Using Constrained Optimization (CONOP) to examine Ordovician graptolite distribution and richness from the Central Andean Basin and their comparison with additional data from North America and Baltoscandia

Abstract

Ordovician graptolite fossils represent one of the first abundant, well-documented records of marine zooplankton. An accurate assessment of their stratigraphic ranges, biogeography, and taxon richness is crucial to studies on the Great Ordovician Biodiversity Event. Unfortunately, our understanding of graptolite richness from high paleo-latitudes is poor and recent work has demonstrated that many high paleo-latitude taxa are often misidentified endemic species. Hence, high paleo-latitude localities likely contain a hidden graptolite richness. This work uses CONOP9 to construct a range chart and correlation model from the graptolite occurrences at thirty-eight Ordovician stratigraphic sections from the Central Andean Basin of South America, and parts of Laurentia and Baltoscandia. Thirteen graptolite biozones spanning the early Tremadocian (Tr1)–middle Dapingian interval (Dp2) are identified. Some taxa in the Central Andean Basin composite exhibit unusual ranges and occur in biostratigraphically unexpected positions in the composite range chart. In order to examine the possible causes of these unusual ranges, sections from Baltica and Laurentia were added to the analyses. These additional data facilitated results that were more consistent with the generally accepted positions of certain taxon. The first raw taxon richness curve of graptolites from the Central Andean Basin obtained with CONOP9 shows a diversification phase in the early Floian that continues to a peak in the late early Floian. A second peak occurs at the beginning of the middle Floian. A decreasing trend is observed from the top of the Didymograptellus bifidus Biozone to the “Isograptus victoriae” Biozone. These results generally agree with the graptolite richness curves published for the same interval in Baltica, Australasia, and the Yangtze region of SW China, but differs from the patterns observed for the Jiagnan and Yiyang regions of SE China. The rarified regional taxon richness curve, obtained with CONOP64, is also in agreement with global curves previously proposed by different authors.