Once upon a time there lived a little crustacean inside a little shell. This is not a usual state of affairs for a crustacean. Most are clad in figure-hugging armor (like lobsters or crabs), but they don't live inside clam-like shells. This one was different. It had both armor and a hinged shell. Inside her cozy little fortress, she lived together with her children. There were nearly a dozen, some hatched, some not. She had long front legs, which she may have used for hauling around her nursery/mobile home. Together, they lived in the deep center of a quiet, soft-bottomed marine basin, near some nice trilobites called Triarthrus. Their days were all numbered.
One day, a great choking cloud of sediment plunged into the basin. It was a turbidity current -- an underwater landslide. It swept swiftly down like an avalanche, suffocating and burying everything in its path.
It wasn't a very happy ending for our heroine, an ostracod crustacean. But it was a very happy ending for paleontologists. The inhabitants of this underwater canyon were all to become part of the glittering Beecher's Trilobite Bed in Oneida County, New York. The animals entombed there, thanks to their rapid burial, empressment under heavy sediment, and some lucky chemistry, were exquisitely preserved in iron sulfide. You may remember this mineral from Geology 101 as as pyrite, or perhaps by its more famous name: fool's gold.
Fool's gold is the pale, pedestrian gold impersonator cursed by many a naive prospector. Though the shiny mineral ordinarily has little value, against a drab, gray mudstone background, this pyrite preserved treasures beyond price. The golden fossils had by chance recorded the tiniest details of the trilobites' soft parts, including antennae, legs, gills, and muscles. It also preserved the beautiful details of our little ostracod and her family, frozen in time together till the end of the Earth.
Fortunately for us and them, her relations survived for another 450 million years -- the majority of the time during which large multicellular life has existed on our planet -- through mass extinctions, asteroid impacts, glaciations, and eras where the Earth went berserk with volcanism, and they are with us still today. Here are a few showing off those long, segmented legs that you can see in the fossil above (note both videos below have sound):
And here is another walking around and feeding in HD. Note the knobby, painted shell and the graceful, brush-like antennae covered in hairs called setae (see'-tee):
Today ostracods are abundant wherever water is found. As you saw, they have little shrimp-like bodies without clearly defined segments sandwiched between their bivalved shell, an appearance that has also inspired the name "seed shrimp". The shell hinges on their backs. Underneath, they may or may not have jointed legs for scuttling or swimming.
Though they're abundant, they're still virtually unknown to humans, which may be because the whole organism is usually no bigger than a few millimeters long. In spite of their ubiquity both now and for the last half billion years, they don't even rate a mention in my DK Big Book of Ocean, or even in their beautiful, fossil-filled Prehistoric Life, which is rather shocking considering that ostracod shells are "by far the most common arthropods in the fossil record." I guess size does matter.
But the squishy bits inside those countless ostracod shells rarely survived the fossilization process. In this case, by a miracle of geology, they did. The fossil at the top of this page and three others like it from Beecher's were studied by a team of scientists from the United Kingdom, Japan, and the United States and their findings were published in Current Biology this month. They named the new species Luprisca incuba.
According to this team, the little fossils -- just 1-2 mm long -- are the only known invertebrate fossils that seem to indicate simultaneous brooding of eggs, embryos, and young. The fossils also indicate that this habit, maintained by some ostracods today, has persisted for at least 450 million years. Most living ostracods lay their eggs outside their bodies, although a few living groups, including the one our fossil ostracod is believed to have belonged to, brood their embryos inside. The number and size of L. incuba's brood is similar to its modern descendants, and the overlap of eggs, embryos and the recently hatched is also known today.
The pyrite also preserves a few other tiny, stunning details of anatomy. Remember those elegant, bristly antennae and that nubbly shell from the film? Well, 450 million years ago, it was thus. At left are the antennae and setae (F) and the shell texture (G) from L. incuba.
Incredibly, the paper describing these new fossils notes that the arrangement of some of the structures on the first antenna is the same as that of the group of living ostracods they are hypothesized to have belonged to, "even down to the tiny ventral seta and two long distal setae." That such details could remain unchanged over 450 million years is, in my opinion, mind-boggling.
However, not all details have remained unchanged. The external shell of L. incuba has none of the characteristic features of the living ostracod group it is believed to belong to, the Myodocopida. Even with organisms I rightly believe can be called "living fossils", some things change, some remain the same.
We are beyond lucky to have fossils of such exquisite detail that we can make such pronouncements on the details of delicate shells and microscopically fine antennae. Had any of us to have been around 450 million years ago to speculate on the fate of our little be-shelled ostracod and her brood, I'm pretty sure that no one would have put money on our still being able to see her and her offspring today, much less count the hairs on her one of her compatriot's antennae. And yet ...
there she is.
Siveter D., Tanaka G., Farrell ., Martin M., Siveter D. & Briggs D.G. (2014). Exceptionally Preserved 450-Million-Year-Old Ordovician Ostracods with Brood Care, Current Biology, DOI: 10.1016/j.cub.2014.02.040