Barocoryth

Barocoryth (meaning heavy helmet in Old Viodoxan), sometimes called the Giant Eldhamite Clam was a genus of massive marine bivalve mollusks that existed during the Late Tudenanian period and possibly as late as Ardatian period. They would have been found throughout many of the world's oceans at the time, with fossilized remains being found in deposits from Draeksla and as far as the USSE. Currently the only identified species and the type species of the genus is Barocoryth draekslorex.

Description
The largest and most complete specimens of Barocoryth suggest that the members of the genus reached as large as three meters in axial length. Some paleontologists believe that they could reach as large as four meters though. This makes it the largest known extinct mollusk and by far the largest bivalve known of. The first specimen of significance was found in Draeksla, a small independent island city off the coast of Druk Yul and was interpreted and researched by paleontologists from the USSE who had reported similar very fragmentary findings in their own Loanian and Tudenanian fossil deposits. It was almost immediately identified as a large bivalve by the paleontologists.

Its shell was physically massive, with the width of the shell being proportionally thicker than any modern bivalve, this has been interpreted by some paleontologists as being a defensive mechanism. While no physical taxa have been discovered that are believed to have had preyed on Barocoryth, particularly adult specimens, it is believed that large species of ray or other creatures may have at least historically predated on the creatures incentivizing the evolution of thicker shells.

Because of the shell's size, it is likely they were sometimes capable of being practical microbiomes for small fish and barnacles which may have used the interior of the shell as shelter from predators. This was likely possible since the clam is interpreted to have had been a filter feeder like virtually all modern bivalves, feeding off of microscopic plants and animals such as plankton meaning that if things were to take shelter inside the shell they would not be in immediate danger.

Extinction
It is believed that the last Barocoryth species went extinct in the Ardatian period but it is possible they went extinct as early as the Loanian. It is unknown precisely why the group went extinct but it likely had to do with increasing competition, ecological pressures, and the fact that these clams would have likely had a very late sexual maturity and growth cycle in addition to the fact that they would have been entirely immobile for most of their subadult and adult life making them vulnerable to environmental changes around them.