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The Cambrian explosion was an extraordinary phenomenon in the evolution of life on the planet that led to the emergence of many animal phyla and the diversification of species. During this period, some 530 million years ago, most of the basic body plans of organisms that have survived to the present day emerged. However, this great explosion of life that changed the evolutionary landscape on Earth may have occurred millions of years earlier than previously thought, a hypothesis now reinforced in a study in the journal Geology.

This is a main conclusion of a new study that analyses the body profiles of organisms—symmetry, segmented bodies, exoskeletons, etc.—from around 545 million years ago by analyzing trace fossils, which are the fossilized marks in rocks and sediments left by the activity of organisms in the past.

The authors of the article are the experts Olmo Miguez Salas, from the Faculty of Earth Sciences at the University of Barcelona, and Zekun Wang, from the Natural History Museum in London (United Kingdom).

Fossil traces of extinct animals

The Cambrian explosion is a unique period in the history of life that poses many unanswered questions. To delve into the biodiversity of this period, most studies in paleontology tend to focus on the study of organisms that had hard parts. However, the study of trace fossils (or ichnofossils) opens up the possibility of discovering what the activity of hard-bodied, soft-bodied or skeletally deficient organisms preserved in the stratigraphic record was like.

"The trace fossil record provides valuable information about evolutionary periods when soft-bodied fauna were dominant," says Miguez-Salas, a Beatriu de Pinós postdoctoral researcher at the UB's Department of Earth and Ocean Dynamics.

"Fossil traces reflect the behavior of the organism that generates them, which is determined by habitat and responses to environmental stimuli. Therefore, they are an indicator of the paleoecological conditions in which the organisms that generated them lived."

The authors have focused on the study of trace fossils in the Ediacaran-Cambrian transition, "a period of recognized palaeoevolutionary interest that was a turning point in the evolution of complex life on Earth," says Miguez-Salas.

In this transition, there was a radical change in biodiversity and in the structure of organisms and ecosystems. "The Ediacaran fauna was dominated by complex, multicellular soft-bodied organisms. The transition to the Cambrian involved the extinction of much of the Ediacara fauna, and a rapid diversification of complex multicellular life forms with hard parts (e.g. exoskeletons). This is the evolutionary core from which most modern animal phyla emerged: what is known as the Cambrian explosion," notes the researcher.

The Cambrian explosion may have happened much earlier

The study quantitatively indicates that organisms with slender body profiles thrived around 545 million years ago. "These organisms probably possessed coelomic hydrostatic bodies, with an anteroposterior axis, muscles and possibly segmentation," the expert says.

"Furthermore, these organisms could move in a specific direction (directional locomotion) and probably possessed sensory capabilities to move and feed on heterogeneous substrates in a habitat dominated by microbial mats. Therefore, the so-called Cambrian explosion and its evolutionary implications may have occurred much earlier than estimated."

These adaptations in body profile and mobility allowed these early animals to thrive in increasingly dynamic and complex environments, an ecological engineering that could promote evolutionary innovations. The methodology of the study was based on the analysis of the linear proportionality exhibited by the trace trajectories of modern and fossilized animals. Subsequently, this scaling law has been applied to locomotor traces of Ediacaran-Cambrian fossils (e.g. Archaeonassa, Gordia, Helminthopsis and Parapsammichnites).

Although some previous studies had described trace fossils associated with mobile benthic bilateral organisms in the Ediacara fauna, detailed quantitative approaches were lacking and there were still many unknowns about the body shape of these organisms (length, width, cephalization, etc.). The findings of the new study establish an innovative quantitative approach to analyzing the fossil locomotion traces from ancient times, early animal anatomy and paleoecological dynamics.

"This new discovery opens the door to quantitatively study future Ediacara trace fossils discovered in the coming years and to corroborate that the Cambrian explosion did not happen in the Cambrian, but many millions of years earlier.

"Moreover, the scaling laws obtained in this study enable the study of the morphological evolution of different faunal phyla generating fossil locomotion traces, not only during this evolutionary period, but also during other evolutionary periods of similar importance, such as the great diversification event of the Ordovician," concludes Miguez-Salas.