Copyright � 2007, SEPM (Society for Sedimentary Geology) 0883-1351/07/0022-0017/$3.00

PALAIOS, 2007, v. 22, p. 17–23

Research Report

DOI: 10.2110/palo.2005.p05-050r

PLACUNOPSIS BIOHERMS: THE FIRST METAZOAN BUILDUPS FOLLOWING THE END-PERMIANMASS EXTINCTION

SARA B. PRUSS,1* JONATHAN L. PAYNE,2 and DAVID J. BOTTJER3

1 Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138, USA; 2 Department of Geologicaland Environmental Sciences, Stanford University, 450 Serra Mall, Stanford, California 94305, USA; 3 Department of Earth Sciences, University of Southern

California, 3651 Trousdale Pkwy, Los Angeles, California 90089-0740, USAe-mail: spruss@fas.harvard.edu

ABSTRACT

Outcrops of the Lower Triassic (Spathian) Virgin Limestone Member(Moenkopi Formation) in the southwestern United States contain theoldest known metazoan bioherms formed in the aftermath of the end-Permian mass extinction. These small buildups, up to 1.0 m in di-ameter and 0.2 m high, were constructed by cementing bivalves. Thebivalve bioherms accreted in a shallow, subtidal environment abovestorm wave base atop an oolitic hardground on a carbonate ramp.Similarities in shell microstructure and bioherm morphology betweenthe Lower Triassic buildups reported here and previously describedMiddle Triassic occurrences suggest that the Lower Triassic bio-herms were likely built by a bivalve assignable to Placunopsis. Al-though taxonomic assignment of the bivalve remains uncertain, thepresence of cemented bivalve bioherms in Lower Triassic sections ofthe southwestern United States demonstrates that cementing bivalveswere geographically widespread, even early in their Mesozoic evo-lutionary history. Despite their bioherm-building ecology, cementingbivalves do not occur in Middle Triassic platform-margin reefs, un-derscoring the decoupling of the recovery of framework-buildingmetazoans from the return of large carbonate platform-margin reefsin the wake of the end-Permian mass extinction. These first Mesozoicbioherms built by metazoans represent a significant ecological ad-vance in the evolutionary history of bivalves in that the cementinglife-mode had reappeared before the end of the Early Triassic.

INTRODUCTION

The purpose of this paper is to describe small bivalve bioherms fromthe Lower Triassic (Spathian) Virgin Limestone Member of the MoenkopiFormation, southeastern Nevada. These bioherms are the oldest knownMesozoic bioherms built with metazoans acting as the dominant frame-work builders and binders. The morphology and constituents of the bi-valve buildups resemble Middle Triassic (Anisian–Ladinian) biohermsthat occur abundantly in the Muschelkalk and Lower Keuper of Germanyand eastern France and are constructed by small cementing bivalves as-signed to the genus Placunopsis (e.g., Holder, 1961; Bachmann, 1979;Hagdorn, 1982; Brocard and Philip, 1989; Bachmann, 2002). We followprevious workers in using the genus Placunopsis, although the taxonomyof the Triassic Placunopsis is in question (Todd and Palmer, 2002).

The dominant members of level-bottom Early Triassic bivalve com-munities occupied only a few life modes, including bysally attached epi-faunal suspension feeders and shallow infaunal deposit feeders (Schubertand Bottjer, 1995; Hallam and Wignall, 1997). The presence of bivalvebioherms in uppermost Lower Triassic strata demonstrates that a ce-menting, bioherm-building ecology had developed late in Early Triassictime and, furthermore, that bivalve bioherms were not limited geograph-ically to the Peri-Tethys. The small buildups described here, however,have been observed only in shallow subtidal environments above storm

*Corresponding author.

wave base on carbonate ramps. They did not play a direct role in thereestablishment of vast and complex reef-margin ecosystems of the Mid-dle Triassic (e.g., Flugel and Stanley, 1984; Fois and Gaetani, 1984; Flu-gel, 2002); however, these bioherms provide insight into the evolutionaryhistory of bivalve bioherms as well as the early ecological advances ofMesozoic cementing bivalves.

GEOLOGIC SETTING

The western margin of North America during the Early Triassic was cov-ered by a broad, shallow, epicontinental shelf, which, at its greatest extent,extended from southern Idaho to southern Arizona (Marzolf, 1993). TheMoenkopi Formation was deposited in the western United States in the fol-lowing succession from stratigraphically lowest to highest: Timpoweap, Low-er Red, Virgin Limestone, Middle Red, Shnabkaib, and Upper Red members(Marzolf, 1993), which crop out from Utah to eastern California. The VirginLimestone and Shnabkaib members were deposited during marine incursions;the various Red members were deposited partly in continental environments(Reif and Slatt, 1979). The Virgin Limestone Member crops out as brownishyellow to gray limestone ledges. It is a mixed carbonate-siliciclastic unitcomposed of limestone, dolomitic limestone, calcareous mudstone, and silt-stone (Reif and Slatt, 1979), and it preserves fossil communities dominatedby bivalves, gastropods, and crinoids (Schubert and Bottjer, 1995). A lateSpathian age was assigned to the Virgin Limestone Member based on itsammonoid fauna in Utah (Poborski, 1953) and by its Sr isotope compositionfrom sections west of Las Vegas (Marenco et al., 2003), strongly suggestinga Spathian age for the Virgin Limestone Member in the Muddy Mountainsas well. The bivalve bioherms described herein crop out at the Muddy Moun-tains Overton locality (Fig. 1). Poorly exposed examples are present at theMuddy Mountains Ute locality as well. Data and illustrated specimens pre-sented here are from the Muddy Mountains Overton locality.

METHODS

A field and laboratory study was conducted to evaluate the metazoancomponents of the bioherms and to determine the paleoenvironment inwhich they formed. The limestone bed containing the bioherms was notedon a stratigraphic column (Fig. 2A), and surrounding beds were de-scribed. Where possible, bioherm height and width was recorded andsamples were collected. Thin sections were prepared to investigatemicrofabrics of the bioherms, specifically the microstructure of the meta-zoans. Several silicified samples were treated with dilute HCl to extractindividual shells from the limestone matrix and study the three-dimensional shell morphology. Extracted shells were also examined underSEM (scanning electron microscope).

RESULTS

Field Analysis

The Virgin Limestone Member at Muddy Mountains Overton recordsinitial transgression followed by sea-level highstand (Pruss, 2004). TheVirgin Limestone Member consists of parasequences typically formed by

18 PALAIOSPRUSS ET AL.

FIGURE 1—Locality map with Muddy Mountains Overton locality, Nevada, USA.A) Location of study area relative to California and Nevada, USA. B) Location ofValley of Fire State Park, which contains the North Muddy Mountains. C) Detailedlocality map. Virgin Limestone Member crops out as a gray to yellowish band in acontinuous belt in the North Muddy Mountains. Access to sections is possible viaLogandale Trails Park. (Additional locality information is also available in Shorb,1983; modified from Pruss and Bottjer, 2004a).

FIGURE 2—A) Measured stratigraphic section of the Virgin Limestone Member,Moenkopi Formation, Muddy Mountains Overton locality, showing location of bi-valve buildups in the upper half of the section (modified from Pruss and Bottjer,2004a; Pruss et al., 2005). B) Mound morphotypes in outcrop.

thick fossiliferous and oolitic grainstones, 1–5 m thick, overlain by thickrippled and cross-bedded siliciclastic beds, 0.5–2 m thick. The bivalvebioherms occur near the top of the section within a roughly 11-m-thicklimestone unit (Fig. 3A). The base of this unit consists of a 3.5-m-thick,gastropod and bivalve grainstone with intraclasts. Overlying this bed isan oolite, 0.3–1 m thick, whose top is a hardground surface on whichthe bivalve bioherms occur. The bioherms are overlain by 6 m of fine-grained, bioturbated limestones with interspersed thin beds and lenses ofdisaggregated bivalve bioherm debris (Fig. 2A, 3B). Portions of some ofthe buildups are silicified.

At the Muddy Mountains Overton locality, nine individual bivalvemounds occur in a single bed across 100 m of lateral exposure. It is likelythat additional mounds are present but not exposed at this locality. At theMuddy Mountains Ute locality, about 3 mounds are exposed in a broadlycorrelative bed, but exposure of this bed is even more limited than thebed at the Muddy Mountains Overton locality. The heights of the Pla-cunopsis buildups range from 12 to 26 cm, with the majority of thebioherms attaining a height greater than 20 cm. The widths of the build-ups range from about 0.5 to 1.5 m. Where exposure is continuous, dis-tances between the centers of adjacent bioherms range from 1 to 1.5 m;adjacent bioherms do not appear to connect laterally.

Although three-dimensional exposure of the individual bivalve bio-herms is rare, two distinct bioherm morphologies were noted in outcrop:

a columnar form and a domal form (Fig. 2B). The columnar biohermsconsist of small (�20 cm in height), individual branching columns (Fig.3C); these resemble the pillar morphology recognized from Triassic Mus-chelkalk Placunopsis buildups in Germany (Holder, 1961; Bachmann,1979; Hagdorn, 1982). Other bioherms exhibited a domal morphology,with the hemispheric mounds having rounded dome-shaped tops (Fig.3D). The columnar morphologies observed in the Virgin Limestone bi-

PALAIOS 19EARLY TRIASSIC BIVALVE BIOHERMS

FIGURE 3—Placunopsis buildups, Virgin Limestone Member, Muddy Mountains Overton locality. A) Placunopsis bioherm in relation to underlying and overlying beds(also shown in Fig. 2); a � bivalve-gastropod packstone; b � oolitic limestone underlying Placunopsis buildup unit; c � Placunopsis bioherm; d � overlying micriticbeds. B) Close-up of buildup pictured in Part A. Arrows show light gray micrite infilling pillar structures. C) Placunopsis buildup; a � underlying oolite; b � cementingbivalve horizon. D) Close-up of a Placunopsis bioherm with no pillar structures. Arrows indicate top of bioherm.

valve buildups are broadly similar in form to some columnar stromatolites(e.g., Glaessner et al., 1969). The columnar bivalve buildups commonlycontain abraded and disarticulated fossil debris in the micrite preservedbetween the individual columns, including broken Placunopsis valves aswell as other bivalves and gastropods.

The internal structure of all the bioherms consists of small, irregularlystacked Placunopsis valves (1–10 mm in length, �1 mm thick) that ap-pear in the field as stacked millimeter-size elongate clasts. The Placu-nopsis bioherms broadly resemble stromatolites (Bachmann, 1979) dueto their hemispheroidal, columnar, and laminated macrostructures. Thesmall size and irregular shape of the shells also makes it difficult torecognize them as bivalves in the field.

Thin Section and SEM Analysis

In thin section, the distinct microstructure and feathered margins ofPlacunopsis valves help to clarify the taxonomic affinities of these smallfossils. Accretion of Placunopsis shells began on a truncated oolitic sur-face (Fig. 4A). The initial colonizing Placunopsis shells were somewhattabular, but overlying shells were mainly oriented convex down. Most ofthe preserved valves are cemented right valves, although left valves arealso present. One of the most striking features of the thin sections is thefeathery edge of the valves (Figs. 4A–D), indicative of foliation of theouter shell layers, a common feature of Placunopsis (Bachmann, 1979).Some Placunopsis valves contain small borings filled with micrite (B.Kirkland, personal communication, 2005; Fig. 4B). Tubes resembling spi-rorbids are visible encrusting the tops of some Placunopsis shells (Fig.4C). Placunopsis valves overlying putative spirorbid tubes conform tothe shape of the tubes, suggesting that the overlying bivalves accreted

after the spirorbid-like organism had attached. This relationship indicatesthat these spirorbids encrusted the exposed surface of the bioherm ratherthan dwelling in cavities within the bioherm. The pillar structure notedin outcrop is also visible in thin section. Portions of thin sections showthat Placunopsis valves accreted to form columns and that micrite withfossil debris was deposited in the interstices between columns. Placu-nopsis valves contribute 50%–75% of the rock volume, based on visualestimates of thin sections; the second most common constituent is micrite,which is occasionally diffuse or peloidal. Void-filling cement, spirorbidtubes, and disarticulated fossil debris are less abundant.

The outer layer of a bivalve shell, or ostracum, is composed of foliatedcalcite, which represents the original structure of the shell. The inner shelllayer, or hypostracum, which was originally secreted as aragonite (Carter,1990), now consists of sparry calcite or silica that replaced the aragonite(Fig. 4D). Around the edge of the hypostracum, bladed calcite cement issometimes preserved, with much of the originally aragonitic internal hy-postracum replaced by silica. The hypostracum appears to be preferen-tially silicified in many valves relative to the ostracum, as is the case formany Precambrian carbonates in which preferential silicification of ara-gonite is also common (A. Knoll, personal communication, 2005). Inmany cases, silicification has destroyed the bladed cements around theperimeter of the valves.

Silicified Placunopsis valves dissolved from the carbonate matrix indilute acid were viewed under SEM. Radial ribs on the upper left valveswere clearly visible under SEM (Figs. 5A, B, and D), as was a boringfound on a rib of one of the valves (Fig. 5B). Additionally, the outerfoliated calcite layers, described as the feathery edges of valves in thinsection, were also discernible (Fig. 5C).

20 PALAIOSPRUSS ET AL.

FIGURE 4—Thin section photomicrographs of Placunopsis buildups. A) Oolite that underlies Placunopsis buildup horizon. Arrow indicates erosional surface encrusted byPlacunopsis. Note truncation of ooids. B) Common Placunopsis shell textures. Arrows indicate shell borings. C) Placunopsis valves. Note convex downward position ofsome shells, suggesting these are right valves. Left valves are also present in the photomicrograph, suggesting that subsequent colonization of valves occasionally occurredrapidly after death. Arrows indicate spirorbid-like tubes that encrusted the exposed surfaces of Placunopsis valves. D) Placunopsis valves showing encruster, foliation, andmineralogy; a � large putative spirorbid tube encrusting upper surface of Placunopsis; b � feathery edge of Placunopsis valve indicative of a foliated calcitic hypostracum(outer layer); C1 � hypostracum originally secreted as calcite, now recrystallized calcite; C2 � ostracum originally secreted as aragonite, now replaced by silica. Notearea above letter b, which shows partial silicification of internal area of valve.

DISCUSSION

Evolution of Cementing Bivalves

Such cementing bivalves as true oysters represent a life mode that firstevolved during the Paleozoic but became important in high-energy, ma-rine benthic communities during the Mesozoic. The temporal range ofcementing bivalves extends at least back to Mississippian time, with thefirst known occurrence represented by the pseudomonotid Pachypteria deKoninck, 1885 (Newell and Boyd, 1970). The earliest cementers wereminor constituents of Carboniferous hardground communities (e.g., Haut-mann and Golej, 2004). Other early cementers include the Prospondyli-dae, members of which have been described as oysterlike bivalves fromPermian strata (Newell and Boyd, 1970; Yancey, 1985). One of the oldestbioherms containing bivalves occurs in the Permian Phosphoria Forma-tion of Idaho (Boyd et al., 1990).

Following the end-Permian mass extinction, bivalve diversity as a wholedecreased markedly. Although cementing bivalves were not a conspicuouselement of most Paleozoic communities, they were devastated by the end-Permian extinction. The only cementing bivalves previously known fromLower Triassic strata are possible anomiids found encrusting ammonite shellsin the Ophiceras wordiei beds of the Wordie Creek Formation, East Green-land (Spath, 1930, 1935). Prior to the discovery of the bivalve buildupsreported here, the first occurrence of similar bioherms was in the Middle

Triassic Muschelkalk and Lower Keuper sections of Europe (e.g., Holder,1961; Bachmann, 1979; Hagdorn, 1982; Brocard and Philip, 1989; Bach-mann, 2002). The taxonomic affinity of Middle Triassic Placunopsis is prob-lematic (see below), but most Muschelkalk occurrences have previously beenassigned to the species Placunopsis ostracina von Schlotheim and Placu-nopsis plana Giebel (Todd and Palmer, 2002).

Placunopsis Bioherms: Form and Constituents

The Placunopsis bivalve buildups in the Lower Triassic Virgin Lime-stone Member resemble Middle Triassic Muschelkalk and Lower Keuperbioherms in terms of gross morphology, size, and environmental distri-bution (e.g., Holder, 1961; Bachmann, 1979; Duringer, 1985; Brocard andPhilip, 1989; Bachmann, 2002). Both the Lower Triassic and the Mus-chelkalk bioherms were constructed principally by bivalves similar tothose of the Middle Triassic–Jurassic genus Placunopsis Morris andLyett, 1853, although Todd and Palmer (2002) suggested that MiddleTriassic bivalves assigned to Placunopsis should be placed in a new fam-ily and a new genus. The phylogenetic relationship between Paleozoicand Mesozoic cementing bivalves is poorly understood (Hautmann, 2001;Todd and Palmer, 2002), making it difficult to determine whether thesebivalves evolved a cementing life mode during the Early Triassic or,alternatively, evolved a mound-building ecology within a preexisting, ce-menting lineage.

PALAIOS 21EARLY TRIASSIC BIVALVE BIOHERMS

FIGURE 5—SEM (scanning electroscope microscopy) photographs of silicified valves. A) Upper left valve of Placunopsis with ribbing. Arrow indicates small, probablyendolithic, boring. B) Close-up of boring in Part A. Boring was filled secondarily, possibly with silica. C) Lower cemented right Placunopsis valve, showing thickenedouter edge. Bracket indicates foliated calcitic layers that commonly appear in thin section as feathery edges that curve upward. Arrow indicates a putative spirorbid wormtube. D) Edge of Placunopsis valve, showing layers and ribbing (arrow).

The Placunopsis buildups preserved in the Lower Triassic VirginLimestone at the Muddy Mountains Overton locality exhibit two distinctgrowth forms, although three-dimensional exposure of these buildups isextremely rare. As noted above, a pillar growth form and a domal formlacking discrete pillars were both observed in outcrop. Initiation of build-ups occurred via the initial colonization of a hard substrate by Placunop-sis, and accretion resulted from subsequent generations settling and grow-ing on the remains of former generations (e.g., Bachmann, 1979, 2002).This accretionary mode of growth is reflected in the preservation of most-ly right valves, because the right valve is the larger, lower valve thatcemented to the substrate (Bachmann, 1979), and the left valve commonlydisarticulated after death, leaving a new substrate available for subsequentcolonization. Disarticulation did not always occur, however, inasmuch asleft valves are also preserved in thin section, suggesting that subsequentcolonization of valves at times occurred rapidly after death (see Fig. 4).

The ability to form bioherms was likely advantageous for Placunopsis ina high-energy environment, because this provided a way for bivalves tolive slightly above the sediment-water interface to avoid inundation bysediment. As filter feeders cementing to a hardground, these bivalveswere likely utilizing an Early Triassic environment in which competitionfrom other organisms was low.

The presence of abraded and disarticulated fossil debris within the in-terstices of the bioherms suggests that these buildups were occasionallysubjected to high-energy conditions that scoured the bioherms themselvesand deposited fossil debris in the interspaces of the mounds. Hydrodynam-ics, therefore, likely played a role in the shape and form of the buildups,which are similar to columnar stromatolites (Bachmann, 1979) in high-energy environments (e.g., Hoffman, 1976; Paul and Peryt, 2000). Giventhe small size of the Placunopsis bivalves, they may have responded tohydrodynamic pressures in a way similar to microbial communities that

22 PALAIOSPRUSS ET AL.

build stromatolites. The columnar growth form of some of the larger Pla-cunopsis buildups may have provided a suitable way for Placunopsis tobuild above the sediment while allowing a suspended sediment load to passbetween the columns and prevent burial. The domal form typifies the smallPlacunopsis buildups, which may have been less susceptible to destructionby scouring by currents. Further work is needed to determine what thevariations in bioherm morphology within the same bed represent.

In Placunopsis bioherms of Europe (e.g., Bachmann, 1979, 2002), mi-crobial cementation and binding are common. The only evidence for amicrobial component in these mounds is the clotted or peloidal appear-ance of micrite preserved between valves in the mounds. There is littledirect evidence that microbial communities played an important role inthe cementation of the mounds or in the formation of the micrite; how-ever, the possibility of a microbial influence on the mounds cannot beentirely excluded at this time.

Placunopsis Bioherms: Significance

In the wake of the end-Permian mass extinction, which occurred ap-proximately 252 Ma (Bowring et al., 1998; Kozur, 2003a, 2003b; Mundilet al., 2004), low diversity communities persisted for several million yearsin the Early Triassic (Schubert and Bottjer, 1995; Hallam and Wignall,1997; Kozur, 1998a, 1998b). The ecological impact of the extinction,however, extended far beyond the reduction of community-level diversity.The end-Permian extinction was one of only a handful of events in Earth’shistory that fundamentally altered the structure of marine ecosystems(Bambach et al., 2002). The ecological transition is exemplified by thewidespread molluscan dominance of marine benthic communities thatwas established in the Early Triassic (e.g., Schubert and Bottjer, 1995;Hallam and Wignall, 1997). One ecological proxy for Triassic recoverythat has received particular attention is the reestablishment of reef eco-systems following the extinction of many dominant reef-building taxa atthe end of the Permian (e.g., Flugel and Stanley, 1984; Flugel, 2002). Tounderstand the recovery of reefs in the Middle Triassic, research hasfocused primarily on the diversification of reef-building clades such asscleractinian corals and calcareous sponges, organisms that tend to be thedominant metazoan components of large platform-margin reefs (e.g., Flu-gel, 2002). The framework of many Middle Triassic reefs, however, wasconstructed mainly by problematic organisms such as Shamovella (alsocalled Tubiphytes; Riding, 1993) in association with large volumes ofmarine cements (e.g., Flugel, 2002). It was not until the later MiddleTriassic that organisms with extant relatives, such as scleractinian corals,came to contribute significantly to the framework of many reefs (e.g.,Stanley, 2003). Apparently, the reestablishment of metazoan reef ecosys-tems following the end-Permian mass extinction was a gradual processthat continued through much of the Triassic Period.

The Lower Triassic Placunopsis buildups are older than their Middle Tri-assic counterparts in the Peri-Tethys. The close similarities in shell morphol-ogy, buildup form, and paleoenvironmental preference between the LowerTriassic buildups and those of the Muschelkalk, however, suggest that theseorganisms were close relatives. If the bioherms from the Moenkopi Formationand the Muschelkalk were built by the same taxon, as appears likely, thenPlacunopsis was more geographically widespread and long ranging than sus-pected previously. With so few examples of Placunopsis buildups currentlyknown, it is difficult to determine the dispersal pattern of Placunopsis. Re-gardless of their specific pattern of dispersal, the occurrence of these bivalvesin the southwestern United States points to a broad geographic range, evenearly in their evolutionary history.

Resolution of Paleozoic-Mesozoic bivalve phylogenetics will be nec-essary to reach a satisfactory understanding of the evolution of cemen-tation as a life mode within the bivalves and of bivalve ecology moregenerally in the aftermath of the end-Permian mass extinction. For in-stance, if Early to Middle Triassic Placunopsis are members of the An-omiidae, this would extend the earliest occurrence of the Anomiidae backto the Early Triassic, suggesting that the initial radiation of this family

occurred during the biotic recovery from the end-Permian mass extinc-tion. On the other hand, if the recommendations of Todd and Palmer(2002) prove robust and the Triassic Placunopsis bivalves belong in theTerquemiidae, Placunopsis may have evolved directly from cementingPermian ancestors. A better understanding of the phylogenetic positionof Placunopsis and its relationship to Paleozoic cementing bivalves willdetermine whether the absence of Placunopsis bioherms during much ofthe Early Triassic should be interpreted as a stratigraphic gap in the recordof cementing bivalves or, rather, that the bioherms in the Virgin Lime-stone record the reinvention of a cementing, bioherm-building life modeamong bivalves. Despite the affinities of the constructors, Early Triassicbivalve bioherms represent an important evolutionary step among Me-sozoic cementing bivalves.

The presence of Placunopsis bioherms late in the Early Triassic indi-cates that an increase in the diversity of life modes occupied by bivalvesoccurred earlier than previously suspected. Many of the bivalve assem-blages studied from Lower Triassic sections contain an abundance ofepifaunal and shallow infaunal members (Schubert and Bottjer, 1995;Hallam and Wignall, 1997). It can now be established that the cementinglife mode of bivalves had reappeared by the end of the Early Triassic.The Placunopsis buildups from the Virgin Limestone are also significantin that they appear to represent the first metazoan buildups following theend-Permian mass extinction. The absence of metazoan reefs is one fea-ture of the Lower Triassic rock record thought to reflect the ecologicalimpact of the end-Permian extinction (e.g., Fagerstrom, 1987; Flugel,2002). The occurrence of microbial patch reefs has been well documented(Baud et al., 1997, 2002; Lehrmann, 1999; Pruss and Bottjer, 2004b), anda prominent feature of these reefs is the absence of metazoans acting asframework builders, binders, or bafflers. Placunopsis buildups are smallbioherms on the shallow shelf and, as such, they differ markedly fromlarge platform-margin reefs of the Permian (e.g., Fan et al., 1982, 1990;Weidlich, 2002) and the Middle Triassic (e.g., Flugel, 2002), but thesebuildups do represent the first bioherms constructed solely by metazoanframework builders and binders in the aftermath of the end-Permian massextinction. The Placunopsis buildups were ecologically simple biohermsformed almost entirely by a single taxon, with such ancillary organismsas putative spirorbids present but structurally insignificant.

CONCLUSIONS

Placunopsis bioherms in the uppermost Lower Triassic Virgin LimestoneMember of the Moenkopi Formation are the oldest known buildups con-structed by metazoans during the aftermath of the end-Permian mass extinc-tion. Cementing, bioherm-building bivalves were geographically widespreadearly in their Mesozoic evolutionary history. Moreover, the presence of bi-valve buildups provides additional evidence that ecological recovery follow-ing the end-Permian extinction was underway before the Middle Triassic,although perhaps not much before. The small size of the Placunopsis bio-herms and their occurrence in shallow subtidal paleoenvironments abovestorm wave base on a carbonate ramp highlight the decoupling of the recov-ery of platform margin reefs in the Middle Triassic from the recovery orreinvention of the framework-building life habit among metazoans. Thesebioherms illustrate that the cementing life mode of bivalves was establishedprior to the Middle Triassic and, although ecologically simple, biostromeswere built by metazoans late in the Early Triassic.

ACKNOWLEDGMENTS

We thank the University of Southern California Department of EarthSciences (grant to SBP) and the University of Southern California Womenin Science and Engineering Program (grant to DJB) for support for thisresearch. We thank B. Kirkland for her thin-section expertise and A. Knollfor helpful discussion and comments on an earlier version of this man-uscript. We thank Rachel Wood, Gerhard Bachmann, and two anonymousreviewers for helpful reviews.

PALAIOS 23EARLY TRIASSIC BIVALVE BIOHERMS

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ACCEPTED JULY 6, 2006