly illustrates why Javier and I have been conducting fieldwork in Utah for the last ten years," said Lerosey-Aubril. "The
Marjum strata has all of our attention right now as we know that it
preserves fossils of animal groups, such as tunicates or comb jellies,
that are almost entirely absent from the Cambrian fossil record."
Molecular clock estimates suggest that ascidiaceans originated 450 million years ago. However, at 500 million years old, M. thylakos provides the
clearest view into the anatomy of ancient tunicates and their earliest evolutionary history. Significantly, M. thylakos provides evidence that
most of the modern body plan of tunicates was already established soon
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. "This is an incredible find as we had virtually
no conclusive evidence for the ancestral modes of life for this group
before this." After collecting hundreds of new fossils again this spring,
the researchers are convinced the Marjum Formation has only started to
reveal its secrets.

The authors would like to extend a special thank you to C. Levitt-Bussian
and R.B. Irmis for their assistance during visits to the Natural History
Museum of Utah (NHMU) and for facilitating the study of the specimens
housed in this institution. And to the Bureau of Land Management,
particularly S.E. Foss and G. McDonald, for depositing the holotype of Megasiphon at the NHMU and providing curatorial assistance.

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========================================================================== Journal Reference:
1. Karma Nanglu, Rudy Lerosey-Aubril, James C. Weaver, Javier Ortega-
Herna'ndez. A mid-Cambrian tunicate and the deep origin of the
ascidiacean body plan. Nature Communications, 2023; 14 (1) DOI:
10.1038/ s41467-023-39012-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230706124616.htm

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