Thanks to a landmark study from Harvard scientists, we've now got a glimpse into their prehistoric world, through a fossil dating back 500 million years.
An enigmatic group of marine invertebrates, the tunicates, have just become a bit less mysterious. Thanks to a landmark study from Harvard scientists, we’ve now got a glimpse into their prehistoric world, through a fossil dating back 500 million years. The new species, Megasiphon thylakos, has reshaped our understanding of these ancient sea-dwelling organisms, proving they once thrived as stationary, filter-feeding beings, remarkably reminiscent of their modern counterparts.
500-million-year-old fossil
Taking on numerous forms and lifestyles, tunicates are peculiar creatures. A typical adult tunicate presents a barrel-like form, featuring two siphons that feed and expel water. Despite this bizarre shape, tunicates are actually the closest invertebrate relatives to vertebrates, including humans.
A Tale of Two Lineages
Diving into the tunicate family tree, we find two primary branches: the sea squirts or ascidiaceans, and the appendicularians. The former start life as tadpole-like creatures before metamorphosing into the familiar barrel shape and attaching themselves to the seafloor, whereas the latter maintain their tadpole form, freely roaming the upper ocean layers.
A Fossil Find for the Ages
It’s not every day one comes across a tunicate fossil, let alone one that can shed light on the origins of the entire group. Megasiphon thylakos boasts all the features typical of ascidiacean tunicates, such as a barrel-shaped body and conspicuous siphon-like growths. However, the most striking aspect was dark bands running along the body of the fossil. Advanced imaging techniques revealed remarkable similarities between these bands and the muscles of modern tunicates, crucial for the operation of their siphons.
The Oldest Tunicate Fossil
Megasiphon thylakos stands out not just due to its age, but also its origin — the middle Cambrian Marjum Formation in Utah. This fossil, the only tunicate fossil with preserved soft tissue, is approximately 500 million years old. Rudy Lerosey-Aubril and Javier Ortega-Hernández made its discovery during a 2019 visit to the Utah Museum of Natural History.
Implications for Evolutionary History
Despite the significant evolutionary events that took place after the Cambrian Explosion about 538 million years ago, tunicates have been notably missing from Cambrian rock records. This newly discovered fossil rectifies that gap, providing evidence that tunicates’ modern body plan was already well established shortly after this explosion of life.
Thanks to Megasiphon thylakos, we now have a more profound understanding of the evolutionary history of tunicates. For scientists like Karma Nanglu, the implications of this find are immense. It finally offers definitive evidence for this group’s ancestral modes of life, casting a new light on these peculiar, intriguing marine invertebrates.
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