Creationism in Crisis - 500 Million Year-Old Fossil Reveals How Tunicates Evolved

Artistic reconstruction of Megasiphon thylakos.

Artwork by Franz Anthony.

Discovery of 500-million-year-old fossil reveals astonishing secrets of tunicate origins | Department of Organismic and Evolutionary Biology

Newly-discovered tunicate from the Middle Cambrian Marjum Formation in Utah, USA

Left:
Holotype (UMNH.IP.6079) and only known specimen of Megasiphon thylakos, showing overall morphology including paired siphons and barrel-shaped body

Right:
Counterpart fossil of the same specimen

Nanglu, K., Lerosey-Aubril, R., Weaver, J.C. et al. (2023)

Tunicates are interesting for a number of reasons, not least of which is that they are believed to be ancestral to all the chordates, which includes us and all other vertebrates. However, it's not obvious to look at them how they evolved into the chordates in the first place, until you examine the juvenile form, which is motile, unlike the adult which is sedentary, anchored to a rock by what had been its head.

The juvenile form had the beginning of a notochord, or collections of neurons along its back that evolved into the central nervous system, encased, in the case of the vertebrates in the bony spinal column with a cranium at the head end.

Another interesting thing is that it was not clear what ancestry they had and how they evolved from it. Modern tunicate form two groups: the sedentary (when adult) appendicularias and the free-swimming ascidiaceans or "sea squirts", but it was not clear which form, if either, was the stem tunicate. That mystery came some way to being solved recently with the discovery of a rare fossil from 500 million years ago, in the Middle Cambrian Marjum Formation in Utah, USA, and the study carried out on it by four Harvard biologists led by Dr Karma Nanglu, a postdoctoral researcher in the Department of Organismic and Evolutionary Biology.

Just how, is explained in a Harvard news release:

Karma Nanglu says his favorite animal is whichever one he’s working on. But his latest subject may hold first place status for a while: a 500-million-year-old fossil from the wonderfully weird group of marine invertebrates, the tunicates.

This animal is as exciting a discovery as some of the stuff I found when hanging off a cliffside of a mountain, or jumping out of a helicopter. It’s just as cool.

Dr Karma Nanglu, first author
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology
Harvard University, Cambridge, MA, USA.

In a new study in Nature Communications, Nanglu and coauthors describe the new fossil, named Megasiphon thylakos, revealing that ancestral tunicates lived as stationary, filter-feeding adults and likely underwent metamorphosis from a tadpole-like larva.

Tunicates are truly strange creatures that come in all shapes and sizes with a wide variety of lifestyles. An adult tunicate’s basic shape is typically barrel-like with two siphons projecting from its body. One of the siphons draws in water with food particles through suction, allowing the animal to feed using an internal basket-like filter device. After the animal feeds, the other siphon expels the water.

There are two main tunicate lineages, ascidiaceans (often called “sea squirts”) and appendicularias. Most ascidiaceans begin their lives looking like a tadpole and mobile, then metamorph into a barrel shaped adult with two siphons. They live their adult life attached to the seafloor. In contrast, appendicularians retain the look of a tadpole as they grow to adults and swim freely in the upper waters.

This idea that they begin as tadpole-looking larva that, when ready to develop, basically headbutts a rock, sticks to it, and begins to metamorphosis by reabsorbing its own tail to transform into this being with two siphons is just awe-inspiring.

Megasiphon’s morphology suggests to us that the ancestral lifestyle of tunicates involved a non-moving adult that filter fed with its large siphons. It’s so rare to find not just a tunicate fossil, but one that provides a unique and unparalleled view into the early evolutionary origins of this enigmatic group.

Given the exceptional quality of preservation and the age of the fossil, we can actually say quite a bit about the evolutionary history of the tunicates. This is an incredible find as we had virtually no conclusive evidence for the ancestral modes of life for this group before this.

Dr Karma Nanglu

Interestingly, tunicates are the closest relatives of vertebrates, which includes fish, mammals, and even humans. How this odd-looking creature could be related to vertebrates is hard to imagine were it not for that tadpole beginning. Tunicate’s close relationship to vertebrates makes studying them critical for understanding our own evolutionary origins. Unfortunately, it’s not easy to do as tunicates are almost completely absent from the entire fossil record, with only a handful of fossils appearing convincingly as members of the group.

With so few fossils, scientists relied mainly on what could be learned from modern tunicate species. Because no one knew the morphology and ecology of the last common ancestor of the tunicates, scientists could only hypothesize that it was either a benthic animal with two siphons, like the ascidiaceans, or a free-swimming animal like the appendicularians.

M. thylakos had all the basic hallmarks of an ascidiacean tunicate, a barrel-shaped body and two prominent siphon-like growths. But the feature that stood out to the team was the dark bands running up and down the fossil’s body.

Comparisons between M. thylakos and a modern tunicate. High powered images of M. thylakos allowed the researchers to conduct a side-by-side comparison to a modern ascidiacean. The researchers used dissected sections of the modern tunicate Ciona to identify the nature of Megasiphon’s dark bands. The comparisons revealed remarkable similarities between Ciona’s muscles, which allow the tunicate to open and close its siphons, and the dark bands observed in the 500-million-year-old fossil.

M. thylakos is the only definitive tunicate fossil with soft tissue preservation that has been discovered to date. It is the oldest of its kind originating from the middle Cambrian Marjum Formation in Utah. The fossil was recognized as a tunicate by co-authors research associate, Rudy Lerosey-Aubril, and Professor Javier Ortega-Hernández (both in the Department of Organismic and Evolutionary Biology) while visiting the Utah Museum of Natural History (UMNH) in 2019.

The fossil immediately caught our attention, although we mostly work on Cambrian arthropods, such as trilobites and their soft bodied relatives, the close morphological similarity of Megasiphon with modern tunicates was simply too striking to overlook, and we immediately knew that the fossil would have an interesting story to tell.

Javier Ortega-Hernández, co-author
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology
Harvard University, Cambridge, MA, USA.

Fossils from the Marjum Formation date from shortly after the Cambrian Explosion, one of the most significant evolutionary events in Earth’s history which occurred approximately 538 million years ago. During this time the most major animal groups appeared in the fossil record for the first time radically changing marine ecosystems. Tunicates, however, are noticeably absent in Cambrian rocks even though they are diverse and abundant in modern oceans.

There are many Cambrian fossil sites with exceptional preservation in the United States, but these are often overlooked compared to those from the Burgess Shale in Canada and Chengjiang in China.

The discovery of Megasiphon perfectly illustrates why Javier and I have been conducting fieldwork in Utah for the last ten years. The Marjum strata has all of our attention right now as we know that it preserves fossils of animal groups, such as tunicates or comb jellies, that are almost entirely absent from the Cambrian fossil record.

Rudy Lerosey-Aubril, co-author
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology
Harvard University, Cambridge, MA, USA.

Molecular clock estimates suggest that ascidiaceans originated 450 million years ago. However, at 500 million years old, M. thylakos provides the clearest view into the anatomy of ancient tunicates and their earliest evolutionary history. Significantly, M. thylakos provides evidence that most of the modern body plan of tunicates was already established soon after the Cambrian Explosion.

After collecting hundreds of new fossils again this spring, the researchers are convinced the Marjum Formation has only started to reveal its secrets.

The scientists’ findings are described open access in the journal Nature Communications:

Fig. 2: The tunicate Megasiphon thylakos nov. from the mid-Cambrian (Drumian) Marjum Formation of Utah and comparisons with modern benthic tunicates.

a Holotype (UMNH.IP.6079) and only known specimen of Megasiphon thylakos, showing overall morphology including paired siphons and barrel-shaped body drawing immediate comparisons with modern benthic tunicates (c–e). b Counterpart to a. c Ciona intestinalis (Phlebobranchia). d Ascidiella sp. (Phlebobranchia) e Molgula manhattensis (Stolidobranchia).

Nanglu, K., Lerosey-Aubril, R., Weaver, J.C. et al. (2023)

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Fig. 3: Anatomical details of Megasiphon thylakos nov.

a The holotype (UMNH.IP.6079) preserves details of multiple internal anatomical structures, including prominent longitudinal muscle bands which diverge into thin, individual fibers as they enter the apical regions of the siphon. b Line drawing of (a). c Dissection of the modern ascidiacean Ciona intestinalis. The longitudinal muscles are easily visible, extending from the base of the animal (panel bottom) into both the oral and apical siphons. d Close up of boxed area in (a), showing the thin longitudinal muscle fibers. e Line drawing of (d). f Close up of boxed area in (c), showing the arrangement of muscles around the siphon. Note the pitchfork-like frayed arrangement of the longitudinal muscles around the siphon, highly reminiscent of the organization of Megasiphon thylakos. g Line drawing of (f). ac atrial cavity, as atrial siphon, lm longitudinal muscles, mf muscle fibers, os oral siphon, tm transverse muscles. White arrowheads in (a) and (b) indicate crossover points between adjacent longitudinal muscles.

Nanglu, K., Lerosey-Aubril, R., Weaver, J.C. et al. (2023)

Abstract

Tunicates are an evolutionarily significant subphylum of marine chordates, with their phylogenetic position as the sister-group to Vertebrata making them key to unraveling our own deep time origin. Tunicates greatly vary with regards to morphology, ecology, and life cycle, but little is known about the early evolution of the group, e.g. whether their last common ancestor lived freely in the water column or attached to the seafloor. Additionally, tunicates have a poor fossil record, which includes only one taxon with preserved soft-tissues. Here we describe Megasiphon thylakos nov., a 500-million-year-old tunicate from the Marjum Formation of Utah, which features a barrel-shaped body with two long siphons and prominent longitudinal muscles. The ascidiacean-like body of this new species suggests two alternative hypotheses for early tunicate evolution. The most likely scenario posits M. thylakos belongs to stem-group Tunicata, suggesting that a biphasic life cycle, with a planktonic larva and a sessile epibenthic adult, is ancestral for this entire subphylum. Alternatively, a position within the crown-group indicates that the divergence between appendicularians and all other tunicates occurred 50 million years earlier than currently estimated based on molecular clocks. Ultimately, M. thylakos demonstrates that fundamental components of the modern tunicate body plan were already established shortly after the Cambrian Explosion.

Nanglu, K., Lerosey-Aubril, R., Weaver, J.C. et al.
A mid-Cambrian tunicate and the deep origin of the ascidiacean body plan. Nat Commun 14, 3832 (2023). https://doi.org/10.1038/s41467-023-39012-4

Copyright: © 2023 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

A missing link, a specimen showing evolution since the Cambrian biota, and life on Earth almost half a billion years before they believe Earth was magicked into existence out of nothing.

Overall, another dreadful day for creationists, if only they weren't so practiced at remaining stoically ignorant of anything that doesn't comply with their requirements and might cause them to wonder if they could be wrong.

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