Horseshoe crab and Cambrian arthropod

Crushing It Through Time: How Ancient Arthropods Mastered Shell-Crushing Like Modern Horseshoe Crabs

Reese Marlowe in Science & Nature August 2025 • 5 min read.

"New research reveals surprising similarities in the shell-crushing abilities of ancient arthropods and modern horseshoe crabs, shedding light on the evolution of predatory techniques and the Cambrian "arms race.""

Modern predatory arthropods exhibit a remarkable diversity of feeding strategies and specialized structures for capturing and consuming prey. Among the earliest examples of these adaptations are gnathobases, tooth-like projections found on the appendages of fossil arthropods. These structures suggest that even the earliest arthropods were equipped for complex feeding behaviors, but only one extant group still rocks these unique sets of 'teeth': horseshoe crabs. Their unique chewing method makes them interesting study subjects.

Limulus polyphemus, the American horseshoe crab, stands out as a living relic with gnathobases on its walking appendages. While its modern biology is well-understood, including its fondness for crushing shells, the feeding mechanics of this species have remained largely a mystery. Intriguingly, L. polyphemus is often considered an analogue of extinct arthropods with similar gnathobases, such as eurypterids and Cambrian species like Sidneyia inexpectans. But until now, no one has ever done a side-by-side comparison.

Now, researchers have applied advanced computational techniques to model the feeding mechanics of both L. polyphemus and S. inexpectans. By using 3D finite-element analysis (FEA), scientists were able to compare the shell-crushing capabilities of these creatures, separated by over 500 million years of evolution. The results offer insights into the feeding ecology of Cambrian arthropods and the early evolution of predatory behaviors.

Decoding the Ancient Bite: How the Study Was Conducted

To compare the feeding mechanics of Limulus polyphemus and Sidneyia inexpectans, researchers created detailed 3D models of their feeding appendages and applied finite element analysis (FEA). FEA is a computational method used to predict how a structure will respond to various forces and stresses. In this case, it allowed the scientists to simulate the act of shell-crushing and observe the resulting strain and stress distributions in the arthropods' feeding structures.

Here's a breakdown of the process:

Modern Horseshoe Crab Model: A female L. polyphemus specimen was scanned using micro-CT to create a detailed 3D reconstruction of its cephalothoracic appendage set V, which includes the gnathobases used for crushing.
Muscle Simulation: The researchers digitally dissected the muscles involved in mastication and calculated their cross-sectional area (MCSA) from the CT scans. MCSA values were then converted into maximum muscle force estimates to inform the FEA model.
Finite Element Analysis: The 3D model of the horseshoe crab coxa (the segment bearing the gnathobases) was imported into FEA software. Material properties were assigned based on known values for sclerotized cuticle, and muscle forces were applied to simulate adduction of the coxae. The gnathobases were constrained to mimic the contact with prey during crushing.
Validation: A live horseshoe crab was induced to process food while a force-sensitive resistor measured its bite force. This real-world data was compared to the reaction forces predicted by the FEA model to ensure the accuracy of the simulation.
Fossil Arthropod Model: Due to the nature of fossil preservation, a 3D model of Sidneyia inexpectans was digitally reconstructed based on published images and morphological details from various specimens. The model was scaled to match the volume of the horseshoe crab coxa.
Comparative Analysis: The same FEA parameters and constraints used for the horseshoe crab model were applied to the S. inexpectans model. Von Mises microstrain distributions were compared between the two models, and statistical tests were used to assess the similarity of strain patterns along the appendages.

By comparing the microstrain patterns and magnitudes in the two models, the researchers could infer how effectively Sidneyia inexpectans processed food compared to its modern analogue. Pretty cool, right?

A 500-Million-Year-Old Solution

This research demonstrates a remarkable functional similarity in the feeding apparatus of Sidneyia inexpectans and Limulus polyphemus, separated by vast spans of geological time. The findings suggest that shell-crushing capabilities evolved early in arthropod history, playing a role in the Cambrian explosion and the subsequent diversification of marine life. The arrival of durophagous predators likely fueled an evolutionary "arms race," driving the development of more robust shells and other defenses in prey species. By combining paleontology, biomechanics, and computational modeling, this study sheds new light on the ancient origins of ecological interactions that continue to shape our world today. Now that’s what I call interdisciplinary.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1098/rspb.2018.1935, Alternate LINK

Title: Computational Biomechanical Analyses Demonstrate Similar Shell-Crushing Abilities In Modern And Ancient Arthropods

Subject: General Agricultural and Biological Sciences

Journal: Proceedings of the Royal Society B: Biological Sciences

Publisher: The Royal Society

Authors: Russell D. C. Bicknell, Justin A. Ledogar, Stephen Wroe, Benjamin C. Gutzler, Winsor H. Watson, John R. Paterson

Published: 2018-10-24

Everything You Need To Know

What are gnathobases and why are they important in understanding arthropod feeding mechanisms?

Gnathobases are tooth-like projections found on the appendages of fossil arthropods. These structures are significant because they suggest that even the earliest arthropods were equipped for complex feeding behaviors. Interestingly, horseshoe crabs are the only extant group that still possesses and utilizes these unique sets of 'teeth'. Their chewing method, employing these gnathobases, makes them an interesting study subject for understanding the evolution of feeding mechanisms in arthropods.

How did researchers compare the feeding mechanics of Limulus polyphemus and Sidneyia inexpectans in this study?

The study compared the feeding mechanics of the modern horseshoe crab, Limulus polyphemus, and the ancient arthropod, Sidneyia inexpectans. Researchers created detailed 3D models of their feeding appendages and applied finite element analysis (FEA) to simulate shell-crushing and observe stress distributions. For Limulus polyphemus, they used micro-CT scans and muscle simulations, while for Sidneyia inexpectans, they digitally reconstructed the model from published images. This allowed for a direct comparison of their shell-crushing capabilities despite being separated by millions of years.

Can you explain what finite element analysis (FEA) is and how it was used to study shell-crushing in ancient arthropods?

Finite element analysis (FEA) is a computational method used to predict how a structure will respond to various forces and stresses. In the context of this research, FEA was used to simulate the act of shell-crushing by Limulus polyphemus and Sidneyia inexpectans. By applying FEA to the 3D models of their feeding appendages, scientists could observe the resulting strain and stress distributions, allowing them to compare the effectiveness of their shell-crushing capabilities. This approach helped in understanding the biomechanics of feeding in these arthropods.

What are the broader implications of this research for understanding the evolution of predatory behaviors and marine ecosystems?

This research provides insights into the early evolution of predatory behaviors and the ecological dynamics of ancient marine ecosystems. The discovery of functional similarity in the feeding apparatus of Sidneyia inexpectans and Limulus polyphemus suggests that shell-crushing capabilities evolved early in arthropod history. The arrival of durophagous predators like these arthropods likely fueled an evolutionary "arms race", driving the development of more robust shells and other defenses in prey species, thus shaping the biodiversity and ecological interactions we see today.

What aspects of the feeding process were not included in the models, and how could future research expand on this study?

The models focused primarily on the gnathobases and cephalothoracic appendage set V of Limulus polyphemus, and similar structures digitally reconstructed for Sidneyia inexpectans. A more comprehensive analysis might include the entire feeding apparatus, incorporating the musculature and skeletal structure, to improve the accuracy of the simulation. Additionally, factors such as the variety of prey, environmental conditions, and individual variations within species could further refine the models and provide a more holistic understanding of their feeding mechanics.