Strange Symmetries #03: Eerie Early Echinoderms

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.

A colored line drawing of Sokkaejaecystis, an extinct early echinoderm. It has a body shaped roughly like a flattened boot, with spikes and flanges growing from around its margin. What looks like a long tail-like appendage growing from the sole of the boot shape is actually a starfish-like feeding arm at the animals' "front" end. It's depicted with orange-brown coloration with brighter yellow on the spikes and flanges, and darker brown irregular stripes over its body.
Sokkaejaecystis serrata

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.

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.

A colored line drawing of Maennilia, an extinct early echinoderm. It has a flattened body shaped like a lumpy trapezoid, with a single short starfish-like arm growing from its left side and a long thin segmented tail-like appendage growing from the right side of its back end. It's depicted with mottled red and yellow coloration like a camouflage pattern.
Maennilia estonica

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.