Facivermis yunnanicus: Paleontologists Solve Mystery of Cambrian Worm-Like Creature
Facivermis yunnanicus lived some 518 million years ago in the Cambrian period.
It had a long body and five pairs of spiny arms near its head, leading to suggestions it might be a missing link between legless cycloneuralian worms and a group of fossil animals called lobopodians, which had paired limbs all along their bodies.
But the new study reveals the creature was itself a lobopodian that lived a tube-dwelling lifestyle anchored on the sea floor, and so evolved to lose its lower limbs.
“A key piece of evidence was a fossil in which the lower portion of Facivermis yunnanicus was surrounded by a tube,” said Dr. Richard Howard, a researcher at Yunnan University, the University of Exeter, and the Natural History Museum, London.
“We don’t know the nature of the tube itself, but it shows the lower portion of the worm was anchored inside by a swollen rear end.”
“Living like this, its lower limbs would not have been useful, and over time the species ceased to have them.”
“Most of its relatives had three to nine sets of lower legs for walking, but our findings suggest Facivermis yunnanicus remained in place and used its upper limbs to filter food from the water.”
“This is the earliest known example of secondary loss — seen today in cases such as the loss of legs in snakes.”
The Cambrian period is seen as the dawn of animal life, and the scientists were fascinated to find a species evolving to be more primitive even at this early stage of evolution.
“We generally view organisms evolving from simple to more complex body plans, but occasionally we see the opposite occurring,” said Dr. Xiaoya Ma, a researcher at Yunnan University and the University of Exeter.
“What excited us in this study is that even at this early stage of animal evolution, secondary-loss modifications — and in this case, reverting ‘back’ to lose some of its legs — had already occurred.”
“We’ve known about this species for about 30 years, but it’s only now that we’ve got a confident grasp of where it fits in the evolutionary tree.”
“Studies like this help us understand the shape of the tree of life and figure out where the adaptations and body parts we now see have come from.”
“For several years we and others have been finding lobopodians from the Cambrian period with pairs of appendages along the length of the body — long, grasping ones in the front, and shorter, clawed ones in the back,” said Dr. Greg Edgecombe, from the Natural History Museum.
“But Facivermis yunnanicus takes this to the extreme, by completely reducing the posterior batch.”
Richard J. Howard et al. A Tube-Dwelling Early Cambrian Lobopodian. Current Biology, published online February 27, 2020; doi: 10.1016/j.cub.2020.01.075