April 8. The amazing swimmer with a large penis

Brachiopods, you recall, are bivalved shelled animals – bivalve means they have two shells, like clams, but brachiopods are not clams. They are filter feeders, extracting food from the water using tentacle-like organs called lophophores. Brachiopods are still with us today, so we can project back into the fossil record using modern examples, since soft parts are seldom preserved in fossils.

Modern ostracod

But, in 2005, scientists led by Mark Sutton at Imperial College London reported on some Silurian brachiopods that did indeed have soft parts preserved. They come from the Herefordshire, England, lagerstätte – another remarkable assemblage of fossils. In this case, the fossils are encased in volcanic ash that fell into the shallow marine shelf where the animals lived. They were entombed so quickly, and by such fine-grained material, that soft organic parts did not decay. But the fossils are also so tightly entombed that you can’t get them out, not with a hammer, and not even by trying to carefully pick the non-fossil stuff away. What’s more, some of the most interesting fossils are almost microscopic. To get at the information in these rocks, scientists have made multiple thin slices of the rocks, scanned them, then did painstaking digital reconstructions. It’s like an MRI or cat scan, but one where every slice is a real physical slice of the fossiliferous rock.

The were able to identify the pedicle, the fleshy stem that attached brachiopods to the sea floor, as well as the tiny, soft lophophores.

So what about that amazing swimmer with a large penis? That’s Colymbosathon eplecticos, the scientific name that apparently means exactly that—amazing swimmer with a large penis. It’s an ostracod – a tiny crustacean, sometimes called a seed shrimp because it’s so small, typically a millimeter long. Ostracods generally have genital equipment that’s large compared to the animal’s size, and in some species, individual sperm can be six times the length of the entire animal – they keep them tightly coiled up until the mood is right. Well, I doubt if ostracods had moods, but you know what I mean. This Silurian specimen is the oldest penis of any animal ever found! And the discovery shows that ostracods have been remarkably successful – at least if you define success as surviving for hundreds of millions of years without much change.

The rocks these critters were found in are part of the Wenlock series that we discussed a couple days ago. See the links below to Sutton’s paper for some pretty cool photos of these things – remarkable because delicate soft parts have been preserved for 425 million years.

—Richard I. Gibson

Modern ostracod photo by Anna33 at en.wikipedia under Creative Commons license

References and further reading:
Ostracods and more from Herefordshire (photos)

Sutton et al.