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Home » New study suggests cambrian explosion was actually a gradual process

New study suggests cambrian explosion was actually a gradual process

The Cambrian Explosion, occurring approximately 540-500 million years ago, has long been considered a pivotal event in the history of life on Earth, marking the appearance of major animal groups. However, recent research conducted by the University of Oxford in collaboration with the University of Lausanne challenges the notion of a sudden burst of diversity, proposing instead a more gradual process.

At the heart of the Cambrian Explosion were the euarthropods, a diverse group encompassing insects, crustaceans, spiders, trilobites, and various other forms. Accounting for over 80 percent of all animal species on the planet, euarthropods have played a crucial role in shaping Earth's ecosystems since the emergence of over 500 million years ago.

The Oxford University Museum of Natural History and the University of Lausanne jointly undertook an extensive analysis of early fossil euarthropods, utilizing a variety of fossil preservation types. Published in the Proceedings of the National Academy of Sciences, their findings challenge the prevailing views on the pace of euarthropod during the early Cambrian, suggesting a more nuanced and gradual radiation.

This new analysis disrupts the dichotomy of two major hypotheses regarding early animal evolution. The first hypothesis proposes a slow, continuous evolution of euarthropods starting 650-600 million years ago, aligning with earlier molecular dating estimates. In contrast, the second hypothesis posits the almost instantaneous appearance of euarthropods 540 million years ago due to exceptionally elevated rates of evolution.

Model of the Cambrian stem lineage euarthropod Peytoia, based on from the Burgess Shale. Top left: Closeup of the mouth parts and frontal appendages. Bottom right: Overall view of the body. Credit: Model and image: E. Horn.

The latest research proposes a middle ground between the two prevailing hypotheses on the origin and diversification of euarthropods. Contrary to the idea of a slow evolution starting 650-600 million years ago or a sudden appearance 540 million years ago, the study suggests that the origin of euarthropods likely occurred around 550 million years ago, aligning with more recent molecular dating estimates. Subsequent diversification unfolded over the next 40 million years.

Professor Allison Daley, leading the study at Oxford University Museum of Natural History and the University of Lausanne, emphasizes the importance of integrating various types of fossil evidence. While each type has its limitations, their combined analysis provides a comprehensive and coherent narrative of the euarthropods' origin and radiation during the lower to middle Cambrian. This challenges the traditional view of the Cambrian Explosion as a sudden event, proposing instead a gradual process over the span of approximately 40 million years.

Understanding the timing of the origin of Euarthropoda is crucial for interpreting the group's evolution. By identifying which groups developed first, researchers can trace the evolution of physical characteristics, such as limbs.

Exceptionally preserved soft-bodied fossils of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis from the Burgess Shale, Canada.Top left: Frontal appendage showing segmentation similar to modern-day euarthropods. Bottom right: Full body specimen showing one pair of frontal appendages (white arrows) and mouthparts consisting of plates with teeth (black arrow) on the head. Credit: A. Daley

The absence of euarthropods from the Precambrian Period, prior to around 540 million years ago, has been attributed to potential biases in fossil preservation. However, the comprehensive fossil study suggests otherwise. Dr. Greg Edgecombe FRS from the Natural History Museum, London, not involved in the study, notes that the idea of biases in fossil preservation can now be rejected. The authors present a compelling case that the late Precambrian and Cambrian periods are similar in terms of fossil preservation, leading to the conclusion that arthropods had not yet evolved during that earlier time.

Harriet Drage, a co-author and Ph.D. student at Oxford University Department of , underscores the significance of the fossil record for understanding the early history of life. With a well-preserved fossil record during the early to middle Cambrian, the study emphasizes the wealth of evidence supporting the origin of euarthropods during a period when fossil preservation was at its peak.

Source: University of Oxford

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