Evolution of brachiopods
Brachiopods are marine invertebrates, which were highly widespread and successful throughout the Palaeozoic era. There is currently about 100 living genera and 5000 fossil genera. Brachiopods based on their morphology are a sister group to Deuterostomes. They are seen as relatives because one cilium per cell drive both lophophores. Brachiopods emerged during the Cambrian and Ordovician periods approximately 525 million years ago, but we are unsure of their exact origins. They reached their highest point of diversity during the Devonian era which was 400 million years ago.
Brachiopods had great diversity and were preserved very well and in great abundance. They are excellent index fossils for studying changes through time and because of the wide range of substrate preferences, they great paleoenvironmental indicators. The most prominent hypothesis of Brachiopod origin is that they emerged from the folding of a Halkieria-like organism. Halkieria are composed of a scaled body and at either end have two protective shells.
The support for this theory comes from the gene expression pattern which is extremely similar as seen throughout the development between the two species. Furthermore, the ventral and dorsal valves represent a posterior and anterior shell. This consideration makes the axes of symmetry in course with that of a bilaterian phyla. This also is concordant during embryological development due to the manner in which the body axis folds to bring the shells from the dorsal surface to their latter mature position.
Another theory for Brachiopod origin is that they arose by the shortening of tube-like organisms that were composed of several shell plates. Tommotiids have shells at their rear which is a similar arrangement when being compared to Brachiopods.
Halkieria: the precursor to brachiopods
FUN FACT!
There were around 15,000 species of Brachiopods at one point, now there are only 300-400 species left!
Brachiopods have had to battle against a few major extinctions and major diversifications and proliferations. Over the last 100 million years, there has been a steady decline of Brachiopods.
A major reason for their decrease during the Mesozoic period was the emergence of bivalves. Brachiopods until that point did not have much competition. Early on, they had to deal with Ordovician radiation which spread to more benthic habitats offshore and then eventually carbonate mounds. This occurred throughout the Paleozoic era.
By the end of the Permian extinction, 90% of all living species were wiped out and Brachiopods were not immune to this. It is approximated that at least 15,000 species made up the Brachiopoda phylum and currently only 300-500 species are left today. The exact cause of the Permian-Triassic extinction is difficult to figure out because it dates 250 million years ago.
Scientists have hypothesized that the increase in volcanism, led to an extreme increase in methane released from the sea floor. The Siberian Traps were the largest volcanic event known to date. The basaltic lava covered an area about 2 million square km. This resulted in changing sea levels, anoxic ocean conditions and overall global warming due to the rise in greenhouse gases.
Extinction
Kingdom: Animalia
Superphylum: Lophotrochozia, Clade: Brachiozo
Phylum: Brachiopoda
