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Tracing the origin of the planktic graptolites (Hemichordata, Pterobranchia) through the Furongian Gap (Upper Cambrian)

Applicant Dr. Jörg Maletz
Subject Area Geology
Term from 2022 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 495780956
 
Final Report Year 2025

Final Report Abstract

The Furongian represents a late Cambrian time interval during which considerable changes in the evolution of many groups of organisms took place, including the origin of the planktic Graptoloidea (Pterobranchia, Graptolithina) from their benthic ancestors. It appears that planktic graptoloids suddenly originated at the base of the Ordovician and little is known about their early evolutionary changes during this time interval. This dramatic change in the graptolites led the Graptolithina to the exploration of a completely new environment, the water column of the wide oceans of the planet. Here, they were apparently the first macroplanktonic organisms and did not have any competition. However, the so-called 'Furongian gap' in the upper Cambrian prevented the understanding of the evolutionary changes of the graptolites in the middle to late Cambrian and their diversification, leading to their exploration of the planktic life style. Thus, the Guole Biota of South China provided one of the very few insights into the evolution of the graptolites during the 'Furongian gap' interval. Here, the diverse fauna of benthic graptolites was found, that together with the earliest planktic graptolites in the Xiaoyangqiao section at Dayangcha, North China and in the Green Point section of western Newfoundland, the stratotype of the Cambrian-Ordovician boundary, provided insight into the evolutionary changes necessary for this event. The origin of planktotrophy in the graptolites and the associated modifications in their tubarium formation led to the earliest macro-plankton populating the expanses of the world’s oceans, before nektic organisms and plankton feeders outcompeted these organisms in the early Devonian. Further information from graptolite fauna of Miaolingian to basal Furongian ages from North China and from the Marjum Formation of Utah (USA) led to a better understanding for the evolutionary connection between Cambrian benthic graptolites and their extant descendants (Rhabdopleura, Cephalodiscus). Especially, the revision of late Cambrian faunas from the North China region - described more than 60 years ago - is leading to a better understanding of their tubarium construction and a considerable revision of these faunas, based on the development of bithecate forms. The evolution of the 'nematophorous sicula' as the most important taxonomic indication of the planktic life style of the graptoloids can be supported, even though earliest possible planktic taxa (Rhabdinopora proparabola) is provided with a 'float structure' and is lacking the nema, that appears only slightly later in the evolution of the clade.

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