Giant sea scorpion species discovered in New Mexico

Such sea scorpions are exceedingly rare worldwide, scientists say

Scientists have discovered a giant ancient sea scorpion species in New Mexico that lived between 307 and 303 million years ago.

Hibbertopterus lamsdelli was over a metre long and likely lived in a marine-influenced estuary fed by a river delta, according to a new study published in the journal Historical Biology.

It belonged to an extinct group of aquatic arthropod invertebrate animals and likely fed on small crustaceans, invertebrate larvae and gastropod eggs, said scientists, including those from the New Mexico Museum of Natural History in the US.

Such hibbertopterid sea scorpions are exceedingly rare worldwide, with this newly earthed fossil being only the fourth – and “the most reliable” – such specimen reported from the US.

“Hibbertopterus lamsdelli is important because these fossils are so rare,” study co-author Simon Braddy told Sci.News.

“It is also the most reliable American hibbertopterid – this group of giant sweep-feeding eurypterids (sea-scorpions),” Dr Braddy added.

“It is unlikely that they fed on large prey. Instead, they used their anterior appendages to explore the substrate for shallow, infaunal animals such as small crustaceans and worms,” researchers wrote in the study.

While these sea scorpions were generally rare as fossils, researchers said they are locally abundant in American and European sedimentary rock deposits pertaining to this time period.

The fossils studied in the research were unearthed from the Atrasado Formation at Kinney Quarry, Bernalillo County in central New Mexico.

Researchers say the fossil was collected from the top of Kinney Quarry’s bed 3, a 15-16 cm thick, mostly ochre-colored, laminated, bituminous limestone to calcareous siltstone referred to as the “fish bed.”

They said the fossil is preserved as a layer of carbonized cuticle and consists of a part and incomplete counterpart.

The uncovered fossil likely represents a sea scorpion’s molt, a process involving the shedding of its exoskeleton.

The fossil was transported a short distance and had partially slipped out, a sign that the exoskeleton had not completely disarticulated from the body.

“It was just over 1m long and similar to Hibbertopterus scouleri, from Scotland, but with a wider body segment before it’s tail-spine, which itself was shorter, with more parallel keels on the underside.” Dr Braddy said, according to Sci.News.

“These acted like sled rails to reduce body drag when it hauled itself out of the water during seasonal nuptial (mating) walks,” he explained.

Unusual spines found on the sea scorpion’s legs likely acted to help spread the load during their excursions.

“Hibbertopterus had walking legs with spinose extensions at the base (Laden) to spread their load, and the ventral keels on their telson [last abdomen segment] functioned like sled rails to reduce body drag,” scientists wrote in the study.