Starfishes are a familiar part of any day spent at the seaside. With their five fleshy 'arms', they are as much a part of the tide-pool panorama as crabs and mussels. Less familiar are the brittle-stars, with their spiny, snake-like arms.

Brittle-stars (or ophiuroids) are quite different from regular starfishes (technically, asteroids). The fleshy arms of asteroids are often more-or-less continuous with the central part of the body, and are relatively immobile. Asteroids don't move their arms to get around. Instead, they use thousands of tiny, extensible 'tube feet', borne on the undersurfaces of the arms, to carry the whole animal along. The arms of ophiuroids make a complete contrast. Rather than plump and fleshy, they are thin and almost solid, joined to a discrete, disc-like body and supported on an internal 'backbone' of calcite blocks, articulated just like the vertebrae in our backbone. The arm of an ophiuroid can coil like a snake: armed with five of these, the brittle-star is a surprisingly lithe and mobile mover.

So far, so good – but has this distinction always been clear? In the June issue of the Zoological Journal of the Linnean Society, Juliette Dean of the University of Cambridge and the Natural History Museum, London, UK, looks at the problems of telling asteroids and ophiuroids apart from their fossils. Both kinds of starfishes have a rich fossil record going back more than 500 million years – but starfishes back then need not have been anything like starfishes now. One fossil in particular – an 450-million-year-old, coin-sized creature called Stenaster – has the general outline of an asteroid, but looks much more like an ophiuroid when examined close-up. How, then, do you classify Stenaster? Is it an ophiuroid, an asteroid, or a member of some completely different and extinct starfish group?

Dean confronts a problem familiar to that of all palaeontologists. The range of animal form we see around us nowadays is likely to be only a small fraction of that which once existed. This means that when we confront the forms of the past, we are forced to interpret them in the light of what we know in the present day. As far as fossil starfishes are concerned, we will look at the fossils and see things that look like asteroids; things that look like ophiuroids; and things we can't really make sense of in the light of the residents of a modern tide pool.

Stenaster looks like a mixture of asteroid and ophiuroid, so it falls into the last category. Dean adopts the view that ophiuroids and asteroids, while different, shared a common ancestor, deep in the mists of time. This assumption – of common ancestry – allows one to frame very specific questions about Stenaster's place in starfish evolution.

For example, says Dean, Stenaster could represent a stage in evolution before ophiuroids and asteroids became distinct groups. On the other hand, it could be a primitive ophiuroid that had not shed general ancestral features reminiscent of asteroids; finally, it could be a relatively advanced ophiuroid that had come to resemble asteroids by virtue of a similar lifestyle. Researchers have, at one time or another, come to all three conclusions.

Dean resolves this issue with a comprehensive re-analysis of Stenaster fossils, and by close comparisons of this creature with other fossil starfishes – those known to be asteroids; those known to be ophiuroids, and those suspected of being offshoots from the common starfish stock, and so are neither one thing nor the other. Her answer is clear – Stenaster is an ophiuroid. But why the thick, asteroid-like arms? Dean thinks that Stenaster represents a strain in ophiuroid evolution in which some ophiuroids adopted an asteroid-like habit, moving over surfaces with tube feet, rather than by flexing and coiling their arms. It can only ever be informed speculation, because we are, as yet, unable to travel back 450 million years and watch the living animals doing their thing – and, of course, no living ophiuroid behaves in a way sufficiently similar that it can be used as a 'model' of what Stenaster might have got up to.