#soluta

Represented today by starfish, brittle stars, sea urchins, sea cucumbers, and crinoids, the echinoderms have a characteristic five-way radial symmetry that makes them barely even recognizable as bilaterians. Their true ancestry is only revealed by their genetics and their larvae, which still retain bilateral symmetry – and the way they metamorphose into adults is bizarre, essentially growing a whole new radial body from within the left side of their larval body.

(Sea cucumbers and sand dollars are superficially bilateral as adults, but evolved this secondarily on top of their existing radial symmetry. And some adult echinoderms like starfish also seem to retain a little bit of "behavioral bilaterism", generally preferring to move with a specific arm always acting as their "front" end.)

The first known echinoderms appeared in the fossil record during the early Cambrian, about 525 million years ago, but the common ancestor of the whole group probably actually originated a few tens of millions of years earlier in the mid-to-late Ediacaran. Early echinoderms seem to have started off as flattened animals that sat on the seafloor filter-feeding, and with this largely immobile way of life their bodies started to shift into asymmetry, no longer constrained by the locomotory advantages of being bilaterally symmetric.

In fact, for these early sedentary filter-feeders being radial was actually much more advantageous, able to distribute sense organs all around their bodies and grab food from any direction without having to reposition themselves, converging on the lifestyle of non-bilaterian cnidarian polyps. The evolutionary transition from bilateral to asymmetrical to pentaradial seems to have happened incredibly quickly during the Cambrian Explosion, and all modern echinoderms probably evolved from a group called the edrioasteroids, maintaining their new base body plan even when they later began taking up more mobile lifestyles again.

But during the process of all that some very alien-looking lineages split off at various stages of anatomical weirdness.

Stylophorans had asymmetrical bodies with a single feeding arm at the front, and varied from irregular boot-like shapes to almost bilateral heart shapes depending on their specific ecologies. The highly asymmetrical forms were probably spreading their weight out over soft soupy mud in quiet waters, while the more bilateral forms may have been more streamlined to deal with stronger water currents.

Sokkaejaecystis serrata was a stylophoran that lived during the late Cambrian, about 501-488 million years ago, in what is now South Korea. It was tiny, only about 1cm long (~0.4"), and its boot-shaped body was surrounded by spines and flanges that spread out its surface area and probably also made it much more awkward for small predators to attempt to eat.

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Meanwhile the solutes started off as immobile animals living attached to the seafloor via a stalk-like appendage. But fairly early in their evolution they switched to a more active mode of life, modifying their stems into tail-like "steles" that were used to push themselves along.

Maennilia estonica lived in what is now Estonia during the late Ordovician, about 450 million years ago. It was quite large for a solute at about 12cm long (~4.7"), with a sort of vaguely-trapezoidal body, a short feeding arm, and a long thin stele.

Both of these strange early echinoderm lineages were surprisingly successful, surviving for a good chunk of the Paleozoic Era alongside their more familiar radial relatives. The solutes lasted until the early Devonian about 400 million years ago, and the stylophorans continued all the way into the late Carboniferous about 310 million years ago.

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Cambrian Explosion Month #11: Phylum Echinodermata – Increasing Asymmetry

During their early evolution, echinoderms started developing unusual asymmetric body plans – and some of them were so strange-looking that for a while it wasn't clear if they even were echinoderms.

A group called solutes were superficially bilateral in appearance, with a feeding arm at the "front" and a long tail-like appendage at the "back". But their bodies didn't really have any true axis of symmetry, slightly irregularly shaped and with the "arm" offset to one side.

The ancestors of this lineage probably branched off from a common ancestor with modern echinoderms shortly after the cinctans, and they seem to have been suspension-feeding animals that lived attached to the seafloor via stem-like appendages. But fairly early on in their evolution they instead switched to a mobile free-living mode of life, modifying their stalks into "tails" used to push themselves along.

Castericystis vali was one of the earliest know solutes, found in the Marjum Formation in Utah, USA (~505 million years ago). Growing up to about 8cm long, it seems to have been transitional between attached and free-living, with juveniles temporarily attaching themselves to surfaces – sometimes even being found stuck onto the "tails" of other individuals of their own species.

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The stylophorans, meanwhile, pushed an asymmetrical body plan to an even bigger extreme. All members of this group had an irregularly-shaped body and a single feeding arm, but their forms could vary wildly – early species were bizarrely "boot-shaped", while others returned to an almost-bilateral shape.

For a long time it was uncertain which end of their bodies was which, or what sort of animal they even were – they were generally considered to be a weird group of echinoderms based on their calcite skeleton, but some studies instead considered them to be early ambulacrarians or even early chordates. Their echinoderm affinities were only actually fully confirmed in 2019 with the discovery of fossils with preserved soft tissue impressions, including distinctive echinoderm tube feet on their appendages.

But their exact position within the echinoderm evolutionary tree is still rather uncertain. They may be stem-echinoderms related to the solutes, or they might be close relatives of blastozoans or crinoids which secondarily lost their radial symmetry and convergently developed a similarly asymmetrical body plan.

Ceratocystis prosthiakida was one of the earliest known stylophorans, living about 510 million years ago in southeast Germany. Just 2.5cm long (1"), it had a wide flat boot-shaped body with a prominent three-way ridge on its upper surface. It would have been able to move itself across the seafloor using its feeding arm, although the motion would have been "backwards" relative to the orientation of its body.

The odd shapes of stylophorans like Ceratocystis was probably an adaptation to living on very soft sediments – an increased surface area spreading out their weight to act like a snowshoe, and highly exaggerating their asymmetry in the process. Later more symmetrical forms may have instead been adapted for streamlining and stability in areas with strong water currents.

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Despite their sometimes awkward appearances both of these weird echinoderm groups were surprisingly successful after the Cambrian. Solutes became widespread during the Ordovician and lasted until the early Devonian (~400 million years ago), while stylophorans continued all the way until the late Carboniferous (~310 million years ago).

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