The Paleozoic oceans were home to the extinct order Eurypterida, commonly known as sea scorpions. These massive, ancient arthropods dominated aquatic ecosystems for hundreds of millions of years, adapting to marine, brackish, and freshwater environments. Determining what sea scorpions ate is complex, as the order encompassed a vast range of body sizes and ecological roles. Scientists piece together their ancient diets by examining fossilized remains, the structure of their feeding appendages, and trace evidence in the rock record.
Defining the Eurypterids
Eurypterids belonged to the subphylum Chelicerata, making them relatives of modern horseshoe crabs, spiders, and scorpions. They first appeared during the Ordovician Period, approximately 480 million years ago, and persisted until the Permian-Triassic extinction event around 252 million years ago. Their peak diversity occurred during the Silurian Period, where they became significant players in the aquatic food web.
The order exhibited a remarkable size range. While most species measured less than 20 centimeters, some reached gigantic proportions, such as Jaekelopterus rhenaniae, which exceeded 2.5 meters in length, representing the largest arthropod known to have ever lived. All eurypterids shared a body plan featuring a fused head and thorax (the prosoma) and an abdomen (the opisthosoma), often ending in a tail-like telson.
Relevant to their feeding was the presence of six pairs of appendages on the prosoma. The first pair, the chelicerae, were pincer-like claws adapted for grasping and manipulating prey. Their form varied significantly based on diet. Other appendages were often modified into walking legs or large, paddle-like structures used for swimming and controlling movement.
Reconstructing Their Ancient Diet
The diet of sea scorpions varied significantly based on species size, morphology, and environment, ranging from scavengers to apex predators. Smaller eurypterids, such as those in the genus Eurypterus, were generalists that likely scavenged or hunted small, soft-bodied invertebrates. These creatures probably consumed small benthic macroinvertebrates, including worm-like organisms, small crustaceans, and mollusks found on the seafloor.
Some specialized groups within the Stylonurina suborder evolved into “sweep-feeders.” They used comb-like structures on their appendages to rake through the substrate, a strategy that allowed them to capture small organisms ranging from 1.6 to 52 millimeters in size. For example, Cyrtoctenus may have captured mesoplankton directly from the water column or by sifting through sediment.
In contrast, massive Pterygotid eurypterids, including Jaekelopterus and Pterygotus, were active apex predators. These giants hunted larger, heavily armored prey, such as early jawless fish (agnathans) like the armored ostracoderms. They also targeted other large arthropods, including trilobites, and evidence suggests cannibalism occurred among the largest species.
Robust Pterygotids, such as Erettopterus and Pterygotus, likely preyed upon heavily armored fish like thelodonts and osteostracans. Their immense size and powerful appendages allowed them to engage with the increasingly armored fish of the Silurian and Devonian Periods. This variety of prey demonstrates that sea scorpions were ecologically flexible and opportunistic hunters.
Predatory Mechanics and Fossil Evidence
Scientists reconstruct the feeding habits of eurypterids by analyzing specific physical evidence preserved in the fossil record, since direct observation is impossible. One key piece of evidence comes from coprolites, or fossilized feces, which have yielded fragments of fish scales from agnathans and pieces of trilobite exoskeletons. The presence of these identifiable remains directly confirms the consumption of these organisms by eurypterids.
The morphology of the chelicerae provides further insight into their predatory mechanics and the type of prey they could handle. The massive claws of Pterygotids, for instance, had a relatively low mechanical advantage. This suggests they were adapted for rapid grasping and puncturing rather than slow, powerful crushing. These claws were lined with backwardly directed denticles that would have been effective for snaring and slicing struggling or slippery prey, like fish.
Other eurypterid groups, such as Megalograptus and Mixopterus, possessed highly specialized raptorial appendages used for trapping. Three-dimensional kinematic models of these limbs show they could grab, hold, and rip prey, or even form a capture basket to secure an organism. This suggests an ambush hunting style, where the predator would use its paddle-like appendages for slow, controlled movement before rapidly striking.
Trace fossils, known as praedichnia, also offer clues about predatory interactions by showing damage traces on the shells of contemporary organisms. Evidence of such traces on the exoskeletons of trilobites and pteraspid fish suggests direct attacks by sea scorpions. Once captured, the prey was processed near the mouth using robust, spiny structures called coxal gnathobases and a plate-like structure called the metastoma, which helped push food into the digestive tract.