How the Starfish Got Its Arms

More than 500 million years ago, a small plated animal not even an inch long sat on the seafloor of what is now Spain.

It was kinda oval-shaped and covered in upward-pointing spines.

Paleontologists named this little creature Ctenoimbricata and it’s one of the oldest echinoderms that we’ve found.

This group includes animals like starfishes, brittle stars, sea cucumbers, and sea urchins.

And scientists know that it was an echinoderm based on the material that made up its internal skeleton - a calcium carbonate matrix called stereom - which is a feature shared by all of the members of this group.

But today’s adult echinoderms all share another special characteristic.

They have radial symmetry - with a mouth at their center and their other body structures repeating around it in sets of five, like the spokes on a wheel.

Yes, even sea cucumbers!

Except Ctenoimbricata didn’t have radial symmetry.

It had right-left bilateral symmetry like us, where the left half mirrors the right half, and its mouth faced upwards at one end of its body.

So how did the ancient ancestors of starfish go from having bilateral symmetry to having radial symmetry?

And how did the starfish get its arms?

Well, it might’ve been caused by a series of changes in the environment that saw the ancestors of starfish adapt from living life face-up to living it face-down.

The story of echinoderms might actually go back a little further than Ctenoimbricata, to a 555 million year old fossil found in Australia.

It’s named Arkarua and it’s an impression that shows what appear to be repeating sections around a central axis.

But there’s no evidence that it had stereom - the stuff that makes up the internal skeleton of all echinoderms, so it could’ve just been another round animal like a jellyfish or a sponge.

This makes Ctenoimbricata one of the first for-sure echinoderms.

It had no arms and was a so-called “basket” feeder - taking water into and out of its upward facing mouth and filtering out suspended bits of organic stuff with a basket shaped structure.

In contrast, most modern starfishes are active predators that use their chunky arms to explore the seafloor for food.

Each arm can bend and curl, thanks to supportive plates made of stereom and special connective tissues that can go soft or stiff.

And rows of hydraulic tube feet found along canals on the underside of each arm help living starfishes move around and bring food to their mouths.

So, a starfish’s arms are essential to its mobile, seafloor-cruising way of life.

And yet the first echinoderms had no arms - not just Ctenoimbricata, but other species from the Cambrian Period, too.

They were all bilaterally symmetrical basket-feeders, with their upward-facing mouths positioned at one end of their bodies.

The first radially symmetrical echinoderm known from the fossil record is Camptostroma, which showed up between 516 and 513 million years ago in Pennsylvania.

It had the five-part symmetry common to all modern echinoderms.

And Camptostroma was paving the way for the echinoderms we see today.

It shared a common ancestor with a group called the blastozoans, which evolved many variations on a body plan built around five repeating parts.

They fed by filtering out suspended food from the water column and channeling it through upward-facing feeding grooves to their mouths.

But why did some echinoderms become radially symmetric and develop this new body plan in the first place?

“Why” questions are tough to answer in paleontology, but it might be that, basically, an animal with bilateral symmetry is equipped to move in one direction, often leading with its head.

If you’re just going to sit still, capture food, and not move around, the radial body plan may help you better access food in 360 degrees.

So, okay, this body plan might have some advantages.

But why five-part radial symmetry, specifically?

Why not some other number?

It’s still an open question, but there’s a hypothesis that five parts, instead of any other number, gives you the strongest possible arrangement of the plates that give structure to echinoderms.

And how about arms, which showed up in some blastozoans?

One of these groups, called eocrinoids, had a bunch of spaghetti-like feeding arms spread out around an upward-facing mouth.

Eocrinoids first appeared in the fossil record roughly 517 million years ago.

They became super common during the Cambrian, so something about their body plan was working.

Early Cambrian eocrinoids stuck lightly to sediment or just rested on it while suspension-feeding.

But by the Late Cambrian, they attached to new harder substrates and some used root-like holdfasts.

For example, Lichenoides buried part of itself in a burrow and may have used fibers made of collagen to anchor itself to the substrate.

And it’s that period of time called the “Cambrian substrate revolution” that may have driven them and other blastozoans to adapt.

As burrowing animals evolved, the ocean bottom was becoming a lot softer.

And the water near the seafloor became more murky with sediment being churned up.

Some eocrinoids, like Gogia, developed stem-like stalks that raised their arms higher into the water column, further from the sediment.

Heading into the Ordovician Period, around 485 million years ago, eocrinoids diversified into thousands of forms.

Sea levels were high then, and the area north of the equator was almost all ocean.

There was an explosion of marine biodiversity of all kinds....

Including the similar-looking and closely related group - the crinoids, also called sea lilies - which showed up and diversified into many species with different suspension-feeding body types.

Crinoids became the most abundant echinoderms of the Ordovician, and some were the ancestors of today’s stalkless feather stars and stalked sea lilies.

If anything, the eocrinoids and crinoids with stalks were moving away from the low-profile body plan of a starfish.

So, what happened – where did starfishes come from?

Well, there’s a fossil gap between the early stalked echinoderms and forms that look like today’s starfish.

And a study published in early 2021 could provide some clues about what happened in the gap.

It looks like some of the crinoids might’ve taken a different approach to feeding - instead of staying face-up suspension feeders, they went face-down.

A fossil found in Morocco and dated to 480 million years ago is the most primitive starfish-like animal known.

And it kinda looked like a flipped-over stalkless sea lily.

Cantabrigiaster had the thick arms of a starfish but, like sea lilies, lacked small hard plates along the edges of its arms.

And, judging from the orientation of its arm canals and tube feet, Cantabrigiaster fed face-down on the sediment.

So, the first starfish may have originated from a sea lily that faceplanted, putting its mouth down on the seafloor.

And it might’ve been pressure to find a new feeding niche that drove this switch from face-up to face-down, since the Ordovician witnessed an explosion in suspension-feeding, with lots of competition for floating bits of food.

Plankton diversified, followed by a host of suspension feeders like certain types of brachiopods, rugose corals, and bryozoans.

The Ordovician period also saw the advent of more active suspension feeders, like the giant, filter-feeding anomalocarid Aegirocassis.

And while it’s impossible to know why Cantabrigiaster developed its adaptations, its ecological niche of feeding on bits from the seafloor might’ve reduced competition with active filter-feeders.

It also may have opened up the possibility of using its tube feet for moving around.

Modern starfishes move using their tube feet in a way that’s similar to what crinoids do to waft food into their mouths.

But we still need more fossil evidence to help us figure out whether Cantabrigiaster really was a transitional animal between crinoids and sea stars or not.

Either way, the Paleozoic Era continued to be dominated by the crinoids and other echinoderms.

And once they developed their multi-armed, face-down body plan, starfish had a foot in the door.

Or, an arm in the door?

The Paleozoic came to a dramatic end about 252 million years ago with a mass extinction event.

Research suggests that this dying off was caused by global warming and a serious decrease in the ability of ocean water to hold oxygen.

Echinoderms were hit hard and whole groups went extinct.

But starfishes and their skinny-armed cousins called the brittle stars made it through, along with a few crinoids, sea cucumbers, and sea urchins – the groups we still see today.

Paleontologists wonder whether the ability to move around might’ve helped starfishes fare better than the stationary crinoids did.

Beginning about 240 million years ago, and continuing through the Late Triassic and Jurassic, starfishes diversified rapidly to become among the most common echinoderms, second only to the brittle stars.

Today, starfishes and brittle stars are the most successful groups of echinoderms in terms of abundance, diversity, and geographic distribution.

Together they contain around 4000 known species.

And crinoids are still around, too, but their species number in the 600s, in contrast to the 6000 known species that’ve lived throughout Earth’s history.

So, as odd as the starfish body plan seems, something about it was successful -- from their ancient bilateral ancestors, to the radial crinoids with their upward-facing arms, to the predatory, downward-facing starfish.

And their arms play a whole range of roles – from feeding to sensing their environments, taking in oxygen, and housing some of their vital organs.

The evolutionary past of starfishes from bilateral to radial animals is also echoed in their development.

All modern echinoderms start their lives as swimming larvae with bilateral symmetry and then settle on a substrate to transform.

The left side grows into five parts, while the right side gets absorbed, and we end up with an adult radial echinoderm.

The story of how the starfish got its arms reminds us that even animals that might be familiar to us today can have incredibly deep histories - ones that stretch back almost half a billion years.