#brachiozoa

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.

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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.

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Lingulosacculus nuda might represent another possible link between brachiopods and hyoliths.

Discovered in the Mural Formation in Alberta, Canada (~524-522 million years ago), it was about 4cm long (1.6") and had a long conical shell that was either poorly-mineralized or completely unmineralized.

Its position within brachiozoan evolution is uncertain. It was originally proposed as a stem-phoronid, but other analyses place it as a stem-brachiopod, an early linguliform brachiopod related to lingulellotretids, or possibly a stem-hyolith.

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The earliest true hyoliths were the conical orthothecids, which mostly lived resting on top of the seafloor feeding on organic detritus around themselves. A few have also been found vertically oriented in the sediment, suggesting they may have been filter-feeders collecting particles from surrounding water currents.

Later members of the lineage, the hyolithids, developed distinctive long tusk-like spines on each side of their opercula. These moveable structures have been termed "helens" and were probably used as stilts, holding up the front end of the animal slightly above the seafloor.

There's also some evidence that helens gave hyoliths the ability to move themselves around, albeit probably rather awkwardly. They may have dug out shallow scrape-like "burrows" in the surface of the seafloor to shelter from stronger currents that could overturn them, and clusters of individuals found around carcasses of larger animals indicate they might have been opportunistic scavengers.

Haplophrentis reesei was a typical hyolithid, known from Utah and Idaho, USA, about 509-504 million years ago. A closely related species, Haplophrentis carinatus, is also known from Canada (~508 million years ago) and southwest China (~516-513 million years ago).

Up to about 6cm long (2'4"), this hyolith was one of the first found to preserve soft tissue feeding organs, showing up to 16 tentacles in a lophophore-like arrangement.

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Whatever they actually were, the hyoliths were highly successful animals for a time, found abundantly worldwide for most of the Cambrian. They later began to decline, but still hung on throughout the rest of the Paleozoic and only finally went completely extinct during the catastrophic "Great Dying" mass extinction at the end of the Permian, about 252 million years ago.

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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.

Brachiopods probably descended from tommotiids with stiff cone-like bodies, and some early evolutionary branches like Lingulosacculus and the hyoliths seem to retain that long conical shape in their larger lower shell valves.

Later lineages compacted their bodies and developed more similarly-sized valves, and Yuganotheca elegans might be an example of that sort of transition.

Known from the Chinese Chengjiang fossil deposits (~518 million years ago), this lophophorate animal was about 4cm long (1.6"). It had a unique body plan with two unmineralized brachiopod-like shell valves around a chunky-tentacled lophophore, set above a "collar" region, a rigid conical tube-like body, and a long fleshy pedicle.

It would have lived partially buried in soft seafloor sediment with its pedicle acting as an anchor, while its stiff conical body raised its shells and lophophore up higher into the water column. The surface of its shells, collar, and cone were also coated with sand grains in a similar manner to the tubes of some horseshoe worms.

It may have been a stem lineage brachiopod, potentially representing a stage of their evolution where bivalved shells had developed but the rest of the body hadn't yet become fully enclosed by them. It's also been proposed as a more basal brachiozoan, close to the common ancestor of brachiopods and horseshoe worms – or it could just be a weird side branch of early brachiopods that experimented with a different body plan.

Meanwhile, Kutorgina chengjiangensis was a true brachiopod that was an early representative of the rhynchonelliforms – a major lineage characterized by having calcareous shells with complex hinges.

Also known from Chengjiang, this brachiopod had a shell about 1.5cm long (0.6") with a fringe of hair-like setae, a spiral-shaped lophophore, and a thick stalk-like pedicle with a ribbed texture that attached it onto hard surfaces. Its anatomy shows a mixture of features from both rhynchonelliform and linguliform brachiopods, helping to confirm the shared ancestry of these two very different groups.

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Linguliforms are another major lineage of brachiopods, some of which have changed very little in outward appearance since they first appeared in the Cambrian. They're distinguished from rhynchonelliforms by having apatite shells with simpler hinge mechanisms, and they use different sets of muscles to open and close their valves.

Neobolus wulongqingensis was an early linguliform brachiopod that lived in Yunnan, China, about 512 million years ago. Its shells could grow up to 6mm long (0.25"), but generally averaged closer to 2mm (0.08") – and it's especially notable for being one of the oldest known fossilized examples of a clear parasite-host ecological interaction.

Tubes of an unidentified worm-like animal are preserved encrusted onto many specimens of this brachiopod, often with multiple tubes on a single shell. All of them were aligned so that the parasite could reach into the flow of water being pumped into the host's interior, "stealing" incoming nutrients.

Infested Neobolus were also noticeably smaller than parasite-free individuals, indicating that the worms' presence was actively harmful to them and stunted their growth.

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…And that brings us to the end of this month, but not to the end of the Cambrian Explosion series. We're only halfway done, so later this summer we'll be returning to the early Paleozoic for part 2 – this time focusing entirely on the ecdysozoans and the rise of the arthropods.

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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.

Tommotiids were early brachiozoans found worldwide between about 530 and 500 million years ago. They were originally known only from tiny isolated shell plates that made their full anatomy difficult to reconstruct, and they were thought to be armored slug-like animals similar to halkieriids. But in the late 2000s articulated specimens were finally discovered, revealing that some tommotiids actually looked very different from that concept.

Eccentrotheca helenia was the first tommotiid found with its shells articulated, discovered in South Australian fossil deposits dating to around 520 million years ago. About 5mm long (0.2"), it was a sessile tubular animal with irregular shell plates spiralling up around its body forming a stiff cone.

It would have lived with its base attached to the seafloor, while its filter-feeding lophophore tentacles pointed up into the water column – and it may have been closely related to phoronids as part of an early stem lineage.

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Other tommotiids were probably more closely related to the ancestors of brachiopods and hyoliths (making them paraphyletic and more of an "evolutionary grade" than a defined lineage), and forms like Tannuolina maroccana had fewer shell plates in a more symmetrical arrangement.

Known from Morocco around 520 million year ago, this tommotiid was about 4mm long (0.16") with multiple overlapping shell plates on one side of its body and a single pair on the other side. Small holes in the surface and edges of some of its shells may have had hair-like sensory setae protruding through them.

Its shell arrangement suggests that brachiopods’ two valves may have originated through gradual reduction and specialization of earlier complex tubular Eccentrotheca-like patterns.

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