The Caytoniales (Figs. 1-2) are an extinct order of
seed plants
A spermatophyte (; ), also known as phanerogam (taxon Phanerogamae) or phaenogam (taxon Phaenogamae), is any plant that produces seeds, hence the alternative name seed plant. Spermatophytes are a subset of the embryophytes or land plants. They inc ...
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, around .
They are regarded as
seed fern
A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), 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, includ ...
s 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 angiospe ...
-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 Gristhorpe bed of the
Cloughton Formation in Cayton Bay,
Yorkshire
Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other English counties, functions have ...
, 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 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. 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
A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garde ...
.
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. '' (Fig. 3).
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 a single very small fragment of shale collected from
Cape Stewart
A cape is a clothing accessory or a sleeveless outer garment which drapes the wearer's back, arms, and chest, and connects at the neck.
History
Capes were common in medieval Europe, especially when combined with a hood in the chaperon. Th ...
"
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
The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats, through multicellular marine and freshwater green algae, terrestrial bryophytes, lycopods and ferns, to the complex gymnosperms and angiosperms ( ...
).
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 attrib ...
'' 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 fern
A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the pro ...
'', 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