Mazon Creek Fossil Fauna
Shark Egg Cases
Within the taxonomic realm, Paleaeoxyris has had two lives: first within the flora, and now among the fauna. Its discovery some 175 years ago amidst plant material in the Mazon Creek concretions, coupled with its resemblance to a Weeping Willow or Ash leaf, prompted early researchers such as Leo Lesquereux (1880) to classify it as a plant. Similar specimens were placed in the genera Vetacapsula, Spiraxis, Fayolia, and Spirangium; species were then erected in these genera. Not all authorities agreed. By 1888, a paper stating these fossils to be "eggs of Elasmobranch fishes" had already been published. Opinions remained divided until the relationship between Palaeoxyris and modern forms of spiral shark egg capsules was demonstrated by Zidek (1976).