As early as 535 million years ago, various species of annelids probably lived in the water and soil of the oceans. This is suggested by discoveries of three-dimensionally preserved fossils of tiny annelid worms from southern China. The specimens, which were only a few millimeters long, already had appendages of different lengths – an indication that they were different species, each with their own ecological niche. Accordingly, the ancestors of today’s annelids had already differentiated in the early Cambrian.
Annelids, also known as annelids, are among the ecologically most widespread animal phyla in the world. These include the bristle worms (Polychaeta), which are found primarily in the sea, the few bristle worms (Oligochaeta), which include earthworms, and the leeches (Hirudinea), which mostly live in freshwater lakes. Some annelids are excellent swimmers, others burrow through the ground, and others live a sedentary life at the bottom of the sea. But when did this diversity arise? This question is difficult to answer because, due to their soft bodies, the worms rot quickly after death and are rarely preserved as fossils. Most of the annelid fossils found so far are two-dimensional casts of the bodies that have been dated to less than 521 million years old.
Mineralized bodies
But now a team led by Xiaofeng Xian from the Chinese Academy of Sciences in Nanjing has discovered seven three-dimensional body fossils of annelids, which are around 535 million years old and date back to the earliest epoch of the Cambrian, the Fortunian. The finds from southern China represent the oldest known fossil evidence of this animal phylum.
As Xian and his colleagues found, the fossilized bodies consist predominantly of the mineral calcium phosphate. “An outer skin or internal structures have not been preserved,” report the researchers. “This suggests that fossil mineralization began after the internal structures had already decayed, but before the more resistant outer skin had completely decayed. This eventually degraded, leaving behind a mineralized internal form.”
Two new species
Based on these forms, which have been preserved over millions of years, the researchers identified two new species. Both have elongated, segmented bodies with clear grooves between the segments as well as paired outgrowths on the individual segments, so-called parapodia, which, however, differ in their length. According to Xian and his colleagues, this suggests that the specimens found occupied different ecological niches and moved differently.
The team named the species with shorter appendages Kuanchuanpivermis brevicruris. Based on the shape of its body, the researchers suspect that this species lived on the seabed, similar to annelids with a similar shape that live today. The species with longer appendages was named Zhangjiagoivermis longicruris. With its long bristles, it was probably adapted to life in the open ocean. This would make Z. longicruris the earliest known species of annelid to move freely in the sea.
“These findings indicate that annelids had already developed a polychaete-like body structure in the Fortunium and that early representatives of this clade had split off from their living sister group by the Fortunium and differentiated into forms with both short and elongated parapodia,” explains the research team. Since the different species had already differentiated 535 million years ago, the evolution of annelids could have begun even before the Cambrian species explosion around 541 million years ago.
Source: Xiaofeng Xian (Chinese Academy of Sciences, Nanjing, China) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2538071123