Thoracic structure and enrolment style in middle Cambrian Eccaparadoxides pradoanus presages caudalization of the derived trilobite trunk

Abstract

The ability to enrol effectively evolved several times among trilobites. Here, we show that, unlike most redlichiid trilobites that could not enrol, both morphotypes of Eccaparadoxides pradoanus from the middle Cambrian of Spain enrolled so as to enclose most of the ventral surface beneath the exoskeleton and possessed specialized articulating devices that facilitated this behaviour. The holaspid thorax of all E. pradoanus was divided into two principal regions. The boundary between these marked a transition from anterior segments with short pleural spines, fulcra and ridge-and-groove inner pleural regions to posterior segments with longer, acuminate pleural spines that lack fulcra and inner pleural regions. Devices that aid articulation, such as fulcra with short articulating pleural surfaces, the petaloid articulating facet and long articulating half rings, are concentrated in the anterior region. These features, and the large number of specimens preserved in various degrees of enrolment, suggest an enrolment procedure in which the rear part of the trunk, containing both the posterior thorax and the pygidium, rotated as a single unit without significant internal flexure. As these posterior trunk articulations were apparently not required to permit enrolment, concentrating flexure in the anterior may have presaged the caudalized condition seen in many derived trilobite groups that encapsulated, in which a larger proportion of the trunk segments were allocated to the mature pygidium, and therefore unable to articulate.

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

“…complete enrolment without open gaps [6]. The overall exoskeletal configuration in GSC 137152 bears a close resemblance to that observed in encapsulated individuals of the redlichiide Eccaparadoxides pradoanus from the middle Cambrian (Drumian) of Spain [9], and also to rsbl.royalsocietypublishing.org Biol Lett 9: 20130679 completely enrolled specimens of the olenellids Olenellus chiefensis and Olenellus gilberti [12] from the early Cambrian (Upper Dyeran) of Nevada [16] (figure 2a; see electronic supplementary material, figure S1). The second example, an individual of Mummaspis occidens (GSC 137153) [13], comprises the dorsal view of a cephalon plus the articulated first to fifth trunk tergites (figure 1b).…”

“…This interpretation is further supported by the burial pattern that follows the axial convexity of the dorsal exoskeleton, indicating that the appearance of the specimen is not a result of disarticulation or breakage. GSC 137153 shows a similar disposition to ventrally flexed specimens of E. pradoanus [9] and is also comparable to a flexed specimen of the olenellid Nephrolenellus geniculatus from the early Cambrian (Upper Dyeran) of Nevada [18] (figure 2a; see electronic supplementary material, figure S1). …”

“…When completely enrolled, the configuration of the terrace lines on the dorsal and ventral articulating surfaces of adjacent pleurae would face each other at approximately right angles ( figure 2b). 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.…”