eldritch echinoderms – Nix Illustration

Strange Symmetries #04: Even More Echinoderms

Early echinoderms seem to have gone through an asymmetrical phase before starting to evolving their characteristic radial symmetry. The first truly radial forms had three-way symmetry, but soon a group called the edrioasteroids upped that count to five.

First appearing in the fossil record around 525 million years ago in the early Cambrian, edrioasteroids were mostly shaped like discs or domes, and were immobile filter-feeders that lived permanently attached onto surfaces like the seafloor or the shells of other animals. Unlike most modern echinoderms their pentaradial symmetry was actually created by taking a tri-radial body plan and forking two of their arms near the bases to create a total of five.

A colored line drawing of Thresherodiscus, an extinct early echinoderm. It's a domed disc-shaped creature that looks like it has a starfish merged onto its upper surface – but the "arms" branch many more times than five, and not totally symmetrically, creating an erratic forking pattern. It's depicted with a dark purplish body and brighter orange arms. — *Thresherodiscus ramosus*

Thresherodiscus ramosus was an unusal edrioasteroid that lived in the shallow seas of what is now central Canada during the late Ordovician, around 460-450 million years ago. Up to about 4cm in diameter (~1.6″), its arms split additional times at irregular intervals, creating a complex asymmetrical branching pattern across its upper surface.

The tips of its arms protruded slightly over the rim of its body, and along with the erratic extra branching this may have been an adaptation to increase its food-gathering surface area.

Another group of early pentaradial echinoderms known as the blastozoans were characterized by erect feeding appendages called brachioles. But some blastozoans abandoned their five-way symmetry in favor of much stranger arrangements, sometimes having as few as two arms – and, in some cases, two mouths.

A colored line drawing of Amygdalocystites, an extinct early echinoderm. It has an oval body with a curving stem coming out from its right side, with the stem ending in a circular holdfast. Two "food grooves" run along its top edge, roughly in line with each other on each side of where its not-visible mouth is located. Each food groove has a single row of long tendril-like feeding appendages growing from its left edge. It's depicted with a red and yellow color scheme, with a striped stem and irregular stripes on its body giving a sort of flame-like pattern, and purple tips on its feeding appendages. — *Amygdalocystites radiatus*

Known from the same general area and time period as Thresherodiscus, Amygdalocystites radiatus was part of an Ordovician-to-early-Silurian lineage known as paracrinoids, which attached their irregularly-shaped bodies to the seafloor via a stem.

About 5cm long (~2″) Amygdalocystites had just two asymmetric arms forming “food grooves” along its upper edge, each lined with numerous long brachioles along just one of their sides. It probably orientated itself so its body was facing down-current, which would have created eddies that brought suspended food particles within easier reach of its brachioles.

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.

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.

Heckerites

While most modern echinoderms display the group’s characteristic five-way symmetry, there were plenty of much much stranger-looking forms back during the Paleozoic.

And some of the most confusing of them were the paracrinoids, which evolved an incredibly diverse range of body shapes during their group’s relatively short 40 million year existence during the Ordovician.

Despite the name these echinoderms weren’t particularly closely related to true crinoids, instead being part of a completely extinct lineage known as the blastozoans. Their ancestors had been radially symmetric, but paracrinoids largely abandoned that body plan, instead developing irregularly shaped and often asymmetric bodies ranging from round to flattened. They had between two and five “food grooves” on their upper surfaces, derived from the ambulacra, lined with numerous feeding appendages along only the left side of each.

They were shallow-water animals, living either attached to the seafloor by a long stem or anchored into the sediment by a shorter one, suspension feeding with their appendages and transporting the food particles towards the mouth located between the bases of the food grooves.

(…And speaking of mouths, some paracrinoid species appear to have had two of them.)

Heckerites multistellatus here lived around 458-445 million years ago, during the Late Ordovician. It inhabited the then-subtropical seas of the Baltica region, with fossil material known from what is now Estonia, southeastern Norway, and northwest Russia.

About 10cm tall (4″), it lived on the seafloor in sheltered waters protected from strong waves by large reefs, and is unusual even among its weirdo relatives for features such as retaining feeding appendages on both sides of its food grooves – although irregularly arranged and with fewer on one side than the other. Its body was shaped rather like a flattened bean, with two food grooves diverging from roughly the centre of the top margin, chunky skeletal plates forming a border around its edges, and a short stem at is base.

It also had an unusually large “anal pyramid” on the opposite side of its body from its mouth, and this may have been used for respiration as well as waste expulsion, similar to modern sea cucumbers.

Sollasina cthulhu

Ophiocistioids were a group of weird and poorly-understood echinoderms which lived between the early Ordovician and the late Triassic, about 475 to 233 million years ago. Related to modern sea cucumbers, they were squat dome-shaped creatures with clusters of tentacle-like scaly tube feet, and have been compared to the bizarre fictional monsters of H. P. Lovecraft’s Cthulhu Mythos.

So it’s not really surprising that one of them has been named Sollasina cthulhu.

But unlike its namesake this “monster” was actually tiny, only 3cm across (1.2″). It was discovered in the fine-grained Wenlock limestones of the UK, and dates to the late Silurian, about 430 million years ago. Its exceptionally well-preserved state makes it the first ophiocistioid with known fossilized internal structures, including evidence of its water vascular system.

Unfortunately this high level of detail comes at a cost — the tiny Wenlock fossils are preserved in three dimensions inside hard concretions and are almost impossible to extract or interpret from split-open cross-sections, and highly expensive CT scans don’t give a good enough resolution. So the only way to actually “see” them is to destroy them, grinding away a tiny layer at a time and taking a photograph at each step, then assembling a digital reconstruction from the hundreds of slices.

Syringocrinus

Syringocrinus paradoxicus from the Upper Ordovician of North America (~450 mya). Measuring up to around 6cm long (2.3″), it was part of an extinct group of marine animals known as solutes – characterized by irregularly-shaped bodies covered in calcite armor plates, the structure of which suggest they were echinoderms despite their complete lack of any proper symmetry.

It had two appendages, one a short “arm” that was probably used for feeding on food particles suspended in the water, and the other forming a longer stalk-like “tail” that may have served to propel it along the seafloor.

Solutes were once thought to be closely related to the equally weird-looking stylophorans, but some versions of the echinoderm family tree place them much further apart, suggesting their superficial similarities may have been due to convergent evolution instead.