Abstract: Growth periodicity is a phenomenon occurring in fossil and modern corals. The most apparent feature is growth banding, and environmental changes are broadly accepted as controls on this phenomenon. If environment controls the growth, then all corallites within a colony should repeat the same growth pattern, as individuals are clones and must have shared the same environment. A study on several species of Alveolitidae (Anthozoa, Tabulata) from the Late Devonian (Early Frasnian) of the Holy Cross Mountains (Poland) shows that the growth pattern varies between neighbouring individuals within the same corallum. This contradicts observations of closely related Favositida as demonstrated on Pachyfavosites sp. from the Givetian of Avesnois, France, where neighbouring individuals repeat the same pattern. Therefore, environmental control on growth rhythm in Alveolitidae can be excluded; the causes of differences between individuals remain unknown.
If the environmental changes are the controlling factor of growth periodicity, then all corallites in a corallum should repeat the same growth pattern. The aim of this study is to verify whether the growth periodicity is the same in neighbouring corallites in tabulate corals and therefore to conclude whether this phenomenon is controlled environmentally or genetically. Such a question is crucial, as corals (not only tabulates) are used for palaeoclimatic reconstructions, and the influence of environment must be well recognized to make such reconstructions reliable.
To date, all investigations have presented data for coralla, and not for individuals (tabulates: Young and Kershaw 2005; rugose corals: Berkowski and Belka 2008); only Scrutton and Powell (1980) traced individuals within a corallum, but these authors did not correlate growth patterns of individuals. This study is using the tabulae spacing for comparison of growth dynamics in neighbouring individuals and is a following up on the preliminary report published by Zapalski (2007).
The Devonian basin of the Holy Cross Mountains is divided according to palaeobathymetry into the North Łysogóry palaeolow and the South Kielce palaeohigh (Racki 1992; Szulczewski 1995) with the intermediate Kostomłoty Transitional Zone. Within the Kielce palaeohigh, three subregions have been distinguished (Racki 1992), that is, the Northern, Central and Southern Subregions, where the Central Subregion represents the most shallow environment with carbonate sedimentation (Kowala Formation). The Kielce palaeohigh is flanked to the south by the Chęciny-Zbrza basin. The Kowala section and quarry, located in the Southern Kielce Subregion exposes late Devonian (Frasnian to Famennian) to Carboniferous sediments (Racki 1992) including the Lower Frasnian (P. transitans Zone; Pisarzowska et al. 2006) biostromal and biohermal limestone, containing numerous tabulate corals (Zapalski 2012).
The investigated material comes from the Early Frasnian of the Kowala railroad section and quarry, Holy Cross Mountains, Poland (Fig. 1). Racki (1992) divided the whole section into informal lithological sets (A and B belong to Upper Sitkówka Member, and set C belongs to Kadzielnia Member of the Kowala Formation). Detailed biostratigraphy is published by Szulczewski (1971), and Pisarzowska et al. (2006) present the detailed event- and biostratigraphy.
This study focuses on three species of the genus Alveolites (Favositida: Alveolitinae), namely, A. suborbicularis Lamarck, 1801; A. compressus Milne-Edwards and Haime, 1853; and A. regularis Sokolov, 1952. Zapalski (2012) presents the details on the taxonomy of tabulate corals occurring in the Frasnian deposits of the Kowala railroad section and quarry.
One additional specimen of Pachyfavosites (Favositida: Favositidae) is also studied. The favositid Pachyfavosites sp. is collected from Givetian strata of Avesnois (France), and the growth pattern of this species has been analysed previously by Zapalski et al. (2007), in which the details concerning the locality and stratigraphy for this material are presented.
The most obvious feature in cyclomorphosis is the growth banding with higher and lower concentration of skeletal material. It is observed as dark bands, which are the high-density bands (HD) and the light bands, which comprise the low-density bands (LD).
The tabulae spacing is the observed feature of growth banding. It is often assumed that tabulae are secreted at even time intervals, and therefore, their spacing reflects the time scale. In such case, dense tabulae packing assumes lower growth rate (e.g. Young and Kershaw 2005; Zapalski et al. 2007). Likewise, the distribution of dissepimenta in extant scleractinian corals reflects best growth rate (Carricart-Ganivet 2011). It was measured on the thin sections longitudinal to the axes of corallite growth in the neighbouring individuals. The measurements follow the scheme of Zapalski et al. (2007, p. 277, fig. 3). Histograms showing the distance between the following tabulae (Fig. 2) were prepared, and polynomial trend curves approximating the trend were provided. Set of histograms was prepared for each corallum. These diagrams were prepared on Igor Pro 4.0 software (WaveMetrics, Lake Oswego, OR, USA).
Alveolitid coralla differ significantly from other tabulates. Corallites have secondary bilateral symmetry, and they are strongly flattened. They are connected with neighbouring individuals through pores on short walls only, while in other tabulates, connections are on all walls and corners (Hladil 1981, 1989). Moreover, corallites are usually oblique to the colony surface. Skeletons of alveolitids are thus complicated anatomically, and consequently, it is difficult to perform such a study because their corallites are usually bent or irregularly meandering, and thin sections usually cut them longitudinally only on short distances. Therefore, out of the collection of several hundreds of thin sections of alveolitids, only very few were suitable for this study.
Abbreviations. The abbreviations used throughout the paper are: HD for high-density zones, LD for low-density zones.
Repository. Specimens abbreviated ZPAL are housed at the Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland, and the specimen GFCL is stored at the Laboratoire de Paléontologie stratigraphique, Lille, France.
Material analysed. One corallum, one longitudinal thin section, rubble of Croix de Bourges outcrop, Avesnois, France; Givetian (for details see Zapalski et al. 2007). Thin section GFCL 4715.
Macroscopic features. In longitudinal section, growth banding demonstrates that borders between density bands are gradual and the base of the HD zone can be defined.
Spacing of tabulae. The spacing of tabulae marks growth banding. It can also be detected by measurements, and matching of growth rhythms between individuals is possible. The four first maxima appear on all three histograms and can be correlated (Figs 3A, 4); also, the minima correlate well.
Remarks. If the growth pattern in corals is controlled environmentally, then all corallites should display the same tabulae distribution with similar, correlatable growth cycles. Such a correlation is visible in the here-analysed specimen of Pachyfavosites sp.
Material. One corallum, specimen ZPAL KOW 4.001, five longitudinal thin sections. Kowala Railroad Section, Holy Cross Mountains, Poland; lithological set A, Early Frasnian.
Macroscopic features. Banding is present in most of corallum. The borders between HD/LD/HD are gradual (Figs 3B, 5).
Tabulae spacing. The spacing of tabulae shows visible growth banding. The cycles in neighbouring corallites correlate. Figure 5 shows a diagram of tabulae spacing; in both corallites spaces, 9 and 25 correspond to LD, while spaces 4–5 and 13 to HD. The couplets of HD–LD are 4–6 mm in thickness, and they start on the same levels in neighbouring corallites. Visible growth banding can also be seen on another corallum of the same species (specimen ZPAL T.25 KOW 8.002).
Material. One corallum, specimen ZPAL T.25 KOW AX.001, one longitudinal thin section. Kowala Railroad Section, Holy Cross Mountains, Poland; lithological set A, Early Frasnian.
Macroscopical features. On the longitudinal section through corallum, the growth banding is barely visible only in some parts of the specimen.
Spacing of tabulae. The spacing of tabulae is strongly variable within a single corallite and shows a weak cyclic pattern (Figs 3C, 6). The trend curves on the histograms have their minima and maxima irregularly placed. Each corallite appears to have its own rhythm of tabulae formation, and cycles in the neighbouring individuals cannot be correlated; the peaks appear usually at different instants of time. Nonetheless, several similarities were observed. The first LD (space 4) on the upper histogram (Fig. 6) is weakly visible on the middle histogram and well visible on the lower one. On all three histograms (Fig. 6), a LD occurs around spaces 27–28 on each of the histograms. Other LD and HD occur on each histogram in different places, and therefore, they cannot be matched. It can be concluded that the neighbouring corallites show differing growth patterns.
Material. One corallum, specimen ZPAL T.25 KOW R.075, one longitudinal thin section. Kowala Quarry, Holy Cross Mountains, Poland; rubble of lithological set C, Early Frasnian.
Macroscopic features. Banding is not visible.
Spacing of tabulae. Tabulae spacing is highly variable and without any obvious trend (Figs 3D, 7). As in Alveolites compressus above, the neighbouring corallites show strongly different growth pattern.
Differences in growth pattern – possible causes
The first and most striking feature of the growth pattern is the differences in neighbouring corallites in two of the species analysed, that is, A. compressus and A. regularis. Except for small irregularities shown by Scrutton and Powell (1980) on favositids, such a feature has not been reported previously in tabulates. Data from a single coralla (Young and Kershaw 2005, text-fig. 1A) have been averaged for a colony even if measured in the same way (i.e. Gao and Copper 1997).
Studies of scleractinians and tabulates have suggested environmental changes as a factor controlling growth rate (e.g. Scrutton and Powell 1980; Langdon 2002; Reynaud et al. 2004, Corrège 2006; Zapalski et al. 2007). The size of the diameter of corallites is in a scale of millimetres; therefore, the individuals shared the environmental conditions, that is, salinity, temperature, chemical composition of seawater; hence, the environmental parameters cannot be the cause of the irregularities that are observed here.
One possibility for the differences in growth pattern in neighbouring corallites of Alveolites may relate to incorrect measuring, but in Pachyfavosites sp. cycles in neighbouring corallites are correlatable. Also in scleractinians, measuring dissepiments is considered as best reflecting time scale (Carricart-Ganivet 2011). Another possibility is that such changes may have been caused by changes in genotype (mutations or epigenetic changes) of a given individual during its ontogenesis. This cause cannot be confirmed, or rejected, and therefore, it is speculative. Diagenesis was not a factor causing artificial banding, as the specimens are well preserved, with anatomic details.
The activity of dinoflagellate symbionts in extant scleractinian corals controls the CaCO3 precipitation; their random distribution within the soft tissues (Marshall and Wright 1998) may cause differences in skeletal formation. A similar situation could have applied in the discussed case, where variations in amount of these symbionts in neighbouring individuals could change the growth pattern (P. Copper, pers. comm. 2007).
Gill (1982) suggested the mechanical necessity of banding alternation in neighbouring individuals (originally described on scleractinians), where individual use neighbours as a fulcrum for levering during the growth period; then they alternate. This hypothesis may be supported by the fact that alveolitids have different corallum architecture (Hladil 1981, 1989) comparing with favositids, thus forcing different mode of growth. However, the surface of the investigated coralla is rather smooth and does not show differences in relief caused by single corallites, and therefore, such a levering seems improbable.
The last hypothesis considered here is that each individual has a constant supply of carbonate for building the skeleton. In such a case, the individual may build either thicker or thinner walls, or more abundant or fewer tabulae (or septal elements). Therefore, as presented in Figure 8, the histograms of neighbouring individuals may be inverted. Changes in corallite diameter and changes in distribution of septal elements and pores may additionally complicate the process.
Testing of this hypothesis cannot be undertaken with use of classical methods such as thin sections and serial sectioning: because of uncertainty about what is being measured (i.e. wall thickness or tabulae spacing) it is impossible to have both measurements on the same individual. A possibility is to use well-preserved specimens not filled by sediment (such as these described by Iven et al. 1997); computed tomography (micro-CT) would allow a detailed three-dimensional view on corallum. With such a view, it would be possible to investigate wall thickness changes and tabulate spacing, which is the distribution of skeletal material.
In alveolitids, the correlation between growth patterns of neighbouring individuals is weak, but favositids have well-marked cycles. If such observations could be repeated on more abundant material, the traditional ‘environmental’ interpretation of growth banding would probably have to be revised. Because of scarce material, a definitive answer to the question concerning the reason of this kind of growth pattern cannot be given here.
Alveolites regularis and A. compressus had uncoordinated growth within a colony, while A. suborbicularis had faint coordination between individuals; Pachyfavosites sp. did grow with coordination between individuals. In the latter two species, the coordinated growth was probably controlled by the environment; in the cases with lack of coordination, the environmental control can be excluded.
Acknowledgements. We are grateful to Benoît L. M. Hubert (Lille) for giving specimen from Avesnois for study, to Paul Copper (Sudbury) for some suggestions on early stages of writing this manuscript and to Ewa Roniewicz (Warsaw) for inspiring discussions. Adam T. Halamski (Warsaw) kindly read the manuscript, and thin sections were prepared by Pascal Deville (Lille); we are deeply indebted to both. Jindřich Hladil (Prague), James E. Sorauf (Binghamton) and an anonymous journal referee gave several suggestions that allowed us to improve the manuscript. Wojciech Zapalski and Andrzej Korczak-Komorowski (Warsaw) are thanked for kind field assistance. John Brenner (Wokingham) benevolently improved the language.