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Caytoniales †

Extinct seed fern with possible affinities to angiosperms

The Caytoniales are an interesting group of highly derived seed ferns that may have important connections to the origins of flowering plants. These plants were thought to be small plants or small trees, growing in periodically, waterlogged habitats. Their leaves are palmately compound, and angiosperm-like. They have fertile branch systems exhibiting cupules containing several seeds. The cupule may have similarities to the angiosperm carpel, but more evidence is needed to assign a link between the angiosperms and the Caytoniales.

Ecology & Form

Stems

Stems are not commonly found

Leaves

Sagenopteris †

Presl, 1838
Found in South and North America, Greenland, and Europe
Palmately compound with 4-6 leaflets
Lanceolate with entire margin
Reticulate venation, but lacking orders of venation found in angiosperm leaves
Stomata are haplocheilic and abaxial only

S. acuminata [lectotype]

S. colpodes

S. hallei

S. phillipsii

S. pualensis

Reproductive Structures

Caytonia †

Thomas, 1925
Fertile branch system with seed-bearing cupules

- Planated, bilaterally symmetrical branch system bearing lateral opposite cupules
- Interpreted as a megasporophyll by Thomas (1925)
- Cupules round and recurved, with a lip-like projection near the point of attachment
- Several, orthotropous ovules per cupule
  - Ovules possess a single integument (unitegmic); they are flattened and bilateral (=platyspermic)
  - Micropyle faces the cupule opening

C. oncodes

C. orientalis

C. senomanica

C. sewardii

C. tuberculata

Caytonanthus †

Harris, 1937
Found in Greenland, UK, Hungary, Russia, Poland, India, Antarctica, and Argentina
Pollen organ

- Slender axis bears flattened pinnate lateral branches
  - Each branch bears one to three elongate synangia
- Pollen is Vitreisporites-type

Geologic Range

Middle Triassic (Anisian) - Late Cretaceous (Campanian) [Xu et al., 2024]

Classification

└Lignophytes

└Caytoniales †

Relationship to Angiosperms?

- Caytoniales may be the sister group to the angiosperms (Hilton & Bateman, 2006)
- Leaves are angiosperm-like
- The cupule would form a second integument of the seeds, and other tissues would create the carpel.
- These scenarios suggest that closure of the fertile leaf (megasporophyll) is longitudinal, which is supported by the conduplicate folding in certain extant flowering plants and even in some Early Cretaceous angiosperms.
- Specimens from China (Paracaytonia †) show a spiral arrangement of cupules along the reproductive axis, suggesting that the cupule‐bearing organ in Caytoniales is not a megasporophyll, but a branch or stem. (Wang, 2010)
  - This causes problems for their status as an angiosperm ancestor, in which the angiosperm carpel is derived from a leaf (not a stem).

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