Stromatoporoid reefs Like modern corals, stromatoporoids were gregarious
filter feeders which congregated into closely packed patches. They were adaptable and could thrive at a variety of depths, light levels, and fluctuating sea level regimes. In these regards, they were more similar to corals than to modern calcareous sponges, which generally occupy a narrow selection of rocky habitats with high nutrient supply and low light levels. Unlike corals, stromatoporoids usually settled on soft substrates, so their 'reefs' occupied only a single level rather than a multi-tiered vertical framework of built-up skeletons. Flat, horizontally-extensive 'reefs' are formally known as
biostromes. Stromatoporoid reefs had fairly low diversity, with only a few species making up the majority of an assemblage by volume. The most diverse stromatoporoid assemblages were biostromes on
carbonate platforms at intermediate depths, away from muddier basins or saltier shallows. The predominant species were usually laminar or low domical in form. High domical species and other complex forms only developed in calmer settings, where there is little risk of toppling. In a stable environment, stromatoporoids could grow to very large sizes exceeding several meters in width or height. The largest singular stromatoporoid fossil ever reported is a 30-meter (98 feet) wide
Actinostroma expansum from the Frasnian-age
Shell Rock Formation of
Iowa. Despite their preference for soft flat sediment, stromatoporoids occasionally contributed to built-up skeletal mounds (
bioherms) with successive waves of burial and recolonization or regrowth.
Shelf-margin stromatoporoid bioherms are particularly well-developed in the Devonian. Notable examples can be found in the
Canning Basin of
Australia, the
Miette Complex of
Alberta, the
Eifel Region of
Germany, and southern
Belgium. Raised bioherms would have been strengthened by microbial carbonate and other reef-building organisms living between the sponges.
Ectosymbionts and encrusters As hard sessile objects, stromatoporoids were used as a substrate for
ectosymbionts, organisms which attach or encrust onto the outer surface of the skeleton. Most encrusting organisms were
cryptobionts, meaning that they inhabited shaded spaces and cavities. These hidden areas could be found in gaps between the base of the stromatoporoid and its substrate, or on the underside of shelf-like projections. Displaced or toppled sponges had the potential to host cryptic encrusters on any part of the skeleton.
Bryozoans,
tabulate corals,
crinoids,
brachiopods, and clusters of coiled '
spirorbids' all occupied the cryptic niche. The rim of the hidden areas hosted the greatest diversity of encrusters, most of which were filter feeders reliant on a current to feed. More exposed areas were also encrusted by corals (both tabulate and
rugose), crinoids, bryozoans, and
tentaculitids.
Endosymbionts , a domical stromatoporoid in the order Actinostromatida. An endosymbiotic boring (Osprioneides'') is visible at the top left. From the
Silurian of
Saaremaa Island, Estonia. Like many modern or prehistoric reef-builders, stromatoporoids were host to
endosymbionts, organisms living fully within the skeleton. The most abundant were
syringoporids, a type of tube-building tabulate coral. Stromatoporoid fossils with syringoporid burrows are so common that some historical sources have misclassified them as a distinct genus,
Caunopora. Syringoporids were able to grow at the same rate as their host in order to prevent being overgrown. Other tabulate corals, rugosan corals, and
algal fossils have been found wedged between growth zones within the stromatoporoid skeleton. Worm
borings such as
Trypanites are also common endosymbionts, though they apparently only took root in the skeleton after the sponge had died. A persistent question for stromatoporoid ecology is how they were able to compete with corals in shallow, brightly lit areas. One hypothesis is that heavy laminar growth forms were more resistant to damage from waves and storms, yet laminar stromatoporoids were equally common in deep or undisturbed waters. Another hypothesis argues that stromatoporoids benefited from a
mutualistic (mutually beneficial) relationship with endosymbiotic microbes. Modern
scleractinian corals are
mixotrophs, deriving energy from both tiny prey items and
zooxanthellae,
photosynthetic algae which live within their cells. Zooxanthellae additionally assist the corals' biochemical processes, allowing for expeditious growth rates. No equivalent organisms are known in modern sponges, though some demosponges do host a high volume of
cyanobacteria within their skeletons. Several lines of evidence suggest a mixotrophic lifestyle for stromatoporoids, though none are unambiguous. Their ratios of oxygen and carbon
isotopes overlap with corals to an extent. If one assumes that latilaminae (growth interruptions parallel to laminae) are annual (like tree rings), stromatoporoid growth rates can reach 2 to 10 mm per year, equivalent to corals and much higher than modern calcareous sponges. On the other hand, most stromatoporoid growth forms emphasized stability and horizontal breadth rather than a vertical 'race for sunlight', as exhibited by sessile organisms which rely on photosynthesis, such as land plants and corals. == Gallery ==