Wall structure and growth of fusulinacean foraminifera

Paleontological Society Wall Structure and Growth of Fusulinacean Foraminifera Author(s): Scott A. Hageman and Roger L. Kaesler Reviewed work(s): Source: Journal of Paleontology, Vol. 72, No. 2 (Mar., 1998), pp. 181-190 Published by: Paleontological Society Stable URL: http://www.jstor.org/stable/1306707 . Accessed: 20/09/2012 01:21 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Paleontological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Paleontology. http://www.jstor.org J. Paleont., 72(2), 1998, pp. 181-190 Copyright ? 1998, The Paleontological Society 0022-3360/98/0072-0181$03.00 WALL STRUCTUREAND GROWTHOF FUSULINACEANFORAMINIFERA SCOTT A. HAGEMAN ANDROGER L. KAESLER NaturalSciences,ParkCollege, Parkville,MO 64152, and Departmentof Geology,NaturalHistoryMuseum,and PaleontologicalInstitute, The Universityof Kansas,Lawrence66045 electronmicroscopyof tests of Triticitesventricosusand Schwagerinasp. shows that the microgranular ABSTRACT-Scanning wall was secretedratherthan agglutinated.Grainsrangein size from 0.5 to 6.0 ,um,but grainslargerthan4 JImare rare.Evidencethat grainswere secretedincludestightpacking,uniformsize and shape,and apparentlyhomogeneouscomposition.Furthermore, scanning electronmicroscopyconfirmsthe perforatenatureof the antethecaand the natureof the keriothecalwall. Alveoli tapertowardthe tectumand pass throughthe tectumas tiny pores,enhancingcommunicationbetweenchambersand with the externalenvironment. The traditionalmodel for the additionof chambersis rejected.Threeways in which new chambersmay have been addedto the test that are consistentwith observationsmadehere: 1) The antethecamay have thickeneddifferentially,creatinga substrateto which the keriothecaof the new chamberwas attached;2) Partof the tectum,septum,and keriothecaof the previouschambermay have been resorbedbefore calcite formingthe new chamberwas secreted,perhapsnecessarybecausethe tectumwas an unsuitablesubstrateon which to attachthe keriothecaof the new chamber;3) A combinationof the abovemay have occurredwhereinthe antethecathickened differentiallyand was resorbedlocally, providinga suitablesubstrateon which to attachthe keriothecaof the new chamber.The last model is favoredhere becauseit best explainsthe shapeof the septaand the configurationof the keriothecaof the chambersof most specimens.Nevertheless,the othertwo models are consistentwith the morphologyof some specimens. INTRODUCTION using scanning electron MORPHOLOGICAL microscopy have led to significant advances in understanding skeletal microstructure and mode of growth of fossils (Crick, 1989; Simkiss and Wilbur, 1989; Carter, 1990). The wall structure of fusulinacean foraminifera, however, has been studied very little by this method of investigation. Calcite cement that has overgrown nearly all the internal surfaces of the test typically obscures details and especially the boundaries of features secreted by the fusulinid. Moreover, study of the addition of chambers by fusulinids is hampered by the inability clearly to discern in SEM micrographs the tectum, the outer covering of the wall, which is commonly visible in thin section. Because the tectum marks the boundary between chambers, its location is critical to understanding growth (Figure 1). The purpose of this study is to test hypotheses about the wall structure and the mode of addition of chambers in fusulinids. Results of this study show that 1) the grains that comprise the fusulinid test were secreted rather than agglutinated; 2) the keriotheca is a perforate structure with pores that penetrate the tectum, allowing communication between chambers and with the outside environment; and 3) the traditional model of the formation of chambers does not account for the observed relationships of the tectum, keriotheca, and septa. These observed relationships has led to new models for the formation of chambers. The superfamily Fusulinacea is one of the most successful groups of foraminifera, comprising eight families and 166 genera (Loeblich and Tappan, 1988). Fusulinaceans are more abundant and better preserved than any other group of macroscopic marine fossils in the Pennsylvanian and Permian rocks of many areas (Thompson, 1964, p. C359). The abundance and evolutionary development of the superfamily in upper Paleozoic sedimentary rocks make fusulinids valuable for biostratigraphy, and interregional correlation is possible due to worldwide zones based on genera. INVESTIGATIONS WALL STRUCTURE The classification of calcareous foraminifera is based on six major types of wall structure (Henbest, 1937; Thompson, 1951; Loeblich and Tappan, 1988). The suborder Fusulinina is characterized by a microgranular test that is now widely thought to have been secreted but has been regarded previously by some authors as agglutinated (Brady, 1876; Cushman, 1948; for further discussion see Henbest, 1963; Green et al., 1980; Haynes, 1981). Foraminifera with microgranular walls became extinct at the end of the Permian and left no descendants. Thus, both actualistic and taxonomic uniformitarian approaches to the study of fusulinid wall morphology and paleobiology fail. Fusulinid tests are characterized by microgranular walls made of closely packed, equidimensional, subangular grains of calcite. The grains are usually a few micrometers in length (Reitlinger, 1950; Tappan, 1971; Green et al., 1980; Zheng and Yang, 1991; Yang and Zheng, 1993). Different arrangements of grains result in layering in the walls of the test, which, together with the form of the test, are the basis for distinguishing higher taxa within the suborder (Lipps, 1973). Fusulinids have three principal types of wall structure (Figure 2) (Dunbar and Henbest, 1942; Ross, 1982). The profusulinellid wall (Figure 2.1) consists of three layers that include a protheca, tectum, and tectorium. The fusulinellid wall (Figure 2.2) contains an upper and lower tectorium. The keriothecal wall (Figure 2.3) lacks tectoria. In place of the protheca is a thicker and modified, alveolar layer comprising a series of large, thinly walled, polygonal tubes that are subdivided into smaller tubes near the tectum. This modified layer resembles a miniature honeycomb and is termed the keriotheca (Dunbar and Henbest, 1942; Ross, 1982). The large, planispiral, fusiform test of fusulinids shows little external detail. Study of axial and sagittal sections is therefore essential because internal features, especially the wall structure, are the most important morphological features for identifying taxa (Haynes, 1981, p. 120). Whether the fusulinid test was built from grains secreted in layers or was agglutinated was questioned for years, but most fusulinid workers are convinced that the microgranular wall was secreted. However, agglutination is still mentioned (Haynes, 1981, p. 117). This study plans to demonstrate conclusively that secretion is the only form of test construction for fusulinids. Reasons for uncertainty in test microstructure are due to limited resolution of optical microscopes and alteration, typically including recrystallization of a high proportion of the calcareous tests (Haynes, 1981). Such secondary alteration as recrystallization, dolomitization, silicification, and compaction has commonly led to changes in the composition and texture and to 181 182 JOURNALOF PALEONTOLOGY, V. 72, NO. 2, 1998 ' of a fusulinid in partial sagittal and axial sections with a detailed insert of the wall structure (adapted from Dunbar and Condra, 1927). FIGURE I-Morphology /^--"-*--- - ^ >N<~ protheca 1 Profusulinellid / upper ^^ -1-I.- tectorium protheca lower et . -.. z Pusullnellld tectorium tectum XT0 TTTu~TfX A ta secretedwall. Largergrainsof sparrycalcitecement(6 to 14 ,im long) line an unfilledchamber(KUMIP2,506,574); 1,250X. tectorium tectum </ " FIGURE3-Broken specimen of Triticites ventricosus from the Hughes Creek Shale Member. The small grains (2 to 5 ,xm long) comprise the ^upper lower keriotheca distortionof the wall of the test, all of which result in the loss of information(Cummings,1955). Microgranularwall--Early workers disagreed whether fusulinid tests were agglutinated or secreted (Henbest, 1937; Haynes, 1981). Because thin-sectionpetrographygives insufficient resolution,it has not been especiallyhelpfulin determining the natureof the test (Lipps, 1973). Brady (1876) believed that the test was built of minuteparticlesembeddedin a calcareous cement, and Cushman(1948) judged the test to be wholly or partly agglutinated.Fusulinidworkerswho concluded that the wall was secretedinclude Moller (1878), Plummer(1930), Galloway (1933), and Rauzer-Cherousova (1936). Wood (1949) and Cummings(1955, 1956) showed that the wall is formedof equidimensional,subsphericalgrains of calcite that are closely packedbut withoutdetectablecement. Greenet al. (1980) used high-voltagetransmissionelectron microscopyto study the internal structureand space between micrograins in Triticites moorei. The presenceof small intergranular voids, which range in size from less than 50 A to 1,000 A and probablycontained organic matter,suggests a secreted test. The idea that the test was constructedof secretedgrains in layers is now widely accepted (Henbest, 1937; Wood, 1949; Reitlinger, 1950; Cummings, 1956; Greenet al., 1980). Keriotheca.-Studies of thin sections have suggestedthatthe keriotheca is a perforate structure comprising alveoli, hence its name meaninghoneycombwall (Douville, 1906; Hayden,1909; Dunbarand Condra,1927;White, 1932;Henbest,1937;Thompson, 1951; Skinner and Wilde, 1954). Such early keriothecal FIGURE 2-Types of fusulinid walls. 1, three-layered,profusulinellid fusulinidsas Triticitesand Schwagerina,however,were thought wall. 2, four-layered,fusulinellidwall. 3, two-layered,keriothecalwall. to have had an imperforatewall. Proponentsof the imperforate 3 Keriothecal FIGURE4-Scanning electronmicrographsshowing the secretedgrainscomprisingthe test. 1-3, Schwagerinasp. from boreholeMaune2-36 [36- 21-34]. 1, stereopairs7? apartof a sagittallybrokenspecimenexposingthe proloculus,spirotheca,and septa (KUMIP2,506,582), 35x. 2, detail of grainscomprisingthe left side of proloculuswall (see Figure4.1), 2,000X. 3, detail of grainsof a septum(KUMIP2,506,588), 1,250X. 4-8, Triticitesventricosusfromthe HughesCreekShaleMember.4, polishedandetchedaxial sectionthroughthe proloculus(KUMIP2,506,562),10X. 5, detailof polishedandetchedspecimenshowingindividualgrainscomprisingthe left side of proloculuswall (see Figure4.4), 5,000X. 6, axially brokenspecimenthatexposes septalfluting,tunnels,and chomata(KUMIP2,506,580), 20x. 7, detailof grainscomprisingthe flutedseptumand spar in the three open spaces surroundingthe septum(rightarrowin Figure4.6), 1,250x. 8, grainscomprisinga crackedchoma (left arrowin Figure4.6), 2,500x. 183 AND KAESLER-FORAMWALLSTRUCTURE HAGEMAN U 'A 14h 2 :,c~ia.. '._i '- -'^^,~--."< #9 -&5^%">< Ps ' t ; *4,~ ^ -* , ",r t r'V I I A ,' .- 11))1 4w I 0~~~~~~~~ , I 4,~~~~~~~~~~~~~~~~~~~ I 0 . ,:- ftc -I tAd *1It 1'1 I: . * 4 & z * e l, J r: ;;? 44 ,..... "^r-" .. .. -. : .? ?>*; ?,-li?'.-:i . sa- I 54% "" .ti- '* 184 JOURNAL OF PALEONTOLOGY,V. 72, NO. 2, 1998 specimens have weathered free from the shale matrix and were collected by sweeping them into a sample bag with a whisk broom. o Sparry calcite Preparation.-The fragmented specimens of Schwagerina sp. * Mean of secreted were mounted on scanning electron microscopy (SEM) stubs grains using double-sided tape. The specimens of T. ventricosus were E * Mean of spar . 10.00 prepared in three different ways in a search for a good method of mounting fusulinids onto SEM stubs. Some specimens were encased in epoxy, cut and polished into axial and sagittal sections, and mounted to the SEM stubs with epoxy. The epoxy0 encased fusulinids did not photograph well because the bound( 0 0 O aries of the walls were difficult to distinguish from the epoxy. m. 5.00 o0 Other specimens were not embedded but were glued to a glass '0 o o 3 slide with epoxy, cut, and polished. The slides were then frozen ID l in order to remove specimens from the epoxy and slide. The zO?oo O 0 o na00 o specimens were then mounted onto SEM stubs with a small amount of epoxy. This method worked if care was used in ap- OLOC1 O E ..) i n nnf plying the epoxy. The presence of epoxy on the polished surface U.WJ or sides of the specimen, of course, resulted in poor photographs. 0.00 5.00 10.00 The remaining specimens were broken with a hammer or scissors and mounted onto SEM stubs with epoxy. This method Lengthof grains, pm reveals the microstructure quite well but does not provide any FIGURE5-Relationships between sizes of secretedand sparry-calcite control in observing specific features. Before being mounted, all grainsof Triticitesventricosus.SEM micrographswere used to mea- the specimens were cleaned by using an ultrasonic cleaner and sure 120 secretedgrainsand 40 grainsof spar.Some datapointsrep- etched in one-percent hydrochloric acid for five seconds to reresentmore thanone grain. veal individual grains. C Secreted grains * I aI I I wall structure believed that the alveoli abruptly taper toward the tectum so that they close, do not penetrate the tectum, and thus are not represented by pores on the external surface (Dunbar and Condra, 1927; White, 1932). Most authors, however, now believe that the openings in the keriotheca pass through the tectum as tiny pores that lead to the next chamber, the next whorl, or to the outside environment (Thompson, 1951; Zheng and Yang, 1991), a view that is supported by this research. GROWTH The method by which fusulinids added chambers has not been controversial. The traditional view has held that a new chamber is added to the existing antetheca, the anterior wall of the last chamber, simply by attaching calcite of the new chamber to the pre-existing one without any modifications (Dunbar, 1963; Thompson, 1964). In the process the antetheca becomes a septum between the last two chambers when an additional chamber is added, and the new chamber's leading edge becomes the antetheca. The addition of chambers has been studied very little. Deprat (1912) discussed the different shapes of the antetheca and septa. Dunbar and Condra (1927) dismissed most of these observed differences as being due to improperly prepared specimens, but they were not able to explain all of the shapes that Deprat observed. Haynes (1981, p. 120) reported that the antetheca may be planar, bow out in a curve, or fold. MATERIALS AND METHODS Sampling.-Well-preserved, broken fragments of immature specimens of Schwagerina sp. were recovered from drill cuttings sampled at ten-foot intervals from a bore hole, the Maune No. 2-36 [36-21-34] at 2,700 to 2,800 feet below the surface in central Kansas. The fragmented, immature specimens could not be identified to species and did not allow firm biostratigraphic conclusions, but they are probably from the Beattie and Grenola Limestones of the Council Grove Group. The Hughes Creek Shale Member of the Foraker Limestone, approximately 1.5 km east of Paxico, Kansas, on Interstate 70 was sampled because it contains abundant, well-preserved specimens of Triticites ventricosus (Meek and Hayden, 1858). These THE MICROGRANULAR WALL Well-preserved specimens were a priority because minute features of fusulinids are easily obscured by diagenesis, especially by sparry-calcite cement and recrystallization. Fortunately, some of the specimens had unfilled chambers or chambers filled with a single crystal of sparry calcite rather than a drusy lining, which facilitated observation of the structure of the wall. Evidence of a secreted wall would include grains that are closely packed, fitted, equidimensional, sharp edged, and homogenous in appearance suggesting a uniform composition (Cummings, 1955; Tappan, 1971). Evidence of an agglutinated test would include grains that are: loosely packed with gaps, cement, or organic matter separating them; rounded; and variable in size and composition (Lipps, 1973). The specimens studied have equidimensional grains in a fitted network with a uniform size, shape, and, apparently, composition as judged from crystal habit (Figure 3). If the tests were agglutinated, the packing could be tight but with cement-filled gaps and with a wider variety of grain sizes, shapes, and possibly compositions. Specimens were studied with these criteria in mind to determine if their tests were secreted or agglutinated. One immature specimen of Schwagerina with well-defined spirotheca, septa, and proloculus clearly lacks sparry calcite in the pore space (Figure 4.1). The wall of the proloculus consists of well-packed, subangular, fitted grains that are typically 2 to 4 ,Im long with no spar between the grains (Figure 4.2). The surface of the septum of another specimen of Schwagerina sp. has grains that are closely packed, subangular, and consistently 3 to 4 ,Im long (Figure 4.3). Studying a specimen of Triticites ventricosus shows that the proloculus, chomata, and axial fillings comprise granular calcite (Figure 4.4, 4.5). Many of the chambers have been filled with secondary, sparry calcite. On the left side of the wall of the proloculus, grains are tightly packed and fitted, strongly suggesting that they were secreted (Figure 4.5). In the background, a large crystal of spar approximately 10 pm across extends into the hollow proloculus and shows the size difference between the secreted grains and the sparry calcite cement. A freshly broken surface of T. ventricosus (Figure 4.6-4.8), also with spar-filled 185 HAGEMANAND KAESLER-FORAM WALLSTRUCTURE i: r 1 ?--:r h 'C ?r' ?7.,. CC CC' LC*' , &I ... c' ,......... :: ?I, r, ? c 1 ? -- ? ?r?;- c? ,, ;* *I. t co?l I, :? f.; : ' '? electronmicrographsshowing the perforatenatureof the keriothecaof Triticitesventricosusfrom the Hughes Creek Shale Member.1, split image of the keriothecafrom a cut, polished,and etchedaxial section (KUMIP2,506,563), upper50x, lower 250x (areain the white box from the upperimage). 2, spirothecawith counterclockwisegrowthshowing open alveoli, from a cut, polished, and etched sagittal section (KUMIP2,506,568), 350x. 3, split image of the keriotheca,septum,and the exteriorof the spirothecawith clockwise growthfrom a cut, polished,andetchedsagittalsection(KUMIP2,506,610);uppershows a largealveolusthatopens on the outersurfaceas a pore (arrowP), 170X; lower shows wheretectum(T) stops (arrow)and only keriotheca(K) is in contactwith the suturethatextendsdown into the septum,340x (area in the white box from the upperimage). 4, split-image,exteriorview of the antethecaand spirothecafrom an etched,whole specimen(KUMIP 2,506,614); upper shows the small pores covering the outer surfaceof the spirotheca(arrow),100; lower shows the grains comprisingthe antethecaand a spar-linedpore (arrow)in the antetheca,1,OOO (areain the white box from the upperimage). FIGURE 6-Scanning 186 JOURNALOF PALEONTOLOGY, V. 72, NO. 2, 1998 HAGEMANAND KAESLER-FORAM WALLSTRUCTURE chambers, clearly shows the difference between grains comprising the wall and the spar that fills the chambers. The apparently secreted grains are all tightly packed, angular to subangular, and consistently 0.5 to 5.0 ,xm long, whereas the crystals of chamber-lining spar are 10 to 15 ,pm long. A choma of the same specimen that was cracked during preparation comprises tightly packed, angular to subangular grains that are 2 to 4 ixm long (Figure 4.8). A septum of another specimen that was used to study the addition of chambers also reveals grains that are 2 to 4 ,um long and appear to have been secreted (Figure 7.3, 7.4). SEM micrographs that show distinct grains were used to measure randomly the secreted grains of the wall and sparry calcite (Figure 5). A grid was constructed to lay over micrographs. Lengths and widths of 120 secreted grains and 40 sparry calcite grains were measured. The grains comprising the wall are different in size from the spar. Secreted grains range from 0.5 to 6.0 IJm long and have an average length of 2.5 Ixm, but 95 percent (114 grains) are from 1 to 4 pIm long. Grains of sparry calcite cement are considerably larger than grains in the secreted wall and range from 2 to 16 xim long and an average length of 7.6 ,um. Only 4 grains (10 percent) were smaller than 5 ,xm in length; thus 90 percent of the grains of sparry calcite are from 5 to 16 ,Im long. When viewed in thin section, the keriothecal wall is seen to have light and dark bands, 75 to 100 ,xm long and 15 to 20 pxm wide, that are perpendicular to the surface of the test. These are also visible in some SEM micrographs (Figure 6.1-6.3). The bands represent walls and pores in a two-dimensional, edge view of the honeycomb structure, but authors have not always agreed on which bands correspond to pores and which to wall (Dunbar and Condra, 1927; White, 1932). Whether the alveoli extend as tiny pores through the wall into the next chamber or to the outside world has also been debated. Thompson (1951), however, demonstrated that the alveoli taper and pass through the tectum. He heated spar-filled specimens, oxidizing iron in the pore filling of the alveoli so that they became visible in thin section. Zheng and Yang (1991) also concluded that the pores penetrate the spirotheca and allow communication between whorls and with the outside. When viewed in SEM micrographs, the dark bands are alveoli that are filled or lined with sparry calcite (Figure 6.1-6.3). When the fusulinid was alive, the alveoli were filled with cytoplasm, possibly bearing symbiotic, photosynthetic algae (Ross, 1972) and connected to the inner chambers. The alveoli are 15 to 20 Ixm wide, 75 to 100 pxmlong, and taper abruptly toward the tectum. Figure 6.1 resembles the typical two-dimensional views seen in thin sections and emphasizes the difficulty in determining whether the keriothecal openings pass through the tectum to the outside or into the next whorl and in ascertaining which bands correspond to pores and which to wall. Crystals of sparry calcite line every surface, obscuring details and complicating the interpretation. SEM micrographs of etched specimens, however, provide a three-dimensional view, and some show that the alveoli are open and pass through the tectum as tiny pores (see Figure 6.3 in which the arrow indicates one such pore). More- FIGURE 7-Scanning 187 over, numerous pores in the spirotheca appear to correspond to the underlying alveoli that comprise the honeycomb wall (Figure 6.4). The perforate nature of the antetheca is also demonstrated by the presence of large, spar-filled pores (Figure 6.4, arrow), but they are not as abundant as the pores in the spirotheca. These SEM micrographs (Figure 6.3, 6.4) confirm Thompson's (1951) idea that the alveoli extend from the inner surface of the wall and pass through the tectum to the next whorl or to the outside during life to allow exchange of nutrients and to provide other cytoplasmic communication with the external environment. In the past, secondary, sparry calcite cement has confounded the interpretation of the wall, making its structure difficult to see in thin section and leading to misinterpretation even with scanning electron microscopy (Figure 7.1). Grains of spar, however, are typically coarser than the calcite of the wall (Figure 5). In some thin sections the open spaces in the keriotheca appear as dark bands (White, 1932), probably because of the manner in which large crystals of sparry calcite refract the light. Some authors have described or named sparry calcite as part of the wall instead of a diagenetic feature, leading to even more confusion in the interpretation of structure. Henbest (1937, p. 215), for example, was probably describing spar when he noted a keriothecal structure he called columnar jointing on a microscopic scale. Similarly, it seems that Zheng and Yang (1991, p. 223, pl. 1) and Yang and Zheng (1993, p. 323, fig. 7) did not differentiate between the grains comprising the wall and the secondary sparry calcite. While Zheng and Yang (1991) demonstrated the nature of the keriothecal wall but unfortunately, their interpretation labeled the spar that lines the alveoli as brachycolumns which suggests a two-dimensional interpretation. Indeed, the walls appear to be columnar (Figure 7.1), but that is due to the polished, etched, and orientated section of the wall of a three-dimensional pore being observed in a two-dimensional, edge view. To label a diagenetic feature that appears to be columnar as part of the wall is misleading, and the term brachycolumn should be suppressed because it refers to a diagenetic feature and stresses the twodimensional appearance of the wall rather than its three-dimensional honeycomb reality. Yang and Zheng (1993) analyzed the layers of the wall, but once again it appears they may not have differentiated sparry calcite grains from secreted grains. ADDITION OF CHAMBERS The traditional model to explain the addition of new chambers and the consequent formation of septa suggests that when a new chamber was added, the fusulinid merely attached the calcite of the new chamber directly to the organic-rich tectum of the previous chamber (Figure 8.1) (see, for example, Dunbar and Skinner, 1937). Scanning electron micrographs and thin sections, however, do not support this mode of growth. Instead, they indicate that the antetheca may have been modified as a new chamber was added (Figures 6.2, 6.3, 7.2). The modification seems to have involved a partial or complete removal of the tectum from the upper portion of the antetheca, which may have been initially quite thin (Figure 7.5, arrow). Specimens on which electronmicrographsshowingthe modificationsof the antethecaassociatedwith additionof chambersby Triticitesventricosus from the Hughes Creek Shale Member. 1, split image of a broken, etched specimen (KUMIP 2,506,603); upper 40X; lower shows spar covering every surface, 400X (area in the white box from the upper image). 2, spirotheca with counterclockwise growth showing septa and the keriotheca from a cut, polished, and etched sagittal section (KUMIP 2,506,566), 200x 3, spirotheca with clockwise growth showing a septum from a cut, polished, and etched sagittal section having modification of the septum (arrows mark end points of resorption) (KUMIP 2,506,610), 250x. 4, detail of secreted grains comprising the septum (see Figure 7.3), 2,000X. 5, thin antetheca (arrow) from a cut, polished, and etched sagittal specimen possibly with a malformed penultimate chamber (KUMIP 2,506,613), 100x. 6, polished spirotheca from a sagittally sectioned specimen with counterclockwise growth showing the modification of septa (arrow), 30x. 188 JOURNALOF PALEONTOLOGY, V. 72, NO. 2, 1998 1 Traditionalmodel I I I II I i I I =t- , 31 P 7^ -g 2 Differential-thickeningmodel 3 Resorptionmodel _ ,' O,> 4 Combinationmodel I the antethecaecould be studiedwere rarebecausethe antethecae were typically absent, poorly preserved,disturbedby compaction, or lost duringpreparation.This may have resultedin part fromtheirbeing initiallyquitethin.Unfortunately,the only specimen found that clearly reveals the thin antethecawas one that may have failed properlyto develop its last chamber(Figure 7.5). Thus, this thin antethecamay be exceptional,the resultof teratologicaldevelopment,ratherthan being normal. The modificationof the antethecabeforethe additionof a new chamberis not partof the growthprocess accordingto the traditional model (Figures 6.2, 6.3, 7.2, 7.3, 7.6). SEM micrographs,thin sections, and publisheddescriptionsall indicatethe need for a revised model of growth. A prime example comes from the work of Dunbar(1963, p. 29) in which he showed the traditionalmodel beside an actualspecimenthatclearlydoes not fit the model. Otherauthorsseem also to have illustrateddiagramaticallyand labeledincorrectlythe featuresof the wall. For example, alveoli have been drawnextendinginto the septa, indicatingthat both septa and antethecaehad a keriothecalstructure(White, 1932, p. 7; Haynes, 1981, p. 121). These specimens, however, indicate that pores in the antethecawere largerthan those of the keriothecaand very sparsely distributed(Figures 6.4, 7.3). Althougha revisedmodel of the additionof chambersis clearly needed, no single model can account for all the apparent methodsof chamberadditionobservedin the specimensstudied. Threenew models have been developedto explain the apparent methodsof growthobserved (Figure8.2-8.4). Differential-thickeningmodel.-In this model the antetheca was initially thin. It either continuedto grow or, at some stage after the new chamberwas formed, began to thicken. When anothernew chamberwas to be added,the tectumwas resorbed or fragmentedalong its zone of attachment,forming a secure foundation(Figure8.2). Wall-resorptionmodel.-In this model of growth,the antetheca was initially secretedat its final thickness,equivalentto the thicknessof a septum.When a new chamberwas to be added, the tectumand portionsof the keriothecawere resorbedproviding a tectum-free,calcite foundationfor the attachmentof the calcite that formedthe new chamber(Figure8.3). Combinationmodel.-This model combines the two previously describedmodels. It accountsfor both thickeningof the antethecaduringthe additionof the new chamberandresorption of part of the tectum and keriotheca(Figure 8.4). This model assumes that an initially thin antetheca(Figure 7.5) is characteristicof fusulinidsand thatclearingaway some of the organicrich tectumand calcite of the antetheca(as it becomesa septum) would providea firmersite for attachmentof calcite formingthe new chamber. The combinationmodel is favoredhere becauseit seems best to fit the observed evidence. The specimens reveal a modified 8-Models of additionof chambers.1, traditionalmodelwiththe FIGURE new septumformedby the antetheca'sbeing attacheddirectlyto the tectumof the previouschamber(tectumshownas a heavy blackline). model with the septumthickeningby addi2, differential-thickening tionalgrowthin the lower part(arrows)while the tectumin the upper partwas resorbedor fragmented,thus formingan areafor the site of attachmentof the new chamber.3, wall-resorption modelwith the septum modifiedand the adjacenttectum(shown as a dashedline) modified, forminga tectum-freesurfacefor attachmentof the calciteof the new chamber.4, combinationmodelwith partof the septumthickened (arrows)and some of the septumandtectum(shownas a dashedline) resorbed.This also formsa tectum-freesite for attachmentof the new chamber. HAGEMANAND KAESLER-FORAM WALLSTRUCTURE area where the tectum and calcite of the septumhave been resorbed(Figures6.2, 6.3, 7.2, 7.3, 7.6) and an antethecathatwas initially thin (Figure7.5). In additionto explainingthese observations, this mode of growth also seems intuitivelyto provide the greatest strength.The strength,which was due to the increased surface area and better mineralogicalsubstratefor attachment,may have been coincidentalor adaptive,as has been suggestedfor otherlarge,benthicforaminifera(see, for example, Yan et al., 1994). The additionaltime and energyrequiredfor a fusulinid to implement the combinationmodel may not have been a concern.The energy involved in resorptionandreprecipitationseems unlikelyto have been appreciablygreaterthanthat needed for extractingand precipitatingnew calcite from sea water or food and may have been inconsequentialwhen compared with the advantagesconferredby increased strength(Blaxter, 1989; Smil, 1991). Resorptionof part of the test is common among fusulinids. Many genera,for example,resorbedpartsof their septato form tunnels that are thoughtto have allowed for intracellularcommunication,migrationof the nucleus (Dunbar,1963), or brood chambers (Ross, 1972). The resorbedcalcite presumablywas then secondarilyredepositedas chomata. The antethecaeof the studied specimens were poorly preserved and did not provide conclusive evidence that they were initially thin. If antethecaewere not initiallythin, then the wallresorptionmodel would be favoredfor the additionof chambers. CONCLUSIONS 189 Natural History Museum at The University of Kansas as specimens 2,506,562 to 2,506,615. REFERENCES K. 1989. Energy Metabolismin Animals and Man. CamBLAXTER, bridgeUniversityPress, Cambridge,336 p. BRADY,H. B. 1876. A monographof the Carboniferousand Permian Soforaminifera(the genus Fusulina excepted). Palaeontographical ciety, London, 166 p. J. G. (ed.). 1990. Skeletal Biomineralization:Patterns,ProCARTER, cesses and EvolutionaryTrends,Volumes 1 and 2. Van Nostrand Reinhold,New York,832 p. and 101 p. R. E. (ed.). 1989. Origin,Evolution,and ModernAspects of CRICK, Biomineralizationin Plants and Animals. PlenumPress, New York, 536 p. R. H. 1955. Nodosinella Brady (1876) and associated upCUMMINGS, per Palaeozoicgenera.Micropaleontology,1:221-238. . 1956. Revision of the upperPalaeozoictextulariidforaminifera. Micropaleontology,2:201-242. CUSHMAN, J. A. 1948. Foraminifera their Classification and Economic Use. HarvardUniversityPress, Cambridge,Massachusetts,605 p. DEPRAT,J. 1912. Etude des fusulinides de Chine et d'Indochineet classificationdes calcairesa fusulines.MemoiresService Geologique de l'Indochine,1(3):1-63. DOUVILLE,H. 1906. Sur la structuredu test dans les fusulines. C. R. Academiedes Sciences, Paris, 143:258-261. DUNBAR,C. 0. 1963. Trendsof evolution in Americanfusulines, p. 25-44. In G. H. R. von Koenigswald,J. D. Emeis, W. L. Buning, and C. W. Wagner(eds.), EvolutionaryTrendsin Foraminifera.Elsevier,Amsterdam. 1927. The Fusulinidaeof the Pennsylvanian , ANDG. E. CONDRA. System of Nebraska.NebraskaGeological Survey Bulletin, second series, 2, 135 p. The grainsof microgranular calcite in the fusulinidwall were secreted ratherthan agglutinated.This conclusion, which con, ANDL. G. HENBEST.1942. Pennsylvanian Fusulinidae of Illinois. firms the interpretationof most recent authors,is based on the Illinois State Geological Survey Bulletin, 67, 218 p. 1937. PermianFusulinidaeof Texas. Unitight packing of grains,their uniformsize and shape, and their , ANDJ. W. SKINNER. apparentlyhomogeneous composition. Grains comprising the versity of Texas Bulletin,3701:518-825. wall, which rangefrom 0.5 to 6.0 ,um long with 95 percentfrom GALLOWAY,J. J. 1933. A Manual of Foraminifera.PrincipiaPress, 1 to 4 Ijm long, are consistently smaller than grains of sparry Bloomington,Indiana,483 p. calcite cement, which are 2 to 16 ,Im long with 90 percent GREEN, H. W., J. H. LIPPS,AND W. J. SHOWERS.1980. Test ultrastructure of fusulinidforaminifera.Nature,283:853-855. longer than 5 ,xm. H. H. 1909. Fusulinidaefrom Afghanistan.Geological SurHAYDEN, The keriothecais perforatedby pores that stem from the alvey of IndiaRecords,38:250-256. veoli. These pass from the interiorside of the spirotheca,taper HAYNES, J. R. 1981. Foraminifera.John Wiley and Sons, New York, abruptlytowardthe tectum,and penetratethe spirotheca,allow433 p. ing communicationbetween chambersand with the externalen- HENBEST,L. G. 1937. Keriothecalwall structurein Fusulina and its vironment. influence on fusuline classification. Journal of Paleontology, 11:212230. New models of chamberadditionare neededbecause the tra. 1963. Biology, mineralogy,and diagenesisof some typical late ditionalmodel does not adequatelyexplainfeaturesof the septa. Paleozoic sedentary foraminiferaand algal-foraminiferalcolonies. Three revised models are proposed to explain the additionof CushmanFoundationfor ForaminiferalResearchSpecialPublication, chambers:1) the differential-thickening model, 2) the wall-re6, 44 p. the combination The last of these and model. 3) sorptionmodel, LIPPS,J. H. 1973. Test structurein foraminifera.Annual Review of seems best to explain the observedmorphology. Microbiology, 27:471-488. ACKNOWLEDGMENTS We are grateful to C. G. Maples, who providedinvaluable assistanceduringdiscussionsof our ideas and who reviewedthe manuscript.P. Enos, C. A. Ross, and G. A. Sandersonalso reviewed the manuscriptand contributedto its improvement.B. E. Cutler,I. J. Rowell, and K. R. Evanshelped with preparation of specimensand with SEM work.A. J. Rowell, R. A. Robison, and N. G. Lane assistedus in obtainingreferences.R. W. Lange provided specimens from the subsurface,which he collected while drilling oil wells. The study was funded in part by the KansasGeological Foundation,Sigma Xi (GIAR 93/06 19077), and a Departmentof Geology summerresearchfellowship.The University of Kansas electron microscopy laboratoryprovided SEM micrographs.J. Kernsof The PaleontologicalInstituteprepared the line drawings.The fusulinids studied have been reposited with the Division of InvertebratePaleontology of the LOEBLICH,A. R., AND H. TAPPAN. 1988. Foraminiferal Genera and TheirClassification.Volumes 1 and 2. Van NostrandReinhold,New York,970 p. and 212 p. MEEK,F B., AND F V. HAYDEN. 1858. Remarks on the Lower Creta- ceous beds of Kansas and Nebraska,together with descriptionsof some new species of Carboniferousfossils from the valley of the Kansas River. Proceedingsof the Academy of NaturalSciences of Philadelphia, 10:256-264. MOLLER,V. VON. 1878. Die spiralgewundenen Foraminiferen des russischen Kohlenkalks. Akademie Imperiale der Science, St. Petersburg, Memoir7, 25(9):1-147. H. J. 1930. Calcareous foraminifera in the Brownwood Shale near Bridgeport. University of Texas Bulletin, 3019, 21 p. PLUMMER, RAUZER-CHERNOUSOVA, D. M. 1936. On the question of the stratigraphicsignificanceof the upperPaleozoic foraminifers.Akademiia Science USSR Classe des Science Matematichee naturali,Geologicheskogo Seriia 1:61-86. E. A. 1950. Foraminiferaof the middle Carboniferous REITLINGER, deposits of the centralpart of the Russian platform(excluding the 190 JOURNAL OF PALEONTOLOGY,V. 72, NO. 2, 1998 - . 1964. Fusulinacea,p. C358-C436. In R. C. Moore(ed.), Treatise family Fusulinidae).AkademiiaNauk SSSR GeolgicheskiyInstituta on InvertebratePaleontology,Part C, Protista2, Geological Society Trudy,126, GeologicheskogoSeriia,47:1-127. Ross, C. A. 1972. Paleobiologicalanalysis of fusulinacean(Foramiof Americaand Universityof KansasPress, Lawrence. niferida)shell morphology.Journalof Paleontology,46:719-728. M. P. 1932. Some TexasFusulinidae.Universityof TexasBulWHITE, . 1982. Paleozoic foraminifera-fusulinids,p. 163-176. In T. W. letin, 3211, 107 p. Broadhead(ed.), Foraminifera.Universityof TennesseePress, KnoxWOOD,A. 1949. The structureof the wall of the test in the foraminifville. era; its value in classification.Geological Society of London,QuarSIMKISS, K., ANDK. M. WILBUR.1989. Biomineralization.Cell Biolterly Journal,104(2):229-255. and Mineral San 337 California, ogy Deposition. Diego, p. YAN, S., R. G. BLACK,ANDJ. H. LIPPS.1994. MorphologicaloptiJ. AND G. L. WILDE. 1954. Fusulinid wall structure. JourSKINNER, W., mization in the largest living foraminifera:implicationsfrom finite nal of Paleontology,28:445-451. element analysis.Paleobiology,20:14-26. SMIL,V. 1991. GeneralEnergetics:Energyin the Biosphereand CivYANG, X., AND H. ZHENG. 1993. The spirotheca of the foraminifer ilization.John Wiley and Sons, New York,369 p. H. 1971. Foraminiferida,p. 615-623. In McGraw-HillEnQuasifusulina. Lethaia, 26:319-325. TAPPAN, cyclopediaof Science andTechnology,5th edition,Volumeelem-fus, ZHENG,H., ANDX. YANG. 1991. The SEM study of wall ultrastructure of Triticitescellamagnus.Stratigraphyand Paleontologyof China, 1: McGraw-Hill,New York. M. L. 1951. Wallstructuresof fusulinidforaminifera.ConTHOMPSON, 183-225. tributionsfrom the CushmanFoundationfor ForaminiferalResearch, 2:86-91. ACCEPTED 7 JULY1997 J. Paleont., 72(2), 1998, pp. 190-201 Copyright ? 1998, The Paleontological Society 0022-3360/98/0072-0190$03.00 TWO NEW GENERAOF UPPERSILURIAN ACTINOSTROMATID STROMATOPOROIDS CARL W. STOCK' ANDJUDITH A. BURRY-STOCK2 'Departmentof Geology,The Universityof Alabama,Tuscaloosa35487-0338 <[email protected]>and 2Programin EducationalResearch,The Universityof Alabama,Tuscaloosa35487-0231 <[email protected]> ABSTRACT-Two new genera of Upper Silurianstromatoporoids in orderActinostromatida are described.Genus Bicolumnostratum is characterized Stock,with type species B. micum(Bogoyavlenskaya), by two kindsof pillarsandnonalignedcolliculi,andis assigned to familyActinostromatidae. GenusAcosmostromaStock,withtypespeciesA. ataxiumStocknew species,containsirregularmicropillars and microcolliculi,and is assignedto family Densastromatidae. Two additionalnew species areAcosmostromaglascoense Stock and A.? cobleskillenseStock.A fourthspecies is A. tenuissimum(Parks).Bicolumnostratum is knownfromLudlow-andPridoli-agestrata, whereasthe occurrencesof Acosmostromaare strictlyPridoliin age. INTRODUCTION TROMATOPOROIDS IN order Actinostromatida are character- ized by a skeleton composed of vertically oriented, cylindrical pillars, and horizontally oriented, cylindrical colliculi that connect adjacent pillars. In some genera the pillars and colliculi are so small as to be called micropillars and microcolliculi. The placement of the colliculi in relation to the pillars is such that in tangential thin section a pattern commonly called a "hexactinellid network" (e.g., Steam, 1966, text-fig. 3) is seen. In at least one genus, Acosmostroma Stock new genus, the orientations of the micropillars and microcolliculi are not regular in at least parts of specimens, and the observer has difficulty discriminating between the two. In the Systematic Paleontology section the terms micropillar and microcolliculus (plural: microcolliculi) are employed in many places. These microstructures, first defined by Stock and Holmes (1986, p. 561), are differentiated from pillars and colliculi on the basis of the smaller size of the micropillars and microcolliculi. Essentially, if pillars and colliculi are small enough to comprise microstructural elements within the macrostructures of stromatoporoids in order Syringostromatida as presented by Steam (1993), they are called micropillars and microcolliculi, even though they may be the primary skeletal elements of stromatoporoids in order Actinostromatida. Most of the specimens described herein were collected from either the Glasco Member of the Rondout Formation in eastern New York or the Cobleskill Member of the Rondout Formation of central New York. Both members are part of the Pridoli Series of the Silurian System. One specimen was collected from the Jersey Shore Member of the Keyser Formation of western Virginia, which is also Pridoli. Information on the collecting localities is presented in the Appendix. Additional information on the stratigraphy of the Pridoli of New York was presented by Stock (1979, text-figs. 5, 6). Denkler and Harris (1988) provided the most recent summary of Upper Silurian and Lower Devonian stratigraphy in Virginia. Stock assumes responsibility for the taxonomic portion of this study, whereas Burry-Stock is primarily responsible for statistical procedures. METHODS Measurement.-Procedures and terms used to describe morphology employed here are the same as those outlined by Stock (1979, p. 307; 1982, p. 657). Statistics.-In the present paper statistics are used as a supplemental tool, to bring some objectivity to the use of skeletal dimensions in species identifications; they are not used in lieu of qualitative decisions. Qualitative conclusions were reached