What Is a Rugose Coral? Horn-Shaped Fossils From a Vanished Group

Each fossil is the calcium carbonate skeleton secreted by a soft polyp, comparable in general anatomy to a living sea anemone, sitting in a cup at the open end. Solitary forms grew as a horn-shaped cone, attached at the point and widening upward, which is the shape that gives the group its common name, while colonial forms grew as masses of tubes packed together. The interior contains vertical partitions radiating from the centre, and their arrangement is the diagnostic feature, since they were inserted in a specific pattern that differs from the pattern in modern corals. Horizontal floors divide the cup from the abandoned space below, since the polyp moved upward as it grew and sealed off what it had left. The wall carries fine growth lines, and those turn out to be unexpectedly informative.

The banding on the outside of these fossils has been read in several ways:

• •Daily increments, deposited as the polyp grew, which are countable under magnification
• •Monthly and annual bands superimposed on the daily ones, produced by tidal and seasonal cycles
• •A count of days per year, obtained by dividing annual bands into daily ones
• •Results indicating around 400 days in a Devonian year, which is more than today
• •Confirmation of the independent physical prediction that tidal friction has been slowing the Earth's rotation, lengthening the day
• •A rare case of a fossil testing a claim from physics rather than from biology

These corals, together with another extinct group that built honeycomb-like colonies and with calcifying sponges and algae, constructed reef systems across the Palaeozoic that were extensive and in places larger than modern reefs. Those structures did the same ecological work as reefs now, supplying habitat, shelter and feeding grounds for a large associated community, and the fossil reefs preserve that community in place. They are studied intensively because they record how reef ecosystems assemble and how they respond to disturbance, and the record includes several episodes where reef building collapsed worldwide and took millions of years to resume. Those gaps, sometimes called reef eclipses, follow the major extinction events and are among the clearest evidence that complex ecosystems do not simply reassemble once conditions improve. The buried reef structures are also economically significant, since many form reservoirs holding oil and gas.

These are among the fossils an amateur collector is most likely to encounter, since Palaeozoic limestone is widespread and the shapes are conspicuous. Solitary forms weather out as horn-shaped objects a few centimetres long, and the radiating partitions are visible in the open end or in a broken cross section. Colonial forms appear as masses of polygonal or circular tubes in cut and polished rock, and they are frequently visible in building stone, where a polished limestone floor or facade will show cross sections through entire colonies. Distinguishing the group from the other extinct reef builders is done on the tube shape and the partition pattern, since the honeycomb-like group has thin shared walls and small pores between neighbouring tubes. Identification to species requires thin sections cut at specified orientations, which is a specialist business, and most collected material is identified only to the level of the group.

The group disappeared at the end of the Permian in the most severe extinction known, alongside the other major Palaeozoic reef builders, and nothing in the group survived. Modern stony corals appear in the fossil record some millions of years later, and the relationship between the two is a long-standing question, since the partition-insertion patterns differ in a way that suggests they are not simply descendants and the mineral form of their skeletons differs too. The favoured explanation is that modern corals arose separately from soft-bodied anemone-like ancestors that survived the extinction without skeletons and acquired the ability to build them afterwards, which is supported by the observation that some living corals can survive as soft-bodied forms when conditions prevent calcification. If that is right, the capacity to build a reef was invented independently more than once, which is a reassuring thought and a very slow process.

Horn-shaped skeletons with radiating partitions inserted in a pattern unlike modern corals, sealed off below by floors as the polyp moved upward. Daily growth lines counted against annual bands give around 400 days in a Devonian year, confirming that tidal friction has slowed the Earth. The group died out at the end of the Permian and modern corals appear later, probably from soft-bodied survivors rather than as descendants.