March 18. The oldest starfish

A couple weeks ago we talked about cystoids, an extinct class of the echinoderms. Today let’s focus on some echinoderms that you’ll be more familiar with: starfish. The problem with studying ancient starfish is that they tend to fall apart – they don’t fossilize well. Professor Tony Martin’s blog, which I linked in the post on trace fossils on March 11, has a photo of a great trace fossil, the resting mark of an Ordovician starfish, so we know they were around at least that long ago.

Stenaster huxleyi, from the Ordovician of Newfoundland,
drawn by Elkanah Billings in 1865.
Animal is about 4” across.

I checked quite a bit and so far as I can tell, there are no known Cambrian starfish. So the echinoderms form another group, a phylum, that was established during the Cambrian explosion but diversified greatly during the Ordovician. Starfish were part of that diversification. Starfish were described from the Tremadocian, the lower Ordovician, by W.K. Spencer in 1951, published in the transactions of the British Royal Society, and I think no older starfish have been found so far.

It appears to me that specialists in echinoderms really don’t know the ancestry of starfish. Maybe it was a soft-bodied critter originally, and evolved to produce a skeleton of sorts during the Ordovician, but it doesn’t seem clear to me that we know what starfish evolved from. Perhaps it was a quick evolution, something like the development of the trilobite eye, which seems to be almost instantaneous in the fossil record.

However they came about, starfish have survived for almost 500 million years. They’ve changed some – the Silurian Period, which we’ll take on next month, saw the development of starfish with more than five arms – and those multi-armed sea stars survive to this day as well.

Will they continue to survive? Who knows? Last fall, 2013, the popular science press was full of a mystery story of millions of starfish dying. Along parts of the west coast of North America, as much as 95% of the starfish population has vanished. It appears to be some disease that makes a starfish turn to goo – quoting one news story. It affects a dozen different species – that’s unusual – and as of a month ago, February 2014, scientists were still trying to figure it out. It’s a big deal, because starfish are the main thing keeping opportunistic organisms like mussels in check. Stay tuned. And for the record, while radiation from Japan’s Fukushima nuclear plant hasn’t been ruled out entirely as a cause, it’s considered to be quite unlikely. See the links below.

—Richard I. Gibson

Starfish deaths 
Starfish epidemic 
Oldest multi-armed starfish (and lots more)