Sphaeroidal enrolment in middle Cambrian solenopleuropsine trilobites

Abstract

Fifty specimens belonging to species of Solenopleuropsis and Pardailhania from Spain and France demonstrate sphaeroidal enrolment in Cambrian trilobites for the first time. These solenopleuropsines show novel coaptative structures in different regions of the exoskeleton: in the cephalon there are vincular furrows and notches; in the thorax an articulating facet is developed at the pleural margins, with a ball and socket connection on the adaxial most portion, and an articulating half-ring axially; the pygidium possesses an articulating facet. The interaction of these coaptative structures resulted in a sphaeroidal enrolment that was a progressive act from the first articulation between the occipital ring and the first segment to the pygidial articulating facet. A similar type of sphaeroidal enrolment is observed in the Devonian trilobite Phacops. Both Cambrian and Devonian trilobites developed a vincular furrow in the ventral surface of the cephalon to close their bodies tightly. In both cases, this is probably a convergent adaptation to protect against predators and obrution. Indeed, the enrolled trilobites are very common in obrution deposits restricted to shallow and soft muddy substrates.

Middle Cambrian echinoderms from north Spain show echinoderms diversified earlier in Gondwana

“…All localities have been sampled in detail with trilobites providing biostratigraphical control. One level near the top of the Murero Formation at Purujosa (Moncayo Natural Park, Iberian Chains) (for stratigraphic details see Esteve et al, 2010) contains eight different echinoderms: the cinctan Gyrocystis platessa, eocrinoids (Gogia gondi, a new Lichenoididae, and "eocystitids"), stylophorans (Ceratocystis sp. and two new cothurnocystids), and isorophid edrioasteroids.…”

Pelagic or benthic? Mode of life of the remopleuridid trilobite Hypodicranotus striatulus

“…Through enrolment, some benthic trilobites could entirely encapsulate the non-mineralised parts of their bodies, protecting them against predatory attack (e.g., Esteve et al 2010Esteve et al , 2011Feist et al 2010). We argue that this defensive strategy provided a secondary benefit for trilobites and other arthropods with enrolment or volvation abilities.…”

Enrolment and trunk segmentation of a Cambrian eodiscoid trilobite

“…For instance, in the Purujosa assemblage from the middle Cambrian strata of North Spain, two or more enrolment types are represented within the genera Solenopleuropsis and Conocoryphe (Esteve et al . ). The enrolled specimens of T. yanjiazhaiensis documented herein show that one species could achieve spheroidal enrolment in different ways during its life cycle.…”