brachiopod – Nix Illustration

Strange Symmetries #10: Shellraiser

Brachiopods (also known as “lamp shells”) superficially look very much like bivalves, but these two groups aren’t very closely related to each other – although they’re both lophotrochozoans, their last common ancestor probably lived sometime in the Ediacaran at least 560 million years ago, and their similarities in appearance are due to convergent evolution.

The two valves of their shells are also arranged differently. Bivalve shells grow on their left and right sides and are usually symmetrical, but brachiopods form their shells from the upper and lower surfaces of their bodies.

As a result brachiopod shells are usually unequal in size and shape but have their own plane of bilateral symmetry down the center – but some of them still managed to become asymmetrical anyway.

Torquirhynchia inconstans lived during the Late Jurassic, about 161-145 million years ago, in the warm shallow seas that covered what is now Europe and Iran. Around 3cm across (~1.2″) it had a strongly ridged shell with an asymmetrical closing edge, positioned high on one side and low on the other.

This unusual uneven arrangment is thought to be an adaptation to living on soft sediments. Asymmetrical brachiopods like Torquirhynchia may have lived with one side of their body mostly buried into the seafloor, and twisted their shell edges so the still-exposed half was raised up to better function for water circulation and filter-feeding.

Cambrian Explosion Month #31: Phylum Brachiopoda

While modern brachiopods superficially resemble clams, they’re not actually very closely related to each other. Clams are bivalve molluscs, related to snails and squid, while brachiopods are lophophorates related to bryozoans and horseshoe worms.

Their two shell valves are also arranged very differently – while bivalve shells originate from the left and right sides of their bodies, brachiopods grow theirs on the top and bottom.

They first appear in the fossil record in the early Cambrian, about 530 million years ago, but they may have actually diverged from a tommotiid-like ancestor as far back as the late Ediacaran. Only around 300 species survive today, but during the Paleozoic they were some of the most abundant filter-feeding and reef-building animals with tens of thousands of fossil species known. Different species tended to have strict habitat and temperature preferences, and so their fossils are also useful indicators of how ancient climates changed over time.

Cambrian Explosion Month #30: Phylum(?) Hyolitha

Hyoliths were a group of small shelled animals that first appeared in the fossil record just after the start of the Cambrian, about 536 million years ago. They had conical calcareous shells with a lid-like operculum, and some species also featured long curling spines that made them look like ice-cream cones with mammoth tusks.

They were so odd that for a long time their evolutionary relationships were unknown. They were generally accepted to be lophotrochozoans, but some studies considered them to be part of their own unique phylum while others tended to place them as being closely related to molluscs.

It wasn’t until 2017 that well-preserved soft tissue fossils revealed a tentacled feeding structure that resembled a lophophore – and hyoliths finally found their place in the lophotrochozoan family tree as close relatives of brachiopods and horseshoe worms, possibly even being a stem lineage within the brachiopod phylum.

However, this isn’t universally accepted and some recent studies continue to dispute it. The feeding organ of a different hyolith fossil has been interpreted as not being a lophophore, classifying the group as an early lophotrochozoan stem lineage, while an analysis of shell microstructure has instead suggested realigning them with molluscs. I’m grouping them with brachiopods here, but future discoveries might still make this obsolete.

Cambrian Explosion Month #29: Phylum Phoronida & Early Brachiozoans

The last group we’re looking at this month are the brachiozoans, a lineage that includes modern horseshoe worms and brachiopods along with the extinct hyoliths.

Horseshoe worms, or phoronids, are represented by about 15 living species and are usually considered to be their own phylum, but some analyses classify them as a sub-group of brachiopods instead. Like other lophophorates they have a “crown” of filter-feeding tentacles around their mouths, and similarly to some bryozoans they build protective chitinous tubes around their bodies.

There are no definite body fossils of phoronids at all, although there are a few possible trace fossils of their tubes and the enigmatic fossil hederelloids might be related to them.

But some Cambrian fossils might give us a hint about their evolutionary history.