MSGID: 1:317/3 64a79508   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Discovery of 500-million-year-old fossil reveals astonishing secrets of   
   tunicate origins    
      
     Date:   
         July 6, 2023   
     Source:   
         Harvard University, Department of Organismic and Evolutionary   
         Biology   
     Summary:   
         Researchers describe a 500 million-year-old tunicate fossil   
         species. The study suggests that the modern tunicate body plan   
         was already established soon after the Cambrian Explosion.   
      
      
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   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 fossilfrom 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," said Nanglu, postdoctoral researcher   
   in the Department of Organismic and Evolutionary Biology at Harvard   
   University.   
      
   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," sais 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. thylakoshad 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.   
      
   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.   
      
   "Megasiphon's morphology suggests to us that the ancestral lifestyle   
   of tunicates involved a non-moving adult that filter fed with its large   
   siphons," said Nanglu. "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."  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-Herna'ndez (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," said Ortega-Herna'ndez,   
   "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."  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," said Lerosey-Aubril. "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."   
   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.   
      
   "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," said Nanglu. "This is an incredible find as we had virtually   
   no conclusive evidence for the ancestral modes of life for this group   
   before this."  After collecting hundreds of new fossils again this spring,   
   the researchers are convinced the Marjum Formation has only started to   
   reveal its secrets.   
      
   The authors would like to extend a special thank you 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 the specimens   
   housed in this institution. And to the Bureau of Land Management,   
   particularly S.E. Foss and G. McDonald, for depositing the holotype of   
   Megasiphon at the NHMU and providing curatorial assistance.   
      
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   Journal Reference:   
      1. Karma Nanglu, Rudy Lerosey-Aubril, James C. Weaver, Javier Ortega-   
         Herna'ndez. A mid-Cambrian tunicate and the deep origin of the   
         ascidiacean body plan. Nature Communications, 2023; 14 (1) DOI:   
         10.1038/ s41467-023-39012-4   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230706124616.htm   
      
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