500-million-year-old fossil reveals early evolution of tunicates

Karma Nanglu, a postdoctoral researcher at Harvard University, has found a new favorite animal: a 500-million-year-old fossil from the fascinating group of marine invertebrates known as tunicates. Nanglu describes the discovery of the fossil, named Megasiphon thylakos, as equally thrilling as his adventures hanging off cliffs or jumping out of helicopters.

Tunicates are peculiar creatures with diverse forms and lifestyles. Typically, adult tunicates have a barrel-like shape and two siphons. One siphon draws in water and food particles, allowing the animal to filter-feed using an internal basket-like structure. After feeding, the other siphon expels the water.

The researchers’ study, published in Nature Communications, reveals that ancestral tunicates lived as stationary, filter-feeding adults and likely underwent metamorphosis from tadpole-like larvae. There are two main lineages of tunicates: ascidiaceans, often called “sea squirts,” and appendicularians. Ascidiaceans start their lives as mobile tadpoles and later transform into barrel-shaped adults that remain attached to the seafloor. On the other hand, appendicularians retain their tadpole-like appearance as they grow into adults and swim freely in the upper waters.

Nanglu finds the process of metamorphosis in tunicates awe-inspiring. As they reach maturity, the tadpole-like larvae attach themselves to a rock and begin the transformation by reabsorbing their own tails. This metamorphosis results in the development of an adult tunicate with two siphons.

Comparisons between the new Cambrian tunicate Megasiphon thylakos (a,b) with some modern tunicates (c,d,e). In particular, M. thylakos shares the rounded vase or barrel-like body and prominent pair of siphons of the modern ascidiacean tunicates. Given the fact that M. thylakos is half-a-billion years old, this suggests that ancestrally, tunicates lived much like modern ascidiaceans: they had a non-moving adult form with siphons for filter feeding, a body plan that was arrived at after metamorphosing from a tadpole-like juvenile. The modern species represented are c: Ciona, d: Ascidiella, e: Molgula. Credit: Rudy Lerosey-Aubril (a,b) and Karma Nanglu (c,d,e)

It is truly fascinating that tunicates, despite their peculiar appearance, are actually the closest relatives of vertebrates, including fish, mammals, and even humans. The connection between these odd-looking creatures and vertebrates becomes more understandable when considering their tadpole-like beginnings. Studying tunicates is crucial for unraveling our own evolutionary origins.

Unfortunately, the scarcity of tunicate fossils poses a challenge. The fossil record contains only a handful of convincing tunicate fossils. Therefore, scientists heavily rely on studying modern tunicate species to glean insights. Until now, the morphology and ecology of the common ancestor of tunicates remained largely speculative, with two main hypotheses: either a stationary benthic animal with two siphons, like ascidiaceans, or a free-swimming creature like appendicularians.

Megasiphon thylakos, the newly discovered fossil, exhibited typical ascidiacean features, such as a barrel-shaped body and prominent siphon-like growths. However, what caught the researchers’ attention were the dark bands running along the fossil’s body.

By utilizing high-powered imaging techniques, the scientists conducted a detailed comparison between Megasiphon thylakos and a modern ascidiacean species called Ciona. Dissected sections of Ciona’s anatomy helped identify the nature of the dark bands in Megasiphon. Astonishingly, the comparisons revealed striking similarities between the muscles responsible for opening and closing Ciona’s siphons and the dark bands observed in the 500-million-year-old fossil.

According to Nanglu, the morphology of Megasiphon suggests that the ancestral tunicate lifestyle involved a stationary adult that filter-fed through large siphons. The rarity of not only finding a tunicate fossil but also one that provides a unique glimpse into the early evolutionary origins of this mysterious group is remarkable.

Megasiphon thylakos stands as the only definitive tunicate fossil with preserved soft tissue discovered thus far. It is the oldest-known tunicate fossil originating from the middle Cambrian Marjum Formation in Utah. The recognition of the fossil as a tunicate occurred when researchers Rudy Lerosey-Aubril and Professor Javier Ortega-Hernández, both from the Department of Organismic and Evolutionary Biology, visited the Utah Museum of Natural History in 2019.

Details of the anatomy of Megasiphon thylakos. M. thylakos had two prominent siphons and a barrel shaped body. It also had prominent longitudinal muscles running from the tips of the siphons to the base of the body. These are comparable with modern tunicates, including Ciona intestinalis, which is dissected in c and f. Even the micrometer sized individual muscle fibers can be compared between this 500-million year old fossil and modern tunicates. Credit: James Wheeler (a,d) and Karma Nanglu (b,c,e,f,g)

“The fossil immediately grabbed our attention,” remarked Ortega-Hern├índez. “Although our main focus is on Cambrian arthropods like trilobites and their soft-bodied relatives, the striking morphological similarity between Megasiphon and modern tunicates was impossible to ignore. We knew right away that this fossil had an intriguing story to tell.”

Fossils from the Marjum Formation date back to shortly after the Cambrian Explosion, a significant event around 538 million years ago that saw the emergence of major animal groups, reshaping marine ecosystems. Despite their diversity and abundance in modern oceans, tunicates are noticeably absent from Cambrian rocks.

While exceptional fossil sites with preserved specimens exist in the United States, they often receive less attention compared to the renowned Burgess Shale in Canada and Chengjiang in China. Lerosey-Aubril explained the focus on Utah, saying, “The discovery of Megasiphon perfectly illustrates why Javier and I have been conducting fieldwork in Utah for the last ten years. The Marjum strata has captured our attention because it holds fossils of animal groups like tunicates or comb jellies, which are almost entirely missing from the Cambrian fossil record.”

Molecular clock estimates suggest that ascidiaceans, one of the main tunicate lineages, originated around 450 million years ago. However, M. thylakos, at 500 million years old, provides the clearest insight into the anatomy and early evolutionary history of ancient tunicates. Importantly, M. thylakos reveals that the majority of the modern tunicate body plan was already established soon after the Cambrian Explosion.

Nanglu emphasized the significance of the find, stating, “Given the exceptional preservation and age of the fossil, we can now glean a great deal about the evolutionary history of tunicates. This discovery is incredible because, until now, we had very limited conclusive evidence about the ancestral modes of life for this group.”

Having collected hundreds of new fossils this spring, the researchers are convinced that the secrets of the Marjum Formation have only just begun to unfold.

Source: Harvard University, Department of Organismic and Evolutionary Biology

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