Old Sea Squirt Could Push Back Origins of Vertebrates

In 2019, a small fossil that looked like a finger was given to Karma Nanglu, a paleontologist at Harvard University who studies ancient periods in Earth’s history called the Cambrian and Ordovician periods.

The fossil had been kept in a museum in Salt Lake City for many years. The people who found it in a layer of rock from the Cambrian period in western Utah thought it might be a type of marine animal called a sea squirt or tunicate.

What are tunicates

Tunicates are marine creatures with a common ancestor with all vertebrates, including humans. Nanglu was excited but cautious because there were no known tunicate fossils from the entire 500 million years of recorded history.

In a scientific paper, Nanglu and his colleagues explain that the 500-million-year-old fossil they studied looks very similar to current tunicates.

It has two openings called siphons that help filter organic particles from the water and complex muscles that control these siphons. Nicholas Treen, a developmental biologist from Princeton University who was not part of the research, describes the fossil as resembling a tunicate that recently died and fell onto a rock.

This discovery provides insights into the timing and development of early tunicates and could even change our understanding of when the ancestors of vertebrates, including humans, first appeared.

Current number of 3000 species

Today, there are around 3,000 different species of tunicates living in various ocean habitats. Most of them go through a two-part life cycle. They start as larvae that look like tadpoles and can swim freely, but then they settle down and transform into stationary adults.

The larvae of tunicates have a structure called a notochord, which is the early form of a spinal column. This is an important characteristic of a larger group called chordates, which includes all vertebrates.

Surprisingly, only a few tunicate fossils are known to scientists, and it’s not fully understood why. The most well-known tunicate fossil, called Shankouclava anningense, is 520 million years old but is controversial because it has a siphon and appears to filter feed. Yet, it doesn’t resemble any living tunicates we know of today.

Megasiphon thylakos – the new fossil discovered

The researchers, led by Nanglu, named the fossil Megasiphon thylakos due to its large siphons and pouch-like body (thylakos comes from the Greek word for pouch).

Modern tunicates utilize these siphons to assist in filter-feeding and have longitudinal muscle bands along their bodies and circular muscles around the siphons to control them.

Nanglu notes that the musculature of Megasiphon closely resembles that of a well-studied modern tunicate called Ciona intestinalis.

During the maturation of a tunicate, the muscles in its siphon develop from the same cells that form the heart tissue in modern vertebrates. The presence of these muscles in the Megasiphon suggests that it may have already possessed a structure resembling a heart, even though the internal organs are not preserved in the fossil.

Evolution of the early chordates

The fossil’s discovery addresses a long-standing question in the evolution of early chordates: whether the common ancestor of all tunicates was a free-swimming organism or anchored to the ocean floor.

The resemblance of Megasiphon to existing sessile tunicates strongly supports the latter hypothesis. This finding suggests that the two-part life cycle and metamorphosis ability observed in tunicates today may have been a characteristic inherited from their ancestors.

The discovery of such a well-preserved and recognizable tunicate from this period challenges the current understanding of vertebrate origins, which is believed to be around 450 million years ago.

Half-billion-year-old sea squirt could push back origins of vertebrates, including humans

By Amanda Heidt
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“If a tunicate body plan was already established [at 500 million years ago], and the tunicates are actually the sister group of vertebrates, then vertebrates may be older as well,” explains William Jeffery, a developmental biologist from the University of Maryland, College Park, who was not involved in the research.

Megasiphon likely coexisted with a fishlike creature called Metaspriggina, which lived as far back as 505 million years ago. Metaspriggina possessed eyes, muscular blocks, and probably a notochord.

Metaspriggina and Megasiphon exhibit many characteristics considered typical of vertebrates, indicating an early origin for this group around 500 million years ago. However, Nanglu emphasizes that fossil evidence is insufficient to reshape our understanding of vertebrate evolution completely.