Weird Ancient Shrimp from the Cambrian Period Has Dozens of Legs and Dagger Spines

During the Cambrian Period, around 518 million years in the past, a weird, shrimp-like animal flashed over 800 "knives" on its feet; the fierce-looking creature had 54 legs with a maximum of 15 dagger-shaped spines. This was revealed by the new study published in the BMC Evolutionary Biology online journal last June 1.

This ancient scavenger was named Xiaocaris luoi, which has the literal meaning of "Luo's small shrimp." It was a small animal with a length of only two centimeters or an eightieth of an inch. However, according to the study authors, the vicious and probably deadly weapons on its legs could mean that its meals always ended up shredded to pieces. 

Harvard University Organismic And Evolutionary Biology assistant professor, Harvard Museum of Comparative Zoology invertebrate paleontology curator, and study co-lead researcher Javier Ortega-Hernández says that the small animal is quite equipped for scavenging. He says that its limbs suggest that it probably shredded organic matter and soft animals on the seafloor that serve as its food. Food items could include ancient worms.

This shredding predator was discovered when its fossil was found among the Chengjiang biota in Yunnan, China. This area has many fossils that lived during the same Cambrian period. When retired professor Huilin Luo unearthed a chunk of rock during the 1980s, only a single species was found, which was an ancient arthropod named Jianshania furcatus. The new shrimp X. luoi was named after Lao.

In the new study, the researchers scanned the specimen using computed X-ray tomography and discovered the new species. According to co-lead researcher of the study and Yunnan University Key Laboratory for Palaeobiology deputy director and professor Yu Liu, the animal is a relative of our present-day centipedes, spiders, insects, and shrimps. It is different from its modern descendants, however, by having many more legs than the extant arthropods. He says that arthropods typically have one leg pair per segment of the trunk, and millipedes have two. Xiaocaris luoi, on the other hand, possesses four.

Among its other weird characteristics are its stalked eyes and a head shield shaped like a boomerang. It has two slender antennae with 18 segments to sense its surroundings chemically and physically.

Ortega-Hernández says that the animal does not look like any other living animal today. An example of its weirdness is the hard exoskeleton it possesses that gives it protection from predation by the equally-ugly claw-faced sea monster Lyrarapax unguispinus.

It is curious to note that even if the legs of Xiaocaris luoi are not uniformly sized,  all of them have a similar shape, which means that the animal does not have specialized legs for specific behavior or purposes, like mating. Liu says that this simple anatomical design denotes a 'primitive' stage in the evolution of arthropods.

Ortega-Hernández says that the animal nonetheless used its legs to feed itself efficiently and that it probably moved them in a sort of "eternal wave" movement for swimming around and hovering over the seafloor, in the same way, that sea monkeys of today move