Chain corals
Tabulate corals are colonial corals. Some tabulate corals grew as spherical mound shapes, some grew like small shrubs with many branches, while a particular group grew in clusters of splitting and recombining, twisted, chain shapes. The entire cluster of chain-like segments is called a corallum, while the individual tubular chambers within the corallum are called corallites. These types of tabulate corals can be found in Silurian strata in Kentucky.
Description and identification
All chain corals have thin (1–3 millimeter) corallites (tubular chambers) that are attached laterally in chain-like walls or ridges. Individual corallites in the chain are circular to oval (elliptical) shaped in cross section. In some chain corals, larger corallites (termed autocorallites or macrocorallites) connect with each other, and in others, larger corallites alternate with much thinner tubes and tissues (termed tubules, microcorrallites, and mesocorallites). These smaller tubes and tissues are one-fourth to one-tenth the width of the larger tubes, so are only 0.25–0.1 millimeters in width, which is tiny! The intervening smaller tubes and tissues are called coenenchyme tissues, because they link the major corallites together. The presence or absence of coenenchyme is the first criteria used to differentiate different types of chain corals (Buehler, 1955; Laub, 1979; Hill, 1981). The number of corallite tubes in a segment of chain is termed a rank. Some chain corals are characterized by smaller or longer ranks than others. The open spaces between splitting or merging ranks are called lacunae. The shape of lacunae are another distinguishing characteristic for some chain corals. The term “polygonal” is used for lacunae shaped like polygons, but also includes elliptical and looping shapes (see, for example, Buehler, 1955). Many circular lacuna have a star shape formed by the pinching and swelling of the oval-shaped corallites bounding the space. Other features that may help to distinguish different types of chain corals are the width of corallites, presence or absence of septae within corallites, and the shape and spacing of tabulae within corallites and smaller tubules.
The differences between chain-coral genera and species are subtle. Many features used to distinguish genera (and especially species) are frequently less than 1 millimeter in width, so require good specimens and a good eye or magnifying glass. Weathering of fossils, dolomitization, and silicification can smooth or obliterate smaller features, which complicates identification. In fact, Hill (1981), in the coral treatise, wrote that identification of genera for these types of corals is difficult because degrees of differences (rather than concrete, explicit differences) are inevitably used for subdivisions. Unless specifics are required, it is probably better to refer to weathered or mineralized specimens simply as chain corals or halysitid (Halysites-like) corals, rather than trying to identify the genera or species.
Table 1 shows the characteristics of known fossil chain corals in Kentucky. Many other genera and species of chain corals occur in other areas.
Table 1. Different genera and species of chain corals reported from Silurian strata in Kentucky (data from Buehler, 1955; Stumm, 1964; Laub, 1979; Hill, 1981; Young and Noble, 1987). *H. catenularia has been reported from the Brassfield Dolomite in Ohio, but not Kentucky. It may also occur in the Louisville Limestone (see text).
In Kentucky, chain-coral fossils are commonly silicified or dolomitized, which makes it difficult to see if a specimen has coenenchyme between major corallites, which are the first-level criteria for determining if a chain coral may be Halysites or Cystihalysites, or is Catenipora or Quepora. In practice, a secondary criteria, the shape of lacunae (the gaps between corallite chains) are used to determine genera in the field. Specimens with mostly polygonal lacunae and elongated loops (especially contorted loops) are usually identified as Halysites sp. (although they can also be Catenipora sp.). Specimens dominated by mostly circular, star-shaped, to elliptical (not contorted) lacunae are probably Quepora sp. Long chains that don’t often split or rejoin may be Cystihalysites sp.
See images of chain coral fossils
Chain corals in life
Silurian chain corals lived in warm, shallow, well-lit, tropical seas. They grew in low-relief, flat-topped, hedge-like mounds or as small hemispherical mounds on the seafloor. Catenipora microporous and Quepora huronensis commonly occur as small hemispherical mound shapes in the Louisville Limestone. Most of the chain-coral mounds found in Kentucky are less than a foot across (some only several inches in diameter), and only a few inches or less in height. In life, tiny coral polyps (less than 0.5–3.0 millimeters) lived in each of the corallite chambers. Chain corals are often found in clusters on the same bedding plane. They may occur with other types of corals, including favositid tabulate corals, rugose horn corals, and rugose colonial-mound corals.
Chain corals as index fossils for the Silurian
Chain corals are mostly restricted to the Silurian Period in Kentucky, so are good field indicators of Silurian strata. An exception is rare eroded fragments of chain corals found in the basal Devonian Jeffersonville Limestone, where they were reworked from erosion of the underlying Silurian Louisville Limestone. Quepora has also been found in Upper Ordovician strata (Hill, 1981) in some areas, but to date, none have been reported from the Upper Ordovician strata of Kentucky.
Comparison to other splitting and merging fossil shapes
Some tabulate corals, like Cladopora, exhibit splitting and merging (anastomosing) networks, which when composed of thin branches, can look somewhat similar to chain corals. In chain corals, the splitting and merging chain shape thickens and thins around regularly-spaced oval corallites along chain segments (ranks). In corals like Cladopora, thin branches are smooth and do not pinch and swell. In three-dimensional specimens, the splitting and rejoining pattern in chain corals is on top of erect tubes forming looping and winding ridges. In corals like Cladopora, there are no connected ridges or thin winding walls because the vine-like pattern is formed by actual branches. Additionally, the pattern of gaps (lacunae) between tubes is different in chain corals, and corals like Cladopora.
Chain corals named for Louisville
Two different Kentucky chain-coral genera have had the species name “louisvillensis.” The species were both named for the Louisville Limestone (in which they were found), which was named for the city of Louisville, Ky. Usually, if you see the same species name in similar-appearing genera, it means that the species was reassigned to another genera, and one of the genus names is no longer valid for that species. In this case, however, Stumm (1964) described both a Halysites louisvillensis and Quepora louisvillensis as distinctly different and valid corals. Laub (1979) studied Kentucky and Ohio halysitids again and thought H. louisvillensis was the same (conspecific) as Halysites nitidus, a species of Halysites previously found in the Brassfield Dolomite. Still later, Young and Noble (1987) considered both H. louisvillensis and H. nitidus junior synonyms of Halysites occidens, a previously described species with similar-shaped lacunae and similar-size corallites. Hence, only one chain coral, Quepora louisvillensis, retains the Louisville moniker today.
What happened to Halysites catenularia?
The type species for the genus Halysites is H. catenularia (also spelled catenulatus). It has been reported in Silurian strata around the world, and was historically the only chain coral reported from Kentucky (Davis, 1887; Butts, 1915; Foerste, 1931; McFarlan, 1943). Many specimens previously assigned to H. catenularia in Kentucky were subsequently reassigned to Quepora huronensis and Quepora louisvillensis, however, because they lacked coenenchyme (intervening tubules) like other species of Halysites (Stumm, 1964). For example, the specimen of Halysites catenularia pictured in Foerste’s (1931) “Silurian of Kentucky” would likely be Q. huronensis today. Stumm (1964) did not include H. catenularia in his review of valid coral species from the Louisville area, although several subsequent publications did include H. catenularia in the upper Louisville Limestone (e.g., Conkin and Conkin, 1992). Some specimens identified as H. catenularia from Kentucky might also be the similar-appearing H. occidens (see previous paragraph). Halysites catenularia may occur in the Louisville Limestone in addition to Quepora and other chain corals, but if it occurs, it is not as common as was previously reported. H. catenularia is a valid chain-coral species, and a few specimens have been reported from the Lower Silurian Brassfield Formation in Ohio (Laub, 1979), but any halysitid coral from the Louisville Limestone or Brassfield Dolomite in Kentucky should be examined carefully to determine which chain coral it might be, and the assumption should not be made that it is H. catenularia.
