Fossil of the month: Constellaria

In the Ordovician rocks of central Kentucky, the most common fossils are branches of a type of organism called a bryozoan. Many bryozoans have similar outward appearance, which can make them difficult to identify to genus or species. This month’s fossil, however, is the easy-to-identify bryozoan, Constellaria. Constellaria is covered by star-shaped patterns, not found in other bryozoa. The “stella” in its name is Latin for “star.”

Description. Bryozoans are colonial organisms, and build colonial structures called zoariums. Constellaria zoariums are usually branching (called ramose), or frondose (leaf-like or cabbage like shapes) structures, and less commonly may be encrusting on other objects (Boardman and others, 1983).  The structures are rarely found whole, but broken branches and fronds (typically no larger than a few inches in width and length) are abundant in several rock units in central Kentucky. Branches and fronds are ornamented with raised star-shaped (stellate) patterns. The star-shaped patterns are a type of feature called maculae. They may consist of separated or connected rays. No other bryozoan has maculae in star-shaped patterns, which make this bryozoan easy to identify. The star shapes usually occur on small bumps raised above the surface of Constellaria. The bumps are called monticules.  Between the star shapes, the surface of Constellaria is covered by tiny (pin-sized) pores called zooecia. In life, each zooecia housed a tiny bryozoan organism (individual member of the colony), called a zooid. Colonies may consist of thousands or more zooecia and zooids. Aside from the star shapes, Constellaria is also distinguished from other bryozoa based on internal structures, such as the microstructure of its zooecia walls (Boardman and others, 1983).

Constellaria parts and simplified detail of the living bryozoan.

Species

At least six species (and one subspecies) of Constellaria are known from Kentucky. Twenty-eight species are recognized worldwide (Bryozoan.net). Species of Constellaria are based on subtle differences in the shape of its branches or fronds; the relative size of star-shaped maculae; and the heights, spacing, and distribution of its monticules; as well as details of internal structures. Constellaria florida is the most common species reported in Kentucky. Its branches widen upward into frond-like (frondose) structures with distinct rays on closely spaced, raised monticules (McFarlan, 1931; Karklins, 1984).

 

Differences in the shape, size, and distribution of branches, fronds, star-shapes, and monticules used to described different species of Constellaria found in Kentucky. Based on descriptions in Nickles (1905), McFarlan (1931), and Karklins (1984).

Three different species of Constellaria found in central Kentucky. C. teres are from the University of Kentucky Department of Earth and Environmental Sciences (UKEES) paleontological collection, no. 1379 (all 4 samples). C. fischeri are UK EES specimens 7300 and 8264. C. florida samples are UK EES 6293, KGS unnumbered, and UK EES 5031. All samples are from central Kentucky.

 

Range

Constellaria ranges worldwide from the Upper Ordovician to Lower Silurian (Boardman and others, 1983). Silurian examples are only known from China and Russia. In Kentucky (and surrounding states) Constellaria is reported from Upper Ordovician units, from the Grier Member of the Lexington Limestone to the Bull Fork Formation. The Lexington Limestone occurs in the central Bluegrass region of central Kentucky. Overlying units occur in the outer Bluegrass region of central and north-central Kentucky. The most common species, Constellaria florida, is abundant in the Calloway Creek Limestone and Fairview Formations. The Upper Ordovician units in which Constellaria is found are 445-555 million years old.

 

Range of Constellaria species found in central Kentucky. Data from Nickles (1905), McFarlan (1931), Cressman (1973), Weir and others (1984), Karklins (1984), and Wright and Stigall (2013). Gray boxes are units in which the species has been found. Black boxes mark units in which the species is most common or abundant. Dashes mark units where the species is uncertain but may be found. Oldest strata is to the left, and youngest strata is to the right

 

Paleoecology

During the Late Ordovician Period, Kentucky was covered by shallow, tropical seas. Constellaria is a marine bryozoan fossil and is found with other marine fossils including brachiopods, corals, cephalopods, gastropods, and other bryozoa. It commonly occurs in what has been called the Constellaria-Cyclonema biofacies, which includes other branching bryozoans, Cyclonema gastropods, and atrypid brachiopods (Holland and Patskowsky, 2004).

The living Constellaria was a filter-feeding organism. It formed branching colonies (centimeters to tens of centimeters tall) with outwardly flattening branches or fronds. Branching thickets of Constellaria (and other branching bryozoa) covered parts of the sea floor at different times in the Late Ordovician of central Kentucky.

Constellaria thickets are interpreted to have lived in shallow, subtidal water depths based on the fossils it is found with and the types of limestone in which it occurs (Holland and others, 1996; Holland and Patzkowsky, 2004). In the Late Ordovician of central Kentucky, this range would perhaps have been 15 to 60 ft deep (Brett and others, 2015).

Each frond of a Constellaria colony was covered with hundreds of zooids. Zooids in different parts of the colony probably served different functions such as feeding, waste removal, etc. The hair-like cilia on the tentacles of the tiny feeding zooids generated currents which brought seawater and microscopic food particles into the mouths of the soft-bodied bryozoa zooids. Waste material and water were than expelled from the monticules, sometimes referred to as “chimneys” because they vented material from inside the colony back into the sea (Antsey, 1976; 1981; Taylor, 1979; Key and others, 2002; Jackson and others, 2014).

 

References

Antsey, R.L., 1981, Astogeny and phylogeny: Evolutionary heterochrony in Paleozoic bryozoans: Paleobiology, v. 13, no. 1, p. 20–43.

Boardman, R.S., Cheetham, A.H., Blake, D.B., Utgaard, J., Karklins, O.L., Cook, P.L., Sandberg, P.A., Lutaud, G., and Wood, T.S., 1983, Part G–Bryozoa revised, in Robison, R.A., ed., Treatise of Invertebrate Paleontology: Geological Society of America and University of Kansas Press, p. 379-380.

Brett, C.E., Malgieri, T.J., Thomka, J.R., Aucoin, C.D., Dattilo, B.F. and Schwalbach, C.E., 2015, Calibrating water depths of Ordovician communities: lithological and ecological controls on depositional gradients in Upper Ordovician strata of southern Ohio and north-central Kentucky, USA: Estonian Journal of Earth Sciences, v. 64, no 1, p. 19–23.

Bryozoan.net, 2022, The Bryozoa home page, https://www.bryozoa.net/cystoporida/constellariidae/constellaria.html, accessed October, 2022.

Cressman, E.R., 1973, Lithostratigraphy and depositional environments of the Lexington Limestone (Ordovician) of central Kentucky: U.S. Geological Survey Professional Paper 768, 71 p.

Davis, R.A., 2007 (original website 1999), phylum Ectoprocta (a.k.a., Bryozoa, Polyzoa): College of Mount St. Joseph, Cincinnati, Ohio, website  http://faculty.msj.edu/davisr/potpouri/bryozoa.htm, accessed October, 2022.

Holland, S.M., Patzkowsky, M.E., Witzke, B.J., Ludvigson, G.A., and Day, J., 1996, Sequence stratigraphy and long-term paleoceanographic change in the Middle and Upper Ordovician of the eastern United States, in Witzke, B.J., Ludvigson, G.A., and Day, J., eds., Paleozoic sequence stratigraphy: Geological Society of America Special Papers 306, p. 117–130.

Holland, S.M., and Patzkowsky, M.E., 2004, Ecosystem structure and stability: Middle Upper Ordovician of central Kentucky, USA: Palaios, v. 19, no. 4, p. 316–331.

Holland, S.M., and Patzkowsky, M.E., 2007, Gradient ecology of a biotic invasion: biofacies of the type Cincinnatian Series (Upper Ordovician), Cincinnati, Ohio region, USA. Palaios, v. 22, p. 392–407.

Jackson, P.N.W., Key, M.M. and Coakley, S.P., 2014, Epizoozoan trepostome bryozoans on nautiloids from the Upper Ordovician (Katian) of the Cincinnati Arch region, USA: an assessment of growth, form, and water flow dynamics: Journal of Paleontology, v. 88, no. 3, p. 475–487.

Karklins, O.L., 1984, Trepostome and cystoporate bryozoans from the Lexington Limestone and the Clays Ferry Formation (Middle and Upper Ordovician) of Kentucky: U.S. Geological Survey, Professional Paper 1066-I, 105 p., plus plates

Key, M.M., Jr., Thrane, L., and Collins, J.A., 2002, Functional morphology of maculae in a giant ramose bryozoan from the Permian of Greenland, in Jackson, W., Jones, B., and Jones, S., eds., Bryozoan studies 2001: Lisse, Swets and Zeitlinger Publishing, p. 163–170.

McFarlan, A.C., 1931, The Ordovician fauna of Kentucky, in Jillson, W.R., ed., Paleontology of Kentucky: Kentucky Geological Survey, ser. 6, v. 36, p. 49–165.

Nickles, J.M., 1905, The Upper Ordovician rocks of Kentucky and their Bryozoa: Kentucky Geological Survey Bulletin, v. 5, 64 p.

Weir, G.W., Peterson, W.L., Swadley, W C, and Pojeta, J., 1984, Lithostratigraphy of Upper Ordovician strata exposed in Kentucky: U.S. Geological Survey Professional Paper 1151-E, 121 p.

Wright, D.F. and Stigall, A.L., 2014, Species-level phylogenetic revision of the Ordovician orthide brachiopod Glyptorthis from North America: Journal of Systematic Palaeontology, v. 12, no. 8, p. 893–906.

 

Text, photographs, and illustrations by Stephen Greb