Caught in the Cambrian Explosion: The mystery of the early evolution of the Pterobranchs (Hemichordata, incl. Graptolithina) and their evolutionary innovations
Final Report Abstract
Even though the Pterobranchia (Cephalodiscida and Graptolithina) secrete a highly durable housing construction or domicile (the tubarium), their fossil record is largely restricted to the tubaria of the planktic Graptoloidea, while the fossil record of their sister-group, the Enteropneusta, is close to non-existent. Pterobranchs are present until today in a few inconspicuous taxa (Atubaria, Cephalodiscus, Rhabdopleura), but during the Ordovician Biodiversification Interval the planktic Graptoloidea were among the most highly evolved and diverse colonial organisms in the marine realm with an origin deep in the time of the Cambrian diversification (or Cambrian explosion) interval. The oldest definitive pterobranch fossils include Sokoloviina costata from the Rovno Horizon of Ukraine of basal Cambrian (Fortunian) age. While Sphenoecium from the Cambrian Series 2, Stage 4 definitely represents a colonial organism, older material identified as ‘pterobranch’ cannot demonstrate coloniality unequivocally. The pterobranchs may have been the earliest group of animals to evolve coloniality during the Cambrian Explosion interval and, thus, differentiate between sexual and asexual reproduction in a single taxon. Coloniality definitely led to their evolutionary success in the early Palaeozoic. The Hemichordata can be regarded as useful examples to understand the fossil preservation of organically preservable (non-mineralized) fossils through geological history and with this, the interpretation of many fossil lagerstätten. Especially the Pterobranchia vary considerably in preservation due to numerous taphonomic effects and these effects can be compared due to the commonality of their fossils in diverse Palaeozoic sediments. Their fossils consist of organic materials of three different types, the zooids (cellular tissues), the stolon system (recalcitrant tissues) and the tubaria (recalcitrant tissues). These materials differ considerably in their taphonomical changes and final preservational potential. While the soft-bodied zooidal or cellular tissues, similar to the bodies of the phylogenetically related enteropneust worms are nearly impossible to preserve, the pterobranch tubaria are extremely common in the early Palaeozoic fossil record. Diagenesis, metamorphism and tectonics modify pterobranch tubaria within the sediment. Tectonic distortion and subsequent weathering may have the largest influence on the destruction or preservation of these fossils in fossil lagerstätten. Graptolite reflectance is used commonly in Palaeozoic sediments to determine the thermal maturity of the encasing sediments. Maturity investigations show that all organic materials behave similarly to geological modifications and, thus, can used to interpret taphonomical changes in fossil lagerstätten in a consistent way. The most surprising result of this research project includes the discovery of the trilobite Fritzolenellus in eastern North America, providing an important key to the early evolution of the Olenellidae through the record of early ontogenetic stages of this taxon. The record may lead to a better understanding of the investigated biostratigraphic interval in the Lower Cambrian of western Newfoundland, as well as the sedimentary and tectonic evolution of this part of the eastern rim of Laurentia.
Publications
- 2016. Sphenoecium mesocambricus (Öpik, 1933) in the Middle Cambrian of Krekling, Norway. Norwegian Journal of Geology 96 (4), 311-318
Wolvers, H. M. & Maletz, J.
(See online at https://doi.org/10.17850/njg96-4-02) - 2017. Fossils explained 70. Graptolites: fossil and living. Geology Today 33 (6), 237-244
Maletz, J.
(See online at https://doi.org/10.1111/gto.12213) - 2017. Graptolite Paleobiology. 323 pp., 16 pls. Wiley–Blackwell. ISBN 987-1-118-51572-3
Maletz, J.
(See online at https://doi.org/10.1002/9781118515624) - Graptolites in the making – On the origin and early diversification of graptolites. In: Zhang, Y. D., Zhan, R. B., Fan, J. X. & Muir, L. A. (eds.). Filling the gap between the Cambrian explosion and the Great Ordovician Biodiversification Event (GOBE). Proceedings of the International Geoscience Programme (IGCP) Project 653 Annual meeting October 8th -12th, 2017, Yichang, China. Zhejiang University Press, pp. 107-111
Maletz, J.
- 2018. Tracing the evolutionary origins of the Hemichordata (Enteropneusta & Pterobranchia). Palaeoworld. Special Issue on the GOBE
Maletz, J.
(See online at https://doi.org/10.1016/j.palwor.2018.07.002)
