What Is a Tabulate Coral? Honeycomb and Chain Patterns in Old Limestone

The skeleton consists of many narrow tubes packed together, each having housed one small polyp, with the whole mass forming a sheet, a dome or a branching structure. The name comes from the horizontal floors that cross each tube at intervals, since the polyp moved upward as it grew and sealed off the space below, and those floors are the most consistent feature of the group. Vertical partitions of the kind that dominate other corals are reduced or absent, which is the main distinction. Colony form varies enormously and produces the distinctive patterns, with tightly packed hexagonal tubes making a honeycomb, tubes joined in rows making a chain pattern, and fine tubes in a branching mass making something resembling organ pipes. Small pores or connecting tubes link neighbouring individuals, indicating that the polyps were connected rather than merely adjacent.

Several forms are common enough to have their own popular names:

• •Honeycomb coral, with tightly packed polygonal tubes sharing thin walls, which is the most familiar form
• •Chain coral, where tubes are joined in curved rows that look like links in cross section
• •Organ pipe forms, with fine parallel tubes joined by horizontal platforms at intervals
• •Branching colonies that grew as antler-like masses
• •Encrusting sheets that spread across whatever hard surface was available
• •Large domed colonies that formed a substantial part of reef frameworks

Together with rugose corals, sponges and calcifying algae these animals built extensive reef systems across the Palaeozoic, and their remains dominate some limestone units. Their value to geologists goes beyond that, since colony growth records environmental conditions in a directly readable form. Growth bands record time, and counting fine daily increments against annual bands supplied the evidence that the year contained more days in the distant past, which confirmed that tidal friction has slowed the Earth's rotation. Colony shape responds to water energy and light, so the form of a fossil colony indicates the depth and exposure of the place it grew, and assemblages are used to reconstruct ancient environments in detail. Their reefs also form reservoir rock holding oil and gas, which is why the group has been studied by industry as well as by academics.

The pores and connecting tubes linking neighbouring individuals indicate that a colony functioned as something more than a collection, since material could pass between polyps and resources were presumably shared. That is the same arrangement living colonial corals use, where connecting tissue distributes food from well-fed parts of a colony to shaded or damaged parts, which allows a colony to survive local injury and to grow towards better conditions. Whether these extinct animals hosted symbiotic algae in the way most modern reef-building corals do is an important and unresolved question, since that partnership is what allows modern corals to build reefs quickly in nutrient-poor water. The evidence is indirect, resting on growth rates, on the depths the fossils are found at and on isotopic work, and it points towards at least some of them having had a comparable arrangement.

The group declined through the later Palaeozoic and disappeared at the end of the Permian, along with the rugose corals and the great majority of the reef community. Nothing survived, and the modern reef-building corals that appear in the fossil record afterwards belong to a different group with a different skeletal arrangement, probably arising separately from soft-bodied ancestors. The gap between the extinction and the reappearance of substantial reefs is measured in millions of years, which is among the clearest demonstrations that a complex ecosystem does not simply resume once conditions allow. That interval, and the comparable gaps after other major extinctions, are studied closely now because the question of how long a damaged reef system takes to rebuild has acquired an immediate relevance that it did not have when the work began.

Packed narrow tubes crossed by horizontal floors, with the vertical partitions of other corals reduced or absent, forming honeycomb, chain and organ pipe patterns. Small connecting pores show the polyps were linked. Daily growth increments counted against annual bands gave the evidence that the year once held more days. The group vanished at the end of the Permian and modern reef corals are not its descendants.