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Rapid growth of cambrian sea predator revealed

Around 500 million years ago, during the Cambrian Explosion, Earth witnessed a remarkable diversification of , marking the emergence of the first sizable marine predators. Among these creatures was Amplectobelua symbrachiata, a member of the Radiodonta group—relatives of contemporary arthropods. Towering at almost one meter in length, Amplectobelua symbrachiata is easily identifiable by its formidable spiny feeding appendages.

Despite the increasing recognition of the diversity within radiodonts, our understanding of the growth patterns and evolving ecological roles of these creatures has remained notably limited—until now.

A collaborative effort by an international team of researchers from Northwest University in China, Cambridge University in the UK, and the University of British Columbia in Canada has brought fresh insights. The team meticulously examined hundreds of fossilized feeding appendages of Amplectobelua symbrachiata, sourced from the renowned Chengjiang fossil deposits in China. The specimens varied in length from less than 1 cm to over 10 cm.

Utilizing a combination of statistical methods and growth modeling techniques commonly employed in modern fisheries analyses, the researchers achieved a groundbreaking understanding of how the proportions of different parts of the feeding appendages changed with size. For the first time, they quantified the growth rates of Amplectobelua symbrachiata.

The analyses uncovered a distinctive and swift growth strategy employed by Amplectobelua symbrachiata, setting it apart from its modern arthropod counterparts.

Boxplots show comparisons of maximum, minimum and mean growth ratio between stages in total group euarthropods (data gathered from literature). Note how A. symbrachiata displays among the highest maximum growth ratios of any euarthropod in the literature (except for chironomid Chironomus, coenagrionid Ischnura cruzi, and lycaenid Maculinea arion). The diagram is collated from various body parts of large number of modern and extinct euarthropods. Credit: Science China Press

Amplectobelua symbrachiata, with its appendages displaying isometric growth, maintained consistent proportions in the shape of individual podomeres and the relative length of spines throughout its life, from small juveniles to large adults. This suggests a uniform function for the appendage across the animal's lifespan. However, the larger size of adult A. symbrachiata appendages opened up new possibilities for securing food sources.

Dr. Yu Wu, a leader from Northwest University, Xi'an, emphasized that the larger appendages of adult A. symbrachiata, coupled with faster swimming speeds, enabled the capture and subduing of larger prey. The similarity in form, according to Dr. Wu, should be understood in the context of scaling up muscle power and forces with size.

Using ELEFAN, a model based on size-frequency distributions commonly applied in studies of modern arthropods and fish, the researchers estimated growth and mortality parameters for Amplectobelua symbrachiata. Prof. Pauly from the University of British Columbia, who led the ELEFAN analyses, highlighted that these findings indicate A. symbrachiata's exceptional activity and rapid growth compared to modern marine arthropods. The analysis of size-frequency data peaks suggested that this rapid growth occurred over very few growth stages.

Boxplots Note that the position of the L and K data pair for Amplectobelua symbrachiata is far above the large ellipsoid's main axis (dotted line), suggesting a much faster growth than for recent crustaceans, as well as the Ordovician trilobite Triarthrus eatoni. Credit: Science China Press

The large growth ratio observed may have been facilitated by the unique body plan of radiodonts like Amplectobelua symbrachiata, characterized by a soft, non-arthrodized body, with the feeding appendages representing the most reinforced part of the skeleton.

A. symbrachiata's distinctive position as a large, active apex predator in ecosystems with substantial predation, coupled with its soft-bodied and unsclerotized nature, seems to have favored an active, fast-growing life history strategy. Dr. Stephen Pates of Cambridge University emphasized the potential influence of high predation pressure during the Cambrian period, creating a scenario where rapid growth to a substantial size offered advantages in both survival and prey capture.

(A) Modified Wetherall plot to estimate asymptotic length (L) using data from all sites. (B) Estimating the best K value (associated with highest Rn value), for L = 14.1 mm, given the DARL L/F data from Jianshan locality; n = 134). (C) length-converted catch curve for the estimation of instantaneous mortality (M). (D) von Bertalanffy growth curve, with L = 14 mm and K = 0.33 yr-1, superposed on the original (Year n+1) and the ‘restructured' L\F data (Year n). Credit: Science China Press

Prof. Fu, one of the corresponding authors, highlighted that this unexpected life history strategy might be a product of the escalatory ‘arms race' theorized to shape the Cambrian Explosion. The study, published in the journal National Science Review, underscores the importance of analyzing numerous fossil specimens to uncover not only the appearance but also the growth dynamics of ancient organisms.

Collectively, these findings illustrate how the animals in today's oceans represent just one snapshot of . Over the last half a billion years, diverse morphologies, body plans, and life history strategies have emerged due to varying ecological, environmental, and evolutionary pressures—insights derived from data-rich interdisciplinary studies like this one.

Source: Science China Press

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