The enigmatic Esconichthys apopyris lived during the late Carboniferous, around 308 million years ago, in a lush tropical estuary covering what is now Illinois, USA.
Up to about 8cm long (~3"), it had a prominent pair of eyes, two pairs of elongated external gills bearing long feathery projections, a slender limbless body lined with muscle segments, and a single low fin running along the underside of its tail.
There also seem to have been two different body types that might represent separate species: the "flathead" form with wide-set eyes, and the "snubnose" form with close-set eyes.
Often nicknamed "blades", "ghosts", or "grasshoppers" by fossil collectors, specimens of this little animal are the most common vertebrates found in the Mazon Creek fossil beds — and yet we don't actually know what it is. In the past it was proposed to be a larval lungfish or amphibian, but its anatomy doesn't quite fit any known group.
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