Examining fossils of an ancient worm-like creature uncovers new features and evolutionary history

A Tube-Dwelling Early Cambrian Lobopodian

Written by: Richard J. Howard, Xianguang Hou, Gregory D. Edgecombe, Tobias Salge, Xiaomei Shi, Xiaoya Ma

Summarized by: Jace Chastain is a 3rd year student studying geology at the University of South Florida. He is looking to work in the oil or natural gas industry after graduating. Aside from geology, he enjoys writing and watching movies.

What data were used? Researchers found nine new fossils of a worm-shaped animal called Facivermis from the evolutionary group Lobopodia. These organisms lived in the Cambrian Period and were found in the Yunnan Province of China. Previously found fossils similar to these were also re-examined. All specimens were housed at the Yunnan Key Laboratory for Paleontology. 

Methods: The Facivermis fossils were examined using a microscope with both polarized and non-polarized light. Polarized light is created when light is passed through a thin slit to limit it to a certain orientation. When polarized light is used in a microscope, it can allow the user to see details that would otherwise not be seen. To detect fine details on the specimens, an electron microscope was used. One fossil was investigated using X-ray spectroscopy, which measures the way X-rays interact with different chemical elements to determine what the fossil is composed of. Phylogenetics (determining the evolutionary history) of Facivermis was done using Bayesian inference (a statistical technique in which the probability is updated as more information becomes available), parsimony (the assumption that the simplest path of evolution is most likely correct), and maximum likelihood estimation (a technique in which under a statistical model, the observed data is considered the most probable). Read more about phylogenetic methods on this  Time Scavengers blog post. Finally, drawings were created by tracing the outlines in the fossils. These drawings can be seen in Figure 1. 

Figure 1. Anatomy diagrams were created by tracing over the Facivermis fossils. D illustrates fossil A, E illustrates fossil B, F illustrates fossil C. Labels with arrows in D, E, and F point to notable features.
an, annuli (ring, similar to earthworms); g, gut; l1–5 and r1–5, left/right lobopods (limbs) 1–5; ph, posterior hooks; ps, posterior swelling; t, tube; ta, terminal anus. The alimentary canal (route where food passes through the body) is highlighted in orange where visible, with the surrounding body cavity in dashed lines. Scale bars, (A and B) 5mm; (C) 3mm.

Results: Facivermis was a worm-like animal living during the Cambrian Period from the phylum Lobopodia. As can be seen in Figure 1, it has a long worm-shaped body with a bulb at the end, and a small head at the front with feathery limbs branching off of the body. Additionally, this study discovered preserved tubes in some of the fossils that the researchers believe formed around the body for protection, like feather-duster worms today. In the past, paleontologists believed Facivermis was a stationary predator that used its long appendages to catch food. However, thanks to this study, it was discovered that Facivermis did not do this. Using the hooks on its bulb-like end, it attached itself to the sea floor, but the arms were used for filter-feeding (pulling floating food particles from the water) rather than predation. Due to the phylogenetic analysis described in the methods section, the researchers determined that Facivermis was part of the family Luolishaniidae (Figure 2). They also determined that the worm-shaped body was unique to Facivermis and considered that the lack of armor compared to its ancestors supported the idea that it lived in tubes to hide.

Figure 2. Phylogenetic tree showing the evolutionary relationships of organisms closely related Facivermis and its place in the family Luolishaniidae. Facivermis has a soft, unarmored body similar to the hallucigeniids to which it is closely related.

Why is this study important? Understanding where Facivermis lies on the evolutionary tree of life allows us to get a clearer picture of how life evolved, especially lobopodians. It also gives us a rare example in the Cambrian of a filter-feeding role being filled. The worm-like shape of Facivermis is important because it is autapomorphic (i.e., unique to this species), and the authors’ interpretation is that feeding off of food particles suspended in the water column was not the origin of paired appendages, as was previously thought.

The big picture: This study shows that Facivermis had a tube similar to the feather-duster worms of today, which it used as protection in a similar way. It also was a filter-feeder, like feather-duster worms, which is a more uncommon ecological niche and gives us insight into how the Cambrian ecosystem worked. The Cambrian was one of the earliest periods in complex life’s history on Earth and contained creatures so different from what is alive today that our knowledge is very limited. A discovery like this gives us a lot of insight into those ancient animals. Overall, this clarified Facivermis’ place in evolutionary history, and disproved a previous hypothesis as to the origin of paired appendages.

Citation: Howard, R. J., Hou, X., Edgecombe, G. D., Salge, T., Shi, X., & Ma, X. (2020). A tube-dwelling early Cambrian lobopodian. Current Biology30(8), 1529-1536.