Palaeoecology of a post-extinction reef: Famennian (Late Devonian) of the Canning Basin, north-western Australia

P A L A E O E C O L O G Y O F A P O S T - E X T I N C T I O N R E E F :

F A M E N N I A N ( L A T E D E V O N I A N ) O F T H E C A N N I N G

B A S I N , N O R T H - W E S T E R N A U S T R A L I A

by R A C H E L W O O D

ABSTRACT. Reefs were decimated by the Frasnian/Famennian (Late Devonian) mass extinction event (371 Ma), and areassumed to have survived only as depauperate calcimicrobial communities dominated by disaster taxa. Description ofFamennian proximal reef-slope communities within the Windjana Limestone, Canning Basin, north-western Australia,shows that, notwithstanding the loss of large metazoans, novel ecologies were established in this setting by a rich biotaof survivor and progenitor taxa. Diverse calcimicrobes together with algae, crinoids, bryozoans, brachiopods, andabundant sponges (stromatoporoids, inozoans, sphinctozoans, lithistids and hexactinellids) formed a reef framework ofeither elevated platy structures up to 4 m in diameter and 0·35 m thick, or mounds up to 15 m in diameter. Thisframework was dominated by a complex intergrowth of calcimicrobes, where Rothpletzella formed the primaryframework, Ortonella, and Girvanella were secondary encrusters, and Shuguria spp. occupied small crypts 2–30 mmin diameter. Contiguous columnar stromatolites up to 50 mm in height and 1 m in width grew upwards from substratesheltered beneath large sheltered primary cavities: based on minimum growth rates of 50–100 mm/year these areestimated to have been between 500 and 1000 years old. The elevated platy community is inferred to have grown inconditions of episodic siliciclastic sediment input; the reef mounds grew during either episodes, or in localised areas,of low sedimentation. At least 14 morphospecies of spicular sponges are now identified from the Windjana Limestone,where only two were previously documented. These fore-slope reef communities exposed in Windjana Gorgeflourished in high-energy carbonate environments dominated by coated grain sediments, and where rapid, earlylithification was pervasive.

Such observations demonstrate that no protracted interval of time was necessarily required for post-extinction‘recovery’ in regions where some reef-building taxa survived and suitable carbonate habitats persisted or returned.Moreover, they show that new ecologies, rather than remnants of the pre-extinction community, could be establishedrapidly. The reef-slope communities of the Windjana Limestone offer little evidence to support the ideas ofresurgence or invasion of taxa from deeper waters after the Frasnian/Famennian extinction. Indeed, there is evidenceto suggest that similar microbial-sponge communities were already established in margin and reef slope communitiesin the latest Frasnian. As such, the most dramatic ecological changes caused by the extinction occurred in back-reefcommunities.

KEY WORDS: mass extinction, Late Devonian, Famennian, Canning Basin, reefs, palaeoecology, calcimicrobes.

M A S S extinctions are apportioned a special status within the study of evolution, and research into theircauses and effects has intensified since publication of the seminal paper on the end-Cretaceous bolideimpact by Alvarez et al. (1980). Less work has focused, however, on post-extinction recovery, and noconsensus has yet emerged as to the importance of mass extinctions in either shaping the history ofbiodiversity or the degree to which they may influence the development of ecosystems (see review inConway Morris 1999).

Mass extinctions in the fossil record, as with other evolutionary events, are usually analysed by changesin taxonomic diversity through time. This requires sound biostratigraphy and correlation, as well as securetaxonomy. Although skeletal marine organisms provide the best fossil record, this is still very incompleteat the species level and subject to considerable sampling and preservational biases (e.g. see McLaren1988). For example, the best time resolution possible is usually limited to 103–104 years, and socompilations of global diversity data are often biased towards the most abundant, widespread and

[Palaeontology, Vol. 47, Part 2, 2004, pp. 415–445, 1 pl.] q The Palaeontological Association

geologically long-lived species (Jablonski 1995). And while such data can identify mass extinctionepisodes, they do not reveal details of timing or local geographical information.

Perhaps most importantly, poor phylogenetic knowledge of clades that survive extinction can lead to anunderestimation of the number of surviving taxa which may magnify not only the apparent severity of theextinction event (Erwin and Pan 1996; MacLeod et al. 1997) but also estimates of ‘endemic’ extinctions(Smith and Jeffrey 1997). This is a particularly acute problem with morphologically simple reef organismssuch as corals, sponges, and calcified algae, where an abundance of convergent characters results in anotably rudimentary understanding of ancestor-descendant relationships.

Mass extinctions are complex phenomena, and often involve the preferential elimination of some partsof an ecosystem, such as phytoplankton at the base of the foodchain, that cause a variety of cascadingecological effects. Yet compilations of numbers of taxa may reveal little of ecological importance forunderstanding extinction events (Jackson 1995), and indeed changes in diversity may not be astraightforward expression of the magnitude of ecological change (Droser et al. 1997, 2000). It istherefore surprising that few analyses of post-extinction recovery have concerned the changing palaeo-ecology of communities, even though the effect of mass extinctions cannot be appreciated fully withoutsuch considerations.

This paper describes in detail the palaeoecology of a reef-slope community that formed immediatelyafter the Frasnian/Famennian (Late Devonian) mass extinction event (371 Ma), as exposed in theFamennian Windjana Limestone within the celebrated succession at Windjana Gorge, in the CanningBasin, north-western Australia (Text-fig. 1). The Canning Basin is a key area for understanding the globalchanges at the Frasnian/Famennian boundary (see Becker et al. 1991; House 1996; Copper 2002; Georgeand Chow 2002), and Windjana Gorge, in particular, presents a unique, apparently continuous sequence ofreef-associated facies from the mid-Frasnian to early Famennian. The crisis at the end of the Devonian wasone of the most significant during the Phanerozoic (Sepkoski 1982), and this locality also offers arguablythe finest outcrops available to study detailed ecological relationships within any Late Devonian reefsystem. Analysis of such in situ communities overcomes many of the potential problems associated withunderstanding the ecological effects of mass extinction events, such as the presence of reworked fossils or

416 P A L A E O N T O L O G Y , V O L U M E 4 7

TEXT-FIG. 1. Geological map showing Windjana Gorge within the Late Devonian reef complexes of the north CanningBasin, north-western Australia (modified from Playford 1980 and George et al. 1994).

inadequate sampling, and eliminates the risk of biases introduced by exceptional preservation or facieschanges (MacLeod et al. 1997). The study of a continuous carbonate sequence also introduces taphonomiccontrols (Bottjer and Jablonski 1988).

There is a consensus that reef communities are particularly susceptible to the factors that cause massextinction, and that recovery occurs more slowly than in other communities, with reefs often taking2–10 myr to reappear in the geological record (e.g. Scrutton 1988; Talent 1988; Copper 1989; Kauffmanand Fagerstrom 1993; Jablonski 1995; Erwin 1996; McGhee 1996; Hallam and Wignall 1997). Implicitwithin many of these analyses is the assumption that reefs take longer to re-establish not because of thecontinuation of environmental stresses hostile to reef formation, but because of the time necessary torestore the high biodiversity and complex ecological interactions that characterise tropical reef com-munities. Such a contention is not, however, supported by consideration of the Windjana Gorge reefsequence (Wood 2000b). While received opinion is that reefs persisted only as localised, highlyimpoverished calcimicrobial communities after the Frasnian/Famennian mass extinction (e.g. Scrutton1988; Geldsetzer et al. 1993; Copper 1994, 1997, 2002; Fagerstrom 1994; McGhee 1996; Hallam andWignall 1997), this study demonstrates that in fact a complex and novel community constructed by adiverse biota was established in the reef-slope environment in the immediate aftermath of the event.

T H E L A T E D E V O N I A N M A S S E X T I N C T I O N

Extinction of reef biota

The Mid to Late Devonian (Givetian–Frasnian) is considered to represent possibly the largest globalexpansion of continental carbonate platforms (Wilkinson and Walker 1989; Kiesling et al. 2000) and reefs(Copper 1989) in the Phanerozoic. During this interval, climate was equable and sea level high: at theiracme during the Eifelian–Givetian, reefs periodically extended to latitudes as high as 458S and possibly608N (Copper 2002). Substantial Late Devonian reefs are found throughout Europe, western North Africa,South China, south-east Asia, and extensive reef tracts, some exceeding in size those of the present day, areknown from Canada, central Asia, and the Canning Basin, north-western Australia.

The crisis at the end of the Devonian is one of the major extinctions of the Phanerozoic, with 57 per centof genera going extinct (Sepkoski 1995). This extinction can in fact be resolved into three distinctstep-down events that occurred during the Givetian, Frasnian/Famennian (F/F), and end-Famennian(Text-fig. 2). Of these, the F/F event represents a significant period of extinction comparable in magnitudeto the end-Cretaceous event (Sepkoski 1995). Most extinction occurred during two intervals, thePalmatolepis rhenana and Palmatolepis linguiformis conodont zones of the late Frasnian, and these areknown as the early and late Kellwasser events (Buggisch 1991). Extinction is notable in both benthic andpelagic marine groups, but is generally accepted to be most severe in equatorial shallow-marineecosystems; recent estimates suggest that 60–85 per cent of skeletal and reef-building genera wentextinct (Copper 2002). The miospore record also shows major extinctions and floral turnover, suggestingthat a significant terrestrial crisis was coincident with the marine event (Bless et al. 1992; Streel et al.2000).

Many reefs and associated platforms were drowned during the mid–late Givetian Taghanic Event(House 1985), but reef-building had recovered by the early Frasnian. In striking comparison to thewidespread distribution of Frasnian reefs, estimated to cover some 5 million sq. km, Famennian shallow-water reefs are relatively rare, known only from the Canning Basin, Alberta and Omolon, north-easternRussia, and the north Caspian region, and probably covered less than 0·5–1 million sq. km (Copper 2002).However, other estimates suggest that no reduction in the aerial extent of carbonate platformsaccompanied this substantial decline in reef development. Based on lithofacies maps, Kiesling et al.(2000) estimated shallow carbonate platforms to have covered approximately 1·18 · 107 sq. km duringboth the Frasnian and Famennian, although carbonate production itself declined by more than 70 per centafter the F/F boundary (Bosscher and Schlager 1993; Kielsing et al. 2000; Copper 2002). Many reefs inEurope drowned during the major early Frasnian transgressions, but a global decline in the abundance ofreefs occurred within or below the rhenana Zone, below the F/F boundary (Copper 2002). Reefsdisappeared or reduced in size and abundance during a protracted stepdown series lasting more than

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1·5 myr but culminating at the F/F boundary, and show a variable regional response, although detailsremain unclear due to the lack of precise dating of reef-growth cessation in a number of areas (Copper2002).

Although the F/F mass extinction removed a substantial proportion of invertebrate reef-builders anddwellers, diversity had began to decrease over a long interval during the Givetian and Frasnian from amaximum in the mid-Givetian (Fagerstrom 1994) and did not terminate until the Devonian/Carboniferousboundary. During the Frasnian numerous reef-associated brachiopod families were eliminated, mostnotably from tropical (Copper 1997; Racki 1998) and mid- and outer-shelf settings (Bratton 1996). In theFrasnian of the Canning Basin, however, atrypid brachiopods show relatively low generic diversity(Grey 1978). Bryozoans suffered substantial extinction at the Givetian/Frasnian boundary, but werelittle affected by the later Devonian crises. Large and complex foraminiferans appear to have gonepreferentially extinct during the F/F event.

Corals suffered major extinction. However, the most severe extinction in rugose corals (Scrutton 1998)and stromatoporoids (Stearn et al. 1999) occurred at the end of the Givetian; half of tabulate, rugosan, andstromatoporoid genera were also lost. Only 39 per cent of Devonian coral suborders and superfamiliessurvived into the Carboniferous (Scrutton 1997). Rugosan diversity peaked in the Eifelian, plummetedaround the Taghanic Event, but recovered rapidly to diversify in the Frasnian to yield cosmopolitan faunaslargely within a single suborder, the Columnariina (Scrutton 1988; Poty 1999). This radiation was abruptlycurtailed coincident with the F/F boundary: of the 43 Frasnian rugosan genera, only 14 are known from the


TEXT-FIG. 2. Reef communities collapsed during the Late Devonian in a series of extinction events. The reduction inglobal abundance of reefs is probably related to the loss of carbonate environments owing to a combination ofregression and global cooling. Carbonate production curve (Bosscher and Schlager 1993); sea-level curve (Johnson

and Klapper 1992); rCO2 estimate curve (Berner 1994) (from Copper 1997).

Famennian. The crisis appears to have had a differential effect on rugosan species, removing nearly allshallow-water but only a few solitary, deep-water forms (Sorauf and Pedder 1986). These rugosanssubsequently diversified in the Famennian, mainly from deep-water Frasnian species that invaded vacatedshallow environments; colonial taxa had re-appeared by the late Famennian (Pedder 1982) and likewise,Heterocorallia flourished in the Famennian (Scrutton 1998). Tabulate corals also suffered major extinctionduring the Kellwasser event, perhaps as many as 80 per cent of genera having been removed (McGhee1996). All Famennian survivors, however, crossed the Devonian/Carboniferous boundary (Scrutton 1988).These diversity patterns are similar worldwide, consistent with global, environmentally-driven causalcontrols, although the Canning Basin reefs show comparatively low generic coral diversity (Hill and Jell1970).

Total diversity of stromatoporoid sponges in the Middle Devonian was about 64 genera, with 38recorded from the Frasnian; the end-Devonian extinction reduced the number of genera to about 47 percent, and by the end of the Frasnian 74 per cent of remaining genera were lost (Stearn et al. 1987; Copper2002). The clathrodictyids persisted, but only the labechiid stromatoporoids showed significant recoveryafter the F/F crisis (Stearn 1987), although this group became extinct in the late Famennian (Fagerstrom1994). In the Canning Basin, Cockbain (1984) recognised 12 genera and 25 species of Givetian–Frasnianstromatoporoids, but only two species and genera in the Famennian. Early Famennian genera are alsosmall in comparison to the substantial size achieved by Frasnian representatives, although there waslimited re-emergence of stromatoporoids as reef framebuilders later in the Famennian (Cockbain 1984;George et al. 1997). In contrast to stromatoporoids, hexactinellid sponges appear to have proliferatedduring the F/F interval in sites occurring close to the Devonian equator (McGhee 1982).

Reef-associated calcimicrobes appear to have been little affected by the extinction but, surprisingly, allwent extinct in the mid to late Famennian, a time when other groups were radiating. Only Shuguria(¼Renalcis) survived into the Tournaisian (Chuvashov and Riding 1984).

Causes

The F/F boundary events remain controversial, both in terms of the timing and number of events, as well asthe possible causes. The late Devonian extinctions were clearly complex, with biotas responding in avariety of ways. As a result, several hypotheses have been promoted to explain the extinct events. Somefavour a bolide impact as the cause of extinction (e.g. McGee 1996). The most persuasive evidence is thepresence of shocked quartz, a modest iridium anomaly, and in particular a spectacular 70-m-thick brecciabed in early Frasnian sequences of Nevada and adjacent states (Leroux et al. 1995). This hypothesis hasfound little favour, however, as the dating of the impact does not appear to coincide with the majorextinction intervals (Joachimski et al. 2001), and the disappearance of shallow marine taxa in a series ofstep-down extinctions argues against a single catastrophic event.

The Late Devonian extinctions have been explained most persuasively by scenarios involving rapid sealevel oscillations, possibly with associated anoxia, combined with dramatic climate change, particularlyglobal cooling. Sea level rose during the Late Devonian, but the F/F boundary is marked by a shift to highfrequency sea level fluctuations, possibly regressive cycles that exposed large continental areas (Johnsonand Klapper 1992). Some have argued that the F/F boundary lies within a global relative highstand(Hallam and Wignall 1999); others have recognised a major regression (Racki 1998). The effects ofwidespread extensional tectonism and volcanism in Eurasia have been proposed by Racki (1998) to havehad further disruptive effects.

It has been suggested that the early and late Kellwasser events are associated with a sea level rise andhigh global temperatures, and may have been related to the upwelling of basinal, cold, oxygen-deficientand nutrient-rich waters onto shallow, carbonate shelf settings, particularly during subsequent regression(Buggisch 1991; House et al. 2000). This is evidenced by the development of anoxic/dysoxic sediments(including black shales) in Germany, South China, and France, which show a close correspondence to theextinction events (Hallam and Wignall 1997).

Although the mechanism to explain the widespread generation of oxygen-poor shelf waters is notclear, such a scenario would explain the preferential loss of warm-water taxa (for example atrypid and


pentamerid brachiopods), the preferential survival of dysoxia-tolerant benthos (ostracods and bivalves),deep-water and high-latitude brachiopods, and ammonoids, and the proliferation of deep-water rugosecorals, and hexactinellid sponges (normally encountered in deep, cool waters) near the Devonian equator(see McGhee 1996). Only the labechiid stromatoporoids show significant recovery after the Kellwassercrisis, perhaps owing to their preference for cold waters (Stearn 1987). The stepped extinction pattern ofcricoconarid families during the Kellwasser crisis might also be indicative of progressive deterioration inwater conditions, although little is known of the ecology of this extinct group. Tasmanaceans are usuallyassociated with anoxic and/or cool waters, and chitinozoan blooms may likewise have been triggered bycool or nutrient-rich waters (Hallam and Wignall 1997).

More recent analyses, however, based on highly accurate global species compilations, point to the maininterval of increased extinction being just after the deposition of the Kellwasser black shales (seesummaries in Racki and House 2002). At the end of the Givetian, a precipitous drop in atmospheric CO2

began which is coincident with the Taghanic crisis (Text-fig. 2). This drop in CO2 continued untilc. 320 Ma, marking a dramatic reduction in global temperatures. This, in combination with globalregressions, a shift in chemical weathering patterns of the continents, and the start of substantial soilproduction, all point to a time of massive global disruption (Copper 2002).

Assessment of the effects of cooling, however, requires systematic geochemical data that are not yetavailable. The limited isotopic data currently available (e.g. Playford et al. 1984; Joachimski and Buggisch1994) are not conclusive and do not offer evidence for the circulation of an increasingly cooler ocean.

R E E F S O F T H E C A N N I N G B A S I N

Plate reconstructions for the Late Devonian place north-western Australia in an arid subequatorial beltsome 10–158S (e.g. Scotese 1984; Baillie et al. 1994). Reefs developed along the northern margin of theCanning Basin (Text-fig. 1), on the shallow north-eastern flanks of the fault-controlled, north-west–south-east trending Fitzroy Trough (Yeates et al. 1984), which formed during significant crustal extension in theMid to Late Devonian (Begg 1987; Drummond et al. 1991). This reef-rimmed platform is known asthe Lennard Shelf, and fringed the adjacent mountainous Proterozoic landmass of the Kimberley Block tothe north-east (Playford 1980; Begg 1987; Playford et al. 1989). A great variety of reefs is exposed,including atolls, and fringing-, barrier- and patch-reefs that formed a belt extending for 350 km and up to50 km wide. Reef limestones reach a maximum thickness of 2000 m (Playford 1980; Playford et al. 1989),and developed from the Givetian to the Famennian, so recording a history of some 15 myr of continuousreef-building that evolved in an active extensional regime.

During the Frasnian and Famennian, contemporary faulting is believed to have produced themountainous topography in the adjoining landmass of the Kimberley Block, which became the sourceof vast quantities of coarse-grained siliciclastic sediments. Sandstones and conglomerates were trans-ported across the platform in the form of fan-deltas, which were deposited contemporaneously withcarbonate sediment on the platform and slope but limited the landward development of the carbonateplatform (Playford and Lowry 1966; Read 1973a, b; Holmes and Christie-Blick 1993). As the reef-complexes developed in an active rift basin, probably in a series of half-grabens along its length (e.g. Begg1987; Drummond et al. 1988), it is likely that tectonically controlled uplift and eustacy combined toproduce marked relative sea-level fluctuations throughout reef development.

The carbonate platforms of the Canning Basin were flat-topped, rimmed by reefs, and flanked by steeplydipping marginal-slope deposits composed largely of platform-derived sediments (Text-fig. 3). Fringingreefs developed as narrow, discontinuous rims, commonly only 5–10 m wide (Wallace et al. 1991), thatgrew on precipitous, eroded and scarp-like margins several hundreds of metres (George et al. 1997; Ward1999) above the surrounding basin. Contemporary collapse of the early-cemented platform margin,probably triggered by either over-steepening or episodic tectonic activity, produced rock falls and viscous,clast-rich debris flows that led to the deposition of large allochthonous blocks, and interbedded talusbreccias and turbiditic grainstones within the marginal-slope deposits (George et al. 1994, 1997).

During the Givetian–Frasnian, carbonate platforms in the Canning Basin aggraded almost vertically(the Pillara phase), with backstepping of the reef margins and related pinnacle-reef development in the


Frasnian, but during the latest Frasnian or Famennian a rapid progradation of the reef (the Nullara phase)over associated marginal-slope deposits began (Playford 1980, 1981; Kennard et al. 1992; Becker et al.1993; Becker and House 1997). The Famennian reef margin has been interpreted as an advancing rollovermargin (Playford 1984). These phases have now been interpreted to be a response to a major transgressive-regressive cycle (Southgate et al. 1993), in response to slower subsidence rates (George et al. 1997). Theapplication of sequence stratigraphy to seismic well data has led to the further recognition that each cycleconsists of successively onlapping depositional sequences that reflect short-term sea level cycles (Kennardet al. 1992).

George and Chow (2002) have summarised the regional sedimentological changes that occurred acrossthe F/F boundary in the Canning Basin. Stromatoporoid-dominated reef margins appear to have been indecline before the F/F boundary, as composition of allochthonous blocks in the Napier Range fore-reefsuccession already shows a dominance of Shuguria-microbial limestones in the rhenana Zone. There isno evidence for any break in sedimentation at the conodont-defined F/F boundary in any of studied


TEXT-FIG. 3. Schematic reconstruction of a typical Late Devonian reef margin, Canning Basin, north-western Australia(modified from George et al. 1997).

continuous boundary sequences including Horse Spring, McWhae Ridge, and Casey Falls, or for a singledistinctive event bed, but George and Chow (2002) concluded that the F/F boundary occurs within amappable interval of up to 15 m thick characterised by a marked absence of contemporaneous shallowwater carbonate sediments and Frasnian macrofossil content. Although this unit is expressed by variedlithologies the interval is notably condensed, indicative of reduced sedimentation rates and sedimentreworking. Siliclastic lithologies and abundant sponge debris are locally present, and there is no evidencefor anoxic or hypoxic conditions during deposition. The described F/F lithologies are consistent withstarvation of the slope due to reduced carbonate productivity of the platform during a period of loweredrelative sea level, supporting interpretations of a major transgression immediately prior to the boundary(Kennard et al. 1992; Southgate et al. 1993) and previous analyses suggesting deposition under fullyoxygenated conditions (Becker et al. 1991). These observations are also compatible with other evidencefor several rapid and strong sea-level changes during the latest Frasnian (Becker et al. 1991), and forsubaerial exposure followed by margin collapse in the Napier Range (George et al. 1997; George andPowell 1997).

Windjana Gorge displays a classic section of Frasnian Pillara Limestone (reef-margin, reef-flat, andback-reef sediments), coeval Napier Formation (reef-slope and fore-reef), and Famennian WindjanaLimestone (reef-margin) (Text-fig. 4). The well-bedded back-reef sediments of both the Frasnian andFamennian become progressively more thickly bedded approaching the platform margin, which ischaracterised by massive sediments with abundant subparallel neptunian dykes and other fissures.Margin-slope deposits are steeply-bedded (up to 30 degrees), and contain allochthonous blocks indicativeof collapse of the early-cemented platform margin.

Back-reef ecologies within the late Frasnian Pillara Limestone of Windjana Gorge are dominated bymicrobial communities (Wood 2000a). Proposed microbialites are expressed as weakly laminated,fenestral micrite that show unsupported primary voids, peloidal textures, disseminated bioclastic debris,and traces of microfilaments. These grew as either extensive free-standing mounds or columns, oftenintergrown with encrusting metazoans, or thick post-mortem encrustations upon skeletal benthos. In somecases, microbial encrustations are inferred to have developed in protected cavities formed by progressiveburial of the reef. The calcimicrobe Shuguria also shows a preferentially cryptic habit, encrusting eitherprimary cavities formed by skeletal benthos, microbialite, or the ceilings of mm-sized fenestrae withinmicrobialite. A further calcimicrobe, Rothpletzella (Rothpletz, 1890) (¼ Sphaerocodium) formedcolumns up to 0·3 m high in areas enriched by very coarse siliciclastic sediment. Stromatoporoidsponges with a diverse range of morphologies also formed in situ growth fabrics. Monospecific thicketsof closely-aggregating dendroid stromatoporoid sponges (Stachyodes costulata), and platy-laminarforms (?Hermatostroma spp.) were common, as were a variety of remarkably large stromatoporoids(Actinostroma spp.) that grew as isolated individuals up to 7 m in diameter (Wood 2000a).

The palaeoecology of the Frasnian reef-margin itself has not been documented in detail, but appears tohave been dominated by the calcimicrobes Shuguria Antropov, 1950 (¼Renalcis of Playford 1980, 1981;Webb 1996; Wood 1998, 1999; re-assigned by Riding 1991) in shallow waters, and Rothpletzella in deeperparts (Playford 1981), together with abundant clotted, fenestral micrite and large, tubular and cup-shapedlithistid and hexactinellid sponges.

The location of the F/F boundary has been established in the Windjana Gorge succession by conodontbiostratigraphy (George and Powell 1997). A thin, laterally continuous stromatolite biostrome is present atthe boundary, separating the stromatoporoid-bearing breccias below from relatively poorly fossiliferousstrata above (George and Chow 2002), but as in other boundary sections in the Canning Basin, there is noobvious lithological change.

L O C A L I T I E S A N D M E T H O D S

Reef fabrics within the Famennian Windjana Limestone crop out for up to 900 m on the north and southsides of the western entrance of Windjana Gorge, within Windjana Gorge National Park (Text-fig. 4).Windjana Gorge is dissected by the Lennard River, and is nearly 4 km long, with an average width of200 m. The high gorge cliffs (up to 60 m) are precipitous, but annual flooding of the Lennard River


produces numerous readily accessible, river-polished outcrop surfaces at the base of the cliffs, whichextend for some 200 m from the western entrance on both sides of the gorge.

Specimens were either slabbed and polished, or thin-sectioned. Studied material is lodged in theSedgwick Museum, Cambridge (prefix SMX).

S E D I M E N T O L O G I C A L S E T T I N G

The Windjana Limestone lithologies are characteristic of energetic, mixed siliciclastic-carbonate, andhave been described as representing back-reef, reef-margin to proximal slope environments (Playford1981). Beds are up to 1·5 m thick, composed of medium–coarse quartz-feldspathic sandstone, peloidallimestone and mudstone, coated-grain (ooids and peloids) packstone, and some grainstone and wacke-stone. The peloidal limestone is composed of fine sand-sized, round to subangular peloids, intraclasts andskeletal debris, and abundant coarse, angular, quartz-feldspathic clasts. Many neptunian dykes and fissuresare common, which are filled with oolites and early cements.

The reef communities described here grew within proximal-slope settings characterised by silty–sandy,oolitic or grainstone beds dipping from 10 to 30 degrees. These may represent turbidites and debris flows.Some horizons contain eroded, possibly embayed, allochthonous clasts of reef talus ranging from 10 to250 mm in diameter.

Within this setting are common beds up to 2·5 m thick consisting of pelleted, fenestral limestones. Theirregular fenestrae from 10 to 110 mm in length and from 2 to 15 mm in width form stromatactis structuressubparallel to the sedimentary surfaces (Text-fig. 5). The bases of the stromatactis are at a high angle ofapproximately 45 degrees to vertical, with flat bases and irregular ceilings. The stromatactis formsswarms, which are noteworthy for a regular increase in size from the base towards the top of themound-like structures (Text-fig. 5A). The fenestrae are lined by radiaxial calcite, followed by a spar infill(Text-fig. 5B).


TEXT-FIG. 4. Late Devonian reef complex, Windjana Gorge area. A, geological map. B, diagrammatic cross-section withhypothetical development of the reef (modified from Playford 1981).

C O M M U N I T Y P A L A E O E C O L O G Y

The community found in reef-slope settings within the Windjana Limestone is expressed in twoconstructional forms, described below.

Platy calcimicrobial-sponge community

The most striking Famennian reef community is manifested by diverse calcimicrobes and siliceoussponges that grew to form a previously undocumented reef framework type of laminar, sheet-like growths(Wood 2000b). These structures initiated as small, millimetre-scale laminar growths upon the substrate,dominated by the calcimicrobe Rothpletzella magnum Wray, 1967 (Text-fig. 6). Advancing growth atthe edges produced a broadly circular shape in plan, indicative of unconstrained lateral expansion(Text-fig. 7A). As Rothpletzella extended laterally, it also accreted vertically, and small-scale, paler-coloured projections on upper skeletal surfaces are interpreted to be early growth phases of such advancinggrowth (Text-fig. 7B). Rothpletzella formed broadly flat to rounded upper surfaces but highly unduloselower surfaces that enclosed either a single or a series of primary cavities beneath (Text-figs 8–9).

With continued growth, the laminar Rothpletzella structures increased in diameter and thickness, toreach impressive sizes: up to 4 m in length and 0·35 m in thickness (Text-fig. 10). Also, further platyoutgrowths commonly formed tangential to the ‘primary’ horizontal growths, to form highly complex,elevated and platy morphologies (Text-fig. 11). The plates are often inclined parallel to bedding.

As these structures grew, further taxa colonised the primary cavities and open surfaces: the red algaSolenopora geikiei Wray, 1967, and green alga Ortonella imprimis Wray, 1967 (Text-fig. 12) formedcryptic and laterally-projecting growths, and the calcimicrobe Girvanella ducii Wethered, 1890,commonly encrusted Rothpletzella magnum. This diverse calcimicrobe community was joined by a richvariety of other attached metazoan benthos, including rare, small solitary rugosan corals and cryptic


TEXT-FIG. 5. Stromatactis swarms from the Windjana Limestone, Windjana Gorge, north-western Australia. A, generalview showing the gradual increase in size of stromatactis from the base to top; · 0·15. B, detail; · 0·4.

fistuliporid bryozoans (Text-figs 13C, 14D), the encrusting to nodular stromatoporoid spongesClathrocoliona saginata and Stromatopora lennardensis, the green alga Litanaia perisseia Wray, 1967(Text-fig. 13B), encrusting Wetheredella sp. (Text-fig. 14B) and a problematicum (Text-fig. 13A). Thestromatoporoids are hemispherical or laminar encrusting, and reach up to 150 mm in diameter. Crinoidholdfasts and stems also project downwards from the edges of the platy growths (Text-fig. 14B), andbrachiopods grew vertical or sub-vertical attached to the calcimicrobial sheets (Text-fig. 9B, E).

The cavities are commonly encrusted by cryptic, pendent bushes of the calcimicrobe Shuguriadevonicus Johnson, 1964 (Pl. 1; Text-figs 9F, 14A, C, 16). Primary crypts can be up to 300 mm in diameterand height, and are commonly partially or wholly filled with laminated and graded geopetal sediment,usually graded ooids or peloids, grainstone, or fenestral micrite, with angular silt-grade quartz up to150 mm thick. A few large gastropods and abundant large ostracods 2–5 mm in diameter are commonlypresent within geopetal infills (Pl. 1, fig. 2). Remaining cavity space is first filled with a crust of radiaxialfabrous calcite (Text-fig. 12), sometimes followed by a generation of scalenohedral calcite (Pl. 1, fig. 2),then sparry calcite (Text-fig. 12).


TEXT-FIG. 6. Field photographs of juvenile Late Devonian (Famennian) laminar calcimicrobe-sponge community,Windjana Limestone, Windjana Gorge. A, ‘swarm’; · 0·2. B–C, detail showing development of primary, cement-filled

cavities with pendent Shuguria (arrowed); B, · 0·5; C, · 1.

When the laminar growths reached a thickness of approximately 70 mm, the community was joined byabundant attached lithistid, hexactinellid, inozoan and sphinctozoan sponges (Text-fig. 16A–B). Thesesponges range from 5 to 110 mm in diameter, and up to 300 mm in length. These projected bothdownwards within crypts, and aggregated to form clusters upon the upper surfaces of the sheet-complexes,either vertically or horizontally. Many are vase- or cup-shaped; others are branching, tubular or explanatewith substantial holdfast structures. Encrusting sponges are often found on the upper surfaces of the platy


TEXT-FIG. 7. The growth of laminar Rothpletzella. A, sketch of proposed mode of advancing edge growth in laminarRothpletzella. The form grew laterally while increasing in thickness vertically, enclosing primary framework crypts.B, field tracings of upper surface of laminar Rothpletzella, showing advancing uppermost front of the growing skeleton.

Primary framework cavities are commonly encrusted by cryptic Shuguria.

TEXT-FIG. 8. Field tracings of juvenile calcimicrobe-sponge community, showing laminar growth with multipleoutgrowths and the development of enclosed cavities. Many cavities were lined with radiaxial fibrous cement followedby later sparry calcite; stromatolites developed upon sediment beneath larger cavities, which later became smothered

by sediment. North side of Windjana Gorge, western entrance.

TEXT-FIG. 9. Field photographs of Late Devonian (Famennian) platy calcimicrobe-sponge community, WindjanaLimestone, Windjana Gorge. A, intermediate-sized laminar growths; · 0·08. B, detail of arched platy growth, showingdevelopment of primary radiaxial calcite cement-filled cavity with geopetal infill (G). Note attached brachiopod(arrowed); · 0·4. C–D, laminar growth; C,· 1; D, · 1·1. E, platy growth, showing development of primary cement-filledcavity with geopetal infill (G). Two in situ brachiopods are attached within the calcimicrobial community (arrowed);· 0·4. F, undulose growth of Rothpletzella, with a thin crust of cryptic Shuguria growing from walls and ceilings on a

primary cavity; · 0·3.


growths. Although these siliceous sponges are now replaced by calcite, making exact species determina-tion difficult, at least two sphinctozoan and eight lithistid (?anthaspidellid), and five hexactinellidmorphospecies have been identified (Table 1). The branching form of ?Playfordiella (Text-fig. 15) ispreviously undescribed. Laminated autochthonous micrite is a common encrustation upon projectingsponges (Text-fig. 15).

Contiguous columnar stromatolites from 20 to 70 mm in width and up to 50 mm in height grew up fromthe floor of cavities that formed under the more mature platy growths (Text-fig. 16C). These are red incolour, and show wavy micritic-iron oxide-rich laminae from 0·2 to 1 mm in thickness. Sometimes thesecoalesce into larger domal heads. Many were fully encased within the subsequent geopetal infill.

Calcimicrobial-sponge mounds

The calcimcrobial-sponge community is also expressed as massive, mound-like growths up to 3 m thickand 15 m in diameter. These mounds are dominated by Rothpletzella magnum and Shuguria devonicus


TEXT-FIG. 10. Field photographs of mature Late Devonian (Famennian) platy calcimicrobe-sponge community,Windjana Limestone, Windjana Gorge. Note the thickening of the plates towards the right. A,· 0·04. B, · 0·1.

Johnson, 1964 together with abundant sponges (Text-fig. 17). The sponges grew either vertically inaggregations on the undulose upper surfaces or within the mounds.

Large primary cavities and sub-vertical fissures are common (Text-fig. 17B–D). These are often highlycomplex, composite structures up to 1 m in length and 0·5 m in width. The fissures are often occluded byvery thick encrustations of Rothpletzella magnum Wray, 1967, which appears as near-symmetrical, finelylaminated growths up to 150 mm thick (Text-figs 17C–D, 18). Remaining void space is occluded by layersof fibrous calcite, often with bioclastic-rich geopetal infills (Text-fig. 17C).

D I S T R I B U T I O N O F C O M M U N I T I E S

Thin, juvenile Rothpletzella magnum laminar growths appear to be characteristic of proximal slope-marginenvironments, as they are found in inclined beds and associated with eroded, embayed clasts of thecalcimicrobial-sponge mounds, sandstone-rich and ooid-rich beds, and toppled sponges. Their growth fromsuch small laminar encrustations, through large platy forms, to full-scale mounds may have been hamperedby the high rates or episodic nature of sedimentation. Mature, highly elevated and platy growths are inferredto have grown where swamping by sediments was not prevalent. Reef mound growth is inferred to haveformed in either localised areas where sediment input was minimal, or during periods of quiescence.

E A R L Y D I A G E N E S I S

The Canning reef complexes show a highly complex history of repeated fracturing, infilling andcementation. Many reefs show spectacular quantities of early marine cements within back-reef, reefflat, reef-margin and reef-slope settings, which reflects the abundance of both large, intact growthframework cavities, and syndepositional fissures and fractures that formed within strongly cemented partsof the reef (Kerans et al. 1986; Wood 1998, 2000a).

Rigidity was imparted to Shuguria by the precipitation of microcrystalline cement. The isotopicsignatures of microcrystalline cements support their interpretation as early marine precipitates (Hurley andLohmann 1989), and these may even have formed contemporaneously with Shuguria (Kerans et al. 1986).Radiaxial calcite is often the next precipitate after microcrystalline cement, forming a single or a fewbands particularly within primary reef framework cavities (Pl. 1, fig. 1). These cements formed early


TEXT-FIG. 11. Mature examples of Late Devonian (Famennian) platy calcimicrobe-sponge community, WindjanaLimestone, Windjana Gorge, showing the development of platy tangential platy outgrowths. A, field photograph;

· 0·025. B, field tracings.

within the Famennian reefs, as small-scale fissures are lined with the same generation of radiaxial cementas primary crypts. The spectacular banded fills that account for 20–50 per cent of the total rock volume inmany Frasnian reefs are, however, not well developed in the Famennian, due in part to the smaller cavitysizes. Radiaxial calcite cement is usually followed by scalenohedral calcite (marine cement) and clear,


TEXT-FIG. 12. Photomicrograph of Late Devonian (Famennian) platy calcimicrobe-sponge community, WindjanaLimestone, Windjana Gorge. SMX 36076; longitudinal section showing primary cavity with geopetal infill of angularquartz and bioclastic sand, followed by micrite. The framework is formed by encrusting Ortonella (O) to which isattached Solenopora geikiei (S). Pendent Shuguria grew from the cavity ceilings and walls, encased by microcrystal-line cement. The cavity walls are lined with a few crystals of scalenohedral calcite cement (arrowed), followed by a

thin crust of radiaxial which formed after deposition of the micrite geopetal fill; · 12.

equant blocky calcite (burial or meteoric cement). Scalenohedral calcite occurs as crusts that line theprimary cavity systems beneath the laminar calcimicrobe-sponge community (Pl. 1, fig. 1).

D I S C U S S I O N

Famennian reefs of the Canning Basin

Globally, the earliest Famennian reefs are documented as being either low diversity calcimicrobialcommunities, mud mounds, or bryozoan and stromatolite biostromes (see summaries in Copper 2002;George and Chow 2002; Webb 2002).


TEXT-FIG. 13. Photomicrographs of associated Late Devonian (Famennian) reef biota, Windjana Limestone, WindjanaGorge. A–B, SMX 36079. A, problematicum, consisting of hollow bodies with tapering tubular extensions; · 20. B, thegreen alga Litanaia perisseia Wray, 1967 growing attached to the calcimicrobial community; · 20. C, SMX 36075;cryptic community growing from the undersurface of a laminar inozoan sponge; pendent fistuliporid bryozoan (B),

calcimicrobes, and a lithistid sponge (S); · 7.

The palaeoecology of Famennian reef margin communities in the Canning Basin has not been describedin detail, but these reefs are considered to have formed under high energy conditions as massive, cavity-rich frameworks dominated by the intergrowth of microbialite, Rothpletzella, Shuguria, and Girvanella,with attendant calcified algae including solenoporaceans (Stephens 2002; Webb 2002; pers. obs.). Thewidth of the Famennian reef margin is not clear, but it was likely to have been less than 20 m and of lowrelief with a steep margin (Webb 2002). Back-reefs were dominated by shallow, high-energy carbonatefacies dominated by coated grains, oncoids and fenestral fabrics. A variety of mud and reef mounds grewin fore-slope environments (see summary in Webb 2002).

The proximal-slope reef mounds exposed in Horseshoe Range and Horse Spring Range grew in highenergy conditions at depths only 10 m below the contemporaneous reef margin, where sediments largelyrepresent those washed over the margin from proximal back-reef settings (Webb 2002). The mounds wereelongate and may have graded into the margin itself. They consisted of low-relief structures dominated byeither stromatactis-polymud dominated fabrics, or micrite, Rothpletzella, and Shuguria, with some laminarstromatoporoids and multiple generations of cement. Brachiopods and lithistid sponges are rare, and thereis evidence for rapid and pervasive early lithification.

Other reef mounds are known from reef fore-slope settings in the Canning Basin. Large stromatolite-sponge bioherms of latest Frasnian–early Famennian age have been described from Elimberrie (Playfordet al. 1976). These mounds are up to 1 km in diameter and 100 m thick, and developed in relatively deep


TEXT-FIG. 14. Photomicrographs of Late Devonian (Famennian) platy calcimicrobe-sponge community, WindjanaLimestone, Windjana Gorge. A, SMX 36076; primary crypt with pendent Shuguria and micrite geopetal fill; · 3. B–D,SMX 36075. B, encrusting Wetheredella (arrowed), and basal attachment and crinoid stem, fixed to edge ofRothpletzella growth; · 5. C, Solenopora geikiei (S) and cryptic Shuguria, with geopetal infill with ostracode debris

at base of photograph; · 8. D, calcimicrobial community with encrusting fistuliporid bryozoan (B); · 5.

fore-slope settings. Large, 50-m-diameter stromatolite mud-mounds of similar age have also beendescribed from the Napier Range (George 1999), and small stromatolite-crinoid mounds are knownfrom the Dingo Gap area (Playford 1981; Becker et al. 1991) as well as larger, undescribed sponge-richmounds (Webb 2002). The calcimicrobial-sponge community described herein from Windjana Gorge thusadds a further reef type to an already diverse set of early Famennian proximal slope reef communitiespresent in the Canning Basin.

Both George (1999) and Webb (2002) concluded that a variety of microbially dominated reefcommunities are found in the Canning Basin in both upper Frasnian and lower Famennian strata. Thishas been corroborated by George and Chow (2002), who observed that in the Napier Range, allochthonousblocks of stromatoporoid-bearing limestone were absent from uppermost Frasnian fore-reef strata,suggesting that extinction of stromatoporoids occurred before the F/F boundary with microbial com-munities dominating reef margin ecologies before the beginning of the Famennian. They concluded thatextinction of most skeletal metazoans was caused by a series of local relative sea level fluctuations thatgenerated further environmental stress either due to siliclastic input, restriction of habitat, or generation oftoxic lagoonal waters, superimposed upon a global regime of cooling climate.

Changes in biodiversity

In addition to between ten and 12 taxa of calcimicrobes and algae, the Famennian reef biota of theWindjana Limestone is composed of at least 15 species of spicular sponges, eight taxa of calcifiedsponges (five stromatoporoids, two sphinctozoans and one inozoan), encrusting bryozoans and attachedbrachiopods (see Tables 1–3).

Only two lithistid sponges have previously been reported from the Windjana Limestone (Rigby 1986),although 20 species of lithistids, seven hexactinellids, and one species of heteractinid have been reportedfrom Famennian slope-margin settings of the Napier and Sadler formations in other locations (Table 1). Ofthe tentatively identified taxa, 64 per cent represent progenitors. None of the 19 species of stromatoporoiddescribed from the Frasnian of the Canning Basin survived into the Famennian (Table 2; Cockbain 1984,


TEXT-FIG. 15. Photomicrograph of Late Devonian(Famennian) platy calcimicrobe-sponge community,Windjana Limestone, Windjana Gorge. Pendent,branching sponge ?Playfordiella, encrusted by lami-nated microbialite (M) on the upper surface. CrypticShuguria is attached to the ceiling and walls of a

primary cavity (arrowed); SMX 36078; · 2.

1989); indeed all Famennian calcified sponges are progenitors. Three species of calcimicrobes and algae(23–20 per cent of the post-extinction biota; Wray 1967) are progenitors. Otherwise, the describedFamennian reef community from Windjana Gorge is dominated by Frasnian calcimicrobial survivors(Table 3).

These observations demonstrate the local survival of reef taxa, rather than recovery through subsequentevolution or invasion of new taxa. These Famennian reef communities therefore show no markedreduction in the preservable biodiversity of reef-builders compared to Frasnian counterparts. This canbe in part explained by the fact that the F/F extinction preferentially removed metazoans that occupied theback-reef, i.e. stromatoporoid sponges and corals, rather than those present in the reef margin and reefslope communities that were dominated by calcimicrobes and spicular sponges. It may also be noteworthythat unlike other late Frasnian reefs, the corals from the Canning Basin show relatively low speciesdiversity and were not major reef-builders.


TEXT-FIG. 16. Field photographs of mature Late Devonian (Famennian) platy calcimicrobe-sponge community,Windjana Limestone, Windjana Gorge. A–B, intergrowth of calcimicrobes and sponges; · 0·05. C, development ofcolumnar stromatolites (S) within a primary cavity; · 0·2. D, large cavity system lined with banded adiaxial fibrous

cement; · 0·3.


TEXT-FIG. 17. Field photographs of Late Devonian (Famennian) calcimicrobial-sponge mounds, Windjana Limestone,Windjana Gorge. A, massive growth of Rothpletzella and Shuguria, intergrown with abundant tubular sponges(arrowed); · 0·2. B, massive growth of Rothpletzella and Shuguria, with large fissure encrusted by banded radiaxialcalcite cement; · 0·2. C–D, details of fissures within the framework encrusted by a crust of Rothpletzella (R), followed

by fibrous radiaxial calcite cement; · 0·5.

TEXT-FIG. 18. Field sketch of layered Roth-pletzella growth lining a fissure within thecalcimicrobial-sponge mound framework.The geopetal infill contains abundant ostra-codes. Late Devonian (Famennian), Windjana

Limestone, Windjana Gorge.

Persistence of carbonate platforms

Reefs disappeared prior or close to the F/F boundary almost globally, but reef loss was not simultaneous,and appears to have followed a series of step-down declines. Reduction in reef growth after the extinctionwas due to global lowstand reducing available habitat, and possibly to global cooling, but loss of the largeskeletal invertebrates resulted in changing reef ecologies, and in the Canning Basin this change was mostmarked in back-reef settings. The global lowstand close to the boundary caused the widespread erosion ofexposed former carbonate platforms (Copper 2002).

The observation that tropical biotas are more susceptible to mass-extinction events than temperatebiotas appears to be based on the high susceptibility of carbonate platform biotas to extinction, rather thanall biotas that occupy low latitudes (e.g. Raup and Jablonski 1993; Smith and Jeffrey 1997). In regionswhere carbonate environments continued uninterrupted across extinction boundaries, there can beconsiderably less extinction than in areas where carbonate habitat was lost (Smith and Jeffrey 1997).Times of extensive reef building usually correlate with extensive carbonate platform development, oftenwhen sea levels were high and climates warm, and high resolution stratigraphic analysis has establishedthat shallow carbonate platforms can be more sensitive to global perturbations than temperate habitats(Johnson et al. 1996). During many mass-extinction events, tropical carbonate platform distributionbecame restricted, and carbonate platform accumulation rates plummeted (Bosscher and Schlager 1993);for the F/F extinction, estimates suggest that additionally due to global lowstand conditions, carbonateplatforms became severely restricted. All these observations suggest a prominent role for loss of habitatlinked to climatic or oceanographic deterioration.

The absence of any obvious lithological change across the F/F boundary in the Windjana Gorgesuccession suggests that these areas were too shallow to be affected by encroaching hypoxic bottom waters(Becker et al. 1991), and this may further explain the persistence of so many surviving taxa into theFamennian.

Changes in ecological structure

Complex ecological interactions are evident within the calcimicrobe-sponge community, with some formsadopting preferentially cryptic habits (e.g. Shuguria spp.), while others developed complex mutuallyencrusting intergrowths (Rothpletzella and Girvanella). Most noteworthy, however, is the novelmorphological expression of this community notwithstanding the similarity in composition to that ofthe Frasnian reef margin. The formation of highly elevated, platy intergrowths, present novel ecologicalinter-relationships compared to the Frasnian habits of these taxa. The preference of Rothpletzella forfissures within the community suggests that this taxon preferred a cryptic, shaded habitat.

The stromatolites also show a clear preference for the sheltered areas formed beneath large primary reefcavities. George (1999) has described the occurrence of stromatolites in a wide range of deep-waterdepositional settings from the Upper Devonian reefs of the Canning Basin, including several depositionalsettings within the Famennian, but irrespective of relative depth, all represent areas of either regionally lowsedimentation rate or local settings protected from sediment influx. Such an association of deep-waterstromatolites with reduced sedimentation is well established (e.g. Playford et al. 1976). Based on


E X P L A N A T I O N O F P L A T E 1

Figs 1–2. Longitudinal photomicrographs of Late Devonian (Famennian) platy calcimicrobe-sponge community,Windjana Limestone, Windjana Gorge. 1, SMX 36075; growing edge, showing the framework formed byRothpletzella (R), Ortonella (O), and cryptic Shuguria (arrowed) within small primary crypts. The large cavityto the left is lined with a thin crust of radiaxial followed by scalenohedral calcite cement, with a micritic geopetal fill.Note the formation of small laminar cavities within the sediment beneath the calcimicrobial growth; · 5. 2, SMX36077; arching growth of Rothpletzella (R) forming primary cavities with cryptic Shuguria. Primary cavities arelined with radiaxial calcite cement; · 5.

P L A T E 1

WOOD, platy calcimicrobe-sponge community

biostratigraphy, George (1999) has estimated minimum growth rates of 50–100 mm/year for biohermalstromatolites from the Canning Basin. Given such rates, the maximum age for the largest Famennianstromatolites associated with the Windjana reef communities is estimated to be between 500 and 1000years.

The morphological form of the calcimicrobe-sponge platy complexes is also noteworthy for mimickingthe elevated Frasnian stromatoporoid sponge growth forms that were eliminated by the extinction event,suggesting that vacated ecospace may have been utilised in a similar way. As such, there was also nochange in tiering of the reef community.

Palaeoecological analysis of the Famennian reefs in Windjana Gorge offers little support for either theidea of ‘resurgence’ or ‘reinvasion’ of calcimicrobial biotas into reef communities (Copper 1994, 1997).Description of Famennian reef ecologies from other localities in the Canning Basin also reached a similarconclusion (Webb 2002). Likewise, the persistence of spicular sponges within reef margin ecologies doesnot support the notion of the ‘invasion’ of these taxa into shallow marine habitats from deeper waterenvironments (McGhee 1982, 1996).


TABLE 1. Distribution of spicular sponges across Frasnian/Famennian boundary, Canning Basin, north-westernAustralia. þ, those tentatively identified in the Windjana Limestone in this study; *, those found previously in theWindjana Limestone; 17 per cent of the taxa were survivors, 64 per cent progenitors, and 19 per cent went extinct(from Rigby 1986).

Frasnian Famennian Status

DemospongiaeLithistidaAllassospongia polystromne X ExtinctAnthaspidella multiostia Xþ ProgenitorAttungia wellingtonensis X ExtinctAulocopoides patulum X ProgenitorA. teicherti X ProgenitorAustralospongia turbinata X ProgenitorA. cylindrica X ProgenitorCanningella obconica Xþ ProgenitorC. interrupta Xþ ProgenitorC. expansa X Xþ SurvivorC. magnipora X ProgenitorCockbania palmata X ExtinctFistulosospongia parellella X ProgenitorPlayfordiella cylindrata Xþ ProgenitorP. (?) capitanea Xþ ProgenitorP. (?) sp. Xþ ProgenitorSadleria pansa X ExtinctScheielloides conica X ExtinctS. (?) sp. Xþ ProgenitorSriataspongia cylindrica X Progenitor

HexactinellidaCavospongiella confossa Xþ ProgenitorC. (?) sp. X ProgenitorGaleospongia pleriducta Xþ ProgenitorPelicaspongia cupula Xþ ProgenitorP. (?) sp. X Xþ SurvivorPillaraspongia ellimberia X Xþ* Survivorunidentified hexactinellid root tuft X Extinctunidentified heteractinid X X Survivorsponges indet. X Xþ* Survivor

Persistence of microbial communities

Notwithstanding the near-complete loss of large, heavily calcified metazoans in the F/F mass extinctionevent, the continuation of reef-building across this boundary at Windjana Gorge is testament to theimportance of microbial communities in late Frasnian reef building. In fact, many assumptions as to theslow recovery of reefs after mass extinctions have been based upon mistaken identification of marineinvertebrates as the main reef-builders, where recovery has been confused with the return of a significantskeletal metazoan component to the reef community (Webb 1996; Wood 1999, 2000a). Moreover, adiversity of rigid, early lithified microbial and calcareous algal communities continued to buildsubstantial reefs with associated spicular sponges in the early Famennian wherever environmentalconditions allowed.


TABLE 2. Distribution of calcified sponges across Frasnian/Famennian boundary, Canning Basin, north-westernAustralia; 16 per cent of the taxa were progenitors and 84 per cent went extinct (from Cockbain 1989).


StromatoporoidsActinostroma papillosum X ExtinctA. papillosum var. A X ExtinctA. papillosum var. B X ExtinctA. windjanicum X ExtinctAmphipora rudis X ExtinctA. pervesiculata X ExtinctA. sp. X ExtinctAnostylostroma ponderosum X ExtinctAtelodicyton stelliferum X ExtinctClathrocoliona spissa X ExtinctC. saginata X ProgenitorDendrostroma oculatum X ExtinctHermatostroma schlueteri X ExtinctH. roemeri X ExtinctH. ambigum X ExtinctH. perseptatum X ExtinctPennastroma yangi X ProgenitorPlatiferostroma kueichowense X ProgenitorP. sp. nov. X ProgenitorPseudoactinodicyton dartingtoniensis X ExtinctStachyodes costulata X ExtinctS. australe X ExtinctS. crassa X ExtinctS. dendroidea X ExtinctStromatopora cooperi X ExtinctS. minutitextum X ExtinctS. lennardensis X ProgenitorStromatoporella laminata X ExtinctTrepetostroma bassleri X ExtinctT. mclearni X ExtinctT. laceratum X Extinct

Sphinctozoanssp. indet. X Progenitorsp. indet. X Progenitor

Inozoansp. indet X Progenitor

The persistence of surviving calcimicrobial and algal taxa does not, therefore, support a supposed‘explosion’ of a putative disaster biota (e.g. Geldsetzer et al. 1993; Copper 1994, 1997). Indeed, thepresence of microbially dominated reef fabrics throughout the upper Frasnian reef and fore-reef successionsuggests that some Famennian reef communities may represent a continuation of ecologies already presentin some late Frasnian communities (George and Chow 2002; Webb 2002), rather than being communitiesthat ‘bloomed’ as crisis progenitors in the early Famennian (e.g. Playford et al. 1989; Copper 2002).

Microbial reefs were present after several other mass extinction events, such as during the MidCambrian (Zhuravlev 1996) and Early Triassic (Schubert and Bottjer 1992; Lehrmann 1999). This may bebecause some microbes such as cyanobacteria show a wide tolerance to stressful conditions, such asreduced productivity or reduced oxygenation.

Despite the survival of major Frasnian reef builders such as microbial communities and Shuguria,Lower Tournaisian reefs are very rare, and are known only from Russia. Microbial reefs persisted into theCarboniferous and Permian, particularly on the northern margin of Pangaea (Davies et al. 1989), but reef-building is widely assumed to be represented solely by relatively deep microbial mud-mound developmentin Europe and North America.

The nature of reef communities

Observations presented herein reinforce current ecological thought that considers communities to bechance associations of species with similar ecological requirements, rather than bound by fixed ecologicalinteractions. This notion is supported by several models that adequately predict community compositionon the basis of immigration and extinction, the spatial distribution of environments, and the size of thespecies pool alone (e.g. Hubbell 1997). Communities constantly change through local extinction andrecruitment of their component species, and new communities develop that are composed of speciessampled from the available population (Buzas and Culver 1994). It has been well documented thatformerly co-occurring reef species are now found in associations entirely different from those which they


TABLE 3. Distribution of calcimicrobes and algae across Frasnian/Famennian boundary, Canning Basin, north-westernAustralia; between 47 and 69 per cent of the taxa were survivors, 23–30 per cent progenitors, and 23–30 per cent wentextinct (from Wray 1967).


CalcimicrobesGirvanella wetheredii X ExtinctGirvanella ducii X ProgenitorParaepiphyton caritus X X SurvivorRothpletzella exile X X SurvivorR. magnum X X SurvivorShuguria devonicus X X SurvivorS. turbitus X X Survivor

Red algaeParachaetetes regularis X ? ? SurvivorP. improcerus ? ExtinctSolenopora geikiei X X SurvivorStenophycus teichertii X ProgenitorTharama glauca X ? ? Survivor

Green algaeLitanaia perisseia X X SurvivorOrtonella imprimis X ProgenitorVermiporella myna X Extinct

occupied previously, e.g. during the Pleistocene, Indo-Pacific scleractinian corals responded to fluctuatingsea level by changing community membership (Potts 1984). Ecological associations between reeforganisms are not fixed, and the ecology of communities has clearly changed as new species haveevolved whilst older taxa have persisted. This suggests that many species can occupy a broadly similarniche within reef communities.

Acknowledgements. This work was funded by a Royal Society University Research Fellowship. The Department ofConservation and Land Management (CALM) is thanked for permission to work and collect within Windjana GorgeNational Park. Sharon Capon, Hilary Alberti, Andrew Pluck, and Dudley Simons are thanked for technical support,Clive Oppenheimer assisted in the field, and John Sibbick drew the ecological reconstructions. I thank Ken McNamara(Western Australian Museum), Terry Bloomer (CALM Ranger), Annette George (University of Western Australia),and Nat Stephens (University of California, Davis) for logistical support and advice. J. Keith Rigby is thanked for helpwith sponge identification. I am grateful to Louie and Mary-Anne Dolby (Mt Pierre Station) and Nick Algie ofKimberley Diamond Mines for their kind permission to visit the Bugle Gap and Morown Cliff localities, respectively.This is Cambridge University Earth Sciences Publication no. 7066.

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RACHEL WOOD

Department of Earth SciencesUniversity of Cambridge

Downing StreetCambridge CB2 3EQ, UK

Current addressSchlumberger Cambridge Research

High CrossMadingley Road

Cambridge CB3 OEL, UKe-mail [email protected]

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Palaeoecology of a post-extinction reef: Famennian (Late Devonian) of the Canning Basin, north-western Australia