Stromatoporoidea

Stromatoporoidea is an extinct group of sea sponges that lived from the Middle Ordovician to the Late Devonian periods. They are well-known from fossils and were very important in building reefs long ago. These sponges had dense, layered skeletons made of a mineral called calcite and did not have spicules, which are tiny needle-like structures found in other sponges.

Stromatoporoids often lived close together, forming flat areas called biostromes or taller structures called bioherms on tropical carbonate platforms. Some species had special bumps and star-shaped crevices on their outsides, which helped water flow away from them. Inside, their skeletons had a mesh-like structure with layers, rods, and boxy spaces. They came in many shapes, from flat and layered to dome-shaped, and even rare finger-like or tree-like forms.

These sponges lived alongside other reef-builders like tabulate and rugose corals. Scientists study them to learn about ancient environments because their shapes and where they were found can tell us about the conditions of the past. Some think stromatoporoids had a friendly relationship with algae, similar to modern scleractinian corals, though evidence for this is not complete. For a long time, they were thought to belong to the same group as corals and jellyfish, but they are now classified as sponges, related to modern sclerosponges. True Paleozoic stromatoporoids belonged to seven orders, with most appearing in the Silurian period. They declined in the Late Devonian and seemingly disappeared by the end of that period during the Hangenberg event. Some sponges from later periods have been called stromatoporoids, but they are probably not closely related to the ancient ones.

Morphology

Stromatoporoids were strong sea sponges with thick skeletons made of a special kind of rock called calcite. Unlike other sponges, they did not have tiny needle-like parts called spicules in their skeletons. They grew upward and outward from a single spot where they stuck tightly to the ocean floor or rocks.

Many of these sponges had small bumps on their tops called mamelons. Some also had special cracks or grooves that helped move water away from their surfaces. Inside, their skeletons had layers and pillars, with open spaces that would have been filled with seawater when they were alive. These layers and pillars helped shape each sponge and could look different depending on the species.

Stromatoporoids could grow in many shapes. Some were flat like plates, while others were dome-shaped or even tall and column-like. Their shapes changed based on where they lived and how much sand or sediment covered them. For example, when sand buried part of a sponge, that part stopped growing, and the sponge grew over the sand again, sometimes creating a rough, bowl-like appearance.

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

Stromatoporoids have been a mystery for a long time, with scientists unsure about how they are related to modern animals. For many years, people thought they might be related to certain sea animals called cnidarians, which include creatures like Hydractinia and Millepora. These animals build skeletons that look a little like stromatoporoids.

Later discoveries showed that some living sea sponges have skeletons very similar to stromatoporoids. This led many scientists to believe stromatoporoids were also a type of sponge. They found that each mound of stromatoporoid was one animal, not a group of tiny animals working together. Even though these ancient sponges looked different from modern ones, most scientists now agree they were sponges. A few scientists, mostly from older studies, still thought they might be related to other stone-like growths in the ocean.

Evolution

The first clear examples of stromatoporoids appeared in the Ordovician period. Similar organisms might have existed even earlier, in the Cambrian period, but these are likely examples of convergent evolution rather than true ancestors. These early forms had porous structures inside their skeletons.

A more likely group of ancestors lived during the Ordovician. These were low, heavily calcified sponges known as pulchrilaminids. They shared many features with stromatoporoids but had spine-like projections between their skeleton layers.

True stromatoporoids first appeared in the Ordovician, belonging to a group called Labechiida. These sponges quickly spread around the world during the Middle Ordovician. They formed important reefs in places like South China, eastern North America, and North China.

Over time, new groups of stromatoporoids appeared, including Clathrodictyida and Actinostromatida. While labechiids were very diverse early on, their numbers dropped during a major extinction event in the Late Ordovician. Other groups then became more common.

Stromatoporoids reached their peak diversity in the Devonian period, especially during the Eifelian stage. However, a major extinction event at the end of the Devonian, known as the Kellwasser event, greatly reduced their numbers. Another extinction event, the Hangenberg event, marked the end of stromatoporoids by the end of the Devonian period.

Some fossils thought to be stromatoporoids have been found from later periods, but their identification is uncertain. Many experts believe these are actually different types of sponges rather than true stromatoporoids.

Paleoecology

Stromatoporoid reefs

Stromatoporoids were like modern corals, living close together and feeding by filtering food from the water. They could survive in many different places, at various depths and light levels, and with changing sea levels. Unlike corals, they usually sat on soft ocean floor, creating flat "reefs" called biostromes instead of tall, layered ones. These reefs did not have much variety, with only a few species making up most of the group.

The most diverse groups lived on special ocean areas called carbonate platforms, away from muddier or saltier water. In calm places, some stromatoporoids could grow very big, even over 30 meters wide! Sometimes, they built up mounds called bioherms, especially along the edges of ocean shelves, with help from other small organisms living between them.

Ectosymbionts and encrusters

Stromatoporoids, being hard and fixed in one spot, were used by other small sea creatures as a home. These creatures, called ectosymbionts, attached themselves to the outside of the stromatoporoid skeletons. Many of these hidden creatures lived in shaded spaces between the sponge and the ocean floor or under shelf-like parts of the sponge. Some of these included bryozoans, tabulate corals, crinoids, brachiopods, and spirorbids. More open areas were also covered by corals, crinoids, bryozoans, and tentaculitids.

Endosymbionts

Stromatoporoids also hosted endosymbionts, small organisms that lived inside their skeletons. The most common were syringoporids, a type of coral that built tubes. These were so common that some old studies thought they were a different kind of sponge. Other corals, algae, and worm borings were also found inside stromatoporoid skeletons.

Scientists wonder how stromatoporoids competed with corals in bright, shallow water. One idea is that they grew in flat shapes that were strong against waves and storms. Another idea is that they may have had a helpful relationship with tiny microbes inside them, similar to how some modern corals have helpful algae. Evidence for this includes their growth rates and chemical makeup, which were like corals and much faster than modern sponges.