Stromatoporoids were among the most abundant and important reef-builders of their time, living close together in flat biostromes or elevated bioherms on soft tropical carbonate platforms.

Externally, some species have raised bumps (mamelons) and star-shaped crevices (astrorhizae), which together help vent exhalant water away from the living surface.

Internally, stromatoporoids have a mesh-like skeletal system combining extensive horizontal layers (laminae), vertical rods (pillars), and boxy spaces (galleries), along with other features.

[3][4][5] Some studies have argued that stromatoporoids were mixotrophs (engaged in a mutualistic relationship with photosynthetic algae), similar to modern scleractinian corals.

Prior to the 1970s, stromatoporoids were most frequently equated with colonial hydrozoans in the phylum Cnidaria (which also includes corals, sea anemones, and jellyfish).

[6] A number of hypercalcified Mesozoic sponges have been classified as stromatoporoids, but they are likely unrelated to the Paleozoic radiation, thus making 'stromatoporoids' (in the broad sense) a polyphyletic group if they are included.

In contrast to mamelons, papillae are simply external extensions of internal pillars, rather than stacked deflections of the skeleton's outer surface.

Since most stromatoporoid fossils are only visible in vertical or horizontal cross-section, the internal form of the skeleton is usually the most important region for the purpose of species differentiation.

Hydractinia in particular has a thinly encrusting layered skeleton, augmented by internal rods, external knobs, and radiating nutrient canals.

Moreover, closer investigations of stromatoporoid fossils were able to determine that an individual mound represents a single animal, rather than a colonial congregation of polyps.

[20] Up until the 1990s, some Soviet and Eastern Bloc specialists continued to regard stromatoporoids as cnidarians or cyanobacterial accumulations akin to stromatolites or thrombolites.

One example were the Kazachstanicyathida, an order of archaeocyath sponges with low growth habits and porous internal domes comparable to cyst plates.

In addition, some colonial coralomorphs or algae (Maldeotaina, Yaworipora, and the ‘khasaktiids’) acquired mesh-like encrusting skeletons with a set of internal rods and domes similar to those of early stromatoporoids.

They were low-profile hypercalcified sponges which were similar to stromatoporoids in many respects, with one key difference: pulchrilaminids had spine-like projections (probably homologous with spicules) between the laminae of the skeleton.

The precise timing depends on the status of Plumatalinia, a Late Ordovician Estonian genus often considered a ‘missing link’ between the ancestral labechiids and their presumed descendants, the actinostromatids.

The other four stromatoporoid orders (Amphiporida, Stromatoporellida, Stromatoporida, and Syringostromatida) also originated in the Silurian, though they remained fairly subdued compared to the three older groups.

[30] Actinostromatids, stromatoporellids, and stromatoporids were next in line, with their diversity and reef extent collapsing at the end of the Frasnian in accordance with the Kellwasser event (Late Devonian mass extinction).

[8] Fossils of the Ordovician labechiid Lophiostroma have been reported from sediments as young as the Triassic, but this is another case of poor preservation and uncertain identity.

[7] Labechiid-like fossils are abundant in early Pennsylvanian (Bashkirian) reef deposits of the Akiyoshi Limestone Group in Japan, representing a Panthalassan seamount.

[9] Over 60[33] valid genera of small hypermineralized Mesozoic sponges have been described as stromatoporoids based on their anatomical similarity to stromatoporids, actinostromatids, clathrodictyids, or syringostromatids.

Under this interpretation, the Devonian extinctions merely prompted stromatoporoids to abandon mineralization until the Jurassic, explaining their lack of fossils between the two time intervals.

In contrast to true Paleozoic stromatoporoids, Mesozoic species have recognizable spicules and a more complex microstructure within the laminae and pillars which make up the skeleton.

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.

[36] 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.

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.

[36] 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.

Notable examples can be found in the Canning Basin of Australia, the Miette Complex of Alberta, the Eifel Region of Germany, and southern Belgium.

[36][12] Modern scleractinian corals are mixotrophs, deriving energy from both tiny prey items and zooxanthellae, photosynthetic algae which live within their cells.

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.