Reef builders

The scleractinian corals, important reef builders during the Triassic, suffered a marked decline at the end of the Triassic that was followed by a "reef gap" during part of the Early Jurassic (Hettangian-early Sinemurian), after which corals re-diversified to become the dominant reef builders (Stanley 1988). Stanley (2001, p. 26) viewed this as a "rapid collapse" of reefs at the TJB, concluded it was "the result of a first-order mass extinction" and noted that "Jurassic recovery was slow."

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Fig. 5. The actual stratigraphic ranges of all known taxa (including ammonoids and bivalves) across the TJB in the Ferguson Hill section near New York Canyon, Nevada (modified from Lucas et al., 2007a). The TJB is placed here at the lowest occurrence of Psiloceras tilmanni

The extinctions in the reef community at the end of the Triassic are best documented in Tethys, where the reef ecosystem collapsed at the end of the Triassic, carbonate sedimentation nearly ceased, and earliest Jurassic reefal facies are rare. Earliest Jurassic reefs that are known (particularly in Morocco) are carbonate mounds produced by spongiomorphs and algae (e.g. Flügel 1975). However, coral Lazarus taxa have been discovered in Early Jurassic suspect terranes of western North America, indicating the persistence of at least some corals in Panthalassan refugia (on oceanic islands) during the earliest Jurassic reef gap (Stanley and Beauvais 1994).

Hallam and Goodfellow (1990) argued that sea level change caused the collapse of the reef system, with significant extinctions of calcisponges and scleractinian corals at the TJB. They discounted the possibility of a major drop in productivity as an explanation for the facies change from platform carbonates to siliciclastics. Indeed, a distinct lithofacies change occurs at or near the TJB in many sections, particularly in the Tethyan realm, where facies changes suggest an interval of regression followed by rapid transgression (Hallam and Wignall 1999). At the TJB section in western Austria, for example, a shallowing-upward trend from subtidal carbonates to red mudstones, interpreted as mudflat deposits, is succeeded by deeper water thin-bedded marl and dark limestone (McRoberts et al. 1997). The boundary in parts of the Austrian Alps displays karstification, suggesting a brief interval of emergence. In the Lombardian Alps the TJB is placed (palynologically) in the uppermost Zu Limestone at a flooding surface that marks the transition from mixed siliclastic-carbonate sedimentation to subtidal micrite deposition (Cirilli et al. 2003). Thus, a change in bathymetry resulted in the extirpation of reefs in Tethys, which in large part caused the cessation of carbonate sedimentation. However, the evidence that this was a global event is lacking, and it can be viewed as a regional (circum-Tethyan), not global, extinction driven by sea level changes (Tanner et al. 2004).

Kiessling et al.'s (1999) and Kiessling's (2001) global compilation indicates that the decline of reefs began during the Carnian and that the TJB corresponds to the loss of reefs concentrated around 30oN latitude. Nevertheless, this article has been cited as documenting a TJB mass extinction of reef organisms (e.g. Palfy 2003). However, the timescale used in Kiessling's compilation is very coarse (it is only divided into Ladinian-Norian-Pliensbachian) and shows a steady decline in diversity throughout this time interval to reach a diversity low in the Middle Jurassic (Bajocian/Bathonian). This did not deter Kiessling et al. (1999), however, from identifying a major extinction of reefs at the TJB.

Flügel and Senowbari-Daryan (2001) drew a broader picture of Triassic reef evolution (also see Flügel, 2002). Thus, after the end-Permian mass extinction of the Paleozoic corals, there was a "reef gap" during the Early Triassic (Fig. 6). In the Middle Triassic (Anisian), reef-building resumed with the first appearance of scleractinian corals. The primary Middle Triassic reef builders, however, were stromatoporoids, calcisponges and encrusting algae. This continued into the early Carnian when there was a major evolutionary turnover in reefs that led to scleractinian dominance of reefs by Norian time.

Indeed, Flügel and Senowbari-Daryan (2001) referred to Norian-Rhaetian reefs as the "dawn of modern reefs" because they were characterized by abundant, highly calcified, sessile, gregarious and high-growing corals, as are modern reefs. However, Flügel and Senowbari-Daryan (2001) drew attention to a dramatic extinction of coral species at the TJB, with their analysis indicating that only 1% of Triassic coral species (14 of 321) and 8.6% of sphinctozoid coralline sponge genera (5 of 58) surviving the end of the Triassic. They also noted that the most successful of the Late Triassic corals, the distichophyllids, became totally extinct at the end of the Triassic (also see Roniewicz and Morycova 1989).

Coral reefs are extremely rare in the Hettangian-early Sinemurian, but Lazarus taxa from oceanic island refugia in Panthalassa (Stanley and Beauvais 1994) undermine the case for a simple global mass extinction of corals at the TJB. Furthermore, it is important to stress that by Pliensbachian time the reef ecosystem was well on its way to recovery. The Jurassic reefs continued to be dominated by scleractinian corals, so the disruption of the reef ecosystem was not permanent.

Beauvais (1984) stressed the endemism of scleractinian species during the Liassic, raising the possibility that the apparent TJB extinction of these organisms may be heavily influenced by (Tethyan?) sampling biases. Thus, a sudden extinction of reef organisms at the TJB is well documented in Tethys and reflects a regional change in bathymetry, but not necessarily a global mass extinction of reef organisms.

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  • Zofia
    What were dominant reef builders?
    11 months ago

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