The ocean's first large swimming apex predators had exceptionally rapid growth, fossil study finds

The rapid diversification of animals over 500 million years ago—often referred to as the Cambrian Explosion—saw the appearance of the first large swimming predators in our oceans. Amplectobelua symbrachiata, a member of the group Radiodonta, which are relatives of modern arthropods, was the largest of these, reaching nearly one meter in length, and can be easily recognized by their fearsome spiny feeding appendages.

However, despite recent studies revealing a huge diversity of radiodonts, our understanding of how these animals grew and their changing ecological role throughout their life has been distinctly lacking. That is, until now.

Hundreds of fossil specimens of the feeding appendages of Amplectobelua symbrachiata (ranging from less than 1 cm to more than 10 cm in length) collected from the famous fossil deposits of Chengjiang in China were carefully studied and statistically analyzed by an international team of researchers based at Northwest University, China, Cambridge University, UK, and University of British Columbia, Canada.

Combining statistical methods with growth modeling methods commonly used in modern fisheries analyses, the team determined both how the relative proportions of parts of the feeding appendages changed with size and quantified the growth rates in these animals for the first time. All analyses revealed that Amplectobelua symbrachiata displayed an unusual and rapid growth strategy when compared to its modern arthropod relatives.

Amplectobelua symbrachiata appendages display isometric growth—the proportions in the shape of individual podomeres and the relative length of spines to the appendage length and height were the same for small juveniles as for large adults. This implies that the overall function of the appendage was similar for the animal's whole life. However, the greater size of adult A. symbrachiata appendages would have made new food sources available. "The larger size of

adult appendages, alongside faster swimming speeds, would have allowed larger prey to be captured and subdued. Similarity in form must be placed in the context of the scaling up of muscle power and forces with size," said Dr. Yu Wu, one of the leaders from Northwest University, Xi'an.

Growth and mortality parameters for Amplectobelua symbrachiata were estimated using ELEFAN—a model based on size-frequency distributions commonly used in studies of modern arthropods and fish.

"These results show that Amplectobelua symbrachiata were extremely active animals in Cambrian ecosystems and had exceptionally rapid growth when compared to modern marine arthropods," said Prof. Pauly, British Columbia University, who led the ELEFAN analyses. consideration of peaks in size-frequency data suggests that this rapid growth might have been achieved over very few growth stages.

The large growth ratio may have been facilitated by the unique body plan of radiodonts such as Amplectobelua symbrachiata—the body was soft and not fully arthrodized, with the most toughened part of the skeleton represented by the feeding appendages.

The unique position of A. symbrachiata, as a large, active apex predator in ecosystems with large amounts of predation, without a fully arthropodized or sclerotized body, appears to have facilitated an active, rapid-growing life history strategy.

"The Cambrian is thought to have been a time with high predation pressure, which may have provided a benefit for rapid growth to a large size. Simultaneously, rapid growth to become one of the largest animals in the ecosystem would have also provided benefits in terms of prey capture, making more prey items available," said Dr. Stephen Pates of Cambridge University, UK, who co-led the study.

"This unexpected life history strategy might have been shaped by the escalatory 'arms race' that has been hypothesized to shape the Cambrian Explosion," explained Prof. Fu, one of the corresponding authors of this study. "Only through study and analyses of hundreds of fossil specimens are we able to reveal not just what these animals looked like, but also how—and how quickly—they grew."

Taken together, these results show that the animals in today's oceans provide a single snapshot of what evolution can produce—and that the different ecological, environmental, and evolutionary pressures and histories present over the last half a billion years have led to disparate morphologies, body plans and life history strategies that data-rich interdisciplinary studies such as this can reveal.

The work is published in the journal National Science Review.