Intracolony variation in colony morphology in reassembled fossil ramose stenolaemate bryozoans from the Upper Ordovician (Katian) of the Cincinnati Arch region, USA

Abstract

Clusters of associated colony fragments discovered weathering out of bedding planes in the Upper Ordovician of the Cincinnati, Ohio, region provide a rare opportunity to quantify intracolony variation in ramose stenolaemate bryozoans. Sixteen colonies were reassembled as completely as possible from 198 fragments, and the following colony-level characters were measured: colony dimensions, branch link length and diameter, and branch order. Results indicate that branch link length and diameter systematically decrease as colonies grow via branch bifurcation. Branching ratio (i.e., the number of distal first-order branches divided by the number of immediately proximal second-order branches) appears to be more genetically than environmentally controlled and to be consistent among orders of stenolaemates and perhaps across the phylum. Colonies with endozones mined out by endoskeletozoans result in broken branches as opposed to pristine growing tips. This varies stratigraphically, perhaps in response to the distribution of the boring animals. The rarity of borers and the systematic proximal increase in branch diameter in these colonies suggest the zooids in the proximal portions of the colonies were alive at the time of colony death. If the time and effort can be invested in reassembling colonies, these morphometric data can then be applied to taxonomic, phylogenetic, and paleoenvironmental studies.

Stable isotope profiles of skeletal carbonate validate annually-produced growth checks in the bryozoan Melicerita chathamensis from Snares Platform, New Zealand

“…This approach can also be applied to paleoclimate studies using fossils, but with the following limitations. The bryozoan fossilization process often involves fragmentation (Key et al 2016), so finding complete colonies may be a challenge. If the zooecial cavities are filled with carbonate cements and/or matrix of similar density to the zooecial walls, X-ray images may not show the brightness contrast necessary to see the more subtle growth checks (Wyse Jackson and Buttler 2015).…”

Trepostome bryozoans buck the trend and ignore calcite-aragonite seas

“…All species of trepostomes have long, tubular zooids. Although encrusting, massive, frondose, and bifoliate colonies occur, most are ramose with bifurcating cylindrical branches that form small bush-like colonies (Key et al 2016). These branches have an inner thinner-walled endozone surrounded by a peripheral thicker-walled exozone (Fig.…”

A redescription ofStenoporaand the type speciesStenopora tasmaniensisLonsdale, 1844 (Trepostomata, Bryozoa)

“…Collection of more-complete specimens has revealed single colonies with more than one morphology, where frondescent forms give rise to branches, and individual species of Stenopora are now known to have multiple colony forms-and may include encrusting (sedimentary and hard substrates), delicate to coarse ramose, frondescent, or massive spheroidal colonies (Wass, 1968;Reid, 2003Reid, , 2012. This is also known in other trepostomous bryozoans (Erikson and Waugh, 2002;Waugh et al, 2005) in addition to variation in branching characters within Ordovician ramose trepostomes (Key et al, 2016).…”