Age: 303-306 million years ago, Carboniferous Period
A sea urchin can’t smile. A sea urchin’s mouth looks nothing like a human mouth. When different groups of animals end up with toothy mouths, there is no rule that says that those mouths have to look identical.
A sea urchin’s mouth is normally hidden inside its shell. In this fossilized specimen of Archaeocidaris, the shell fell apart and exposed the star-shaped top of the mouth (highlighted in blue) and the top of a single tooth (highlighted in green).
Sea urchins have five individual teeth arranged in a circle instead of two rows of teeth like most four-legged animals. The sharp ends of all of the teeth meet at the bottom of the mouth. An organ made of multiple mineralized plates and functionally hydraulic tubes moves the teeth. The entire apparatus is called an Aristotle’s lantern.
The mouth in Archaeocidaris is not identical to the mouth in modern sea urchins. Newer generations of mouths in most modern species of sea urchins are taller, more complex, and made of more moving parts.
The mouth is also unrelated to the bony jaw of most other animals with teeth. As different populations of animals have changed through time, different groups independently ended up with their own ways of chewing food.
Specimen Number: TMM 1967TX60
Sources:
Schneider, Chris L., James Sprinkle, and Dan Ryder. “Pennsylvanian (Late Carboniferous) Echinoids from the Winchell Formation, North-Central Texas, USA.” Journal of Paleontology 79(2005):745-762.
Smith, Andrew. Echinoid Palaeobiology. London: George Allen & Unwin, 1984.
