Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Middle Ordovician to the Late Devonian.Stock, C.W. 2001, Stromatoporoidea, 1926–2000: Journal of Paleontology, v. 75, p. 1079–1089. 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.
Morphology 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.
Internal structures
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.
Growth forms
Stromatoporoids can show a variety of growth forms, with low domes or plates as the most common varieties. Whenever an influx of sediment buries the edge of the skeleton, the buried portion ceases growing while the exposed central portion expands outwards to cover the sediment once more. This can create a 'ragged' appearance for some fossils, akin to an inverted stack of bowls or plates with sharp lower edges and smoothly curved upper edges. This is one example of how stromatoporoid growth forms can vary somewhat through the animal's lifespan. A single species can acquire a taller, narrower form to survive high sedimentation rates, while acquiring a flatter and more stable form to survive in energetic shallow waters. Some stromatoporoids appear to grow intermittently in a 'ragged' style even without sediment burial, as indicated by an abundance of encrusters under the overhanging 'shelves'.
Stromatoporoid growth forms include:
Laminar – Sheet- or plate-shaped, with a broad base and a thin, flat skeleton expanding outwards along the substrate. Sheet-like forms with a slightly thicker profile are sometimes labelled 'tabular'. Domical – Dome- or mound-shaped, with a curved surface developing both outwards and upwards from a broad base. Domes occupy a spectrum from low (height less than half of the base's diameter) to high (height up to double the diameter). Some high domical species taper significantly, acquiring a conical shape. Bulbous – Bulb-shaped, with a narrow base widening upwards into a semi-spherical mass. Columnar – Cylindrical, tall (height more than double the diameter) and trunk-like, without branches. In many cases the outer wall of the column is reinforced with laminar outgrowths. One example of a columnar stromatoporoid is Aulacera, a very large aulaceratid from the Late Ordovician of Anticosti Island, Quebec. 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.
Classification
Taxonomy From Stearn et al. (1999) and The Treatise on Invertebrate Paleontology:
Order Actinostromatida [Upper Ordovician? (Katian) – Upper Devonian (Frasnian)] Order Amphiporida [late Silurian (Ludlow) – Upper Devonian (Famennian)] Order Clathrodictyida [Upper Ordovician (Katian) – Upper Devonian (Famennian), Upper Mississippian? (Serpukhovian)] Order Labechiida [Lower Ordovician (Tremadocian) – Upper Devonian (Famennian), Triassic?] Order Stromatoporellida [latest Silurian (Pridoli) – Upper Devonian (Famennian)] Order Stromatoporida [early Silurian (upper Llandovery) – Upper Devonian (Frasnian)] Order Syringostromatida [middle Silurian (Wenlock) – Middle Devonian (Givetian), Upper Devonian? (Famennian)] incertae sedis: Clavidictyon Eostachyodes Lamellistroma Paschkoviella Perplexostroma Praeidiostroma Pseudactinostroma Pseudostromatopora? Taymyrostroma
Affinities Like many fossil invertebrates, stromatoporoids have long been regarded as an enigmatic group with an uncertain relationship to modern taxa. For much of their history of study, stromatoporoid fossils could only be observed externally or through natural cross-sections. Several hypotheses developed in the 19th and 20th centuries based on this limited set of data. A few authors suggested that stromatoporoids were sponges or relatives of Gypsina (an encrusting foraminifera). However, for much of their history the mainstream interpretation was that stromotoporoids were colonial cnidarians, most closely related to mineralized hydrozoans such as Hydractinia and Millepora. Hydractinia in particular has a thinly encrusting layered skeleton, augmented by internal rods, external knobs, and radiating nutrient canals. These features were equated with pillars, mamelons, and astrorhizae, respectively.
In 1970, several living sponges were found to possess a calcareous skeletal framework very similar to Hydractinia and stromatoporoids. The modern sponges, collectively termed sclerosponges, immediately reinvigorated the hypothesis that stromatoporoids were sponges. 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. Among the strongest evidence for sponge affinities was the degree of similarity between astrorhizae and exhalant canals, which were easier to homologize than the more integrated canal system of Hydractinia.
Proponents of the sponge hypothesis admitted that sclerosponges and stromatoporoids were not identical in structure; for example, sclerosponges have spicules while true Paleozoic stromatoporoids do not. Several other enigmatic calcareous fossils (archaeocyathids, disjectoporids, 'chaetetids') have been reclassified as sponges thanks to this new information. The sponge hypothesis quickly met widespread acceptance, with a few detractors. Up until the 1990s, some Soviet and Eastern Bloc specialists continued to regard stromatoporoids as cnidarians or cyanobacterial accumulations akin to stromatolites or thrombolites.
Evolution
Origination