Exceptionally preserved fossils of rotadiscus grandis shed light on independent evolution in early animal groups

The exceptional preservation of fossils from the Cambrian period has significantly contributed to our knowledge of the origin and evolution of modern animal groups.

By analyzing fossils of the extinct invertebrate species Rotadiscus grandis, scientists have gained insights into its place in the animal tree of life. This research reveals that certain characteristics found in living species may have evolved independently rather than stemming from a single common ancestor.

Around 500 million years ago, an extraordinary-looking creature roamed the seafloor, utilizing its tentacles to collect food particles as it moved along.

Known as Rotadiscus grandis, this animal thrived during the Cambrian period, a time of intense evolutionary activity that likely gave rise to many of the major animal groups we observe today.

The remains of Rotadiscus were initially discovered over three decades ago, but the incomplete specimens presented challenges in determining their evolutionary significance.

Now, a recent study published in Current Biology provides a more detailed understanding of Rotadiscus grandis thanks to the rare and exceptionally well-preserved fossils found in China’s Yunnan Province, within the Chengjiang biota. These 518-million-year-old fossils have shed new light on the early evolution of prominent animal groups.

Dr. Imran Rahman, a lead author of the study and a principal researcher at the Museum, remarks, “Rotadiscus doesn’t resemble any living organism we see today. That’s why I find these peculiar wonders so captivating.”

“We have discovered that some features we previously thought were present in the common ancestor of major animal groups, such as a post-anal tail, actually evolved independently. Understanding these processes is essential, and Cambrian fossils play a crucial role in achieving this understanding.”

Why has it been difficult to describe Rotadiscus?

The Cambrian period, occurring over 500 million years ago, witnessed a remarkable explosion of life forms, leading to the emergence of a wide array of species, including early members of major animal groups that exist today, such as chordates encompassing vertebrates.

Many organisms from this era boasted distinct anatomical structures that differ greatly from those seen in present-day species. However, the incompleteness of the fossil record often poses challenges for paleontologists in identifying these species and determining their place in the evolutionary tree.

In previous studies, Rotadiscus and its closest fossil relatives have been associated with various groups, including sea cucumbers and jellyfish. The recent discovery of Rotadiscus fossils in the Chengjiang biota has shed new light on this matter.

The Chengjiang biota is renowned for its remarkably well-preserved Cambrian fossils. This exceptional preservation is attributed to the rapid burial of these organisms by mudflows while they were still alive, effectively entombing them and protecting them from decay caused by bacteria and scavenging by other creatures.

The newly found Rotadiscus fossils are no exception to this extraordinary preservation, as they have revealed previously unseen structures in greater detail, such as potential traces of nerves in the tentacles and a newly discovered double spiral structure.

Imran, the lead researcher, remarks, “The identification of Rotadiscus and understanding its evolutionary significance were our primary objectives in this study. Previously, uncertainty surrounding the nature of Rotadiscus hindered its use as a tool for unraveling the diversity of life during that time.”

Therefore, through this investigation, the researchers aimed to establish the identity of Rotadiscus and elucidate its importance in the broader context of evolution.

Why are these Rotadiscus fossils important?

The remarkably preserved Rotadiscus specimens from the Chengjiang biota have enabled scientists to identify new features, thereby unraveling the animal’s evolutionary significance.

Researchers conducted an analysis of 330 features across approximately 60 different groups of extinct and extant animals in search of similarities that could suggest connections. As a result, Rotadiscus was determined to be an early member of the ambulacrarians, a group that includes acorn worms and echinoderms like starfish and sea urchins.

The inclusion of Rotadiscus and other fossils in the evolutionary tree has facilitated the reconstruction of the evolution of major animal traits. Of particular importance was the presence of a tail extending beyond the anus.

Previously, scientists believed that a post-anal tail was present in the common ancestor of all fish and acorn worms. However, the absence of a post-anal tail in Rotadiscus suggests that this trait may have independently evolved within these distinct groups.

This phenomenon is referred to as convergent evolution, wherein the same characteristic evolves separately multiple times, often in response to similar environmental or ecological pressures.

Imran explains, “By incorporating Rotadiscus and its relatives into our analysis, we can bridge an evolutionary gap. This is crucial as paleontologists strive to comprehend the interconnectedness of the tree of life.”

He further adds, “Moreover, this discovery is exciting because it enhances our understanding of the evolution of other major animal groups, including the one humans belong to. It underscores the vital significance of fossils since the predictions made in this analysis would not have been possible based solely on extant animals.”

“It is by incorporating these extraordinary and enigmatic wonders into our evolutionary frameworks that we can gain deeper insights into the origins and early evolution of major animal groups.”

Source: Natural History Museum