Abstract
The Permian and Triassic were critical times for marine and continental life on Earth. These periods saw the transition from the typically Palaeozoic faunas and fl oras to the more modern communities of the Mesozoic and Cenozoic and witnessed profound climatic and oceanographic changes due to the assembly of the supercontinent Pangaea and other environmental factors. A series of important extinction events are known from this time interval, amongst others the largest mass extinction of all time near the Permian- Triassic boundary. The study of the mechanisms and patterns of extinctions and subsequent recoveries has attracted a wealth of research interest during the last decades. Most publications are focussed on marine invertebrates or on terrestrial vertebrates. However, the effects of the various biotic crises of the Permian-Triassic interval on fishes (primary aquatic vertebrates) are still poorly understood. In addition, patterns of recovery after the end-Permian mass extinction are not yet well resolved. Carbon isotope excursions as well as changes in spore-pollen ratios and the palaeogeographic distribution of taxa during the Early Triassic have been interpreted as evidence for severe climatic changes, but little was known about an important component of climate: temperature. The present dissertation is based on new field data and provides new insights into the reconstruction of relative temperature changes during the Early Triassic but also into the turnover and recovery of fishes (Chondrichthyes, Osteichthyes) during the Permian-Triassic. The thesis is organised in two parts, focussing on (1) changes in seawater temperature during the Early Triassic (Chapter 1), and (2) descriptions of new Early Triassic fish material and evaluation of evolutionary trends within Permian and Triassic fishes (Chapters 2–6). In Chapter 1, a new Early Triassic temperature record is presented based on oxygen isotope values from biogenic apatite (conodonts, actinopterygian teeth). The new data from mid-palaeolatitudes (northern Pakistan) suggests severe temperature changes during the Early Triassic, thus confirming previous hypotheses. The trends in the oxygen isotope record correlate well with those of the carbon isotope record, and the latter have been shown to be of global significance. Cooler climatic conditions coincide with early recovery pulses in ammonoids, conodonts and other marine taxa, whereas a phase of increased temperatures in the middle-late Smithian is associated with a severe turnover among nekto-pelagic clades (e.g. ammonoids, conodonts). The results suggest that climatic changes in the aftermath of the end-Permian event are largely responsible for the observed recovery patterns. An important driver for the observed climatic and biotic upheavals during the Early Triassic is eruptive activity of the Siberian Traps Large Igneous Province, which already played a major role during the end-Permian extinctions. Chapter 2 is an extensive literature report on Early Triassic and some Anisian fishes (Chondrichthyes, Osteichthyes) and provides an overview of their global diversity and palaeobiogeography. In Chapter 3 a new specimen of the rare hybodontoid chondrichthyan Palaeobates polaris is described from the Smithian of Spitsbergen (Svalbard, Norway). The new specimen is more complete than the type material of P. polaris and allows the reconstruction of the lower jaw and the determination of a few potential apomorphies of this durophagous fish. In Chapter 4 new fish material from the Early Triassic (late Smithian) of Bear Lake County (SE Idaho, USA) is presented, which amongst others includes a three-dimensionally preserved skull of the garfish-like actinopterygian Saurichthys. Due to its very peculiar morphology, which allows even fragmentary skeletal remains to be identified at genus level, the Triassic Saurichthys has a well-known fossil record. The following trends are noted in Saurichthys: (1) a change from cosmopolitan distribution and high species richness in the Early Triassic to restricted palaeogeographic distribution and low species diversity in the Late Triassic, and (2) disappearance of plesiomorphic bauplans early in the Triassic (in the marine realm) and successive anatomical specialisations to increase predatory efficiency. Chapter 5 deals with the diversity dynamics and body size changes of Permian and Triassic bony fishes based on a new data matrix adapted from the available literature. The results are compared to already existing data on Permian-Triassic Chondrichthyes. Genus diversity trends in Osteichthyes proceed from generally low levels in the Permian to higher values in the Triassic in combination with increased diversity and abundance of more derived taxa near the Permian-Triassic boundary and onwards. Exceptionally high extinction rates at genus level are absent during the Permian and Triassic, but peaks in origination rates for bony fishes are observed in the Early and Middle Triassic. Significant body size changes are noted at the Middle-Late Permian boundary (size increase) and again at the Early Triassic-Anisian boundary (size decrease). The former is related with the appearance of new taxa of larger body sizes within palaeopterygians and the latter with the emergence of numerous small taxa, mainly within neopterygians and derived palaeopterygians (‘Subholostei’). Strikingly, diversity trends in osteichthyans are opposed to those in chondrichthyans – the latter experienced a dramatic decline in genus diversity during the Middle and Late Permian. Hence, the Permian-Triassic marks an important turnover between the chondrichthyan- dominated communities of the Permocarboniferous to the osteichthyan-dominated (mainly actinopterygians) ichthyofaunas of the Mesozoic and Cenozoic. During the Triassic, basal groups of actinopterygians (palaeopterygians) were subsequently replaced by neopterygians, which include the bulk of modern-day fishes. Chapter 6 is a review of marine predator diversity (Chondrichthyes, Osteichthyes, Tetrapoda) in the Early Triassic and Anisian and aims at elucidating trophic complexity in the wake of the end-Permian mass extinction event. This review shows that large marine predators (>1 metre) were present throughout the investigated interval and that they also had a wide palaeogeographic distribution. In the earliest Triassic (Griesbachian, Dienerian, Smithian), marine apex predators were composed of temnospondyl ‘amphibians’ (trematosaurids), chondrichthyans (e.g. hybodontoids, eugeneodontids) and osteichthyans (e.g. Birgeria, Saurichthys). In the early Spathian, sauropterygian reptiles and largesized ichthyosaurs were added. Although fossils of ichthyosaurs are still not known from older strata, these early forms already show a derived, fish-like body shape and it can be inferred that representatives of these predators were already present earlier in the Triassic. Eugeneodontids disappear from the fossil record and trematosaurids become much rarer near the Smithian-Spathian boundary, suggesting that they were possible victims of the end-Smithian event. Later in the Triassic, ma- rine apex predators were mainly represented by ichthyosaurs, sauropterygians, and some fishes. In conclusion, the Permian and Triassic were evidently important intervals for fishes as well as many other groups. During these periods, once dominant taxa disappeared and newer clades emerged and diversified, many of which are still important in present-day faunas. This transition was, however, not sudden but took millions of years. Nevertheless, mass extinction events in combination with severe climatic and environmental changes played an important role during these turnovers near the Palaeozoic-Mesozoic boundary.
Romano, Carlo