Stromatoporoidea

Extinct clade of sponges

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.

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. 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. Digitate – Hand shaped, with finger like columns clustered together above a broad shared base. Dendroid – Bush shaped, with a thicket of narrow columns branching away from a narrow base. Digitolaminar – A composite form combining both digitate and laminar characteristics. Finger like projections are superimposed onto one or more flat plate like platforms. Irregular – A composite form without an easily characterized shape. Both vertical and horizontal growth habits may be apparent in a single skeleton, with domes, platforms, and columns interspersed throughout the development of the sponge.

Labechiids were by far the most diverse stromatoporoids of the Ordovician, and some paleontologists have even ventured to reconstruct lines of descent in this preliminary stage of stromatoporoid evolution. The next order to appear were the Clathrodictyida, in the early Katian stage of the Late Ordovician. The third stromatoporoid order, Actinostromatida, may have originated in the Late Ordovician or the Llandovery Epoch (early Silurian). 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. Stromatoporoids recovered strongly in the Early and Middle Devonian, reaching their overall maximum diversity in the Eifelian stage. Clathrodictyids, stromatoporellids, stromatoporids, and syringostromatids benefited the most from this renewal of biodiversity. The arrival of the Late Devonian disrupted this apex of stromatoporoid evolution. Syringostromatids were the first to be affected, virtually disappearing from the fossil record at the start of the Frasnian stage. Other groups presented unexpected patterns: the formerly rare amphiporids reached their highest diversity in the Frasnian, while labechiids staged a remarkable comeback in the Famennian stage, acquiring levels of diversity not seen since the Ordovician. 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. valid genera of small hypermineralized Mesozoic sponges have been described as stromatoporoids based on their anatomical similarity to stromatoporids, actinostromatids, clathrodictyids, or syringostromatids. One hypothesis suggests a direct line of descent between Paleozoic and Mesozoic forms. 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. Most paleontologists disagree with this idea, since Paleozoic and Mesozoic ‘stromatoporoids’ differ in several key aspects. 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. 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.

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.

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.