''Palaeoxyris'' is a

morphogenus Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of sc ...

of eggs cases, widely thought to have been produced by

hybodonts Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and Batoide ...

, with a predominant occurrence in ancient freshwater environments. They comprise a beak, a body and a pedicle. They display a conspicuous right-handed spiral of collarettes around the body, and in some cases, the pedicle, resulting in a rhomboidal pattern when flattened during fossilisation. At the end of the beak was a tendril which attached the egg to vegetation during development. The body of the egg ranges in length from , depending on the species. Originally described as plant remains (being named after the plant genus ''

Xyris ''Xyris'' is a genus of flowering plants, the yelloweyed grasses, in the yellow-eyed-grass family. The genus counts over 250 species, widespread over much of the world, with the center of distribution in the Guianas. The leaves are mostly dis ...

''), it took decades before their true nature as animal eggs was revealed. Nearly 30 different ''Palaeoxyris'' species are currently known with a stratigraphic range from the Carboniferous to the Cretaceous. Mesozoic ''Palaeoxyris'' specimens differ from Paleozoic specimens by having the twisting of the collarette end at the beginning of the pedicle, rather than the spiraling continuing down the pedicle as in Paleozoic specimens. Likely hybodont producers of some of the eggs assigned to the genus include the genus ''

Lonchidion ''Lonchidion'' is a genus of extinct hybodont in the family Lonchidiidae. The genus first appears in the fossil record during the Middle Triassic (Ladinian) and was among the last surviving hybodont genera, with its youngest known fossils dating ...

''.

References

Vertebrate trace fossils Egg fossils Carboniferous sharks Permian sharks Triassic sharks Jurassic sharks Cretaceous sharks Madygen Formation {{paleo-cartilaginous-fish-stub