First unequivocal record of the hybodont shark egg capsule Palaeoxyris in the Mesozoic of North America

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The hybodont shark egg capsule Palaeoxyris humblei n. sp. is described here from four specimens collected from flood plain deposits in the Blue Mesa Member of the Chinle Formation of Late Triassic (Norian) age in Petrified Forest National Park, Arizona. This find is the first unequivocal record of Palaeoxyris in the Mesozoic of North America. The species differs from previously known taxa, especially other Triassic forms in size and banding. It also is one of the smallest species assigned to the genus. This find offers the rare opportunity of plausibly assigning these egg capsules to the hybodont nonmarine shark Lonchidion humblei as the most probable source since its disarticulated remains (teeth, fin spines) have been found at a closely adjacent locality. Originally, these eggs were deposited in special spawning grounds in the river that deposited the Newspaper Sandstone Bed. There they became attached by tendrils to a piece of drift wood or perhaps the stem or branches of a horsetail such as Neocalamites or Equisetites that grew in the shallow waters of the stream. However, the stream overflowed its banks and the egg capsules together with masses of plant debris and various invertebrates were washed onto the adjacent floodplain where they were buried and eventually fossilized.
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