Sponges are in a generous mood these days, as far as exciting discoveries are concerned! First Otavia breaks the record for oldest known animal, and now Coronacollina (what a pretty name!) shows up with what looks like the oldest hard skeleton in the animal kingdom.
Hard skeletons* are a real success story in the history of life. From the tough organic support structures of trees to our own strong and versatile bones, they’ve revolutionised (or, in the case of trees, pretty much created) ecosystems. (We also owe them some gorgeous landscapes.) Skeletons really came into fashion during the Cambrian explosion, when incorporating minerals into shells, spikes and other hard parts became commonplace among animals. However, there are a few examples of animals with hard parts that are older, mainly from the very end of the Ediacaran period just before the dawn of the Cambrian. Our spiny new friend does one better than those, hailing from the heyday of Ediacaran creatures.
Coronacollina acula (Clites et al., 2012) is described as a smallish creature similar to the Cambrian sponge Choia. Its 300+ specimens were preserved as imprints that show every sign of having come from a fairly solid animal. The body is kind of cone-shaped with what appears to be threefold symmetry. Most intriguing are the traces of long, thin spikes that radiate from the main body of many specimens. There are up to four of them, fewer than Choia had, and they were clearly made of a hard material in life: the grooves they left are straight as arrows, narrow and sharply defined, unlike a trace left by a soft structure. Like the more numerous spikes of Choia, they may have acted as stabilisers/struts to keep the living sponge from being upended by waves.
(From my perspective, it’s a pity that only the imprints were preserved. I have an occupational interest in biomineralisation, so I’d really like to know what the spicules were originally made of. If Coronacollina is a relative of Choia, odds are they were either organic or, if they were mineralised at all, made of silica. Interestingly, the authors bet on some sort of mineral because the spicules broke so often, as though they were quite brittle. I would’ve thought that mineralised structures would leave more than imprints, but apparently the chances of silica or calcium carbonate skeletons being preserved in coarse sandstone aren’t that great. You learn something every day…)
Clites and colleagues consider the creature important for two reasons: first, because it is the oldest known example of an animal with a hard skeleton. The shadows of its long thin spikes in the rocks foreshadow, so to speak, of the age of skeletons that came with the Cambrian. Second, finding an Ediacaran animal that can be related to something outside its own weird contemporaries is always worth a little celebration! 😉
*The word “skeleton” is used in a very loose sense here. It includes any hardened structure that gives support and/or protection to some part of an organism. Bones, shells, armour plates, teeth, perhaps even the protein meshwork that gives bath sponges shape, can belong here.
Clites E. et al. (2012) The advent of hard-part structural support among the Ediacara biota: Ediacaran harbinger of a Cambrian mode of body construction. Geology advance online publication (doi: 10.1130/G32828.1)