Articulated trilobite ontogeny: suggestions for a methodological standard

Abstract

In order to maximize the utility of future studies of trilobite ontogeny, we propose a set of standard practices that relate to the collection, nomenclature, description, depiction, and interpretation of ontogenetic series inferred from articulated specimens belonging to individual species. In some cases, these suggestions may also apply to ontogenetic studies of other fossilized taxa.

Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis

“…Preliminary data inspection showed that lnCCS does not exhibit the kind of multimodal distribution that would allow reliable stage assignments based on size clustering, because, especially in later ontogeny, within-stage size distributions are apparently largely overlapping (Hunt and Chapman 2001;Webster 2015).…”

“…Preliminary Assessment for Staging.-In arthropods, absolute growth dynamics and segmentation are developmental descriptions based on the ontogenetic succession of stages (or instars). Specimen stage assignment is therefore a critical step in any ontogenetic analysis of absolute growth, and the choice of a given staging criterion needs to be supported by evidence (Hughes et al 2020).…”

“…The study of the development of fossilized organisms can elucidate both the state of developmental characters at ancient phylogenetic nodes and their subsequent evolution, and also permits exploration of how mechanisms controlling developmental processes have themselves evolved (Hughes et al 2017: p. 11). Extracting reliable developmental information from the fossil record requires that the data meet stringent standards (Hughes et al 2020). Rarely, specimens of articulated trilobites that collectively span a wide size range are well preserved in large numbers within a short stratigraphic interval.…”

Development of the early Cambrian oryctocephalid trilobiteOryctocarella duyunensisfrom western Hunan, China

“…Herein we adopt suggestions for a methodological standard in the description of articulated trilobite ontogeny as recommended by Hughes et al (2020). The fossils were photographed with a Canon 5Ds Digital SLR camera equipped with a Canon EF-S 60 mm 1:2.8 macro lens, in lower-angle lighting from the northwest direction and higher-angle lighting from the northeast direction, or, for the specimens smaller than 5 mm in length, with a Leica M205C stereomicroscope with a Planapo 1.0X lens, and the associated Leica Application Suite v. 4.10 software.…”

“…Due both to the fine scale of bedding in these deposits, which makes tracing an individual bedding surface along strike difficult, and the fact that many beds contain fossils, to date many collections made for ontogenetic analysis of Balang Formation trilobites have paid limited attention to recording exactly where in the section specimens studied originated. This limits our ability to infer possible controls on patterns of variation witnessed within the sample (see Hughes et al, 2020). For example, several studies have recognized different meraspid “morphs” of the same degree based on different numbers of pygidial axial rings (e.g., Dai et al, 2017; Du et al, 2020), but we cannot determine if these occurred at all or only some of the stratigraphic levels sampled.…”