Stromatoporids

Stromatoporids are a group of organisms that are exclusively known from the fossil record. The links between estromatoporids and other groups of living organisms is uncertain, although their internal structure suggest that they are related to current marine sponges.
Estromatoporids lived on the bottom of relatively shallow seas. They created colonies that contributed to the construction of ancient reefs, along with corals and other organisms.
The first known fossils of stromatoporids have been found in Ordovician rocks. According to the fossil record, this group experienced a period of maximum diversification during the Silurian and Devonian. After that, stromatoporids underwent a progressively loss of diversification, until they went completely extinct in the Mesozoic.

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Reconstruction of an stromatoporids showing the internal structure of the skeleton in a cross section

The external morphology of estromatoporids can vary greatly: planar forms, tabular, branched, massive, etc. The internal structure of this organisms reveals a succession of calcareous sheets that are interconnected by vertical pillars. Accordingly, the vertical sections of estromatoporid fossils have a characteristic reticulate structure.

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Tabular stromatoporid (left) and bulb-like stromatoporid (right)