Ontogeny and evolution of the Early Cambrian trilobite genus Nephrolenellus (Olenelloidea)

Abstract

The cephalon of Nephrolenellus multinodus and its stratigraphically higher sister-taxon N. geniculatus passed through the same four successive phases of development: entry into phase 2 is defined by a change from a decrease to an increase in the dynamic pattern of distance between the intergenal spine bases relative to cephalic length; entry into phase 3 is defined by the appearance of genal spines; and entry into phase 4 is defined by the effective isolation of glabellar furrow S3 from the axial furrow. Phase transitions were associated with significant changes in allometric growth patterns of the cephalon. Five instars are identified within the early development (phases 1 and 2) of N. multinodus. Despite the general similarity in cephalic ontogeny, significant interspecific differences in patterns of shape change are documented throughout phases 1, 2, and 3 of cephalic development which, with differences in rates of glabellar shape change relative to size (higher in N. multinodus) and ontogenetic loss of glabellar axial nodes (higher in N. geniculates), demonstrate that evolutionary modification to ontogeny was mosaic and complex. Stratigraphic occurrences and a temporal trend towards increased rate of glabellar axial node loss relative to size in N. geniculates are consistent with a hypothesis that N. geniculates was a direct descendant of N. multinodus. However, ontogenetic data are needed for immediate outgroups of Nephrolenellus to determine whether two potential autapomorphies of N. multinodus, which refute such a hypothesis, are indeed unique to this taxon. Nephrolenellus jasperensis is recognized as a junior synonym of N. multinodus.

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

“…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). …”

“…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.…”

“…The identification of complete-but non-encapsulating-enrolment in olenellids illuminates the polarity of the morphological features associated with this strategy (figure 2c). Mummaspis and Olenellus [12,18] demonstrate that some micropygous trilobites were able to enrol, and thus this behaviour significantly precedes the general trend to increase body caudalization observed in younger representatives with derived isopygous bodies [1][2][3][4][5][6][7]10,19,20]. The presence of differentiated exoskeletal sculpture on the articulating surfaces of olenellids-including the petaloid facets on the trunk pleurae (figure 1c) and shearresistant structures on the cephalon and posterior spines (figure 1d)-also precedes caudalization.…”

Evolutionary Lability of Integration in Cambrian Ptychoparioid Trilobites

“…A hypothesis for integration based on Hox expression domains cannot be applied to the whole cranidium because we presently lack complete knowledge of the lateral course of segments away from the glabella. However, ongoing study of the ontogeny of olenelloid trilobites, that retain some traces of segmentation in the abaxial portion of the cephalon during early ontogeny, may help resolve this issue (e.g., Webster 2007). Similarly, understanding of the controls over cuticle calcification in arthropods is improving (e.g., Priester et al 2005;Shafer et al 2006;Inoue et al 2008).…”

Buenellus chilhoweensisn. sp. from the Murray Shale (lower Cambrian Chilhowee Group) of Tennessee, the oldest known trilobite from the Iapetan margin of Laurentia

“…Terminology.-The morphological terminology applied herein largely follows that of Palmer and Repina (1993) and Whittington and Kelly in Whittington et al (1997), with modifications to olenelline terminology proposed by Webster (2007aWebster ( , b, 2009) and Webster and Bohach (2014).…”