Triassic

The widespread extinction of many groups of organisms towards the end of the Permian is one of the great enigmas of the fossil record. This wave of mass extinction affected a variety of marine organisms including: foraminiferida, tabulate and rugose corals, the trilobites, eurypterids and certain other arthropods, the goniatite cephalopods, productid and orthide brachiopods, and many groups of stalked echinoderms. On land the pelycosaur reptiles (paramam-mals) also became extinct. The demise of so many taxa requires some explanation.

This major phase of extinction was succeeded in the early Triassic by the appearance of many new forms. Some of these may have arisen to fill vacated ecological niches, whereas others carved out new niches for themselves in response to the newly developed trophic resources of the Mesozoic Earth. In the early Triassic, there is a great diversification in the Bivalvia (clams) with the appearance of the unionids, carditids, myacea and oysters. Other molluscan taxa were also undergoing diversification with a number of new gastropod families appearing upon the scene including the patellids (limpets), trochids (top shells), littorinids (periwinkles), cerithids (ceriths) and naticids (moon shells). A new ammonoid family also arose, (the phylloceratids). A whole suite of new reptile groups developed, namely the ichthyosaurs, rhyncho-saurs, squamata (snakes) and archosaurs (including a series of orders like the crocodiles, dinosaurs, and pterosaurs). The first mammals also appeared before the end of the period while their ancestors, the paramammals, became extinct.

Extinction hypotheses have included cosmic radiation, pollution by volcanic emanations, world-wide salinity changes in the oceans and catastrophic climatic (especially temperature) fluctuations. The 'poisoning' hypotheses can be viewed with scepticism because of the selective nature of the extinctions, and the same criticism applies to the theory of catastrophies on a cosmic scale. World-wide salinity changes in the oceans would also have had a much more general effect, and would possibly have left some independent evidence in the form of changing proportions of evapor-ite minerals in accumulating salt deposits, but these are not to be found. Finally, temperature or climatic deteriorations (that correspond at the present time with low diversities in high latitudes) do not seem to have caused mass extinctions during the Pleistocene and so cannot be regarded as the sole reason for mass extinctions at the end of the Permian.

The Permo-Carboniferous phase of earth movements resulted from the collision of previously separated continental areas and culminated, during the Permian, in the welding together of the huge supercontinent of Pangea (Fig. j). This collisional phase caused a great reduction in the area of shelf seas and caused those that persisted to be influenced much more by continental conditions. Marine environments that are subject to the least continental influences and remain stable through time tend to support the most diverse populations. Those that are most influenced by continental areas and suffer from marked seasonal changes in higher latitudes undergo higher environmental stresses and have lower faunal diversities. The creation of Pangea produced a supercontinent with restricted shelves. The vast spreads of late Palaeozoic epicontinental seaways were eliminated and the limited shelves became areas of high environmental stress allowing only the more eurytopic (tolerant) groups to survive. The most probable reason for the extinction of so many marine faunas is thus an increase in competition within a reduced area of habitable environments, coupled with the absence of any isolated refuges around Pangea (Schopf, 1974).

On the continents too, the joining together of once-separated areas brought terrestrial organisms into direct competition, while the continental environments themselves would have been extreme as a result of the rise of newly generated mountain chains (the Hercynides) and the establishment of a totally new global climate. All these stresses would have led to the survival of the most highly adaptable groups and the extinction of those most specialized to the pre-existing regimes. Extinctions at the bases of food chains would have had snowballing repercussions throughout the higher taxa (Valentine, 1973).

Following the Carboniferous-Permian (Hercynian) earth movements, northern Europe lay in the grip of predominantly continental conditions throughout much of the succeeding Triassic Period. In Britain and in the area now occupied by the North Sea, sediments of Triassic age are dominated by clastic sands and clays that are often a striking red colour.

The linear fold mountain belt of the Hercynian had been attacked by erosion during the Permian, and had also been broken up into a series of fault-controlled blocks that continued to supply coarse sediments locally at least during the early part of the Triassic. These local massifs included the Meseta in Spain, the Cornubian-Armorican Massifs of Britain and northern France, the

Massif Central, and further to the east, the Ardennes and Bohemia.

At the beginning of the Triassic, which also marks the beginning of the Mesozoic Era, the continental areas of the world are still united in the supercontinent of Pangea (Fig. j) that was later divided in its sub-tropics by the encroaching Tethyan Ocean and in the boreal region by an ancestral Arctic Ocean. Periodically the sea spread over the continental interiors to produce broad, but usually restricted, embayments (Fig. 1). In the Permian, the advance of the Arctic seaway into northern Europe led to the formation of thick evaporite (Zechstein) salts under the present North Sea and northern parts of Germany. During the Triassic, the Tethyan transgression led to the development of the Muschelkalk dolomitic limestones which are also associated with evaporitic minerals.

Within the continental areas, deposition was limited to the subsiding basinal areas where Triassic deposits often succeed those of Permian without any noticeable break. The presence of evapor-ite minerals in many Triassic sequences suggests that the climate was predominantly arid. However, many of the sandier successions show evidence of having been water-laid. They contain fragments too large to have been moved by wind action and the finer grained beds often reveal filled, polygonal cracks that were produced during phases of desiccation. Thus, we have a picture of predominantly arid environments with infrequent but catastrophic flash floods spreading sheets of detritus from the massif areas over broad, subsiding and almost featureless sedimentary basins.

As we might expect animal remains are not very common under these conditions. However, there were times when rivers became less ephemeral and some quite rich reptile faunas became established accompanied by floras of horse-tails (Equisetales) and conifers. For example, the Keuper Sandstone fauna of the English Midlands contains amphibia, lungfish, shelled crustaceans and scorpions in an assemblage that became established during a damp phase when semi-permanent rivers drained the Armorican highlands and flowed northward into the Midlands Basin (Warrington, 1971). Later, in the same area, a marginal marine fauna was established during one of the few Triassic marine transgressive phases.

Fig. 1. Geography of the North Atlantic region during the Late Triassic. Much of the areas shown as 'Desert playa lakes' was Subject to invasion by the sea at various times, and contains local salt deposits and marls (the Keuper Marl) with restricted faunas. (After Hallam and Sellwood, 1976).

61 Triassic Lagoon Scene

The reconstruction of this scene is based on material deposited in Warwickshire during the 'Mid-Triassic transgression'. Marine influences are indicated by the presence of Lingula and rare bivalves including some similar to Modiolus and Pholadomya (Rose and Kent, 1955). More recently the sediments associated with this fauna have been found to contain marine microplankton. However, the presence of the moulds of salt crystals and of desiccation cracks in some of these same sediments suggests very arid conditions with perhaps a Persian Gulf type of climate on the margins

Section to show infauna in lagoon

Pig. 61 Triassic Lagoon Scene a rhynchosaur (Vertebrata: Reptilia: Lepidosauria)

b Macrocnemus (Vertebrata: Reptilia: Lepidosauria)

c nothosaur (Vertebrata: Reptilia: Euryapsida)

d Mastodonsaurus (Vertebrata: Amphibia)

e Plateosaurus (Vertebrata: Reptilia: Archasauria)

f Pholadomya (Mollusca: Bivalvia: Anomalodesmacea)

g Lingula (Brachiopoda: Inarticulata)

of the extended Muschelkalk sea. Of the vertebrate fauna, the fish were represented by animals such as the palaeoniscid Gyrolepis which certainly had marine affinities since it is found in the Muschelkalk of Germany. This genus actually returned to the British scene during the late Triassic marine transgression. Amphibian carnivores were represented by labyrinthodonts and some of these reached enormous sizes with their skulls alone reaching lm in length. Feeding around the margin of the brackish shores were rhynchosaurs, a group of reptiles adapted to eating shellfish with their beaked mouths and flattened crushing teeth (Halstead, 1975). Another group of very widespread reptiles lived as semi-aquatic carnivores; these were the nothosaurs. Their pointed teeth, flattened tail and webbed feet suggest that they were active swimmers, with fish as their main diet. During the temporarily humid phases promoted by the transgressions, herds of thecodontosaurid dinosaurs grazed and browsed along the water courses and left abund-dant footprints and the occasional skeleton. Carnivorous lizards like Macrocnemus preyed upon the young of many of the other groups and may also have eaten the odd insect or two. The vertebrate fauna in our reconstruction is based upon the collection of Walker (1969) and on the reconstruction of Halstead (1975).

A surprising feature of this fauna is the widespread distribution of many of the individual components. Nothosaurs are found all over Europe, in China, Japan, Tunisia, Jordan and India. Rhyn-chosaurs are also found in Europe, India, Africa, North and South America. The lizard Macrocnemus is also found in other parts of Europe and in Texas while the terrestrial thecodontosaurid dinosaur Poposaurus (similar to Plateosaurus in Fig. 61) found in Warwickshire also occurs in Wyoming. This brief view of the geographical distributions of some of these animals indicates the homogeneity of the continental areas by mid- to late-Triassic times and reinforces the theory of continuous land connections between these now dispersed areas. As the Triassic progressed, the massif areas were worn down to mere stumps but a series of large rivers, perhaps far to the north, collected a vast amount of fine background mud, probably derived from a deeply weathered continental area. In east Greenland coals were forming periodically, so that either the climate was becoming more humid towards the north or some large Tigris-Euphrates type of river system was entering the generally arid European basin. Towards the end of the Trias this basin had the appearance of a vast and almost totally flat plain, parts of which were probably submerged by shallow hypersaline waters. Salt brines were received from two sources, first from a tenuous connection with the southern Tethys Ocean and second from Permian salt plugs that punched their way through the Triassic sediment column.

Over southern Europe the influence of the Tethys Ocean was stronger and in Germany, southern France and in central and southern Spain the typical Triassic development can be seen in the characteristic threefold divisions from which the Trias takes its name: the sandy Bunter facies, followed by a dominantly carbonate Muschelkalk that is capped in turn by the more argillaceous Keuper facies. While the Bunter and Keuper facies are mostly similar to sequences developed further north, the Muschelkalk is not. The Muschelkalk fauna in central Germany and Spain shows typical high environmental stress associations. Faunas, although generally scarce, are normally dominated on individual bedding planes by a single species but individuals may be present in enormous numbers; bivalves (for instance Gervillia, Myophoria and Hoernesia) are the most important although some bedding planes may contain terebratulides while others may reveal ceratite ammonites.

Further to the south in Europe we enter the tectonically complex Alpine belt. Here carbonate facies are dominant, particularly in the late Trias, sometimes with the development of major 'reef sequences like those of the famous Dachstein Riffkalk (Fischer, 1964). Some of the 'reef complexes were composed of corals but some were constructed by hydrozoans and others contained abundant megalodont bivalves. Certainly, it is in the sequences nearer to Tethys that one finds the more diverse assemblages of Triassic marine faunas.

A large-scale reconstruction for the European Triassic might show an open oceanic belt running east to west in the south, flanked on its northern margin by a major 'reef belt. This protected a huge 'lagoonal' area that extended northwards over thousands of square kilometres. The lagoonal belt was (as in the Muschelkalk) more influenced by the sea at some times than at others, but mostly remained a vast hypersaline region undergoing considerable evaporation. Slight elevations of sea level spread more marine conditions over this lagoonal area but the main phase of marine transgression did not occur until very late Trias-Early Jurassic times.

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