DECAY EXPERIMENTS ON LIGHTLY SCLEROTIZED ARTHROPODS (BRANCHIOPODA, CRUSTACEA) AND THE DEATH OF THE OLDEST ‘LIVING FOSSIL' SPECIES, TRIOPS CANCRIFORMIS

Our perceptions of the arthropod fossil record have been shaped by both exceptionally preserved faunas and taphonomy experiments. The latter still have considerable potential inform how we interpret fossil arthropods. In order to evaluate the effect of taphonomy on specific arthropod characters, fossil notostracans were investigated with parallel decay experiments on their living relatives. These crustaceans are of special interest because of their supposed ‘primitive’ nature, and the status of Triops cancriformis as the oldest living species (with putative occurrences from the Permian). Experiments focused on evaluating the persistence of specific features relevant to the taxonomy of living species: the shape and position of the eyes, dorsal organ and mandibular grooves as well as the morphology of the differentiated first thoracic limbs. Decay and disarticulation were monitored for carcasses and exuviae, while the effect of desiccation on morphology was investigated experimentally.

The dorsal features of the notostracan carapace persisted throughout decay; in fact, after six weeks of decay, a carcass was still largely indistinguishable from an exuvia. Furthermore, the elongate, ‘antennaform’ first thoracic limb, present in almost all living notostracans, remained identifiable. This is in contrast to all known fossil notostracans, where the first thoracic limb is like the rest of the thoracic limbs. This suggests that the presence of an elongate first thoracic appendage may be a useful in defining a monophyletic group of living notostracans including T. cancriformis. The status of T. cancriformis as a living fossil, based on its supposed identity with Permian and Mesozoic fossils, should therefore be reconsidered.