Paleontology: Oldest representative of a weird arthropod group

Before they sank to the bottom of their shallow marine habitat and were fossilized some 435 million years ago, these arthropods preyed on other denizens of the Silurian seas -- although they were not exactly inconspicuous, possessing a bivalved carapace and multiple abdominal limbs. A group of researchers including LMU biologist Carolin Haug recently recognized these fossils as the oldest representatives yet discovered of an enigmatic and now extinct class of arthropods known as Thylacocephala, and assigned them to the new species Thylacares brandonensis. "Where exactly the thylacocephalans belong among the arthropods is still a matter of intense debate," Haug says, but the new specimens shed light on the phylogenetic affinities of this problematic group of animals.

According to the authors of the study just published, certain aspects of the anatomy of T. brandonensis, together with the results of a detailed investigation of more recent specimens attributable to the group, support the hypothesis that the thylocephalans belong among the crustaceans. Moreover, the anatomy of their posteror appendages and an analysis of the organization of their muscles with the aid of fluorescence microscopy strongly suggest that they can be interpreted as a sister group of the so-called Remipedia. Remipedians, which were first described in the 1980s, are blind crustaceans found in flooded limestone caves in coastal settings in the tropics. These cave systems are typically connected to the sea via subsurface channels and are also open to the surface further inland.

"The main reason why it has been so difficult to work out the precise systematic position of thylacocephalans is that their morphology is so bizarre," says Haug. "For a long time, researchers couldn't even agree which end was anterior and which posterior." Most of the specimens so far described come from strata of Jurassic age, and are therefore 200-250 million years younger than the new species. Representatives of the group typically have unusually large compound eyes and are equipped with several paired and anteriorly located raptorial appendages, which are almost as long as the rest of the animal. This combination of characters strongly suggests that they were adapted to a predatory lifestyle. "Actually the eyes were initially not recognized as such, and instead were interpreted as stomach pouches by some researchers," says Haug. By comparison with its spectacular descendants, the new species T. brandonensis can be described as modest and unassuming. "Representatives of this thylacocephalan species have a more 'normal' morphology," says Haug, "their eyes are smaller and the raptorial appendages are shorter."

The authors of the new study therefore conclude that, like more recent representatives of the group, T. brandonensis also earned its living as a predator, but was less highly specialized than the later forms. Consequently, the morphological specializations seen in the latter probably emerged in the course of the further evolution of the class. "It is quite possible that the extreme degree of specialization seen in specimens from the Jurassic proved to be an evolutionary dead end," Haug suggests, "for at the close of the Cretaceous, at a time when many other groups of animals disappeared from the fossil record, Thylacocephala also became extinct." However, we now know that these predators had previously enjoyed a successful career that lasted for more than 350 million years.