The niche of the infaunal filter feeder (sedentary organisms living buried in the mud of the sea flo

The niche of the infaunal filter feeder (sedentary organisms living buried in the mud of the sea floor, surviving on plankton and organic detritus filtered from the water column) is occupied in the modern day by bivalved molluscs - clams, mussels, and similar species - which have a near-universal distribution in all water bodies across the globe. However, this monopoly was not always in place, and before the end of the Permian period, stranger forms were found in the world’s oceans.Brachiopods are common as fossils, but few groups remain extant. The brachiopoda existed as a rival phylum to the molluscs from the Cambrian period (540 to 480 million years ago) onwards, reaching a peak of diversity around the Devonian (420 to 360 m.y.a.), but they were decimated during the Permo-Triassic extinction event - 250 m.y.a - (where around 90% of marine species were erased by rapid climate changes), and after that point never regained their strength - bivalves had replaced them by the Jurassic, as their extendable siphon tubes allowed them to bury themselves while retaining contact with clean water above the sediment, thus opening up another niche which helped them to better avoid predation.Before the Brachiopod period in the Devonian and Carboniferous, an extinct molluscan class came to prominence in the infaunal niche. The rostroconch was a clam-like creature, but its two shells were not hinged like a clam - instead, they were firmly joined at the apex, like a taco shell.As no living rostroconchs remain, their morphology is mostly a mystery, but it is believed that shell had to be periodically broken into two and ’re-set’ like a damaged bone, in order to allow the animal to grow.Rostroconchs were most widespread in the Ordovician (480 to 440 m.y.a.), but declined due to a suspected global ice age (the snowball earth hypothesis) occurring at the end of that period. When the brachiopods came to the fore, they existed for a while in the background, but their class was dispatched completely at the Permo-Triassic boundary.-TJTInformation on the Rostroconchia is hard to find online, as they have rarely been discussed outside specialist publications. To see some rostroconch fossils, visit http://northtexasfossils.com/rostroconchia2.htm where the beautiful photo of an Apotocardium specimen was found. -- source link

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