The Caytoniales (Figs. 1-2) are an extinct order of seed plants known from

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s collected throughout the

Mesozoic Era The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

, around . They are regarded as seed ferns because they are

seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosper ...

-bearing plants with

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...

-like leaves. Although at one time considered angiosperms because of their berry-like cupules, that hypothesis was later disproven. Nevertheless, some authorities consider them likely ancestors or close relatives of angiosperms. The origin of angiosperms remains unclear, and they cannot be linked with any known seed plants groups with certainty.

History

The first fossils identified in this order were discovered in the

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively ...

Gristhorpe bed of the Cloughton Formation in Cayton Bay,

Yorkshire Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a Historic counties of England, historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other Eng ...

, with the name of the bay giving the name to the group. They have since been found in Mesozoic rocks all over world. It is likely that Caytoniales flourished in wetland areas, because they are often found with other moisture-loving plants such as

horsetails ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass ...

in waterlogged paleosols. The first fossil Caytoniales were preserved as compressions in

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

with excellent preservation of cuticles allowing study of cellular histology.

Description

The woody nature of associated stalks and preserved short shoots are evidence that Caytoniales were seasonally

deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, ...

, shrubs and trees. Caytoniales had fertile branches with seed-bearing cupules . The ovules were located inside fleshy cupules with tough outer

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

. Individual ovules had an apical tube called a micropylar canal, that allowed pollen to pass into the pollen chamber. The outer layers of the cupules were fleshy and fruit-like; it is possible this was to aid in animal dispersal. The cupules are 4-5mm in diameter and about 3 mm long (Fig 1-2), and resemble a blueberry. The extra protection of the reproductive organs gave rise to the idea that Caytoniales were predecessors to

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...

, which have completely enclosed seeds. The pollen grains were small, between 25 and 30 μm in diameter. The size of the pollen grains supports the idea that they were wind-pollinated, and their bisaccate wings may have enabled entry into the seed by a pollination drop mechanism. In both respects they were like pollen of pine trees. They were produced in pollen sacs in coalesced groups of four, attached to branching structures. The pollen sacs hang off the structure in clusters, and are typically 2 cm in length. The most common and widespread part found fossilized are leaves of ''

Sagenopteris ''Sagenopteris'' is a genus of extinct seed ferns from the Triassic to late Early Cretaceous. These are compound leaves consisting of, usually, 4 leaflets arrayed in a palmate manner. The individual leaflets are up to 6 cm in length. The leaflets have anastomosing veins, like those of some ferns, but lacking orders of venation found in angiosperm leaves.

Relationship to Angiosperms

''Caytonia'' was first described by Hamshaw Thomas in 1925. His close examination of the cupules led him to believe this was one of the earliest examples of angiosperms. He mistakenly thought the entire ovule was enclosed in the cupule, unlike typical gymnosperms. He worked meticulously, collecting and cleaning specimens to get the best understanding. He spent weeks boiling fruits in different solutions to try to make them resemble their living states. He proposed that the fruits contained a stigma with a funnel shaped opening in the center in which the pollen grains would get lodged. The entire pollen grain would not be able to enter into the ovule, a defining trait in angiosperms. This theory was disproved 1933 by Thomas' student Tom Harris, who studied the same reproductive organs and found different results. "Most of the fruits were obtained by dissolving in

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepres ...

a single very small fragment of shale collected from Cape Stewart" The maceration of the fruits dissolves the main cells wall and leave the cuticle and interior cell organs. This allowed Harris to look closely at the ovules located inside. Upon close inspection of the ovule, whole pollen grains were found inside the micropylar canal. This is typical of a gymnosperm reproduction, not an angiosperm. Presumably pollination was at an early stage of cupule and ovule development, before full inflation of the cupules. While Thomas's original idea led many scientist to believe that Caytoniales may have been angiosperms, Harris's further research disproved this theory. The enclosure of ovules in Caytoniales has nevertheless been considered an early stage in evolution of the angiosperm double integument, and the carpels formed from an elaboration of their stalk (Fig. 5). Other theories for the origin of angiosperms derive them from Glossopteridales (Fig.5), among other groups (see Evolutionary history of plants).

Gallery

File:Morphology of Caytoniales.jpg, Fig. 2 A) leaf structure B) Venation C) Pollen sacs D) Pollen grain E) Seed structure F) Cupule G) Cupule from side H) Ovule File:Sagenopteris phillipsi Natural History Museum v18596 Retallack 1980.jpg, Fig. 3 Fossil leaves of ''

Sagenopteris phillipsii ''Sagenopteris phillipsii'' are leaves of extinct species of seed ferns. Description ''Sagenopteris phillipsii'' has narrow, palmately arranged leaves with anastomosing venation Whole plant reconstructions Different organs attri ...

'' from the Gristhorpe Bed at Cayton Bay. Natural History Museum specimen photographed by G.J.Retallack File:Caytonia nathorsti Natural History Museum v18582 photo Retallack 1980.jpg, Fig. 4 Fossil ''

Caytonia nathorstii ''Caytonia nathorstii'' is an extinct species of seed ferns. Description ''Caytonia'' has berry like cupules with numerous small seeds arrayed along axes Whole plant reconstructions Different organs attributed to the same original p ...

'', the reproductive structure that encloses ovules. Natural History Museum specimen photographed by G.J. Retallack. File:Reproductive structures of Caytoniales and Glossopteris.jpg, Fig. 5 Diagram comparing ovulate structures in Caytoniales, Glossopteridales, and Angiospermae. The upper illustrations show Caytoniales as angiosperm ancestors, and the lower illustrations show glossopterid ancestors.

References

{{Taxonbar, from=Q148539 Pteridospermatophyta Triassic plants Jurassic plants Cretaceous plants Prehistoric plant orders Triassic first appearances Cretaceous extinctions Fossil taxa described in 1932 Monotypic plant orders