"Long before it's in the papers"
September 02, 2015


Giant sea scorpion fossils turn up

Sept. 2, 2015
Courtesy of BioMed Central
and World Science staff

Fossils of a pre­vi­ously un­known spe­cies of gi­ant sea scor­pi­on, meas­ur­ing over 1.5 me­ters (5 feet) long, have turned up in Io­wa, re­search­ers re­port.

The animal’s length roughly matches that of an­oth­er an­cient sea scor­pi­on re­ported in 2005, but the new­found fos­sil is es­ti­mat­ed as about a 130 mil­lion years old­er, mak­ing it the old­est known spe­cies of sea scor­pi­on. These mon­sters once prowled the seas and are part of the or­der arach­nids, an ev­o­lu­tion­ary group that in­cludes both scor­pi­ons and spi­ders.

Artist's impression of Pentecopterus. (Courtesy of Patrick Lynch/Yale U.)

The find­ings are de­scribed in the re­search jour­nal BMC Ev­o­lu­tion­ary Bi­ol­o­gy.

The au­thors named the new spe­cies Pen­te­copterus dec­o­ra­hens af­ter the “pen­te­con­ter”—an an­cient Greek war­ship that the beast re­sem­bles in out­line and par­al­lels in its pred­a­to­ry be­hav­ior.

The an­i­mal is “in­credibly bi­zarre. The shape of the pad­dle—the leg which it would use to swim—is un­ique, as is the shape of the head,” said lead au­thor James Lams­dell of Yale Un­ivers­ity. 

“Per­haps most sur­pris­ing is the fan­tas­tic way it is pre­served,” he added.

The hard out­er co­vering, or ex­o­skel­e­ton, “is com­pressed on the rock but can be peeled off and stud­ied un­der a mi­cro­scope. This shows an amaz­ing amount of de­tail, such as the pat­terns of small hairs on the legs. At times it seems like you are stu­dy­ing the shed skin of a mod­ern an­i­mal.”

The spe­cies is rep­re­sented by more than 150 fos­sil frag­ments, he said dug out from the up­per lay­er of the Win­neshiek Shale in north­east­ern Io­wa—a 27-meter thick sandy shale lo­cat­ed with­in an an­cient me­te­or­ite im­pact crat­er and mostly sub­merged by the Up­per Io­wa Riv­er.

Some large body seg­ments sug­gest a to­tal length of up to 1.7 me­ters, mak­ing Pen­te­copterus the larg­est known eu­ryp­ter­id from its era, he added. The beast is es­ti­mat­ed as 460 mil­lion years old, mak­ing it ten mil­lion years old­er than the pre­vi­ous old­est rec­ord of the eu­ryp­ter­id group.

The age would put it with­in the so-called Or­do­vi­ci­an Pe­ri­od, which al­so saw the first ev­i­dence of plants and an­i­mals col­o­niz­ing land.

Some fea­tures of Pen­te­copterus hint at the func­tions of cer­tain body parts, the au­thors said. The rear­most limbs in­clude a pad­dle with a large ar­ea, and joints that seem to be locked in place to re­duce flex, sug­gesting they were used to swim or dig.

The sec­ond and third pairs of limbs may have been an­gled for­ward, sug­gesting they were in­volved mainly in prey cap­ture rath­er than lo­co­mo­tion, they added. The three rear­most pairs of limbs are shorter than the front pairs, sug­gesting that Pen­te­copterus may have walked on six legs rath­er than eight.

The ex­o­skel­e­ton’s great pre­serva­t­ion al­so helped the re­search­ers to in­ter­pret the role of fin­er struc­tures, in­clud­ing scales, fol­li­cles and stiff bris­tles called se­tae that covered the rear­most limbs. The setae form ar­range­ments si­m­i­lar to those in swim­ming crabs, where they func­tion to ex­pand the sur­face ar­ea of the pad­dle dur­ing swim­ming. But the smaller fol­li­cle size in sea scor­pi­ons, al­so called eu­ryp­ter­ids, sug­gests that the se­tae could have had a sen­so­ry func­tion, the au­thors said.

Some limbs al­so have spines described as similar to those found on horse­shoe crabs, where they aid in pro­cess­ing food.

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The fossil of a previously unknown species of giant sea scorpion, measuring over 1.5 meters (5 feet) long, has turned up in Iowa, researchers report. Its length roughly matches that of another ancient sea scorpion reported in 2005, but the newfound fossil is estimated as about a 130 million years older, making it the oldest known species of sea scorpion. These monster-like predators swam the seas in ancient times and are part of the order arachnids, an evolutionary group that includes both scorpions and spiders. The findings are described in the open access journal BMC Evolutionary Biology. The authors named the new species Pentecopterus decorahensis after the “penteconter” -an ancient Greek warship that the beast resembles in outline and parallels in its predatory behavior. The animal is “incredibly bizarre. The shape of the paddle-the leg which it would use to swim-is unique, as is the shape of the head,” said lead author James Lamsdell of Yale University. “Perhaps most surprising is the fantastic way it is preserved,” he added. The hard outer covering, or exoskeleton, “is compressed on the rock but can be peeled off and studied under a microscope. This shows an amazing amount of detail, such as the patterns of small hairs on the legs. At times it seems like you are studying the shed skin of a modern animal.” The species is represented by more than 150 fossil fragments, he said dug out from the upper layer of the Winneshiek Shale in northeastern Iowa—a 27-meter thick sandy shale located within an ancient meteorite impact crater and mostly submerged by the Upper Iowa River. Some large body segments suggest a total length of up to 1.7 meters, making Pentecopterus the largest known eurypterid from its era, he added. Pentecopterus is estimated as 460 million years old, making it ten million years older than the previous oldest record of the eurypterid group. The age would put it within the so-called Ordovician Period, which also saw the first evidence of plants and animals colonizing land. Some features of Pentecopterus hint at the functions of certain body parts, the authors said. The rearmost limbs include a paddle with a large area, and joints that seem to be locked in place to reduce flex, suggesting they were used to swim or dig. The second and third pairs of limbs may have been angled forward, suggesting they were involved mainly in prey capture rather than locomotion, they added. The three rearmost pairs of limbs are shorter than the front pairs, suggesting that Pentecopterus may have walked on six legs rather than eight. The exceptional preservation of the exoskeleton also helped the researchers to interpret the role of finer structures, including scales, follicles and stiff bristles called setae. The rearmost limbs were covered in dense setae. These form arrangements similar to swimming crabs, where they function to expand the surface area of the paddle during swimming. But the smaller follicle size in sea scorpions, also called eurypterids, suggests that the setae could have had a sensory function, the authors said. Some limbs also have spines that look like those found on horseshoe crabs, where they aid in processing food.