348 MESOZOIC/Triassic Triassic evaporites There was also strong east-west climatic asymmetry across Pangaea, with eastern Pangaea (at least between latitudes 40 S and 40 N) being relatively warmer and wetter because of the presence of Tethys and the absence of an Atlantic Ocean to facilitate oceanic heat exchange With the Pangaean landmass centered near the equator during the Triassic, and a prominent Tethyan bight, climate models suggest that seasonality was monsoonal Hence, there were only two seasons, wet and dry The abundant rainfall was concentrated in the summer months, and there was little annual temperature fluctuation During the northern hemisphere summer, the northern landmass would have been relatively hot, whereas the southern land mass would have been relatively cool Moisture from Tethys would have been pulled into the northern hemisphere low-pressure cell, producing extensive rains, whereas the southern hemisphere high-pressure cell would have remained relatively dry During the southern hemisphere summer, this process would have occurred in reverse Thus, seasonality across Triassic Pangaea would have been alternating hemisphere-wide wet and dry seasons The warm and highly seasonal climates (wet–dry) of Triassic Pangaea are reflected in its biota The Triassic saw an increase in the diversity of gymnosperms, particularly of xeromorphic scale-leaved conifers and seed ferns and cycadophytes with thick cuticles Similarly, during the Triassic, in the evolution of reptiles, more waterefficient (putative uric-acid-excreting) diapsids diversified at the expense of less water-efficient (probably urea-excreting) synapsids Extinctions The Permian ended with the greatest biotic extinction of Phanerozoic history (here termed the PTB biotic crisis) (see Palaeozoic: End Permian Extinctions) This extinction is best documented in the marine realm (Figure 6), where it is estimated that $90% of the species, and more than half of the families of shelled marine invertebrates, became extinct The magnitude and synchrony of the terrestrial extinction are much less clear The Triassic records the recovery of the global biota from this massive extinction The period also bore witness to further marine extinctions within the Late Triassic, and was terminated by a series of Late Triassic marine and non-marine extinctions The cause of the PTB biotic crisis remains uncertain Some workers have identified a complex and interrelated group of terrestrial events as a possible cause: (1) major marine regression that reduced marine shelfal habitat areas and increased climatic Figure Characteristic extinction/diversity patterns of marine invertebrates across the Permian Triassic boundary After Lucas (2000) The epicontinental Triassic, an overview Zentralblatt fuăr Geologie und Palaăontologie Teil I 8: 475 496 variability, (2) eruption of Siberian flood basalts, (3) release of gas hydrates and erosion/oxidation of marine carbon due to the regression, and (4) elevated atmospheric CO2 due to all of these phenomena resulting in ocean anoxia and global warming During the Carnian, there was a substantial marine extinction of many kinds of conodonts, ammonoids, bivalves, echinoids, and reefal organisms, although the impact on land was less obvious, with evolutionary turnover occurring throughout the Late Carnian and Early Norian A further extinction has been identified at the Norian–Rhaetian boundary with the disappearance of the ubiquitous flat clam Monotis This, in fact, was part of a series of Late Triassic extinctions that included the disappearance of the conodonts, near extinction of the ammonites, decimation of about half of the marine bivalves, and collapse of the reef ecosystem On land, there were also profound extinctions of tetrapods between the end of the Triassic and sometime in the Middle Early Jurassic (Sinemurian), but it has been difficult to establish the exact timing A major carbon isotope anomaly has been identified in both marine and terrestrial environments at the end of the Triassic This major perturbation in the global carbon cycle has been variously linked to a significant fall in sealevel, extraterrestrial impact, flood-basalt volcanism, and/or methane release Flora During the Permian and Triassic, there was a complex and prolonged replacement of the palaeophytic flora by the mesophytic flora This was the global change from pteriodophyte-dominated floras of the Palaeozoic to the gymnosperm-dominated floras that characterized much of the Mesozoic Thus, the