Richard Hofmann

This paper compares the relative contributions of within-habitat diversity [alpha-diversity] and between-habitat-diversity [beta-diversity] to regional diversity [gamma-diversity] in marine benthic communities of the western US before and after the end-Permian mass extinction. We found that presumably cool-water faunas from the Permian Gerster Limestone and the Park City Formation had low alpha- and beta-diversities, comparable to those of low diverse faunas of the Early Triassic. In contrast, tropical Permian faunas had much higher alpha-diversities and a variable pattern of beta-diversity: Whereas faunas of space-limited bioherms show a positive correlation between beta-diversity and gamma-diversity, beta-diversity in level-bottom faunas is elevated only when gamma-diversity is very high (>250 species). This contrasting pattern probably reflects differential effects of interspecific competition on habitat partitioning. In low-competitive level-bottom faunas, species are able to coexist until competition forces species into their ecological optima, thereby increasing beta-diversity. This effect occurs at much lower gamma-diversities in more competitive reef-bound faunas, causing the observed positive correlation between beta- and gamma-diversity. We suggest that differences in the level of interspecific competition and hence diversity partitioning between Permian and Triassic benthic communities result from the higher average metabolic rates in the Mesozoic mollusc-dominated benthos in contrast to their Permian counterparts.

Ichnologic approaches have been key to the understanding of causes and consequences of the end-Permian mass extinction event ever since researchers began to focus on this pivotal episode in the history of life. Trace-fossil occurrences or absences have proven useful to recognize the loss in biodiversity and ecologic function during the extinction event as well as to track subsequent restoration of benthic marine communities. There exists now a considerable body of ichnologic data from many critical Permo-Triassic and especially Lower Triassic sections, which allows drawing a more advanced picture of the extinction event and subsequent recovery. The trace-fossil record of immediate post-extinction strata shows a virtual complete breakdown of bioturbation. First modest benthic activity is evidenced by the presence of simple deposit-feeding traces. Recent studies have advanced the notion that recovery, which is traditionally perceived to have proceeded slowly, was remarkably volatile with the reemerging of fairly complex and diverse ichnofaunas on a global scale soon after the extinction. Surveying the record of some key ichnotaxa, such as Arenicolites, Diplocraterion, Rhizocorallium, and Thalassinoides, also suggests that recovery was neither latitude-dependent nor did ichnotaxa reappear in a strict stepwise manner. It is more likely that the irregular recovery pattern of ichnofaunas results from a combination of a generally strong facies dependence of trace-fossils, a patchy geologic record, local environmental controls, and possibly a variable primary recovery signal. Recent work suggested that biogenic sediment mixing was strongly reduced during the aftermath of the end-Permian mass extinction. This must have had tremendous consequences for marine element recycling and burial, as well as ecosystem functioning. Based on observations from extant ecosystems, it can be hypothesized that Early Triassic sedimentary and geochemical signatures as well as ecologic intricacies mirror the depauperate ecology and low biodiversity left by the extinction. As examples, anoxic signatures, behavior of the sulfur-cycle, and paleoecologic characteristics are reevaluated in the light of the poor biogenic mixing scenario.

The Lower Triassic Werfen Formation of northern Italy represents an important archive for Early Triassic ecosystems. Based on quantitative community analysis using species level identifications, we reconstruct the recovery of benthic ecosystems after the end-Permian mass extinction throughout this unit. The analysis of benthic macrofossil communities shows that incipient recovery has taken place during the Griesbachian and the Griesbachian–Dienerian transition. A probable deterioration in environmental conditions is observed towards the end of the Dienerian. The Smithian part of the Werfen Formation is characterized by high siliciclastic input, which ceases around the Smithian-Spathian boundary. The Spathian marks the definitive phase of recovery in the Werfen Formation. The comparison of this pattern with other palaoegeographical regions suggests that both the Griesbachian recovery and the Dienerian decline were of inter-regional if not global extent, whereas the Smithian diversity low in the Werfen Formation is a local signal. In contrast to the recovery dynamics of ammonoids and conodonts, the Smithian–Spathian boundary interval was no caesura for benthic ecosystems. The Spathian recovery pulse is possibly also an inter-regional event, at least in the palaeotropics. These results are in contrast with the previously proposed scenario of persistent hostile conditions during the Griesbachian time interval and highlight an initial recovery phase restricted to Griesbachian times. Instead, the apparently sluggish recovery of benthic ecosystems was at least partly shaped by set-backs due to short-term environmental perturbations during the Dienerian.

The Dinwoody Formation of the western United States represents an important archive of Early Triassic ecosystems in the immediate aftermath of the end-Permian mass extinction. We present a systematic description and a quantitative paleoecological analysis of its benthic faunas in order to reconstruct benthic associations and to explore the temporal and spatial variations of diversity, ecological structure and taxonomic composition throughout the earliest Triassic of the western United States. A total of 15 bivalve species, two gastropod species, and two brachiopod species are recognized in the study area. The paleoecological analysis shows that the oldest Dinwoody communities are characterized by low diversity, low ecological complexity and high dominance of few species. We suggest that this low diversity most likely reflects the consequences of the mass extinction in the first place and not necessarily the persistence of environmental stress. Whereas this diversity pattern persists into younger strata of the Dinwoody Formation in outer shelf environments, an increase in richness, evenness and guild diversity occurred around the Griesbachian–Dienerian boundary in more shallow marine habitats. This incipient recovery towards the end of the Griesbachian is in accordance with observations from other regions and thus probably represents an interregional signal. In contrast to increasing richness within communities (alpha-diversity), beta-diversity remained low during the Griesbachian and Dienerian in the study area. This low beta-diversity reflects a wide environmental and geographical range of taxa during the earliest Triassic, indicating that the increase of within-habitat diversity has not yet led to significant competitive exclusion. We hypothesize that the well-known prevalence of generalized taxa in post-extinction faunas is primarily an effect of reduced competition that allows species to exist through the full range of their fundamental niches, rather than being caused by unusual and uniform environmental stress.

The Spathian (late Early Triassic) Virgin Formation of south-western Utah (U.S.A.) yields a comparatively diverse benthic fauna that flourished ~2 Ma after the end-Permian mass extinction. In this study, we present quantitative palaeoecological data, which are analysed in the context of their depositional environments. This integrated approach helps to discriminate between effects of the end-Permian mass extinction event and local environmental factors on alpha diversity and ecological structure of the Virgin Fauna. Shallow subtidal environments yield the highest species richness and lowest dominance values as recorded in two benthic associations, the Eumorphotis sp. A-association and the Protogusarella smithi-association, both of which contain 20 benthic species (bivalves, gastropods, brachiopods, echinoderms, and porifers). Tidal inlet deposits yield a low diverse fauna (Piarorhynchella triassica-association) with a very high dominance of filter feeders adapted to high energy conditions. Another comparably low diverse fauna is recorded by the Bakevellia exporrecta-association, which is recorded in deposits of the offshore transition zone, most likely reflecting unconsolidated substrates. A single sample containing five bivalve species (Bakevellia costata-assemblage) is recorded from a marginal-marine setting. The Virgin fauna yields a bulk diversity of 30 benthic species (22 genera) of body fossils and 14 ichnogenera and, thus, represents the most diverse marine bottom fauna known so far from the Early Triassic. Our results suggest that oceanographic conditions during the early Spathian enabled ecosystems to rediversify without major abiotic limitations. However, taxonomical differentiation between habitats was still low, indicating a time lag between increasing within-habitat diversity (alpha diversity) and the onset of taxonomical differentiation between habitats (beta diversity). We suggest that taxonomical habitat differentiation after mass extinction events starts only when within-habitat competition exceeds a certain threshold, which was not yet reached in the Spathian of the investigated area. This interpretation is an alternative to previous suggestions that the prevalence of generalistic taxa in the aftermath of mass extinction events reflects protracted environmental stress. The onset of increasing beta diversity is a potential criterion for distinguishing two major recovery phases, the first ending with habitat saturation and the second ending with the completion of ecosystem differentiation.

Recent advances in the understanding of Early Triassic ecosystems suggest that the recovery from the end-Permian mass extinction was much more variable than traditionally suggested. By comparing low-latitude settings of the western Tethys (Werfen Formation of northern Italy) and eastern Panthalassa (Dinwoody Formation and Thaynes Group of the Western US) using quantitative palaeoecology, we aim to reconstruct ecosystem evolution in different palaeogeographical areas and to identify differences and general patterns in the recovery from the greatest mass extinction in Earth’s history.
In the Werfen Formation, the immediate aftermath of the extinction records relatively diverse and balanced communities. In the course of the Griesbachian, simple, high-dominance communities predominate although some samples may show comparatively high diversities and low dominance values towards the upper Griesbachian. This trend culminates in the presence of moderately diverse and balanced faunas around the Griesbachian-Dienerian boundary interval and is mirrored in the ichnofaunas as well. In the course of the Dienerian, dominance, alpha diversity and community composition strongly fluctuates. In the lowermost Smithian interval, shelly fossils are virtually absent apart from some low-diverse shell pavements. Bioturbation is again significantly reduced. The remaining part of the Smithian is dominated by very low-diverse communities that strongly fluctuate in dominance values. The basal Spathian shows a remarkable increase in new species and the establishment of more balanced communities. Bioturbation becomes again abundant and the return of some “high recovery” ichnotaxa is observed. Alpha diversities rebound to the highest values recorded in the Werfen Formation.
In the Griesbachian and Dienerian rocks of the Dinwoody Formation of the western U.S., faunas locally exhibit relatively high alpha diversities and low dominance values. However, most samples of this time interval record rather highly-dominant, low-diverse communities. During the Smithian (Thaynes Group and the Sinbad Formation), more balanced and slightly more diverse communities became established in outer and inner shelf settings with local occurrences of small, reef-like structures. The near shore habitats of the lower Spathian Virgin Formation record the most diverse and most balanced communities of the western U.S. of the Early Triassic.
It becomes apparent that the recovery patterns differed significantly between the two low-latitude settings. The western Tethys shows several pronounced ups and downs. A relaxation phase is observed at the base of the Griesbachian, which is related to the temporary presence of short-term survivors. A next strong recovery signal is observed around the Griesbachian/Dienerian-boundary. The communities of the same interval from Eastern Panthalassa indicate more stressed conditions except for some near-shore settings. In the western Tethys, a dramatic drop in benthic ecosystem complexity is recorded at the base of the Smithian. The subsequent “low” is likely related to high siliciclastic input, which was possibly accompanied by fluctuations in salinity. Smithian sections from Eastern Panthalassa do not show strong evidence for pronounced environmental stress and the ecological conditions improved with respect to the bulk Griesbachian and Dienerian record. The Spathian of the western Tethys marks a next notable restoration signal, which was, however, not fundamentally different to the first Griesbachian pulse in terms of alpha diversity and dominance of taxa. In eastern Panthalassa, this recovery signal is much more pronounced and the ecological and taxonomical recovery exceeds everything observed in earlier in the Triassic. In both palaeogeographic regions, the Early Spathian marks the establishment of many new taxa that add to the inherited diversity from the Smithian interval. With only little loss on the species and virtually no loss on the genus level, the Smithian-Spathian boundary was no caesura for benthic ecosystems.
In summary, restoration signals varied in their magnitude and stratigraphic position between both regions. To extract a general recovery pattern, additional data from other regions are necessary in order to factor out the local facies controls over the ecological structures of the benthic assemblages. It is furthermore noted that the well documented end-Smithian crisis of nektonic clades are not found in benthic ecosystems. By carefully integrating the variations in the recovery pattern among different clades and different regions, improved models on for environmental disturbances during the Early Triassic will be ultimately achieved.

In a comprehensive study of benthic communities from the late Early Triassic Virgin limestone in Utah, USA, we identified 30 body fossil species and 14 ichnogenera. The Virgin limestone therefore contains the most diverse benthic fauna known from the Early Triassic, challenging previous claims that deleterious environmental conditions still persisted in this region during the Spathian. Analyses of quantitative species level data demonstrate that the ecological structure of subtidal palaeocommunities in the Virgin Limestone is not fundamentally different from that of Middle Triassic shallow-marine habitats. Simple communities are limited to locally stressed environments such as intertidal areas and lagoons, thus providing no indication of post-extinction effects. The high proportion of newly evolved taxa, mainly heteroconch bivalves, additionally suggests that recovery was well underway during the Early Spathian. However, taxonomical differentiation between habitats was still low, indicating a time lag between increasing within-habitat diversity (alpha-diversity) and the onset of taxonomical differentiation between habitats (beta-diversity). The onset of increasing beta-diversity may be useful to distinguish two major recovery phases: the first ending with habitat saturation and the second ending with completion of ecosystem differentiation.

The recovery from the end-Permian biotic crisis is traditionally perceived to have been significantly delayed as a result of persistent deleterious environmental conditions and/or the extreme intensity of the extinction itself with an estimated species loss of 95% in the marine realm.

Previous studies on the Virgin Formation of the Western U.S. put forward the view that harsh environmental conditions still persisted during the Spathian, at least 2 ma after the main extinction. However, palaeoecological analyses on the basis of species-level abundance data have hitherto not been carried out. Thus, very little is known on the actual ecological structure of the Virgin palaeocommunities and their spatial distribution.

We present a quantitative palaeoecological data-set, which is analysed with respect to the identified sedimentary environments. This integrated approach helps to discriminate between possible effects of the end-Permian mass extinction event and local environmental factors on alpha-diversity and ecological structure of the Virgin Fauna. Shallow subtidal environments (see figure 1 for overview) yield the highest species richness and lowest dominance values as recorded in two benthic associations: the Eumorphotis ericius-association and the Protogusarella smithi-association, both of which contain 20 benthic species (bivalves, gastropods, brachiopods, echinoderms, and porifers). Tidal inlet deposits yield a low diverse fauna (Piarorhynchella triassica-association) with a very high dominance of filter feeders adapted to high energy conditions. Another comparably low diverse fauna is recorded by the Bakevellia exporrecta-association, which occurs in deposits of the offshore transition zone encompassing unconsolidated, mostly silicilastic substrates with a low preservation potential for calcareous body fossils. A single sample containing five bivalve species (Bakevellia costata-assemblage) is recorded from a marginal marine setting. The Virgin fauna yields a bulk diversity of 30 species (22 genera) of body fossils and 14 ichnogenera and, thus, represents the most diverse benthic fauna known so far from the Early Triassic.

Our results suggest that oceanographic conditions during the Early Spathian enabled ecosystems to rediversify without major abiotic limitations. However, taxonomical differentiation between habitats was still low, indicating a time lag between increasing within-habitat diversity (alpha-diversity) and the onset of taxonomical differentiation between habitats (beta-diversity). We suggest that taxonomical habitat differentiation after mass extinction events starts only after competition within habitats exceeded a certain threshold, which was not yet reached in the Spathian of the investigated area. This interpretation is an alternative to previous suggestions that the prevalence of generalistic taxa in the aftermath of mass extinction events reflects ongoing environmental stress. The onset of increasing beta diversity is a potential criterion for distinguishing two major recovery phases: the first ending with habitat saturation and the second ending with completion of ecosystem differentiation.

The recovery of benthic ecosystems from the end-Permian mass extinction is generally assumed to have been severely protracted, not starting before the Spathian, approximately 2 m.a. after the crisis. However, the precise succession of fossil communities and their ecological significance has not been studied in detail so far. Bed by bed sampling at previously unstudied localities of the Werfen Formation (Alps, Italy) provided new insights into recovery patterns of benthic biota. Ecological parameters (guild diversity, species richness, dominance, etc.) suggest that the most advanced recovery stages in the investigated time interval (Pre-Spathian Early Triassic) were present during the middle and late Griesbachian. Robust ichnoassemblages in uppermost Griesbachian rocks reflect a significant recovery signal that has not been recognized before. The benthic diversity drastically declines towards the Dienerian/Smithian-boundary possibly as a result of local environmental stress. Our data show that the recovery does not reflect a gradual increase of diversity and ecological complexity throughout the Early Triassic but out-of-phase fluctuations of numerous parameters. For instance, the increase of average body size among bivalves is not contemporaneous with diverse or ecological complex communities. Furthermore, the integration of our new trace fossil data with those from the literature shows that advanced recovery stages were reached quite early after the mass extinction on a global scale. The Early Triassic was possibly a time of volatile environmental conditions that allowed at least local ecological amelioration punctuated by smaller crises that repeatedly replaced faunal association at various stages of recovery.

Although trace fossils have been reported from the Cambrian of Jordan, a detailed analysis of the ichnofauna and its environmental, stratigraphic and evolutionary significance has not been performed yet. Trace fossils from this area are extraordinarily well preserved and provide a distinctive insight in lower Palaeozoic benthic communities. The Burj Fm. comprises three informal members recording a marine incursion on the Arabian-Nubian shield during the Cambrian Stages 4 and 5. The initial transgression is marked by a stressed marginal marine, siliciclastic environment (Tayan Mbr.) and succeeding carbonates (Numayri Mbr.) representing an intertidal to shallow subtidal marine system. The youngest Hanneh Mbr. is in the focus of the present study and records the maximum flooding stage and subsequent regression by the progradation of a tide-dominated delta. Strata of the Hanneh Mbr. yield a relatively diverse ichnofauna including at least 19 ichnotaxa: Bergaueria sucta, Cruziana aegyptica, Cruziana problematica, Dimorphichnus cf. quadrifidus, Diplichnites cf. gouldi, Diplocraterion isp, Gordia marina, Gyrolithes cf. polonicus, Helminthoidichnites tenuis, Monomorphichnus bilinearis, Palaeophycus montanus, Planolites tubularis, Rosselia socialis, Rusophycus aegypticus, Rusophycus salomonicus, Rusophycus cf. leiferikssoni, Rusophycus carbonarius, Skolithos linearis and Treptichnus pedum.
Based on identification of ichno-assemblages and on facies analysis, several depositional environments can be reconstructed. The lower part of the Hanneh Mbr. mainly comprises cross bedded sandstones featuring low diverse Arenicolites-Skolithos and monospecific Diplocraterion-assemblages and is interpreted as a high energy subtidal sandbar complex. Succeeding, rhythmically planar-bedded sandstones and siltstones feature a significant assemblage dominated by Rosselia socialis which represents moderate to low energy conditions during highstand. The middle Hanneh Mbr. is representing a prograding tide-dominated deltaic system including numerous sub-environments. The prodelta and delta-front is characterised by strongly tidal controlled sandstone and siltstone deposition represented by heterolithic sand and mixed flat deposits with diverse trace fossil assemblages. Ripple tops are dominated by simple grazing structures like Gordia and Helminthoidichnites and by subordinate occurrences of Diplichnites. The intertidal zone comprises arthropod-dominated assemblages with Cruziana and Rusophycus. Interbar areas are characterised by low diverse Bergaueria assemblages. The upper Hanneh Mbr. is interpreted as an upper intertidal to supratidal mudflat interfingering with sheet-flood deposits of the delta top giving rise to continental braid plain deposits of the overlying Umm Ishrin Fm.
The trace fossils of the Hanneh Mbr. represent a typical ichnofauna in the aftermath of the agronomic revolution. However, the simple tiering structure in archetypal ichnofacies assemblages and the prevalence of cohesive, muddy substrates reveals the unique character of early Palaeozoic ichnofaunas as a hinge between Precambrian and Phanerozoic nature and extend of bioturbation.

Although trace fossils have been reported from the Cambrian of Jordan, a detailed analysis of the ichnofauna and its environmental, stratigraphic and evolutionary significance has not been performed yet. The Burj Formation (Middle Cambrian) comprises three informal members recording a transgression on the Arabian-Nubian shield by the earliest Phanerozoic.
The initial transgression is characterized by siliciclastic stressed marginal-marine facies (Tayan Member) that evolves into a carbonate dominated shallow marine system (Numayri Member). The maximum flooding stage and following regression is recorded by a shallow marine siliciclastic environment, ending in the progradation of a tide-dominated delta system (Hanneh Member to Umm Ishrin Formation).   
Strata of the upper siliciclastic Hanneh Member, recording tidal flat and shallow subtidal environments in a delta plain to delta front setting, are particularly rich in trace fossils and provide insight into the colonisation and utilisation of infaunal ecospace in Cambrian times.
At least 19 ichnotaxa have been identified, including Archaeonassa isp., Arenicolites isp., Asterosoma isp., Bergaueria sucta, Cruziana aegyptica., Cruziana salomonis, Dimorphichnus cf. quadrifidus, Diplichnites isp., Diplocraterion isp., Gordia marina, Helminthoidichnites tenuis, Helminthopsis isp., Monomorphichnus isp., Palaeophycus tubularis, Phycodes isp., Planolites montanus, Rosselia isp., Skolithos linearis, Treptichnus pedum.
Although a relatively large number of ichnotaxa is observed, individual assemblages are characterised by a rather low diversity. High-energy subtidal sand bars and channels are dominated by vertical structures; these are either monospecific Diplocraterion or Arenicolites-Skolithos assemblages. The tops of abandoned small dunes and ripples are characterised by abundant simple pascichnial worm structures and occasional arthropod trackways such as Diplichnites isp. (Gordia marina assemblage). Arthropod traces, comprising the Cruziana salomonis assemblage are predominantly present in rippled sandstones deposited in interbar and intertidal environments. A distinctive assemblage of Bergaueria sucta is locally present in a similar facies and covers large surfaces when structures produced by arthropods are absent. The Burj ichnofauna records a typical Phanerozoic benthic ecosystem in the aftermath of the agronomic revolution. Although tiering structure is simple, ichnoguild analysis reflects a relatively efficient management of resources and ecospace by Middle Cambrian times.