A new fossil unveils the origins of our distant cousins: the tunicates

Published in Ecology & Evolution
A new fossil unveils the origins of our distant cousins: the tunicates
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There is seemingly no limit to the dizzying array of forms that animal life can take, and this is particularly true of life in the ocean. In a world where, to paraphrase Cousteau, life no longer carries the weight of gravity on its shoulders, any morphology seems plausible, if alien. 

One of the best examples of this are the tunicates, of which there are two major groups. The first are the ascidiaceans, which are most commonly vase-like, glued to rocks or other hard substrates, with two chimney-like siphons pushing out into the surrounding environment (Figure 1a). They use these two siphons to feed on small food particles, drawing water in through their oral siphon and expelling waste through their atrial siphon. They arrive at this unusual morphology as adults only after metamorphosing from a larval stage that looks superficially like a tadpole. This resemblance is an early clue that the tunicates are the most closely related group of animals (referred to as a “sister group”) to Vertebrata, which includes fish, reptiles, birds and of course mammals (including humans).

Figure 1. An ascidiacean tunicate (left) Molgula manhattensis, and an appendicularian (right) Bathochordaeus mcnutti. Image credits: left, Karma Nanglu; right, © 2002 MBARI.

Appendicularians, on the other hand, look like tadpoles for their entire lives (Figure 1b). Some notably build elaborate houses around themselves, composed mostly of mucus. By beating their tails, they draw in water to feed on, before periodically discarding their particle-clogged homes.

The huge anatomical and ecological gulf between these two groups has made it exceedingly difficult to figure out what the last common ancestor of the tunicates looked like. Was it a solitary, barrel shaped filter feeder like the ascidiaceans, or an animal that lived freely up in the water column like the appendicularians? Most problematically, tunicates have an atrocious fossil record. Even when considering sites of exceptional soft-tissue preservation like the Burgess Shale or Chengjiang, their fossil record is essentially empty. In a world where we can trace the origins of almost every major invertebrate group back to the Cambrian, tunicates stand out as a notable exception, a veritable black box of early animal evolution.

In our study, we describe the first convincing tunicate in the entire fossil record to have the fine details of soft tissue preservation needed to understand their lifestyle. This new species, which we’ve named Megasiphon thyalkos, has many morphological similarities with modern ascidiacean tunicates (Figure 2a-c). It has a barrel shaped body, with two prominent siphons that project from the dorsal surface (hence the name, “Megasiphon”). Moreover, it preserves the actual longitudinal musculature which is very similar to modern sedentary tunicates like Ciona intestinalis (Figure 2b,c). Even more notably, we can discern the individual muscle fibers that are less than 1 micrometer thick, arranged in the same pitchfork-like pattern as their modern relatives (Figure 2d,e).

Figure 2. The newly discovered fossil species Megasiphon thylakos (a) has many notable similarities to modern ascidiaceans tunicates such as Ciona intestinalis (b). Aside from the prominent siphons, one of the most striking are the longitudinal muscles, which can be clearly observed in a dissected specimen of C. intestinalis (c). Comparing zoomed-in shots of M. thylakos and C. intestinalis (e) demonstrates how even the micrometer sized individual muscle fibers share a similar arrangement. Photo credits: a and d, James C. Weaver; b,d and e, Karma Nanglu.

These similarities are particularly striking when you consider that this fossil is roughly half-a-billion years old, and prompts us to reinterpret early tunicate evolution. First and foremost, Megasiphon leads us to conclude that ancestrally, tunicates had an ascidiacean-like morphology and ecology, rather than an appendicularian lifestyle. This means that they likely had a larval phase that looked like a tadpole, underwent metamorphosis into an adult form, and then were sedentary for the rest of their lives. This new species also highlights the incredible new insights into animal evolution that are being revealed by Cambrian fossil sites from the US such as the Marjum Formation in Utah. Exploration into these localities and into museum collections such as those of the Natural History Museum of Utah are demonstrating that these under-studied areas are crucial for a comprehensive understanding of early marine diversity.

Figure 3. The quarry at the Marjum fossil locality. Photo credit: Javier Ortega-Hernández.

Fundamentally, this new fossil is exceptional not only for its preservation, but also for its position in the animal tree of life. Tunicates, by virtue of being the closest relatives to the vertebrates, play a unique role in the story of animal evolution. They’re both utterly removed from most of our daily experiences, but undeniably close to our own evolutionary history. Fossils such as Megasiphon, therefore, provide a small glimpse into our nearest relatives and, by extension, our link the rest of animal life from sea stars to squid to jellyfish.

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