A revised ontogeny of the early Ordovician trilobite Leptoplastides salteri (Callaway, 1877)

Abstract

ABSTRACTThe ontogeny of the pelturine olenid trilobite Leptoplastides salteri (Callaway, 1877) from the Shineton Shales, Shropshire, England, was first described in 1925 by Frank Raw. Since that time, scanning electron microscopy and other new technologies have revealed many more details of structure, of early developmental stages in particular, than were available to Raw. Whereas protaspides are not preserved and the state of preservation is less than perfect for the smallest meraspides, we have established that the latter had an array of delicate, long thoracic and pygidial spines, as well as paired procranidial spines, which disappear by meraspid degree 8. Raw's reconstructions of early meraspides, and his measurements of the early stages in development, are here amended in the light of new information. Dorsal spines in the adult are much more highly developed than have been documented in any other olenid. The hypostome is preserved in place in several specimens. Initially conterminant (attached to the doublure), it becomes natant (free) in late meraspid to early holaspid stages of development, with its anterior contour fitting exactly to that of the glabella. The ecology of the widespread Leptoplastides is best known from very extensive sections in South America, which provide a useful basis for comparison. It was well adapted to a range of environments, both oxygenated and dysoxic, and is usually the dominant taxon in the biofacies in which it is found.