Purujosa trilobite assemblage and the evolution of trilobite enrollment

Abstract

Localities bearing abundant enrolled Cambrian trilobites are extremely rare, but here we report a new and exceptional such assemblage with a high diversity of enrolled specimens from the middle Cambrian of Spain. This novel fi nd demonstrates that encapsulated enrollment was a behavioral response available to many of the early benthic trilobite morphotypes. This result provides a new perspective for reviewing the evolution of trilobite enrollment through the Paleozoic. We fi nd a signifi cant and consistent increase in the proportion of enrolled trilobites within assemblages bearing abundant articulated trilobites. Similarly, the mean number of coaptative devices also increased progressively during the Paleozoic. Such devices likely provided later enrolled trilobites with greater structural integrity, and may support an escalatory model of trilobite defense evolution. Likewise, environmental circumstances favoring the preservation of enrolled trilobites may have become more common during post-Cambrian time.on June 4, 2015 geology.gsapubs.org Downloaded from

Humble origins for a successful strategy: complete enrolment in early Cambrian olenellid trilobites

“…This arrangement duplicates the 'petaloid facets' found on the articulating pleural regions in stratigraphically younger trilobites, a coaptative device that facilitates the correct accommodation and gliding of overlapping pleurae during encapsulation [2,3,7,9,19,20]. The cephalic margins in Mummaspis-as well as other olenellids [11,12,18] and redlichiines [8,9]-bear strong parallel terrace lines that, in the anteriormost region, are arranged perpendicular to the body axis (figure 1d). When completely enrolled, these anterior cephalic ridges would have made direct contact with the terrace lines found on the pleural spines of the posterior tergites (figure 2b), resulting in a functional ratchet between these opposite body regions.…”

“…Such interaction would also have restricted the degree of lateral movement during enrolment, thereby increasing the structural integrity of this configuration against adverse conditions [3,5,7,9,19]. Terrace lines on the cephalon and pygidum that enhance enrolment are known from numerous trilobite groups, but have been documented only in more derived forms with well-caudalized exoskeletons [3,5,[7][8][9]19,20].…”

“…This behavioural strategy generally required precise morphological adaptations, including the presence of trunk tergites with a horizontal hinge near the longitudinal axis (fulcrum), a fused tail shield-the pygidium-equal to or larger in overall size than the cephalic shield (caudalization), and structures that facilitate enrolment and prevent lateral shearing (coaptative devices) [3][4][5][6][7]. Although these features are most commonly found in post-Cambrian-and phylogenetically derived-groups [2,[4][5][6][7], recent studies show that some middle Cambrian Redlichiina were functionally able to enrol [8,9], hinting at earlier origins for this adaptation. However, the early fossil record of polymerid (i.e.…”

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.…”

New and revised occurrences of rhynchonelliformean brachiopods from the middle Cambrian of the Iberian Chains, NE Spain

“…, lingulate brachiopods and the orthacean brachiopods described in this paper. Esteve et al (2011) interpreted this level as consisting of multiple obrution events deposited in off shore shelf environments. The trilobite Solenopleuropsis thorali indicates a lowermost Languedocian age (sensu Álvaro & Vizcaïno 1998) for this level.…”