Aug. 11, 2016
Research Highlight
Newly Discovered Fossils Strengthen Proposition that World’s First Mass Extinction Engineered by Early Animals
Fossils from Zaris site in Namibia: left, the discs are fossil remains of the holdfast structures that were holdfast structures for an Ediacaran species called aspidella; middle, bumps on the rock surface are the remains of burrows, called conichnus burrows, that were originally inhabited by anemone-like animals that may have fed on Ediacaran larvae; right, odd annulated and ribbon-like fossils that represent mysterious early animals (likely ecosystem engineers) called shaanxilithes. Image credit: Simon Darroch/Vanderbilt
Newly discovered fossil evidence from Namibia strengthens the proposition that the world’s first mass extinction was caused by “ecosystem engineers” – newly evolved biological organisms that altered the environment so radically it drove older species to extinction.
The event, known as the end-Ediacaran extinction, took place 540 million years ago. The earliest life on Earth consisted of microbes – various types of single-celled organisms. These held sway for more than 3 billion years, when the first multicellular organisms evolved. The most successful of these were the Ediacarans, which spread around the globe about 600 million years ago. They were a largely immobile form of marine life shaped like discs and tubes, fronds and quilted mattresses.
After 60 million years, evolution gave birth to another major innovation: metazoans, the first animals. Metazoans could move spontaneously and independently at least during some point in their life cycle and sustain themselves by eating other organisms or what other organisms produce. Animals burst onto the scene in a frenzy of diversification that paleontologists have labeled the Cambrian explosion, a 25 million-year period when most of the modern animal families – vertebrates, mollusks, arthropods, annelids, sponges and jellyfish – came into being.
“These new species were ‘ecological engineers’ who changed the environment in ways that made it more and more difficult for the Ediacarans to survive,” said Simon Darroch, assistant professor of earth and environmental sciences at Vanderbilt University, who directed the new study described in the paper titled “A mixed Ediacaran-metazoan assemblage from the Zaris Sub-basin, Namibia,” published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
Darroch and his colleagues report that they have found one of the best-preserved examples of a mixed community of Ediacarans and animals, which provides the best evidence of a close ecological association between the two groups.
“Until this, the evidence for an overlapping ecological association between metazoans and soft-bodied Ediacaran organisms was limited,” Darroch said. “Here, we describe new fossil localities from southern Namibia that preserve soft-bodied Ediacara biota, enigmatic tubular organisms thought to represent metazoans and vertically oriented metazoan trace fossils. Although the precise identity of the tracemakers remains elusive, the structures bear several striking similarities with a cone-shaped organism called Conichnus that has been found in the Cambrian period.”