THE LOWER CAMBRIAN EODISCOID TRILOBITECALODISCUS LOBATUSFROM SWEDEN: MORPHOLOGY, ONTOGENY AND DISTRIBUTION

Abstract

Several thousand disarticulated remains together with a few complete enrolled specimens of the lower Cambrian eodiscoid trilobite Calodiscus lobatus (Hall, 1847) have been collected at two outcrop areas in Sweden. The material reveals new details of morphology and morphogenesis during ontogenetic development. Size-frequency analyses show that the material from the Fånån rivulet in Jämtland, central Sweden, represents a natural population dominated by juveniles, whereas the material from Gislövshammar in Scania, southern Sweden, has been sorted during postmortem transport. Three stages of protaspid development can be traced and defined as well as all subsequent ontogenetic stages for the cephalon, hypostome and pygidium. The early meraspid pygidium has a pronounced larval notch, which persists, though becoming progressively less distinct in later meraspides. The number of axial rings in the transitory pygidium increases throughout meraspid development until a third and final thoracic segment is liberated. During ontogeny the articulating half-rings are strongly developed, and both meraspides and holaspides were capable of full sphaeroidal enrollment and outstretched postures. The hypostome undergoes some dramatic modifications; in M0 the anterior margin is axe-shaped, by M1 the area of attachment greatly decreases and the hypostome becomes more elongated and pear-shaped, before attaining its adult form, which has an overall resemblance to that of polymerid trilobites. During ontogeny, the hypostome changes from a conterminant attachment to a natant condition, thereby mirroring hypostomal evolution within trilobites generally. The morphology, ontogeny, enrollment, hypostomal development and the presence of calcified protaspides suggest polymerid rather than agnostoid affinities of the eodiscoids.

Early Cambrian eodiscoid trilobite Hupeidiscus orientalis from South China: ontogeny and implications for affinities of Mongolitubulus-like sclerites

“…The ontogenetic development (e.g. morphological changes, hypostomal development and the presence of calcified protaspides) provides important evidence for analyzing the systematic position of the eodiscoids, supporting the view that they are related to polymerid trilobites (Cederström et al 2009).…”

“…The holaspid period (H) begins after the third thoracic segment being liberated from the transitory pygidium. The ontogenetic changes of the eodiscoid cranidia were fairly gradual and could not be tracked by such an evident developmental marker as was present throughout the whole growth series of the transitory pygidia (Cederström et al 2009). Scatter diagrams of previous bivariate (length vs width) analyses of eodiscoid cranidia (e.g., Zhang & Clarkson 1993, Cederström et al 2009) show a nearly isometric increase, and there are no distinct clusters of growth stages.…”

“…The ontogenetic changes of the eodiscoid cranidia were fairly gradual and could not be tracked by such an evident developmental marker as was present throughout the whole growth series of the transitory pygidia (Cederström et al 2009). Scatter diagrams of previous bivariate (length vs width) analyses of eodiscoid cranidia (e.g., Zhang & Clarkson 1993, Cederström et al 2009) show a nearly isometric increase, and there are no distinct clusters of growth stages. Thus, bivariate analysis could not help to define growth stages of the cranidium since overlapping size ranges between adjacent instars may be generally a common feature for eodiscoids (Cederström et al 2009).…”

“…Scatter diagrams of previous bivariate (length vs width) analyses of eodiscoid cranidia (e.g., Zhang & Clarkson 1993, Cederström et al 2009) show a nearly isometric increase, and there are no distinct clusters of growth stages. Thus, bivariate analysis could not help to define growth stages of the cranidium since overlapping size ranges between adjacent instars may be generally a common feature for eodiscoids (Cederström et al 2009). Although the cranidium and pygidium of a same individual is nearly equal in size (especially width) in isopygous eodiscoids (Zhang & Clarkson 1993), it is difficult to refer disarticulated cranidia exactly to the corresponding growth stages of the pygidia simply on the basis of size comparison.…”

“…Although the cranidium and pygidium of a same individual is nearly equal in size (especially width) in isopygous eodiscoids (Zhang & Clarkson 1993), it is difficult to refer disarticulated cranidia exactly to the corresponding growth stages of the pygidia simply on the basis of size comparison. However, a series of gradual development changes for the cranidium and discovery of complete enrolled and outstretched specimens (Zhang & Clarkson 1993, Cederström et al 2009) could facilitate allocation of disarticulated cranidia approximately to the growth stage defined by the transitory pygidia.…”