March 15. Ostracoderms

The first critters that you’d recognize as primitive fish appeared during the Ordovician. They were armored with bony plates, and their general informal name, ostracoderms, means shell-skinned. For the earliest varieties, we only know them from fossils of these individual scaly plates. They didn’t have a rigid internal skeleton, so they generally fell apart when the animal died.

The Ordovician ostracoderms, thelodonts, which means “nipple teeth,” didn’t have jaws, but they did have scales much like fish today – but the scales were tiny, only a millimeter or two long. They appeared probably during the Middle Ordovician, around 470 million years ago or a bit earlier. Since they were without jaws, and their mouths were on the bottom of the head, we think that these early fish were probably sediment bottom feeders, sucking stuff into their mouths and filtering out food.

I talked about conodonts on March 3, and indicated that once we finally found the conodont animal, it was seen to be a small, eel-like animal. Eels are fish, and the conodont animal was indeed a primitive fish. But ostracoderms were the first ones that really had a fishy look to them. And they were widespread – they’ve been found in Ordovician rocks all over the world.

Neil Shubin, in Your Inner Fish, a book I’ve recommended previously, tells us that the bony plates on ostracoderms’ heads were made of material – calcium phosphate, the mineral apatite – and have structures that are essentially teeth – teeth fused together and on the outside of the animal, but teeth nonetheless, in evolutionary terms. So, Shubin argues, the first hard parts in chordates, the group that includes us and the other vertebrates, were teeth in conodonts, the better to eat you with, and the second hard parts were teeth that evolved into armor – protection from those other gnashing tooth-filled mouths. It’s really a cool story that hangs together quite well, and if you’re interested in this sort of thing, I recommend – again – Shubin’s book, Your Inner Fish.

The entire body of ostracodems was covered in scales, like modern fish, but the head area was more strongly armored by the fused-together plates into a more bony shield. 

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Today, March 15, is the birthday of Wallace Pratt, born in 1885 in Phillipsburg, Kansas. Pratt was a pioneer in petroleum exploration geology. In 1918 he became the first geologist hired by Humble Oil & Refining – a company that would eventually evolve into the giant corporation we know as Exxon today. One of his major contributions was fostering the use of geophysical instruments in oil exploration, and he was also a founding member of the American Association of Petroleum Geologists. He donated 23 square miles of land in West Texas, where he had a ranch, to the National Park Service, forming the core of what today is Guadalupe Mountains National Park. He died in 1981.

—Richard I. Gibson

Drawing of reconstructed ostracoderms by Philippe Janvier under CC-by-A license. The black and white drawing is from an old textbook.