Monoplacophorans

Earth’s oceans today are defined by the disasters and exigencies of the past. When you dip a net in a shallow tropical sea it does not emerge from the waves seething with conodonts…because they died out completely during the Triassic. You could fish from the beach every night from now until the sun burns out and never catch another belemnite nor see an Archelon drag her 5 meter carapace from the sea to lay her eggs. Past disasters (and the constant ebb and flow of evolution) have removed some of the core cast from the great drama. Yet the oceans are vast: sometimes we find that an organism known only from fossils and presumed long lost has been swimming around the Comoro Islands or living in an ancient grove in Hubei. Today’s post involves a “living fossil” of this sort, but this creature was presumed lost for longer than the lobe-fin fishes or the purple frog.

This is a fossil monoplacophoran, a strange ancient superclass of single shelled mollusks which thrived in the ancient oceans of the Palaeozoic (or earlier) but then was known only through fossils. I can understand if you are shrugging about some primitive snail/limpet thing–but, my friend this is no gastropod–it is an entirely different class of mollusk which was presumed to have died out 380 million years ago. A look at the (long and complicated) taxonomy of monoplacophorans on Wikipedia is like looking at a World War I cemetery (extinct taxa are noted with a funereal superscript cross).

Monoplacophoran Diagram

Yet, scientists came to discover that not every name on the list had a cross. The monoplacophorans never fully died out. They just moved to the bottom of the oceans and stayed there for the long ages as continents drifted across the world and dinosaurs came and went. As mammals scurried out of burrows and across the world, the monoplacophorans lived their ascetic lives upon the floor of the ocean. They are still there right now, as you read these words! If you look at a picture of the colorless gray ocean bottom, you will see colorless gray ovals–the monoplacophorans (their very name makes them sound like some implacable cthulu-ish monk)

It is funny to me that ancient fossils in 400 million year old rocks were more accessible to scientists than the bottom of the ocean up until about the time I was born. Yet, since then, the bottom of the ocean has become closer as humankind’s ever-grasping arms have become longer. Lately our robot probes have reported a bit of summery warmth at the cold ocean bottom. And mining cartels are eagerly pushing to vacuum of nodules of precious ore upon the distant seabed. I truly wonder if we could look 380 million years into the future whether we will still find these tough little eremites still going about their business in the crushing depths? Or will the field of taxonomical crosses finally be complete, with these ultimate living fossils turning into yet another victim of our insatiable appetite?

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