Echinoderms are weird. They are supposed to be bilaterian animals, but they have abandoned bilateral (mirror image) symmetry for looking like fleshy stars, spiny boobs, strange flowers or funky sausages. When they first appear in the fossil record during the Cambrian period*, they show up with an even weirder menagerie of body plans ranging from almost bilateral through asymmetric to all sorts of variations and twists on the standard five-rayed body plan that we know and love. (Below: a selection of weird and wonderful Cambrian echinoderms from Zamora et al. )
(We only know that some of these creatures were echinoderms or very close relatives thereof because they have skeletons with a unique spongy microstructure (stereom) only seen in echinoderms.)
I don’t know nearly enough about echinoderms to properly discuss the latest addition to the march of the weirdos, but damn me if I don’t at least give a proper fangirlish SQUEEE! to a new Cambrian echinoderm – with bilateral symmetry! Zamora et al. (2012) actually describe two fossil finds, but one of them is new specimens of a previously known animal. However, the other is brand new, and what a pretty thing, too! Behold Ctenoimbricata spinosa, straight out of science fiction – or a nightmare :-P! (OK, don’t start having Ctenoimbricata nightmares just yet. The whole animal was less than an inch long.)
The creature was reconstructed from fossils found in Middle Cambrian (about 510 million years old) rocks in northern Spain. The shape of its body and the arrangement of its many armour plates most closely resemble an obscure group of ancient echinoderms called ctenocystoids (represented by fossil A in the first picture). Typical ctenocystoids have slight asymmetries manifested as different arrangements of armour plates on their left and right sides. However, some are well-behaved bilaterians. That’s the other point of the paper: new fossils belonging to a previously known ctenocystoid demonstrate its symmetry. The authors think that the similarly symmetrical Ctenoimbricata was an even more primitive relative of ctenocystoids. In their view, echinoderms started out with mirror image symmetry, then became asymmetric, and only then did they evolve the radial symmetry starfish exemplify.
Ctenoimbricata, according to Zamora et al., is the most primitive echinoderm ever found. The fact that it doesn’t have a stalk or arms suggests that it wasn’t a filter feeder. Instead, it probably operated flat on the seafloor, gulping sediment and sifting out the edible bits. Since ctenocystoids are also stalk- and armless, this might mean that the last common ancestor of all echinoderms lived in a similar way, which has apparently been a matter of some debate. Yay!
Incidentally, I had no idea that Europe had such awesome Cambrian fossils. I thought the best sites were all at a minimum of a half-day plane ride away. So: double squee for our tiny spiny sandmower!
*People have argued that a Precambrian fossil called Arkarua may be an echinoderm ancestor, but I wouldn’t bet on that. Just about the only thing those tiny imprints can be shown to share with echinoderms is the five-part symmetry, and it’s not like unusual body symmetries were… unusual for Precambrian animals.
Zamora S et al. (2012) Plated Cambrian bilaterians reveal the earliest stages of echinoderm evolution. PLoS ONE 7: e38296