Stromatoporoidea

Stromatoporoidea

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

Stromatoporoids often lived close together, forming flat areas or tall structures on tropical platforms. Some had special bumps and star-shaped marks on their outsides, which helped water move away. Inside, their skeletons had a mesh-like shape 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 with other reef-builders like tabulate and rugose corals. Scientists study them to learn about ancient environments because their shapes and locations tell us about past conditions. Some think stromatoporoids had a friendly link with algae, like modern corals, but this is not fully proven. For a long time, they were thought to be related to corals and jellyfish, but they are now known to be sponges, related to modern sclerosponges. True Paleozoic stromatoporoids were in seven groups, with most appearing in the Silurian period. They declined in the Late Devonian and seemed to vanish by the end of that period during the Hangenberg event. Some later sponges were called stromatoporoids, but they are likely 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 hard to classify, and scientists are still learning about how they are related to animals today. For a long time, people thought they might be related to sea animals called cnidarians, like Hydractinia and Millepora. These animals build skeletons that look a bit like stromatoporoids.

Later, scientists found that some living sea sponges have skeletons very similar to stromatoporoids. This made many believe stromatoporoids were a type of sponge. They discovered that each mound of stromatoporoid was one animal, not many tiny animals working together. Even though these ancient sponges looked different from modern ones, most scientists now think they were sponges. A few scientists from older studies still had other ideas about what stromatoporoids might be.

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 were probably not true ancestors.

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 stromatopoirds appeared, including Clathrodictyida and Actinostromatida. Stromatoporoids reached their peak diversity in the Devonian period. However, a major extinction event at the end of the Devonian greatly reduced their numbers. Another extinction 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 live in many different places, at different 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.