Leg loss 518 million years ago: Researchers have solved the riddle of a bizarre prehistoric being. It is the earliest example of an animal that has reduced limbs that have become unnecessary in the course of its developmental history – similar to snakes. The worm-like creature therefore lived in a tube in the sea floor and could therefore save on feet that its crawling ancestors had already produced.
The focus was on the mysterious earth age of the Cambrian: In the period from 541 to 485 million years ago, evolution really got going – the archetypal forms of almost all of today’s animal tribes were created in the oceans. However, there were also living beings in the Cambrian that paleontologists cannot clearly place in the tree of life. Such a case was previously a strange creature that has been known from various finds for thirty years: Facivermis yunnanicus was about ten centimeters long, had five pairs of spiky arms in the head area and a strange thickening at the end of its worm-like body. This combination of features has led to a debate about the evolutionary mapping of Facivermis.
A riddle being in sight
Because of the limbs in the head area, some researchers attached great importance to this being for evolution research: They suspected that it was a transitional form in the development of legless worms to the so-called lobopods. These “Rag Feet” had pairs of partly long leg structures along their entire body, with which they crawled over the bottom of the Cambrian seas. The lobopods are therefore considered the ancestors of the arthropods to which today’s crabs and insects belong.
But was Facivermis really a link in the evolution from worms to early crawling animals? A team of researchers led by Richard Howard from the University of Exeter has once again dedicated a study to the mystery creature. The basis was formed by new Facivermis fossils from southern China, which show previously unknown details of the structures and way of life of this animal.
“The main clue came from a fossil showing that the lower part of the animal was in a tube,” reports Howard. The scientists concluded from this: Facivermis lived sedentarily on the sea floor. His pear-shaped thickened tail end probably served to anchor him in the ground and with his arms in the head area Facivermis probably fished food from the water, the researchers explain.
No transitional form – a reverse development is emerging
In this way of life, the being therefore did not need any rear limbs. However, the researchers found evidence that the ancestors of Facivermis had hind legs. In other words: as an adaptation to his sedentary way of life in the sea floor, the rear limbs must have receded in the course of the development history of Facivermis. Other newly discovered features – such as the presence of two eyes on the head – also made it clear that Facivermis was not a transition form from the worms to the first animals with legs. According to the researchers, it was already a representative of the lobopods instead. The features of Facivermis can thus be explained as a so-called secondary loss. The leglessness of the snakes is the clearest example of this evolutionary effect, in which body structures are regressed when they lose their function.
“Mostly, organisms have evolved from simple, increasingly complex body structures in the course of evolution. But in some cases there was also an opposite trend, ”says co-author Xiaoya Ma. “It is fascinating to note that such a return to more primitive forms has apparently already existed in the youth of evolution,” sums up the paleontologist.
Source: University of Exeter, technical article: Current Biology, doi: 10.1016 / j.cub.2020.01.075