Australia: The Land Where Time Began

A biography of the Australian continent 

Permian-Triassic Extinction – Decoupled Taxonomic and Ecological Recoveries

The mass extinction that occurred at the close of the Permian was the most severe of all the mass extinction events that have occurred in the history of the Earth; with >90% of Maine species dying in <0.1 My. Knowledge of its macroecological impact over prolonged time scales is limited. In this paper Song et al. have shown that marine ecosystems dominated by animals that were non-motile shifted to ones that were dominated by nektonic groups following the extinction. Animals at high trophic levels recovered more rapidly than those in the lower trophic levels. In the latest Triassic, about 50 My after the extinction event, the top-down rebuilding of marine ecosystems was still underway, and contrasts with the about 5 My recovery for taxonomic diversity. It is suggested by the decoupling between taxonomic and ecological recoveries that a process of vacant niche filling prior to the attainment of environmental carrying capacity is independent of structure building of ecosystems. The Permian-Triassic (P-Tr) mass extinction event that occurred 252 Ma caused a transformation among marine communities from the evolutionary fauna of the Palaeozoic to the modern evolutionary fauna (Sepkoski Jr, 1984) though in The Early Triassic there was a prolonged delay of recovery (Hallam & Wignall, 1997). Biodiversity data, which were compiled from global fossil databases and case studies, provide much detail on the magnitude and duration of the extinction event (Alroy et al., 2008; Shen et al., 2011; Stanley, 2016), though knowledge of the associated ecosystem changes have remained limited (Roopnarine & Anglelczyk, 2015). Marine ecosystems are generally believed to have taken several million years to recover as survivors and that vacant ecospace was filled by new taxa (Erwin, 1998), recovering occurring in a stepwise manner from bottom to top trophic levels  (Chen & Benton, 2012). This view was contradicted by the discovery of diverse predators in oceans of the Early Triassic such as conodonts, ammonoids, and bony fishes, which suggest there was rapid recovery of pelagic predators (Brayard et al., 2009; Stanley, 2009; Romano et al., 2016). Also, there has been no significant loss of global diversity in benthic marine ecosystems that has been observed across the extinction event, though there has been apparent lack of representation of several functional groups during the interval of the extinction event resulting from mass rarity (skeleton crew hypothesis) or lack of sampling, which suggests that ecological changes at this critical time are more complex than has previously been believed (Foster & Twitchett, 2014). This paper reports the results of the analysis in this study of the changing diversity among 3 functional groups (non-motile, motile, and nektonic animals), based on a new global fossil database that was compiled from the Palaeobiology Database and literatures that have been published.

Taxonomic recovery

Included in the new database of Song et al. are occurrences of marine genera from the Late Permian (Changhsingian, 254.1 Ma) to the Late Triassic (Rhaetian, 201.3 Ma). A total of 51,555 occurrences that were derived from 1,679 literature sources were collected to a sub-stage or stage level resolution. The presence of a given genus at a particular stratigraphic unit or site is the definition of an occurrence. If a genus occurred multiple times in a single collection they were treated as a single occurrence. A key feature of older curves is shown by the subsampled diversity curve (Sepkoski Jr., 1984; Alroy et al., 2008), such as the significant loss of diversity during the P-Tr extinction event and delayed recovery. High temporal resolution also reveals, however, features that are not apparent in earlier curves, such a pause in diversification that occurred in the middle Carnian, which is likely to be the result of the Carnian Pluvial Event (Ruffell, 2016). Following the P-Tr mass extinction event, Generic diversity of marine life finally attained pre-extinction levels in the early Anisian about 5 million years later. Clear evidence for environmental amelioration coincides with this recovery, especially reoxygenation of the oceans and cooling of the climate (Sun et al., 2012; Lau et al., 2016). It is shown by the sampling-standardised curve that a logistic increase in taxonomic diversity during the Triassic, which supported an equilibrium model that was similar to the Palaeozoic plateau in the diversity of marine invertebrates (1,16) and suggests that it appears genetic diversity reached the carrying capacity of the environment. The Middle Triassic biota differs from biota of the Late Triassic in comparison, though not in richness.

Ecological recovery

1.     In this study animals were assigned to 3 functional groups depending on their motile abilities (i.e., non-motile, motile, and nektonic animals) in order to investigate macroecological changes.

2.     Non-motile or passive animals are either benthic or planktonic (Bambach et al., 2002) consisting of epifaunal bivalves, brachiopods, bryozoans, hydrozoans, corals, pelmatazoan echinoderms, foraminifers, radiolarians, and sponges.

3.     Motile or active animals are those such as benthic animals that have independent mobile ability, which excludes free swimmers, and includes annelids, infaunal bivalves, crustaceans, eleutherozoian echinoderms, gastropods, and scaphopods.

4.     Nekton are comprised of the cephalopods, conodonts, fishes and marine reptiles.

Different fates during and after the P-Tr extinction event are exhibited by the 3 functional groups. During the P-Tr extinction event the genetic diversity of nekton suffered very little and gradually increased in the Early Triassic, reaching a peak in the early Anisian. Afterwards, the diversity of the nekton showed a continuous decline until the Rhaetian. Severe losses were suffered by motile animals during the P-Tr crisis, resulting in a low diversity in the Early Triassic prior to rebounding rapidly in the initial Middle Triassic. The most severe extinction was suffered by the non-motile animals, their diversity declining from more than 500 genera in the Late Permian to fewer than 100 genera in the Early Triassic. Following a rapid rebound in the initial Middle Triassic non-motile animals achieved pre-extinction levels.

A change in the structure of ecosystems was a result of the faunal shift to one that was dominated by nektonic groups. The data of Song et al. provided information on the richness and occurrence of genera, not abundance, made it difficult to evaluate the biomass of the various ecological guilds. According to Song et al. this problem is common to all fossil data studies, nonetheless, taxon richness as well as occurrence information have implications for predator-prey relationships (Kowalewski et al., 2002). The proportion of genera of nekton is about 14% in the Late Permian, though following the mass extinction event, it rapidly climbed to a peak of 67% in the Smithian, about 2 My later. In the Rhaetian it continued to decline reaching 11%. The proportion of non-motile genera of animals exhibited a reverse trend to that of nekton: it declined from 71% to 21% throughout early Griesbachian to Smithian times, after which it was followed by a long-term increase and final return to 70 % in the Rhaetian. Contrasting with this, the proportion of occurrence of genera of motile animals shows little change at ~15% from the Late Permian to the Late Triassic. The generic richness proportion among 3 groups shows a trend that is similar to that of the occurrence of data, though with a smaller amplitude.

Data that was obtained from different palaeolatitudinal regions show that the trends in the composition and structure of marine ecosystems are dependent on latitude: The proportion of nekton in tropical middle-latitude regions and high latitude regions shows a similar trend throughout the Late Permian to Late Triassic. Plateaus are exhibited by all 3 curves in the middle Early Triassic.  A relatively higher proportion of regions at higher latitudes, and the peak value of nekton is more than 70% in the regions at middle- and higher-latitudes. This pattern may result from the polewards migration of nekton because there was a higher risk of extinction in tropical ecosystems as sea surface temperature may have reached 40^oC at this time (Sun et al., 2012). Moderately decreasing trends is exhibited by all 3 curves throughout the Smithian and Rhaetian.

Early Triassic Oceans – reversed functional pyramid

A reversed functional pyramid for marine ecosystems in the Early Triassic, which are typically dominated by non-motile animals during the Late Permian and the Middle-Late Triassic, was led to by the P-Tr mass extinction event. In the Changhsingian of the Late Permian the proportion of generic diversity comprising non-motile animals is 68%, which is almost the same value as seen in the Early and Middle Permian (Bambach et al., 2002). It is suggested by this finding that the structure of the marine ecosystem was not changed significantly during the end-Guadeloupian extinction event. In the Early Triassic, exceptionally, the functional period of this time was dominated by nekton. It was shown by rarefaction analysis that the reversed functional pyramid is not an artefact of sampling. The reversed pyramid results from a major decrease of non-motile diversity as well as an increase of nekton diversity. For this faunal turnover intrinsic (biotic) and extrinsic (abiotic) factors may have both been responsible. Potentially, nekton have a fundamental niche that is wider than benthos and plankton because of their ability to move freely. Also, during the P-Tr mass extinction event non-nektonic animals were more severely affected and their recovery was slow, which coincided with a prolongation of harmful conditions in benthic environments (Lau et al., 2016), There was also a shrinkage of the fundamental niche of nekton during the crisis, as testified by their losses in the extinction, though it is probable that their realised niches expanded because of the lack of competition stress (McGowan, 2004). The turnover between nekton and non-motile animals, therefore, reflects the ecosystem succession from a normal one under stable environments to an abnormal one under environmental perturbations that in the benthic realm were at their most severe.

In the Early Triassic there were profound implications for the flow of energy in marine ecosystems due to the reversed functional pyramid. Nekton such as all cephalopods, most conodonts, most fish, and all marine reptiles usually occupy the trophic levels as predators (Kowalewski et al., 2002). It is implied by the rapid diversification of predators that in the Early Triassic the simple ecosystems probably had food webs that were simple and food chains that were short. At least 2 good reasons are known that support this inference:

_1.        the maximum richness of genera for all communities with the exception of nekton were markedly decreased, which had the result of a significant drop in the number of links for the food web due to the positive correlation between links of the food web and the biodiversity of the community (Roopnarine, 2010). The maximum richness of genera of nekton decreased moderately in the Griesbachian and Dienerian, but in the Smithian and Spathian rebounded to much higher levels.

_2.        More flow of energy to the top the trophic level was required by the expanded proportion of predators and this was achieved by the shortening in the length of the food chain (Dickman et al., 2008).

The apparent vulnerable nature of the ecosystem at this time was also supported by the opportunistic organisms in the aftermath of the P-Tr extinction event, such as cyanobacteria, small foraminifers, Claraia bivalves, and linguloid brachiopods (Hallam & Wignall, 1997; Pruss et al., 2006).

Decoupling of taxonomic and ecological recoveries

It was shown by this study that the recovery of global marine ecosystems from macroecological disturbance was decoupled from taxonomic recovery. The evolutionary recovery is judged by the stable global taxonomic diversity that is seen from the early Anisian onwards. Compared with this restoration of global ecosystems, that was represented by a gradual decrease in the proportion of  Nekton after the Smithian and the continued increase in metazoan reef richness, was continuing in the latest Triassic (Rhaetian), about 50 My later, when the next mass extinction occurred at the end of the Triassic (Dunhill et al., 2018). A gradual increase in complexity and stability, characterised the restoration of marine ecosystems, as is seen in the accelerated increase of community diversity in the Smithian, Spathian and Anisian, the appearance of large top predators (marine reptiles) in the Spathian, and the emergence of coral-sponge reefs and new groups of phytoplankton, such as coccoliths and dinoflagellates, in the Middle Triassic which became widespread and diverse in the Late Triassic [see also the study of Kiessling (Kiessling, 2010)].

While there is a logistic increase in the richness of genera, the increased proportion of non-nektonic animals during the restoration of ecosystems occurred in an explicitly logarithmic manner (R = 0.99, P « 0.001). It is suggested by the logarithmic increase the speed of reestablishment of ecological structure was in gradual decline. A rapid increase in diversity of non-motile and motile animals in the Smithian, Spathian and early Anisian primarily caused the initial period of rapid increase. The slow change that occurred in the latter period reflects a slow and prolonged decrease in the diversity of nekton, combined with the facts that benthic diversity remained stable, and there was an increase in the diversity of reef-building communities only incrementally. This trend persisted until the end of the Triassic when the proportion of nekton had declined to a level that hadn’t been seen since the latest Permian. When evaluating the nature and timing of the mass extinction at the end of the Triassic it is important to be aware of this trend. It has been noted frequently that in the last few million years of the Triassic marine taxonomic diversity was in decline, especially among ammonoids (Brayard et al., 2009), in the last few million years of the Triassic and that the mass extinction was not much more than a coup de grace, therefore questioning the status of this crisis as one of the “big 5” (Hallam et al., 2002). According to Song et al. this does not appreciate the long term trend of ecosystem evolution throughout the Middle-Late Triassic. Since the beginning of the Middle Triassic the decline of Nekton, had been underway, that it had not been matched by decline in the benthic groups, which maintained their diversity until the end of the Triassic. Therefore, the mass extinction event at the end of the Triassic is best viewed as an abrupt termination of a trend that had been continuing for about 50 My.

A decline in the raw and subsampled diversity in the middle Carnian is shown by the data in this study. All 3 groups, i.e., non-motile, motile and nektonic animals, were obviously affected by this event. In the middle Carnian the decline of the generic richness is not shown in earlier biodiversity curves (Sepkoski Jr., 1984; Alroy et al., 2008). Song et al. suggest that this trend can probably be explained by this extinction event falling within a traditional time bin and is therefore masked, though a peak of extinction for ammonoids and echinoderms has been seen in the middle Carnian (Benton, 1986; Simms & Ruffell, 1989). However, the extinction event of the middle Carnian did not affect significantly affect the increase in the proportion of non-nektonic animals, which suggests that this extinction event had little impact in the long-term on the ecological structure of marine ecosystems. This inference is supported by the recent finding that environmental perturbations that were associated with the Carnian Pluvial Event, such as climate warming and anoxia in the ocean were not affected as severely when compared to the P-Tr event (Sun et al., 2012; Lau et al., 2016; Sun et al., 2016).

Ecosystem rebuilding in a stepwise manner from bottom to top trophic levels in the Early Triassic (Chen & Benton, 2012), favours the stepwise recovery hypothesis. The quantitative analysis carried out in this study suggests that a bottom-to-top destruction of marine ecosystems was followed by ecosystem restoration in a top-to-bottom order. The non-motile animals were ensured to be the biggest victims in the oceans of the Early Triassic by the selective extinction of non-motile primary consumers in the P-Tr event (Song et al., 2013), followed by their suppressed recovery.  Contrasting with this high-level predators that included cephalopods, conodonts and fish diversified, becoming the most diverse fauna at this time. Therefore, marine ecosystems collapsed in a manner that was explicitly bottom-to-top. An increase in diversity of lower level consumers characterised this ecosystem rebuilding.

Summary

According to Song et al. their data revealed that marine ecosystem structure restoration, about 50 Ma, was an order of magnitude slower than rates of taxonomic diversity recovery, about 5 Ma. The decoupling of taxonomic and ecological recoveries reflect distinctive drivers:

i.                   The close link between the rapid rebound in taxonomic richness and the amelioration of the environment in the initial Middle Triassic indicates that a critical role in controlling taxonomic diversity is played by the environmental carrying capacity, and

ii.                 It is likely that the prolonged recovery of ecosystem structure reflects the complex interactions in ecosystems, e.g., interactions among species within clades, interactions between clades, and interactions between biotic and abiotic factors, that show logarithmic rates of change.

It is suggested, in contrast, that this logistic growth of taxonomic diversity a process of vacant niche filling that was independent to ecosystem structure. This study has reaffirmed the importance of protecting diversity of global ecosystems because, once it is destroyed, restoration takes dozens of millions of years, much longer than human history.

Sources & Further reading

Song, H., et al. (2018). "Decoupled taxonomic and ecological recoveries from the Permo-Triassic extinction." Science Advances 4(10).