Last Friday we took a look at a couple of patterned fossils. I was hoping to trick you into thinking they were plant specimens, maybe some kind of tree bark, but the location tipped you off. These specimens are, of course, aquatic, like most of those collected in the quarries in eastern Iowa.
What we were looking at last Friday was basically fish skin—the armored plates of some Homosteus species (you can see a tiny fragment of a Bothryolepsis fossil as well in the image above). The genus Homosteus is one of those found in the flattened arthrodire placoderms. These ancient armored fish are thought to have been predators with upward-facing eyes who lived on the sea bottom, searching for prey above. These fish have a helpful adaptation for predation—jaws! Some of the placoderms even had true teeth. Jaws and teeth are both features that were just beginning to emerge among vertebrates at this time. Placoderm fossils also provide us with the earliest known evidence of a viviparous lifecycle. One was found having died in the act of giving birth to a single live young, with the umbilical cord intact!
But back to our particular placoderm. Although the fossils we’re looking at today are fairly small, it’s worth noting that the Homosteus, as well as other placoderm genera, could be enormous. The armor plates of a Homosteus head alone can be over a meter in length. The placoderms were not just predators, but potentially super predators—some of the first vertebrate predators to prey upon other predatory vertebrates. The beginning of a grand tradition that continues today.
The winner this week? Amy Smith Ascoli! Congratulations, and thanks for playing! This fossil was shared with us by the University of Iowa Palentology Repository. If you have a fossil you want to share, send your pictures to me at schoerning at ncse.com.