Raptorial appendages of the Cambrian apex predatorAnomalocaris canadensisare built for soft prey and speed

Abstract

The stem-group euarthropod Anomalocaris canadensis is one of the largest Cambrian animals and is often considered the quintessential apex predator of its time. This radiodont is commonly interpreted as a demersal hunter, responsible for inflicting injuries seen in benthic trilobites. However, controversy surrounds the ability of A. canadensis to use its spinose frontal appendages to masticate or even manipulate biomineralized prey. Here, we apply a new integrative computational approach, combining three-dimensional digital modelling, kinematics, finite-element analysis (FEA) and computational fluid dynamics (CFD) to rigorously analyse an A. canadensis feeding appendage and test its morphofunctional limits. These models corroborate a raptorial function, but expose inconsistencies with a capacity for durophagy. In particular, FEA results show that certain parts of the appendage would have experienced high degrees of plastic deformation, especially at the endites, the points of impact with prey. The CFD results demonstrate that outstretched appendages produced low drag and hence represented the optimal orientation for speed, permitting acceleration bursts to capture prey. These data, when combined with evidence regarding the functional morphology of its oral cone, eyes, body flaps and tail fan, suggest that A. canadensis was an agile nektonic predator that fed on soft-bodied animals swimming in a well-lit water column above the benthos. The lifestyle of A. canadensis and that of other radiodonts, including plausible durophages, suggests that niche partitioning across this clade influenced the dynamics of Cambrian food webs, impacting on a diverse array of organisms at different sizes, tiers and trophic levels.

Tubule system of earliest shells as a defense against increasing microbial attacks

“… 60 A recent interesting study further suggested that radiodont euarthropods (despite being a raptorial predator of the Cambrian communities) were seemingly incapable of crushing biomineralized prey using their spinose frontal appendages. 61 These observations suggest that the impact of animal predation on the evolution of shell-bearing mollusks may have been minimal during the initial diversification of molluskan shell biomineralization in the Cambrian Terreneuvian, though predation pressure was most frequently discussed as a driving force for biomineralization.…”

The “elongate chelicera problem”: A virtual approach in an extinct pterygotid sea scorpion from a 3D kinematic point of view

“…For instance, in permutation 5 (Figures 4c,d and 6c,d), this might have been hydro‐dynamically advantageous during swimming. Given the prominence of these large structures, it is plausible that pterygotids somehow folded them ventrally/backward when exhibiting agility (see discussion in Bicknell, Schmidt, et al., 2023). Furthermore, regardless of being euthygnathous or klinognathous, complete flexion of joint 1 and joint 2 would have enabled the chela to approach very close to the oral region for further masticatory work of the gnathobases.…”

The significance of Anomalocaris and other Radiodonta for understanding paleoecology and evolution during the Cambrian explosion

“…Various quantitative methods have been employed to analyze the paleoecology of Radiodonta, including hydrodynamic calculations (Usami, 2006), 3D modeling of appendages and their movement (De Vivo et al, 2021;Bicknell et al, 2023), finite element analysis (Bicknell et al, 2023), computational fluid dynamics (Bicknell et al, 2023), and geometric morphometrics of cephalic carapaces (Daley et al, 2013a;Caron and Moysiuk, 2021). The morphology of the frontal appendages allows three main predatory groups to be distinguished: raptorial predators, sediment sifters and suspension-feeders (Daley and Budd, 2010;Vinther et al, 2014;Van Roy et al, 2015;Guo et al, 2019).…”

“…The raptorial predation mode was used by members of Anomalocarididae and Amplectobeluidae (Daley and Budd, 2010;Daley et al, 2013b). The appendages of Anomalocaris canadensis (Figure 4F) are many-segmented and well-articulated with short endites, enabling them to be mobile, flexible and dexterous, and making them efficient for catching macro-scale prey ranging in size from 20 to 50 mm (Daley and Budd, 2010;De Vivo et al, 2021;Bicknell et al, 2023). The pincer-shape appendage of Amplectobelua (Figure 4G) had a low degree of flexibility but could still grasp prey of relatively small sizes at around 20 mm (De Vivo et al, 2021).…”

“…No radiodonts, even these raptorial taxa, had the ability to regularly consume prey with mineralised exoskeletons, as suggested by Nedin (1999), who envisioned Anomalocaris could use a flexing motion to consume mineralised trilobites. Computational modelling has shown that such motions are not possible with Anomalocaris appendages (De Vivo et al, 2021), nor was their sclerotised cuticle capable of tearing a mineralised exoskeleton (Bicknell et al, 2023). Damages preserved in trilobite and nektaspid exoskeletons (Conway Morris, 1985;Nedin, 1999) and large coprolites found in Cambrian lagerstätten that contain fragments of trilobites (Nedin, 1999;Vannier and Chen, 2005) had previously been attributed to durophagous predation by radiodonts (Nedin, 1999;Klug et al, 2017;Vinn, 2018;Zong, 2021;Zhang et al, 2023).…”