An article published in the journal “Current Biology” reports a study on exceptionally preserved Rotadiscus grandis fossils. A team of researchers led by Dr Imran Rahman of the Natural History Museum in London offered many new insights into these animals dating back to the Lower Cambrian period, about 518 million years ago. Previous studies had left a number of questions about their classification and lifestyle. The new study assigns them to the large Ambulacraria group, which includes the Echinoderms and Hemicordata phyla and is part of the superphylum Deuterostomia together with the phylum of Chordates. If this classification is correct, it indicates that certain characteristics of later animal species emerged independently several times.
The vastness of the superphylum Deuterostomia and the morphological variety of today’s species included in it make it difficult to understand the characteristics of their common ancestors. Fossils dating back to such ancient times are rare and therefore the exceptionally preserved ones of Rotadiscus grandis discovered in the so-called Chengjiang biota, in the Yunnan province, China, are very precious. The image (Courtesy Li et al. All rights reserved) shows photos of the specimen cataloged as YKLP 13090 (A) and an interpretive drawing (B) and of the specimen cataloged as CJHMD 00038 (C and D) and their interpretive drawings (E and F).
The many details obtained from the study of these discoidal animals with a diameter of about 15 centimeters allowed to analyze 330 characteristics of 60 different groups of extinct and extant animals in search of similarities. The result indicates that Rotadiscus grandis was one of the earliest members of the Ambulacraria group, which includes animals ranging from worms to starfish.
Scientists believed that the common ancestors of all Deuterostomia had a post-anal tail. However, Rotadiscus grandis lacks this anatomical feature, suggesting that it evolved independently in various animal groups. This is an example of convergent evolution in which the same feature evolves separately, often in response to similar environmental or ecological pressures. Assessing those pressures in environments more than 500 million years old is another of the difficulties paleontologists face in reconstructing the evolution of a group of organisms.
The new Rotadiscus grandis fossils provided information that fills a gap in the evolutionary reconstruction of the entire superphylum Deuterostomia. The enormous diversification that occurred in the Cambrian period led to very different forms and the controversies regarding the reconstruction of the evolution of the later groups of animals will surely continue. More such well-preserved fossils could provide further information on the evolution of the large group that includes humans as well.