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sizes in Early Triassic animals, they make biological predictions about extinction
selectivity that run counter to the pattern actually observed (Bambach and Knoll in
preparation). Moreover, catastrophic carbon dioxide increase provides two distinct kill
mechanisms—direct physiological inhibition of metabolism and climate change
associated with greenhouse enhancement—so CO
2
increase at the boundary is consistent
with both terrestrial and marine patterns of extinction. Other, as yet unarticulated,
environmental perturbations may account for observed patterns of extinction equally
well, but for now, carbon dioxide deserves a prominent place on the short-list of plausible
kill mechanisms. Accepting this, we can ask what sort of trigger could introduce lethal
amounts of CO
2
into the atmosphere and surface oceans. Knoll et al. (1996) favored
catastrophic overturn of anoxic deep waters, but this idea has not generated widespread
enthusiasm among oceanographic modelers. Oxidation of methane released
catastrophically from hydrates on the continental shelf and slope might also be suggested,
but isotopic constraints on the volume of methane that could have been released suggest
that by itself this mechanism cannot provide the necessary trigger (Berner 2002). The
same problem applies to carbon dioxide release associated with extensive continental
basalt eruption at the Permo-Triassic boundary. The wild card that remains is bolide
impact, an attractive mechanism for which empirical evidence remains controversial
(Becker et al. 2001; Kaiho et al. 2001; Verma et al. 2002).
Delayed rediversification of the marine fauna
.
The Early Triassic aftermath of end-
Permian mass extinction poses its own set of questions, some of which potentially
provide new perspective on the extinction event itself. Given the magnitude of biological
disturbance, it is not surprising that basal Triassic rocks are paleontologically
depauperate. What is surprising is that the exceptional features of the earliest Triassic
biosphere persisted for four to six million years; true ecosystem recovery really didn't
begin until the Middle Triassic (Erwin 2001).
Early Triassic marine diversity was lower than at any previous time since the
Cambrian Period. Characteristically, Early Triassic faunas consist of small, lightly
skeletonized survivors, many with apparently cosmopolitan distributions (Schubert and
Bottjer 1995; Rodland and Bottjer 2001). The gastropods, bivalves, and ostracodes found
in local abundance in Lower Triassic carbonates are commonly only 1-2 mm long
(Lehrmann et al. 2003); ammonoid cephalopods and calciate brachiopods were similarly
small and thin walled. Echinoderm debris formed bioclastic sands, but individual crinoid
columnals and skeletal plates are tiny. Calcified red and green algae were important
carbonate producers in both Late Permian and later Triassic seaways, but with one high
paleolatitude exception (Wignall et al. 1998), neither is known from Lower Triassic rocks
(Flügel 2002). Similarly, all Paleozoic corals disappeared by the end of the Permian, and
the scleractinian corals that would rise to ecological prominence in Mesozoic and
Cenozoic ecosystems appear only in Middle Triassic rocks (Stanley 2003), apparently
evolving carbonate skeletons
de novo
in at least two distinct actinarian stocks (Romano
and Cairns 2000). Thus, no metazoan or algal reefs are known from Lower Triassic
rocks. In contrast, microbialites and precipitated carbonates less obviously formed under
microbial influence are relatively widely distributed (Schubert and Bottjer 1992; Baud et
al. 1997; Kershaw et al. 2002; Lehrmann et al. 1993).
The paucity of skeletal remains doesn't mean that carbonates are rare in Lower
Triassic successions; in fact Lower Triassic limestones and dolomites are widespread and
regionally thick. Facies development, however, resembles that of Neoproterozoic basins,
as well as Upper Cambrian successions that have similarly limited contributions from
skeletal algae and animals—carbonate mudstones, generated at least in part by whitings,
and oolites occur along with the microbialites noted in the previous paragraph.
