A remarkable fossil discovered by paleontologist Karma Nanglu and his team at Harvard University sheds light on the evolution of tunicates and potentially pushes back the origin of vertebrates. The finger-size fossil, estimated to be 500 million years old, closely resembles modern-day tunicates, possessing two siphons for filtering organic particles and intricate musculature controlling these siphons. The findings, published in Nature Communications, contribute to understanding the timing and development of early tunicates and their relationship to vertebrates, including humans.
Tunicates, also known as sea squirts, encompass approximately 3,000 species residing in diverse ocean habitats. Their life cycle typically involves a free-swimming larval stage that undergoes metamorphosis to become a stationary adult. Tunicate larvae possess a notochord, a precursor to the spinal column and a defining characteristic of chordates, the group that encompasses all vertebrates. However, the fossil record for tunicates remains scarce, making the discovery of this exquisitely-preserved specimen of great significance.
The fossil, named Megasiphon thylakos, exhibits a saclike body and large siphons, enabling it to resemble modern tunicates. Its soft tissues are remarkably preserved, providing detailed insights into its anatomy. The fossil's musculature closely resembles that of the well-studied modern tunicate Ciona intestinalis, suggesting that Megasiphon possessed similar functionalities. Notably, the presence of siphon muscles indicates the possibility of a vertebrate-like heart in this ancient organism, despite the absence of preserved internal structures.
The fossil's characteristics support the hypothesis that the common ancestor of all tunicates was a sessile organism rooted to the ocean floor. This finding reinforces the understanding that tunicates' two-part life cycle and ability to metamorphose are ancestral traits of the group. Moreover, the existence of a recognizable tunicate fossil at such an early time raises the possibility of an earlier origin for vertebrates. If the tunicate body plan was already established 500 million years ago and tunicates are indeed the sister group of vertebrates, then the origins of vertebrates may be further back in time. However, additional fossil evidence is needed to substantiate this hypothesis.
While the fossil discovery contributes valuable insights to the story of vertebrate evolution, Nanglu and other researchers emphasize the necessity of further fossil evidence to enhance our understanding of these ancient organisms. Nonetheless, this significant find highlights the ongoing quest to uncover the origins of life and provides crucial clues to the evolutionary history of tunicates and vertebrates.