Carnivorous Crustacean was Ancient Relative of this Eyeball with Legs

Artistic reconstruction of the general bodyplan of the Thylacocephalan species Clausocaris lithographica from about 150 million years ago / Haug et al., BMC Evolutionary Biology 2014
Janet Fang 25 Aug 2014, 23:30

The extinct, carnivorous crustacean Thylacares brandonensis roamed the seas 435 million years ago, grabbing its prey with spiny limbs and crushing it to bits before feasting. The new species was described in BMC Evolutionary Biology this week.

Named after the Brandon Bridge Formation near Waukesha, Wisconsin, where it was discovered, T. brandonesis is the oldest example of the enigmatic Thylacocephala group (a member of the extinct clan is pictured above). These sea critters were mostly found in the Jurassic. Some thylacocephalans are considered so bizarre, scientists confused them with barnacles and shrimp larvae, Science explains

The eyeball with legs pictured above is a Jurassic thylacocephalan from about 150 million years ago called Clausocaris lithographica, found in Solnhofen, Germany. The general thylacocephalan body plan consists of a large shield-like carapace covering most of the body: Its trunk is only visible at the rear, and long raptorial appendages stretch out from beneath. These features suggest that they were either mobile predators, swimming in search of prey, or ambush predators who lurked under cover, grabbing prey with their long claws. 

"T. brandonensis was probably an actively hunting predator, which caught the prey with its front claws and crushed it into smaller pieces with the protrusions nearer its mouthparts,” says Carolin Haug from LMU Munich.

The fossils of this newly discovered species push this group of arthropods back to the Silurian period hundreds of millions of years earlier. Many key Thylacocephala characteristics are found in T. brandonesis, especially the large shield covering the body, though it had plenty of its own peculiar features. Its bulbous eyes are small and stalked, unlike the large, immobile compound eyes of other thylacocephalans. Also, the characteristic appendages are present and structurally similar, but they’re much smaller than those of the Jurassic species.

"This early, Silurian, example of Thylacocephala is in many ways much less extreme than the more recent Jurassic species,” Haug explains in a news release. “It still has normal-sized eyes in contrast to the very enlarged ones that came later, and shorter front claws in T. brandonensis compared to the extremely elongated ones in more recent Jurassic representatives." 

Furthermore, the muscle structure and leg morphology of the new species suggests that it used its claw-like appendages to catch prey in a similar way to modern remipedes -- blind, centipede-looking crustaceans that can be found swimming in saltwater-filled caves today. An example of Remipedia is pictured below: Speleonectes tanumekes is possibly the closest living relatives of the thylacocephalans. 

[Via BioMed Central]

Images: Haug et al., BMC Evolutionary Biology 2014 (top, middle), van der Ham et al. 2011, CC2.5 (bottom) via BioMed Central

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