OUT OF THE MUD: FUNCTIONAL MORPHOLOGY CLEARS LATE ORDOVICIAN (CINCINNATIAN) TREPOSTOME BRYOZOA OF SUPPORT BY MUDDY SUBSTRATA

A common impression among paleontologists is that most lower Paleozoic trepostome bryozoan species with erect, stony skeletons were growing in mud.  It is believed that colonies grew surrounded by mud with living zooids covering only emergent surfaces. Individual colony morphologies and taphonomies demonstrate that this mode of life was not the norm for shallow- water, Cincinnatian species. Erect frondose, branching or massive colonies, when reconstructed and examined carefully, demonstrate astogenies, morphologies and growth vectors that precluded life in the mud even if the skeletons were eventually preserved in that sediment.  

Evidence supporting our conclusions is derived from study of four nearly complete colonies and several large colony fragments from species of Monticulipora, Heterotrypa, Parvohallopora, and ?Amplexopora. It can be organized as follows:

Astogenic—

•  ancestrulae of shallow-water forms generally attached to hardground or firmground surfaces on coarse bioclastic substrates, not on mud;
• colonies frequently employed radial and then upward vectors of growth rather than dominantly upward vectors;
• radial portions of some colonies arch back to the substrate to stabilize the colony.

Morphologic—

• contact marks on outer walls mark growth against adjacent colonies;
• some colonies develop buttressed margins to deflect currents;
• most trepostomes produced open architectures favoring current movement through the structure;
• skeletal archways lowered colony resistance to current by encouraging flow-through;
• domal shapes achieved by shallow water species seem parsimonious;

Skeletozoans—

• skeletons often show self-overgrowth or occupation by a wide array of episkeletozoans such as cornulitids, Cuffeyella arachnoidea, Zygospira modesta, or Platystrophia spp.;
• endoskeletozoan Sanctum laurentiensis bored into colonies in life position whereas Trypanites weisi often bored colonies after death.
• No “bathtub rings” of epizoans are found around reconstructed colonies as could be expected to form at the “mud line” or the living tissue margin on exposed skeletons.

 

Upper Cincinnatian bryozoan colonies could withstand mud influxes and continue growth, but functional morphology of most shallow water taxa indicates they were built for open current flow through, not as mud dwellers.