Fossil provides new insights into the evolution of tunicates

tunicates

This is what the tunicate Megasiphon thylakos, which lived around 500 million years ago, could have looked like. © Franz Anthony

With over 3000 species, tunicates such as sea squirts are widespread in the world's seas today. However, fossils of these invertebrates are rare, leaving many unanswered questions about their origins. However, they are considered a sister group of vertebrates. An extraordinarily well-preserved, 500-million-year-old fossil now shows that representatives of that time already lived sedentarily on the sea floor and showed striking similarities to today's species. The newly described species Megasiphon thylakos probably belonged to the ancestral group of tunicates.

Tunicates are considered to be the closest invertebrate relatives of vertebrates. The largest group with around 3000 species are the sea squirts. In a variety of colours, shapes and sizes, these grow firmly on the sea floor as adult animals. What they have in common is the basic construction plan with two siphons, which typically protrude like tubes from the body. They pump water into themselves through one and filter nutrients from it, through the other they release the water again. In the larval stage, however, sea squirts swim freely through the oceans like tadpoles. Other tunicates remain mobile throughout their lives. But when in the course of the evolutionary history of the tunic did the sedentary nature of the sea squirt develop? So far, this question has not been answered due to a lack of meaningful fossils.

Find in the museum

A team led by Karma Nanglu from Harvard University in Cambridge has now for the first time described a well-preserved fossil that clearly belongs to the tunicates and is dated to be 500 million years old. The extraordinary specimen, which the researchers gave the name Megasiphon thylakos, comes from the Middle Cambrian Marjum Formation in the US state of Utah. The species name refers to the animal's large, protruding siphons and saclike body - thylakos is the Greek word for pouch.

In 2019, two of the co-authors discovered the fossil at the Utah Museum of Natural History and recognized it as a tunicate. "The fossil immediately caught our attention," says Javier Ortega-Hernández, one of the finders. "Megasiphon's close morphological resemblance to modern tunicates was just too striking to overlook, and we knew immediately the fossil would have an interesting story to tell."

Modern physique 500 million years ago

With the help of high-resolution images of the fossil, Nanglu and his team got to the bottom of this story. They noticed that the specimen, which was about 3.2 centimeters long, was criss-crossed by dark bands. Comparisons with hose sea squirts (Ciona intestinalis) living today showed that these were muscle fibers. "In modern specimens, the contraction of these muscles causes the animal to pucker and squirt water," the research team explains. "Their presence in M. thylakos indicates that these typical behavioral characteristics of tunicates developed around 500 million years ago." Accordingly, the 500-million-year-old ancestor of today's tunicates was apparently sedentary as an adult and pumped through its food siphons into itself.

The researchers propose two possible hypotheses for the classification in the tunicate family tree. "The most likely scenario is that M. thylakos belongs to the stem group of tunicates, suggesting that the entire subphyla originally had a biphasic life cycle with a free-swimming larva and a sedentary adult," they write. Alternatively, it would be conceivable that M. thylakos already represents a more developed form, i.e. belongs to the crown group. In this case, however, sea squirts would have to have separated from other tunicates 50 million earlier than previously thought.

Looking for more fossils

This in turn would also have an impact on the dating in the family tree of the sister group of tunicates, the vertebrates: because if the tunicates diversified earlier than expected, the origin of the vertebrates would also shift further into the past, as the paleontologists explain. However, there is still a lack of fossil evidence to further support or refute such hypotheses. To gain clarity, Nanglu and his team plan to study more fossils from the Marjum Formation. "This relatively understudied Cambrian deposit represents a potential source of crucial evolutionary and ecological insights into the dynamics of the Cambrian world and early animal diversification," the team said.

Source: Karma Nanglu (Harvard University, Cambridge) et al., Nature Communications, doi: 10.1038/s41467-023-39012-4

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