Other important feature of horn corals are seen on the calice - the top surface of the calyx. Here, for example, we can make out the septae - the radiating ridges, like spokes on a bicycle wheel, and the columella which forms its hub. Both of these may vary. The septae, for example, may vary in both number and in their arrangement; in some species (as in the example above) there are longer, major septae, alternating with shorter, minor septae. The columella may vary in thickness and in the extent to which it protrudes from the calice. Some species also display fossula, which are gaps between the septae, separating the four quadrants. On some corals, also, the septa protrude through the wall of the coral and form vertical lines on the epitheca, called costae. Again, the arrangement of these may help in identification.
If the interior of the coral is exposed, other distinguishing features may also be visible. Tabulae are thin, horizontal plates that subdivide the calyx and originally created the cells within which the organism lives (image, right). Some corals also show dissepiments. These are divisions seemingly caused by constriction and rejuvenation of growth (perhaps due to environmental conditions). They show where renewed growth has occurred inside the cup, following a period of die-back (see image below, right). |
Features of horn corals
Top - a vertical section showing the table and (unlabelled) the vertical lines of the septae. Bottom: a view of the malice, showing the emergence of the columella and septae, and the dissepiments. |
Syringopora geniculata Carboniferous coral from Ludlow, UK. Left = vertical section Right = top surface Pleurodictyum sp. Devonian coral from Ontario, Canada Left = cross-section Right = surface Emmonsia emmonsii, Devonian from New York, USA Left - vertical section Right = top surface Halysites catenularia Silurian from Kentucky USA Left = vertical section Right = top surface Atopora sp. encrusting Favosites sp. Devonian from New York, USA Left: full specimen Right = detail |