A 500-million-year-old fossil reveals amazing secrets of a strange marine invertebrate

An artistic reconstruction of Megasiphon thylakos, a benthic organism that lives directly on the sea floor. M. thylakos was also sessile (immobile) and spent its time filter-feeding with its prominent siphons. Other species in the vicinity have also been commonly reconstructed in the Margum Formation, the site where M. thyalkos was discovered. Proximal brachiopods (lower middle) and spiny sponges Chuya (center middle) are common to many Cambrian environments. In the background is a semi-chordal animal Oesia, which lived in perforated tubes. Credit: Original artwork by Franz Anthony

Karma Nangelo says his favorite animal is always the one he’s currently studying. However, his latest find may remain at the top of his list for a while: a half-billion-year-old fossil from a wonderfully strange group of marine creatures. invertebratesyield.

“This animal is an exciting discovery like some of the things I find when I’m hanging off a mountainside, or jumping out of a helicopter. It’s just as amazing,” said Nanglo, a postdoctoral researcher in Harvard’s Department of Organic and Evolutionary Biology.

In a new study in Nature Communications, Nangelo and colleagues describe the new fossil, named megaphone thylakos, Revealing that the ancestral tunicates lived as sedentary, filter-feeding adults, they likely underwent metamorphosis from a tadpole-like larva.

Tunicates are truly curious creatures that come in all shapes and sizes with a wide range of lifestyles. The tunica adult is normally barrel-like with two siphons protruding from its body. A siphon draws in water along with food particles through suction, allowing the animal to feed using an internal basket-like filtering apparatus. After the animal has fed, the other siphon expels the water.

There are two main strains of occulta, ascetic (they are often called “sea squirts”) and appendicitis. Most asphids begin life as a mobile, tadpole, and then develop into a barrel-shaped adult with two siphons. They live their adult lives attached to the sea floor. In contrast, the appendix retains the appearance of a tadpole as it grows into adulthood and swims freely in higher waters.

Comparisons between the New Cambrian tunic Megasiphon thylakos (a,b) with some modern tunics (c,d,e). In particular, M. thylakos shares the rounded vase or barrel-like body and a pair of prominent siphons from modern tunicates ascidis. Given the fact that M. thylakos has a half-billion-year lifespan, this indicates that the gallalis lived in their ancestors like modern ascetics: they had a non-motile adult form equipped with siphons for filter-feeding, a body plan that was reached after the metamorphosis from a young tadpole-like . The modern species represented are c: Ciona, d: Ascidiella, e: Molgula. Photography: Rudy Lerozi-Obriel (a, b) and Karma Nangelo (c, d, e)

“The idea that it starts out as a tadpole-like caterpillar, and when it’s ready to develop, it butts its head against a rock, attaches itself to it, and begins metamorphosis by resucking its tail into this organism with a siphonin, which is pretty amazing,” says Nangelo.

Interestingly, tunicates are close relatives vertebratesWhich includes fish, mammals, and even humans. It is difficult to imagine how this strange-looking creature could have been related to vertebrates, if not for the beginning of the tadpole. Tunicate’s close relationship to vertebrates makes studying it crucial to understanding our evolutionary origins. Unfortunately, this is not easy to do because tunicates are almost completely absent from the entire fossil record, with only a handful of fossils convincingly appearing as members of the group.

With so few fossils left, scholars have relied mainly on what can be learned from Neo-Tunicella classify. Since no one knew the morphology and ecology of the last common ancestor of tunicates, scientists could only assume that it was either a benthic animal with two siphons, like ascetics, or a free-swimming animal like the appendicular.

M. thylacos It had all the distinguishing features of a tunica, a barrel-shaped body, and two prominent siphon-like outgrowths. But the feature that caught the team’s attention were the dark bands that run up and down the body of the fossil.

high energy images M. thylacos It allowed the researchers to make a side-by-side comparison with the modern ascetic. The researchers used dissected parts of the tunica tunica Siona To get to know the nature megaphone dark bands. The comparisons revealed notable similarities Siona the muscles, which allow the tunica to open and close its labia, and the dark bands observed in the 500-million-year-old fossil.

“megaphone “The morphology suggests to us that the ancestral lifestyle of the gallalis involved a non-motile, filter-feeding adult using its large aspirators,” Nanglo said. “It is very rare to find not only a fossil of the ascidians, but one that provides unique and unparalleled insight into the early evolutionary origins of this enigmatic group.”

Detail of the anatomy of Megasiphon thylacus. M. thylakos had two prominent siphons and a barrel-shaped body. It also had prominent longitudinal muscles extending from the tips of the siphons to the base of the body. These are comparable to modern tunicates, including the intestinal Ciona, which are dissected in c and f. Even an individual micrometer-sized muscle fiber is comparable between this 500-million-year-old fossil and a modern tunic. Credit: James Wheeler (a,d) and Karma Nangelo (b,c,e,f,g)

M. thylacos It is the only definitive tunica fossil discovered so far with soft tissue preservation. It is the oldest of its kind originating from the Middle Cambrian Margum Formation in Utah. The fossil was identified as a tunic by research co-authors Rudy Lerosi-Obriel and Professor Javier Ortega-Hernandez (both in the Department of Organic and Evolutionary Biology) while visiting the Utah Museum of Natural History (UMNH) in 2019.

Ortega-Hernandez said: “The fossil immediately caught our attention because, although we are working mostly on Cambrian arthropods, such as trilobites and their soft-bodied relatives, the close morphological resemblance to megaphone With modern tunicates, it was too amazing to overlook, and we knew right away that the fossil would have an interesting story to tell.”

Fossils from the Margum Formation date back to shortly after the Cambrian Explosion, one of the most important evolutionary events in Earth’s history that occurred about 538 million years ago. It was during this time that most of the major animal groups appeared in the fossil record for the first time, radically changing marine ecosystems. However, tunicates are notably absent in the Cambrian rocks despite their diversity and abundance in the neo-oceans.

There are several exceptionally preserved Cambrian fossil sites in the United States, but they are often overlooked compared to those in the Burgess Shale region of Canada and Zhengjiang in China. “Discovery megaphone “It perfectly illustrates why Javier and I have been doing fieldwork in Utah for the past 10 years,” Lerossi-Aubrell said. “The margum strata are of most interest to us now because we know that they preserve the fossils of animal groups, such as tunicates or comb jellies, which are almost entirely absent from the Cambrian fossil record.”

Estimates of the molecular clock indicate that the ascidians arose 450 million years ago. But, at 500 million years old, M. thylacos It provides the clearest insight into the anatomy of tunicates and their early evolutionary history. Significantly, M. thylacos It provides evidence that most of the modern body plan of tunicates was already established shortly after the Cambrian explosion.

“Given the exceptional quality of preservation and the age of the fossil, we can actually say a lot about the evolutionary history of tunicates,” Nanglo said. “This is an amazing finding because we had no conclusive evidence of the ancestral lifestyles of this group before this.”

Reference: “The tunica cambrian and the deep origin of the ascetic body plan,” by Karma Nanglo, Rudy Lirozzi-Obriel, and James C. Weaver, Javier Ortega-Hernandez, July 6, 2023, Available Here. Nature Communications.
doi: 10.1038/s41467-023-39012-4

After collecting hundreds of new fossils again this spring, researchers are convinced that the Margum Formation is just beginning to reveal its secrets.

The authors would like to extend special thanks to C. Levitt-Bussian and R.B. Irmis for their assistance during visits to the Natural History Museum of Utah (NHMU) and for facilitating the study of specimens at this institution. and to the Bureau of Land Management, especially SE Foss and G. McDonald, to deposit the mass form of megaphone At NHMU and providing organizational assistance.