Growth characteristics of Protoheliolites norvegicus (Tabulata; Upper Ordovician; Estonia)

Abstract

Protoheliolites is an early heliolitine coral characterized by closely spaced corallites separated in places by sparse coenenchyme. Growth characteristics in the type species, P. norvegicus, are revealed by detailed analysis based on serial peels and thin sections of coralla from the uppermost Katian of north-western Estonia. Colonies of this species had a strong ability to recover from damage and partial mortality, resulting in various forms of rejuvenation, regeneration, fusion and reorganization of corallites; in some cases, this involved relatively large areas of undifferentiated soft parts. The shells of commensal cornulitids became enclosed in host coralla during colony growth. Coralla of P. norvegicus exhibit distinctive growth cycles due to responses to seasonal changes. The production of new corallites by coenenchymal increase usually occurred in low-density bands, in which corallites generally display round to subrounded transverse outlines. In high-density bands, the corallites became crenulated, their wall thickness increased, septal development was more pronounced, and the amount of coenenchyme increased. In addition to these cyclomorphic changes, there were significant astogenetic changes during growth. Compared with the early stage of colony development, distinctive characteristics in the late astogenetic stage include a decrease in the growth rate of the colony, better coordination among corallites, maximum development of corallite crenulations and septa in high-density bands, more numerous coenenchymal tubules and a greater proportion of corallum area occupied by coenenchyme. In general, the role of polyps in determining morphological characteristics of individual corallites, such as tabularium area, corallite crenulations and wall thickness, was subordinate to the astogeny of the colony. Growth characteristics including colony-wide coordination of polyp behaviour and subjugation of individuals to restore the colony following damage suggest a strong astogenetic control and high level of colony integration. Protoheliolites probably arose from a heliolitine genus rather than from a nonheliolitine group as some authors have proposed.

Morphometrics, growth characteristics, and phylogenetic implications ofHalysites catenularius(Tabulata, Silurian, Estonia)

“…Scrutton (1998, 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, 1991; Young and Kershaw, 2005, table 1; Liang et al, 2013). Favositids had annual growth rates ranging from 5 to 18 mm (Scrutton and Powell, 1980; Scrutton, 1998).…”

“…The mode of corallite increase is an important character in considering taxonomic assignments and phylogenetic relationships of tabulate corals (Lee and Elias, 2000; Lee et al, 2007; Liang et al, 2013). The present study shows that small intramural openings in some species of Catenipora are remarkably similar in appearance to those in Halysites catenularius .…”

“…Coenenchymal structures in halysitids and heliolitids are almost certainly not homologous (Young and Elias, 1995, p. 66, 67). Lateral increase is typical in Catenipora (e.g., Bae et al, 2006b, 2013), whereas coenenchymal increase is characteristic of heliolitids (Oliver, 1968; for detailed examples, see Young and Elias, 1995, p. 48, 49, and Liang et al, 2013). A close phylogenetic relationship between Catenipora and coccoseridids, or between halysitids and heliolitids in general, seems improbable.…”