Hexactinellids are sponges with a skeleton made of silica that have been present in the oceans for the last 600 million years.  For a long period of their history, they have formed bioherms, that is, large underwater constructions consisting of sponges growing attached to one another.  At the peak of their development, late in the Jurassic period (ca. 150 million years ago), sponge bioherms—or reefs— spread more or less continuously for thousands of kilometers from the Caucasus, across southern Europe and over to southeastern North America.  Hexactinellids were thought to have stopped building reefs since tens of millions of years, given the absence of sponge reefs in the fossil record over that time span.  However, 15 years ago, hexactinellid reefs were reported living and growing on the continental shelf off the Pacific coast of Canada.  This discovery provided a unique opportunity to acquire knowledge that would allow a better understanding of the formation of Jurassic sponge reefs. 

Foraminifera are sand grain-sized unicellular organisms that possess calcium carbonate shells and that can be found living in and on modern sponge bioherms.  Foraminifera are extremely widespread in all oceans today and their shells are found in extensive numbers in ancient marine sediments going back hundreds of millions of years, inclusive of the Jurassic period.  Many of the foraminiferal species in the modern sponge reefs live in the mud filling the space between the sponges and do not differ much from those present elsewhere on the British Columbia continental shelf.  Some, on the other hand, were found attached to or trapped within skeletons of dead sponges (none were found on living sponges).  Sponge skeletons consist of three-dimensional networks of small silica rods called spicules, the size of individual meshes in these networks being typically less than 1 mm.  Foraminifera found inside the sponge skeletons were often caught within the meshes, bulging around the silica rods, twisting around them or, more strangely, engulfing them as they grow.  Most foraminiferal species living in this dead sponge biotope differ from the assemblages found in the surrounding mud.

Foraminifera can also be found attached to sponge meshwork in Jurassic sponge bioherms.  Over the last century, some authors have extracted from these sponges reasonably well preserved three-dimensional specimens showing the same foraminifer-to-sponge relationship we see in modern reefs, either bulging beyond sponge spicules or engulfing them, even though none of the species present are the same.  As on the modern British Columbia shelf, the Jurassic sponge fauna is similar in composition from one locality to the next but contrasts with the assemblages collected just outside the sponges.  We have here what seems to be a case of parallel evolution, where organisms of different genetic makeup develop similar shape and structures to adapt to similar environments.  Our results imply that the dead sponge meshwork environment has remained the same since the Jurassic even though the species have changed.