Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden

The morphology and early ontogeny of Agnostus pisiformis (Linnaeus 1757) is described on the basis of more than a hundred phosphatized, mostly enrolled specimens, etched from nodular limestones of the Upper Cambrian Agnostus pisiformis Zone, Västergötland, Sweden. Gradual, anameric development. with only slight morphological change suggests that most of the recognized structures can be generalized also for later developmental stages up to the adult. The tergal components, cephalic and pygidial shields and two pleurotergites, form a highly symmetrical test, with all rims fitting tightly together when clasped together edge to edge. Complete enclosure of the test may have protected the soft ventral side, while Agnostus swam actively when the two shields were slightly gaping. Locomotion was effected by the strong exopodites of the second and third cephalic appendages. Most probably the environment in which A. pisiformis lived was a flocculent zone at the bottom of the Alum Sea, where the animals swam or floated around, as is also suggested for the other orsten arthropods. Various morphological details of agnostids are simply plesiomorphic, i.e. retained from the groundplan of the Arachnata. Those features which point to affinitites with the polymerid trilobites are most clearly in the exoskeleton and in the ontogeny. The numerous modifications of the morphology of A. pisiformis from the general polymcrid trilobite body plan and features hitherto unknown from trilobites and even other arthropods confirm the separate rank of the Agnostina within the Trilobita.