Corystosperms are a group of extinctseed plants (often referred to as "seed ferns") belonging to the familyCorystospermaceae (also calledUmkomasiaceae) assigned to the orderCorystospermales[1] orUmkomasiales.[2][3] They were first described based on fossils collected by Hamshaw Thomas from the Burnera Waterfall locality near the Umkomaas River of South Africa.[4] Corystosperms are typified by a group of plants that bore forkedDicroidium leaves,Umkomasia cupulate ovulate structures andPteruchus pollen organs, which grew as trees that were widespread overGondwana during the Middle and LateTriassic. Other fossil Mesozoic seed plants with similar leaf and/or reproductive structures have also sometimes been included within the "corystosperm" conceptsensu lato, such as the "doyleoids" from the Early Cretaceous of North America and Asia.[3] A potential corystospermsensu lato, the leaf genusKomlopteris, is known from theEocene of Tasmania, around 53-50 million years old, over 10 million years after theCretaceous–Paleogene extinction event.[5]
Classic Umkomasiaceae have helmet-likecupules aroundovules born in complex large branching structures (Umkomasia). The pollen organ (Pteruchus) has numerous cigar-shaped pollen sacs hanging from epaulette-like blades, again in complex branching structures.
The architecture and size ofDicroidium leaves is highly variable, going from simple totripinnate, to needle-like to lobed ordissected leaflets, however, all leaves are united by being bifurcated at the base.[6]
TheDicroidium plant (which boreDicroidium leaves,Umkomasia seed bearing structures andPteruchus pollen organs) is thought to have grown as largetrees, with trunks at least 10 metres (33 ft) tall and over 50 centimetres (20 in) wide.[7] Some other possible corystosperms likePachypteris may have grown asshrubs.[8]
The oldest fossils of corystosperms, belonging toDicroidium, Pteruchus andUmkomasia date to the Late Permian in the low-latitudes of easternGondwana, including theUmm Irna Formation of Jordan, as well as the Indian subcontinent,[9][10][11] though possible pollen belonging to the group is also known from the Late Permian of South Africa.[12] During the Middle-Late Triassic,Dicroidium bearing corystosperms were widespread and dominant trees over temperate areas of southern Gondwana (including the Indian subcontinent, South America, Southern Africa, Antarctica, Australia and New Zealand) including wetland and forest environments, where they formed part of thecanopy vegetation.[6][9]Dicroidium-bearing corystosperm ecosystems collapsed as a result of theend-Triassic extinction event. Late survivingDicroidium-bearing corystosperms are known from theEarly Jurassic (Sinemurian) of East Antarctica.[6] During the Jurassic, the possible corystospermsPachypteris andKomlopteris were widespread over both hemispheres,[13] withKomlopteris surviving in the Southern Hemisphere through the Cretaceous and into the earlyEocene in Tasmania, making it the last surviving "seed fern" in the fossil record.[5][13] A group of plants with corystosperm-like reproductive structures, either considered corystosperms or as belonging to the separate order Doyleales, were present in North America and Asia during the Early Cretaceous,[3] with records of the group possibly extending back as far as the Early Jurassic.[14]
The relationships of corystosperms to other seed plants are contentious.[15] It has been suggested that some "peltasperms" may be closely related to corystosperms.[16] A 2016 paper suggested that they may be closely related toGinkgoales,[17] while a 2021 study suggested that based on the structure of their ovulate organs, corystosperms might form part of the "Anthophyte" clade more closely related toflowering plants than to livinggymnosperms, alongside thePetriellales,Caytoniales, andglossopterids.[18]
Corystospermaceae is a family name which was not derived from a specific genus, an arrangement which is discouraged by theICN.[19] In light of this issue, Petriella (1981) proposed two solutions: either retain Corystospermaceae as anomen conservandum, or replace it with a new name, Umkomasiaceae.[20] Both names have been used frequently since then. Petriella additionally named a new order to encompass the family, Corystospermales.[20] Doweld (2001) preferred a different name for the order, Umkomasiales. He also added a second family, Angaropeltidaceae (previously known as Cardiolepidaceae).[2] Later sources generally classify Angaropeltidaceae (or its updated spelling,Angaropeltaceae)[21] within Peltaspermales instead.[22][23]
