Discovery of 500-million-year-old fossil reveals astonishing secrets of tunicate origins
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, ...
"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 co-authors 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.
In a new study in Nature Communications, Nanglu and co-authors 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.
"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.
Artistic reconstruction of Megasiphon thylakos, a benthic organism that lived directly on the seafloor. M. thylakos was also sessile (non-moving) and spent its time filter feeding using its prominent siphons. Also reconstructed in the vicinity are other species commonly found in the Marjum Formation, the site from which M. thyalkos was discovered. Nearby brachiopods (bottom center) and the spiny sponge Choia (center middle) are common in many Cambrian environments. In the background is the hemichordate Oesia, which lived in perforated tubes. Credit: Original artwork by Franz Anthony
Comparisons between the new Cambrian tunicate Megasiphon thylakos (a,b) with some modern tunicates (c,d,e). In particular, M. thylakos shares the rounded vase or barrel-like body and prominent pair of siphons of the modern ascidiacean tunicates. Given the fact that M. thylakos is half-a-billion years old, this suggests that ancestrally, tunicates lived much like modern ascidiaceans: they had a non-moving adult form with siphons for filter feeding, a body plan that was arrived at after metamorphosing from a tadpole-like juvenile. The modern species represented are c: Ciona, d: Ascidiella, e: Molgula. Credit: Rudy Lerosey-Aubril (a,b) and Karma Nanglu (c,d,e)
Details of the anatomy of Megasiphon thylakos. M. thylakos had two prominent siphons and a barrel shaped body. It also had prominent longitudinal muscles running from the tips of the siphons to the base of the body. These are comparable with modern tunicates, including Ciona intestinalis, which is dissected in c and f. Even the micrometer sized individual muscle fibers can be compared between this 500-million year old fossil and modern tunicates. Credit: James Wheeler (a,d) and Karma Nanglu (b,c,e,f,g)