Type species

Tubipora catenularia Linnaeus, Reference Linnaeus1767, from Silurian of Gotland (by monotypy).

Neotype

PMU G 681, from unknown locality, Silurian of Gotland (selected by Thomas and Smith, Reference Thomas and Smith1954, p. 767).

Description

Coralla 60 × 30 to 130 × 80 mm across and 50–120 mm in height. Lacunae polygonal to elongated, size 23.59–89.08 mm² (Fig. 5.1, 5.2). Rank junctions occur at tubules (black arrow in Fig. 5.2) or on lateral corallite walls (white arrow in Fig. 5.2). In transverse section, sporadic septa present in a few corallites (Fig. 5.3). Corallites subelliptical to subrounded (Fig. 5.1–5.7); may be distorted in direction of rank junction on lateral wall (white arrow in Fig. 5.2). Corallum average of transverse corallite length 1.92–2.43 mm, width 1.53–1.96 mm; tabularium area 1.28–2.09 mm², perimeter 4.22–5.38 mm, length 1.50–1.93 mm, width 1.05–1.44 mm; outer wall thickness 0.23–0.32 mm; common wall thickness 0.37–0.59 mm, width 0.79–1.09 mm; tubule length 0.18–0.36 mm, width 0.30–0.45 mm. Coenenchymal tubules mostly rectangular, may be narrow, round, irregular, or absent in common wall (Fig. 5.4–5.7); irregular in rank junctions (Fig. 5.4). Balken structure may be present beside tubule (Fig. 5.3, 5.6) and in common wall (Fig. 5.5). In longitudinal section, corallite tabulae mostly complete, flat or concave, vertical spacing 0.38–0.54 mm, in some cases show cyclomorphic variation in spacing but relatively uniform thickness (Fig. 5.8, 5.9); abnormal spacing and appearance may accompany rejuvenation after damage to corallite (arrow in Fig. 5.8). Tubule tabulae mostly flat or slightly concave, generally more closely spaced than corallite tabulae, show cyclomorphic variation involving positive relation between spacing and thickness (Fig. 5.8). Tubules may develop into corallites (arrows in Fig. 5.9).

Figure 5.

Transverse (1–7) and longitudinal thin sections (8, 9) of Halysites catenularius. (1) Polygonal lacunae; (2) elongated lacunae (white and black arrows point to rank junctions at lateral corallite wall and tubule, respectively); (3) corallite with low ratio of tabularium length to width (note the presence of septal spines, and balken structures in the common wall); (4) corallite with high ratio of tabularium length to width (note the irregular tubule in the junction area of three corallites and rectangular tubule on the right); (5) common wall of two corallites (note the absence of a tubule and presence of balken structures); (6) balken structures in the junction area of three corallites (arrow points to a tiny tubule in the center of the balken structures); (7) rectangular and narrow tubules on the left and right sides of a corallite; (8) growth pattern of tabulae in corallites and tubules (arrow points to an interval of abnormal spacing and appearance of tabulae in corallites related to rejuvenation after damage); (9) interstitial increase (arrows point to the development of juvenile corallites from tubules). (1, 3) GIT 806-8; (2, 4–7) GIT 806-4; (8) GIT 806-11; (9) GIT 806-9.

Remarks

Mõtus and Klaamann (Reference Mõtus and Klaamann1999) considered H. priscus and H. catenularius to be conspecific, which is supported by the present multivariate analysis. Laub (Reference Laub1979) and Young and Noble (Reference Young and Noble1987) synonymized H. junior with H. catenularius, but Mõtus and Klaamann (Reference Mõtus and Klaamann1999) noted that those species have very different diagnostic features. We find that the morphological characteristics in the coralla of H. catenularius are similar to, but distinct from the type specimens of both H. junior and H. senior. Halysites catenularius is intermediate between H. junior and H. senior in tabularium and corallite size, as well as tubule width and tabulae spacing. Balken structure is present in H. junior (Mõtus and Klaamann, Reference Mõtus and Klaamann1999) and H. senior as observed in the transverse sections in Geoscience Collections of Estonia. In the examined coralla of H. catenularius, this structure is commonly found next to the tubules. Based on the differences in corallite size, tabularium size, tubule width, and tabulae spacing, we conclude that H. catenularius, H. junior, and H. senior represent three different species that are closely related.

Cyclomorphism

Hamada (Reference Hamada1959) recognized periodic thickening of corallite tabulae and constriction of the outer wall in some species of Catenipora and Halysites, which he related to annual growth. Elias and Lee (Reference Elias and Lee1993) documented cyclomorphism involving high- and low-density bands in Catenipora rubra Sinclair and Bolton in Sinclair, Reference Sinclair1955, with high-density bands characterized by thicker and/or more closely spaced tabulae. In the same species, Young and Kershaw (Reference Young and Kershaw2005) noted banding represented by thickening of tabulae. They found banding to be indistinct in Catenipora escharoides Lamarck, Reference de Lamarck1816, but noted that septa may be slightly longer and more closely spaced in high-density bands. Gao (Reference Gao1992; Gao and Copper, Reference Gao, Copper, Lessios and Macintyre1997) identified external growth banding and internal density bands based on the spacing of tabulae in halysitid corals. Bae et al. (Reference Bae, Lee and Elias2006b) considered cyclic fluctuations of corallite tabularial area, bounded by minimum values, to record annual growth in Catenipora foerstei Nelson, Reference Nelson1963. In high-density cyclomorphic bands, they observed smaller tabularial areas, thicker tabulae, better-developed septal spines, and thicker corallite walls. Using the same methodology for recognition of growth cycles (i.e., tabularial area), Bae et al. (Reference Bae, Elias and Lee2013) documented cyclomorphism in C. rubra, C. foerstei, and two other species of Catenipora.

In the present study, longitudinal sections of H. catenularius show that corallite tabulae are relatively uniform in thickness, but there are some cycles involving the spacing of tabulae (Fig. 5.9). Tubule tabulae show cyclomorphism with a positive relation between thickness and spacing (Fig. 5.8). Following the quantitative approach developed by Bae et al. (Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013), pairs of successive growth cycles bounded by minimum values of corallite tabularial area were detected from transverse serial peels of three coralla of H. catenularius (Fig. 6). Each cycle is considered to represent annual vertical growth of the colony. The annual vertical growth rate ranges from 4.9 to 9.1 mm (average 6.0 mm) as determined from the six cycles. A comparison of these cycles (see Fig. 6) shows that the growth rate is not related to the average tabularial area or the amount of variation in tabularial area. This differs from Catenipora foerstei, in which the growth rate tends to be positively correlated with variation of tabularial area (Bae et al., Reference Bae, Lee and Elias2006b). In H. catenularius, the average size of lacunae fluctuates during growth (Fig. 6). In some cases, there seems to be a weak negative correlation between changes of average lacuna area and changes of tabularial area.

Figure 6.

Examples of vertical growth cycles, considered to be annual, detected from fluctuations of corallite tabularial area in transverse serial peels of three coralla representing Halysites catenularius (horizontal dotted lines mark cycle boundaries; vertical distance between boundaries is shown). (1) GIT 806-10; (2) GIT 806-14; (3) GIT 806-4. For each corallum, the left curve shows changes of tabularial area and the right curve shows changes of average lacuna size as determined from transverse serial peels. Vertical scale shown on right side of figure.

The inferred average annual growth rate of H. catenularius (6.0 mm) is higher than that reported for Halysites sp. (3.4 mm) from Ontario, Canada, but is about average for halysitid corals in general (Table 4). Scrutton (Reference Scrutton1998, fig. 12) summarized data for tabulate corals indicating that coenenchymal heliolitids had very low annual growth rates of about 2 to 6 mm (see also Nowiński, Reference Nowiński1991; Young and Kershaw, Reference Young and Kershaw2005, table 1; Liang et al., Reference Liang, Lee, Elias, Pärnaste and Mõtus2013). Favositids had annual growth rates ranging from 5 to 18 mm (Scrutton and Powell, Reference Scrutton and Powell1980; Scrutton, Reference Scrutton1998). The average growth rates for halysitid species range from 3.4 to 11.7 mm (Table 4), which is intermediate in comparison with heliolitids and favositids. It is noteworthy that average growth rates determined for the cateniform favositid Manipora amicarum Sinclair, Reference Sinclair1955, are 12.1 mm (Bae et al., Reference Bae, Lee and Elias2008) and 13.3 mm (Young and Kershaw, Reference Young and Kershaw2005, table 1). Nowiński (Reference Nowiński1991) reported a wide range of annual growth, from 2.0 to 20.0 mm, in species representing genera that were considered auloporids by Hill (Reference Hill and Teichert1981). Scrutton (Reference Scrutton1998) suggested that highly integrated colonies, such as coenenchymal tabulates, may have been physiologically most efficient at dealing with sedimentation, thereby compensating for their slow growth rates. Most cateniform tabulates are thought to have had an intermediate level of colony integration (Coates and Oliver, Reference Coates, Oliver, Boardman, Cheetham and Oliver1973). Strategies of halysitids for coping with sediment influxes may have included higher growth rates, as well as their cateniform growth pattern, which permitted trapping and shedding of sediment, and various modes of increase associated with rapid rejuvenation and regeneration of damaged and terminated polyps (Lee and Elias, Reference Lee and Elias1991; Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013; present study).

Table 4.

Inferred annual growth rates based on internal characters in halysitid corals.

It has been proposed that some tabulate corals, possessing small corallites, moderate to high levels of colony integration, growth banding, relatively rapid growth rates, and growth forms similar to modern zooxanthellate scleractinians, were photosymbiotic (Coates and Jackson, Reference Coates and Jackson1987; Gao and Copper, Reference Gao, Copper, Lessios and Macintyre1997; Stanley and Lipps, Reference Stanley and Lipps2011; Zapalski, Reference Zapalski2014; Zapalski et al., Reference Zapalski, Wrzołek, Skompski and Berkowski2017). Other researchers, however, have considered such evidence to be equivocal (Scrutton, Reference Scrutton1998; Young and Kershaw, Reference Young and Kershaw2005). The suggestion that tabulates were zooxanthellate based on a comparison of their stable isotope signatures with those of scleractinians (Zapalski, Reference Zapalski2014) has also been questioned (Jakubowicz et al., Reference Jakubowicz, Berkowski, Correa, Jarochowska, Joachimski and Belka2015). The interpretation of halysitid tabulates is especially difficult, because cateniform growth forms are rare among the scleractinians (Coates and Oliver, Reference Coates, Oliver, Boardman, Cheetham and Oliver1973, table 2; Turnšek and Košir, Reference Turnšek and Košir2004). Distinct, annual, high- and low-density growth bands occur in a small proportion of zooxanthellate scleractinians, mostly having massive or hemispherical growth forms (Pratchett et al., Reference Pratchett, Anderson, Hoogenboom, Widman, Baird, Pandolfi, Edmunds and Lough2015). Such bands are comparable to those in some halysitids. However, growth bands are also present in some azooxanthellate scleractinians, including Lophelia pertusa (Linnaeus, Reference Linnaeus1758), which forms dendritic, hemispherical colonies. The banding in L. pertusa has been interpreted as annual (Mortensen and Rapp, Reference Mortensen and Rapp1998; Risk et al., Reference Risk, Hall-Spencer, Williams, Freiwald and Roberts2005), but may not be annual in all cases (Gass and Roberts, Reference Gass and Roberts2011). A plot of growth rates for modern scleractinians indicates a geometric mean of approximately 16 mm per year (Pratchett et al., Reference Pratchett, Anderson, Hoogenboom, Widman, Baird, Pandolfi, Edmunds and Lough2015, fig. 6). The lowest and highest annual rates, 0.8 and 333 mm (Pratchett et al., Reference Pratchett, Anderson, Hoogenboom, Widman, Baird, Pandolfi, Edmunds and Lough2015, table 4), were both obtained from zooxanthellate colonial species. Scleractinians with low and high growth rates tend to be massive and branching forms, respectively (Pratchett et al., Reference Pratchett, Anderson, Hoogenboom, Widman, Baird, Pandolfi, Edmunds and Lough2015). The range of reported annual growth rates for L. pertusa is 2.4–34.7 mm (Larcom et al., Reference Larcom, McKean, Brooks and Fisher2014). The growth rates of halysitids and all other tabulates (see previous paragraph) fall within the ranges of both zooxanthellate and azooxanthellate scleractinians. Definitive evidence that halysitids and other tabulate corals were photosymbiotic remains elusive.

Formation of tubules

Tubules in H. catenularius were generated from small intramural openings between adjacent mature corallites (arrows in Fig. 7.3, 7.4, 7.7, 7.8), and in some cases beside juvenile corallites (arrows in Figs. 8.9, 9.6). Fission of an opening and fusion between adjacent openings were common (Fig. 7.3–7.12). Stasińska (Reference Stasińska1967, fig. 4) found that in some cases, tubules in Halysites arose from the fusion of two intramural openings. In H. catenularius, the size and shape of small intramural openings changed constantly with height in the corallum. Some openings seem to have disappeared in the common wall, but reappeared after a very short vertical distance. The walls around tubules thickened in some cases, resulting in balken structure (Fig. 8.1–8.5).

Figure 7.

Transverse serial peels showing the formation of a tubule from small intramural openings in Halysites catenularius. (1–11) Development of small intramural openings in the common wall between corallites (e.g., arrows in 3, 4, 7, 8); (12) the resulting tubule in the common wall. GIT 806-8, figured peels spaced 0.05, 0.05, 0.05, 0.05, 0.05, 0.05, 0.05, 0.10, 0.25, 0.05, 0.15 mm apart, respectively. Scale bar shown in 1.

Figure 8.

Transverse serial peels showing Type A interstitial increase in Halysites catenularius. (1–5) Balken structure in the thickened walls around a tubule; (6–8) tubule develops into a juvenile corallite; (9) appearance of small intramural openings (arrows) beside juvenile corallite; (10–12) juvenile corallite attains mature size. GIT 806-1, figured peels spaced 0.30, 0.35, 0.45, 0.50, 0.15, 0.25, 0.05, 0.05, 0.15, 0.20, 1.15 mm apart, respectively. Scale bar shown in 1.

Figure 9.

Transverse serial peels showing Type B interstitial increase in Halysites catenularius. (1–5) A tubule develops into a juvenile corallite between two mature corallites; (6–8) juvenile corallite attains mature size, while new tubules form on either side (arrows in 6, 8). GIT 806-10, figured peels spaced 0.65, 0.20, 0.15, 0.20, 0.05, 0.20, 1.35 mm apart, respectively. Scale bar shown in 1.

Stasińska (Reference Stasińska1967), Webby and Semeniuk (Reference Webby and Semeniuk1969, fig. 8), and Webby (Reference Webby1975, figs. 4, 5) reported that tubules associated with a new corallite were inserted interstitially at about the level where the corallite attained mature size. The present study reveals that tubules could appear earlier, beginning as intramural openings beside juvenile corallites when they reached approximately one-third of their mature corallite size (arrows in Figs. 8.9, 9.6). Young and Elias (Reference Young and Elias1995, p. 68, pl. 11, fig. 4) noted one example in Halysites of a tubule that was already present when an adjacent corallite was initiated. Juvenile corallites usually attained mature size over a short vertical distance, as demonstrated by Webby (Reference Webby1975) and the present study (Figs. 8.9–8.12, 9.5–9.8).

The small intramural openings that developed into tubules in H. catenularius (Fig. 7) are remarkably similar in appearance to intramural openings documented in the common wall between some corallites in four species of Catenipora (Bae et al., Reference Bae, Elias and Lee2013, fig. 1a–d). Some of the openings in Catenipora extended for a short or long vertical distance and then disappeared. Others developed into corallites with initial temporary connections to neighboring corallites, suggesting that they arose by a process of lateral increase (Bae et al., Reference Bae, Elias and Lee2013, fig. 1e, f). The intramural openings that disappeared were interpreted as aborted, incipient lateral offsets that originated from soft tissue on the common wall between corallites. The intramural openings in Catenipora were studied through transverse serial sections, so it is unknown whether they contain tabulae such as those in the tubules of Halysites (Fig. 5.8, 5.9).

Some authors have interpreted the tubules in Halysites as sites of polyps that differed from those occupying corallites, but most have considered them to represent intercorallite coenosteum (= coenenchyme; De Boer, Reference De Boer1973, pl. 3; Webby, Reference Webby1975; Hill, Reference Hill and Teichert1981; Scrutton, Reference Scrutton1998). Flower (Reference Flower1961, p. 42, 47, 48, pl. 8, fig. 2) suggested that in some “advanced” species of Catenipora, small depressions (“mesocorallite cavities” = calices) between corallites on the upper surface of the corallum were filled with solid balken structures during upward growth.

Interstitial increase

Interstitial increase in Halysites (= “coenenchymal increase” of Hill, Reference Hill and Teichert1981, p. F438) involved the formation of a new corallite through the expansion of a preexisting tubule (Webby and Semeniuk, Reference Webby and Semeniuk1969, fig. 8a; Webby, Reference Webby1975, fig. 5). In the coralla of H. catenularius, two types of interstitial increase are recognized depending on the location with respect to other corallites.

In Type A interstitial increase, the generation of a new corallite occurred in the junction area of three or four preexisting corallites. The new corallite developed by the expansion of a tubule that was usually irregular in shape, resulting in the extension of a rank and the associated lacunae (Fig. 8). The transformation of a tubule into a juvenile corallite is first recognizable at about half of its mature size. As the tubule developed into a corallite, a new tubule was initiated in its former place and on the other side of the new corallite (arrows in Fig. 8.9). Type A interstitial increase was the most frequent mode for the addition of corallites and occurred commonly in all the examined coralla of H. catenularius.

In Type B interstitial increase, the generation of a new corallite occurred between two preexisting corallites, which could be in the same rank (Fig. 5.9) or in two different ranks perpendicular to one another (Fig. 9). Similar to Type A interstitial increase, the new corallite developed by the expansion of a tubule (Figs. 5.9, 9). New tubules appeared on either side of the developing corallite (arrows in Fig. 9.6, 9.8). Although the frequency of Type B interstitial increase was much less than Type A interstitial increase, it also occurred in all the examined coralla of H. catenularius.

Lateral increase

Lateral increase in Halysites involved the formation of a new corallite through offsetting from a parent corallite (Webby and Semeniuk, Reference Webby and Semeniuk1969, fig. 8b; Webby, Reference Webby1975, fig. 4). In the coralla of H. catenularius, two types of lateral increase are recognized depending on the location with respect to a normal parent corallite. A third type, previously unreported in Halysites, was associated with regeneration of a damaged and terminated corallite or tubule. The processes of lateral increase in H. catenularius are similar to those in species of Catenipora (Dixon, Reference Dixon1976; Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013).

In Type A lateral increase, the new offset initiated from the lateral wall of a juvenile or mature corallite through a lateral process and developed perpendicularly to the existing rank, thus forming a new rank (black arrows in Figs. 10.2–10.6, 11.6, 11.7). This type of corallite increase was rare and not observed in all the examined coralla of H. catenularius. Dixon (Reference Dixon1976, fig. 2b) recognized this type of increase in Catenipora. According to Stasińska (Reference Stasińska1967, p. 14), offsets rarely appeared on the lateral corallite wall of halysitids, and only in Catenipora. Clearly, however, such offsetting occurred in H. catenularius. Unlike Type A lateral increase in H. catenularius, the type C lateral increase recognized in species of Catenipora (Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013) occurred at the junction of two corallites (see also Dixon, Reference Dixon1976, fig. 2d).

Figure 10.

Transverse serial peels showing Type A lateral increase and Type B lateral increase in Halysites catenularius. (1–3) Initiation of Type A lateral increase by the appearance of a small protuberance (black arrows in 2, 3) perpendicular to the existing rank; (4–6) continuation of Type A lateral increase (black arrows), and initiation of Type B lateral increase by the appearance of a protuberance (white arrows) near the distal end of a corallite; (7–9) development of the juvenile corallites, followed by the joining of ranks. GIT 806-10, figured peels spaced 0.05, 0.10, 0.25, 0.05, 0.15, 0.15, 0.10, 1.2 mm apart, respectively. Scale bar shown in 1.

Figure 11.

Transverse serial peels showing the appearance and development of an agglutinated patch of corallites in Halysites catenularius. (1–9) A temporary agglutinated patch of corallites formed by lateral increase along the outer wall of corallites (arrows in 2–4, 6, 7; note openings resulting in temporary fusion of irregularly shaped corallites in 5–9); (10–16) transformation into normal corallites and ranks, with further lateral increase (arrows in 12–14). GIT 806-13, figured peels spaced 0.20, 0.10, 0.15, 0.20, 1.00, 0.05, 0.30, 0.10, 0.35, 0.30, 0.80, 0.10, 0.05, 0.35, 0.75 mm apart, respectively. Scale bar shown in 1.

Type B lateral increase involved the development of an offset near the distal end of a single corallite (white arrows in Fig. 10.4–10.6; Fig. 12). Additional new corallites formed continuously one after another. Offsetting by this type of increase resulted in the rapid development of new ranks and lacunae during expansion of the corallum. Type B lateral increase was common in all the examined coralla of H. catenularius. This type of lateral increase in halysitids (termed “peripheral increase” by Buehler, Reference Buehler1955, and Hamada, Reference Hamada1959) is also well known from previous reports in Halysites (Stasińska, Reference Stasińska1967; Webby and Semeniuk, Reference Webby and Semeniuk1969; Webby, Reference Webby1975). It is comparable to the type D lateral increase documented in species of Catenipora (Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013).

Figure 12.

Transverse serial peels showing Type B lateral increase in Halysites catenularius. (1–4) Appearance of a lateral protuberance at the distal end of a corallite; (5-8) development of a juvenile corallite; (9) corallite attains mature size. GIT 806-10, figured peels spaced 0.05, 0.05, 0.05, 0.05, 0.05, 0.10, 0.25, 0.40 mm apart, respectively. Scale bar shown in 1.

Unlike the four types of increase described above, the generation of a corallite by Type C lateral increase was associated with the regeneration of a terminated corallite or tubule. Corallites or tubules could be damaged and terminated by sediment influx (Fig. 13.1–13.6). During regeneration, a replacement corallite developed by lateral offsetting from an adjacent parent corallite (Fig. 13.7–13.11). When the new corallite was established, tubules appeared on either side of it (Fig. 13.12). This process allowed quick and successful recovery of damaged surface areas in colonies that were subjected to physical disturbances resulting in influxes of detritus. This type of lateral increase was very rare; damaged corallites usually recovered by rejuvenation without the introduction of an additional corallite (Fig. 5.8). Type C lateral increase in H. catenularius is comparable to type E lateral increase identified in species of Catenipora (Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013).

Figure 13.

Transverse serial peels showing Type C lateral increase and reconnection of a damaged rank in Halysites catenularius. (1–6) Damage between normal corallites and separation by sediment; (7–11) generation of an additional corallite by lateral offsetting and reconnection of the rank; (12) the new corallite attains mature size, with tubules on either side. GIT 806-13, figured peels spaced 0.70, 0.15, 0.05, 0.05, 0.15, 0.1, 0.15, 0.1, 0.1, 0.15, 1.00 mm apart, respectively. Scale bar shown in 1.

Temporary agglutinated patch of corallites

Temporary agglutinated patches of corallites are common in the examined coralla of H. catenularius. They formed by lateral increase along the outer wall of corallites, in areas of abnormal or damaged corallites (Fig. 11.1–11.8; note arrows in 11.2–11.4, 11.6, 11.7). During development of the agglutinated patch, walls between some irregularly shaped corallites opened, resulting in temporary fusion of corallites (Fig. 11.5–11.9). With subsequent growth of the colony and further lateral increase, the agglutinated patch of corallites was able to transform into normal ranks of corallites alternating with tubules (Fig. 11.9–11.16; note arrows in 11.12–11.14).

Agglutinated patches of corallites have not been reported previously in Halysites. Temporary agglutinated patches of side-by-side corallites, comparable to those in H. catenularius, were rare in some species of Catenipora (Lee and Elias, Reference Lee and Elias1991; Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013). They formed by rejuvenation and/or regeneration during recovery from damaged and/or terminated corallites. In one species, C. foerstei, agglutinated patches could also form by increase involving normal, undamaged corallites (Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013). In some cases, this resulted in a cluster of many corallites (Bae et al., Reference Bae, Elias and Lee2013, fig. 2).

Axial increase

The axial mode of increase involved longitudinal fission of a corallite. In H. catenularius, this occurred in association with rejuvenation of a damaged corallite (Fig. 14). A new common wall appeared near the axial area of the damaged corallite (Fig. 14.3, 14.4). It subsequently extended from one lateral wall to the other, dividing the corallite in half (Fig. 14.5–14.8). The new wall was initially undulatory, and subsequently became straight. The resulting two juvenile corallites gradually enlarged into mature corallites of comparable size to the neighboring corallites (Fig. 14.9–14.12). Axial increase was extremely rare and not observed in all the examined coralla of H. catenularius.

Figure 14.

Transverse serial peels showing axial increase in Halysites catenularius. (1–4) Damage of a normal corallite, followed by the entry of sediment; (5–8) rejuvenation of the damaged corallite, accompanied by the development of a new common wall dividing the corallite in half; (9–12) the resulting two juvenile corallites develop to mature size. GIT 806-8, figured peels spaced 0.25, 0.25, 1.15, 0.10, 0.05, 0.30, 0.10, 0.15, 0.15, 0.25, 1.00 mm apart, respectively. Scale bar shown in 1.

The process of axial increase has not been described previously in Halysites, although Young and Elias (Reference Young and Elias1995, p. 71) noted that connections between some corallite tabularia suggest axial increase may have occurred in a specimen of H. alexandricus Young and Elias, Reference Young and Elias1995. Lee et al. (Reference Lee, Jun, Bae, Elias, Hubmann and Piller2007) recognized four types of axial increase in Late Ordovician tabulate corals. Their type 2 axial increase associated with rejuvenation of a damage single corallite corresponds to that in H. catenularius. This type of axial increase is also known in some species of Catenipora (Lee and Elias, Reference Lee and Elias1991; Bae et al., Reference Bae, Lee and Elias2006b, Reference Bae, Elias and Lee2013; Lee et al., Reference Lee, Jun, Bae, Elias, Hubmann and Piller2007). It was an effective method for recovery and increasing the number of corallites after damage.