December 7th

Box #7 in our count down is Ischyodus…

Days 1-5 of the count down.
Days 6-10 of the count down
Days 11-15 of the count down
Days 16-20 of the count down
Days 21-25 of the count down
Ischyodus, a Jurassic fish that is similar to modern day chimaeras.

Ischyodus: the “Power Tooth”

Ischyodus was a fish from late Jurassic Germany, but it has many species under its name that appear to range all the way from the middle Jurassic to the Miocene Period after most dinosaurs died out. In most cases this is because there are not enough fossils to tell two different genera apart, and so they are kept under the same name until more fossils are found. This is especially true for creatures that change very little over the years, and this fish is one of those examples.

The modern day chimaera is a fish that looks very similar to Ischyodus. They live in deep, cold waters where the sun doesn’t shine, and are also known as ghost sharks, rat fish, or rabbit fish. They are relatives to sharks and rays, and some say they are close enough to be a direct lineage or “living fossils” to Ischyodus. Others say that it’s not that simple, but they are close enough to get a good idea of what Ischyodus looked like and how it moved. Smooth, scaleless bodies, thin, rat-like tails, and delicate fins used to “fly” through the water.

Some may take this similarity to mean that Ischyodus must’ve been more comfortable in deep, dark water like modern chimaera, but Ischyodus fossils are known from many shallow sea formations and tropical lagoons and reefs (and even a couple of modern chimaera live at shallower depths). Perhaps Ischyodus did prefer the dark, and came up to shallower seas at night like many modern deep-sea dwellers. Or perhaps we only see the tiny fraction of what is left, and there were many that lived in colorful reefs.

It is a reminder to take any comparison with modern relatives with a grain of salt.

7 thoughts on “December 7th

  1. I first remember learning about Ischyodus in a book about prehistoric cartilaginous fish, and an analysis on the range for specimens that have been assigned this genus name makes things more realistic. And like you said, we should take comparisons with our modern world lightly so that things don’t end up too outdated.
    I will now go ahead and tell you today’s critter from my figurative advent calendar: Kerygmachela, a Cambrian lobopodian from Greenland with well-developed frontal appendages, which suggest it may have been predatory, although its size suggests it would be restricted to tiny prey. Its body consisted of eleven pairs of flaps of gill-like structure and in addition, eleven pairs of legs are located under these flaps. The body ended with a single tail spine. It additionally possessed a well-developed pharynx but its brain consisted only of protocerebrum with branched nerves extending only to parts of its head (including its frontal appendages).

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