Stromatoporoidea

Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Middle Ordovician to the Late Devonian. They can be characterized by their densely layered calcite skeletons lacking spicules. 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. The most common growth forms range from laminar (flattened) to domical (dome-shaped). Spheroidal, finger-like, or tree-like species also occur, though they are rare in most environments.

Stromatoporoids competed and coexisted with other reef-builders such as tabulate and rugose corals. Some stromatoporoid species are useful as environmental proxies, since their form and distribution can help approximate the depositional environment of sedimentary strata. They hosted a diverse fauna of encrusting symbionts both within and outside their skeletons. Some studies have argued that stromatoporoids were mixotrophs (engaged in a mutualistic relationship with photosynthetic algae), similar to modern scleractinian corals. Though this hypothesis is plausible, circumstantial evidence is inconclusive.

Prior to the 1970s, stromatoporoids were most frequently equated with colonial hydrozoans in the phylum Cnidaria (which also includes corals, sea anemones, and jellyfish). They are now classified as sponges in the phylum Porifera, based on their similarity to modern sclerosponges. True Paleozoic stromatoporoids (sensu stricto) encompass seven orders. Two or three of these orders appeared in the Ordovician while the rest evolved in the Silurian. They rediversified subsequent to mass extinctions at the end of the Ordovician and Silurian, but a more profound decline began in the Late Devonian. With a few putative exceptions, they apparently died out during the Hangenberg event at the end of the Devonian. 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. Some Carboniferous sponges have been identified as stromatoporoids with a somewhat greater degree of confidence.

Stromatoporoids are robust sponges with a dense calcite skeleton lacking spicules. Like other sponges, they grow outwards and upwards from a single base attached firmly to the substrate. Most were ambitopic (occupying soft substrate such as mud or sand for most of their life), though some were encrusting (concreted onto hard substrates such as rocks or other organisms). The base was stabilized by a crust-like layer covered with concentric wrinkles. The basal layer has historically been termed an epitheca or peritheca, names used for a similar attachment layer in sessile cnidarians.

In many species, the upper surface of the skeleton is ornamented with small mounds known as mamelons. A few species may supplement the mamelons with radiating cracks or grooves known as astrorhizae. Internally, the astrorhizae diverge as independent tapering tubes that intersect smaller open spaces within the skeletal frame. Astrorhizae are generally equated with the exhalant canals of other sponges, while the mamelons help to channel waste water away from the surface. This mechanism works via Bernoulli's principle, which states that flow pressure increases as speed decreases, such as when the flow is redirected by a vertical barrier. The surface may also be covered with even smaller bumps known as papillae. In contrast to mamelons, papillae are simply external extensions of internal pillars, rather than stacked deflections of the skeleton's outer surface.

By comparison to modern sponges with a similar anatomy, living tissue was likely only present at the outer surface of the stromatoporoid skeleton. By volume, the majority of the organism was a dead mesh of internal cavities and support structures. 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. In all species, the most conspicuous internal features are laminae, layers arranged transversely (parallel to the living surface of the sponge). Laminae have an intermediate width and spacing (on average around four per millimeter) relative to other layers with the same orientation. Significantly thinner layers, when present, are termed microlaminae, while thickened irregular plates are termed pachystromes. Another universal type of internal structure are pillars, cylindrical rods oriented longitudinally (i.e., perpendicular to the laminae). Laminae and pillars are often straight and internally solid, but they can exhibit distinctive textures and distortions in some subgroups.

The cubical open spaces among the laminae and pillar meshwork are known as galleries. In life the galleries would have been filled with seawater, while in fossils the spaces are filled by recrystallized calcite. The galleries may be supplemented by very fine curved plates, termed dissepiments. Some species have more complex skeletons with broader pockets beyond the narrow galleries. Pachysteles are longitudinal walls which demarcate maze-like corridors, as visible in a transverse cross-section through the skeleton. Stacked dome-shaped pockets, known as cysts, are defined by large convex plates, known as cyst plates.