Carbonates in thin section: Bryozoans

Devonian crinoid stems and disarticulated columnals in the bioclastic mix with fragmented fenestrate (f) and encrusting bryozoans (e).

Bryozoa – are a complex but important group of reef and non-reef invertebrates and contributors to bioclastic sediment.

Bryozoa are an incredibly diverse group of metazoans with a geological record extending from Early Ordovician to Recent. There are more than 6000 extant species and probably 2-3 times that number of fossil species. They were once classified as plants, or moss animals, because in many cases that’s exactly what they look like. But they are metazoans, tiny as individuals (zooids, commonly less than 1 mm diameter), but as colonial organisms can construct sizeable, rigid mounds and thickets. The majority are marine; a small group, the order Ctenostomata are non-marine. Bryozoa are important contributors to bioclastic sediment in shallow, cool-temperate water depositional systems. The larger buildups also provide an ecological niche for other organisms.

The zooids are housed within structures built of calcium carbonate, chitin, or a mix of both; the solid skeletal structure of a colony is called a zoarium (pl. zoaria). The exoskeleton constructed by an individual zooid is called a zoecium (pl. zoecia). Most species secrete either calcite or aragonite exoskeletons. Only the carbonate forms have good preservation potential (so it’s worth keeping in mind the likelihood that in fossil bryozoan assemblages there will probably be “missing” species). Many species contain a mix of calcite and aragonite. For modern bryozoans the proportion that secretes aragonite is greatest in tropical realms, generally decreasing in cooler waters at higher latitudes. The Mg content of calcites is also highly variable, ranging from 0 to 16 (mol)% (Smith, 2014 – for a review of some modern bryozoans).

Common forms range from low relief encrusters on bedrock, shells, and seaweed, to rigid frameworks and thickets that are delicately or robustly branched (some coral-like), palmate (shaped like a palm tree), or fenestrate (window-like) and lacy, anastomosing sheets, massive domes, and even corkscrew structures commonly labelled as Archimedes screws. Colonies a few centimetres long may house a million and more zooids.

A few of the common bryozoa forms. Each form has a 3-dimensional aspect that can potentially create large (decimetre-scale), complex structures. Individual skeletal cavities – zoecium – are indicated. In fenestrate forms, the (relatively) large openings in the zoarium do not accommodate the living animal; however, they can be filled with sediment and cements as is the case for the zoecia (see the images below).

Secretion of calcium carbonate confers a degree of rigidity to bryozoa, but in the grand scheme of sediment production and dispersal, they are relatively delicate structures. This is particularly the case for delicately branched and sheet or plate-like zoaria. As fragments in grainstones and rudstones, their identification even to the level of bryozoan order can be difficult. In thin section, fragment identifications generally rely on a relatively ordered arrangement of zoecium that are regularly spaced and of similar size and shape. Note that the size and shape of the zoecium may change as the orientation of the fragment changes.

Modern bryozoa encrusting a gastropod. The zooids are arranged in a brick-like pattern. Hahei, NZ.

Modern rigid, calcareous branched bryozoa. Zoom in to see the zooids along each branch. Specimen is from Florida Keys.

Differentiation from some other phyla

Corals:

The corallites of solitary or colonial corals are significantly larger than bryozoan zoecium
Corals are composed of aragonite, that in most cases will recrystallize to low magnesium calcite during burial diagenesis.
Corallites have well defined internal structure, founded on septa (vertical plates of aragonite) that radiate from a central column – zoecia have no such internal structure.

Barnacle plates:

Basal sections of barnacle plates commonly show an array of large pores that resemble bryozoan zoaria. However, the barnacle fragment may also show its characteristic plicate layered structure. Bryozoa exoskeletons have no well-defined layering.

Echinoderms plates:

There is a superficial resemblance between fenestrate bryozoa and perforated echinoderm plates, where pores in the latter are commonly arranged linearly or in quasi-radial patterns. Echinoderm plates also consist of a single calcite crystal – this will show as uniform extinction across the entire plate. Bryozoa plates consist of a myriad, microscopic acicular crystals that during diagenesis will recrystallize to calcite spar.