PENTASTOMID PARASITES FROM THE LOWER PALEOZOIC OF SWEDEN

Small phosphatised fossils from late Cambrian limestones of Vastergotl and, Sweden, share major external features with larval extant Pentasto mida, such as a prominent head with two pairs of stumpy limbs adapted for attachment, and a slender trunk of four portions. Even such detail s, as paired forehead structures, pores on the inner edges of the head limbs and paired papillae at the rear of the trunk correspond with st ructures of extant pentastomid larvae. Neither the fossils nor the Rec ent pentastomids add any additional body segments during growth (segme nt constancy). Since characters of this kind and in this combination d o not occur elsewhere, the fossils are recognised as true Pentastomida . Major differences, such as distinctly divided head limbs, partial oc currence of vestigial trunk limbs, and a different mode of trunk devel opment during growth can be explained as representing merely the plesi omorphic state of characters of Pentastomida, indicating that the foss ils are representatives of its stem-group prior to branching into the two Recent lineages. The fossils clearly document the marine origin of the Pentastomida, and that their specific morphology and parasitic li fe style were already established in the late Cambrian at a high degre e of diversification, long before the terrestrialisation of their pres ent final hosts, the tetrapods. General arthropod affinities are recog nisable not least in the nature of the limbs, but the morphology of st em- and crown-group pentastomids gives no clues for closer relationshi p with any of the major (eu)arthropod taxa.