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 carpal, but more evidence is needed to assign a link between the angiosperms and the Caytoniales.

Ecology & Form

Stems

  • Stems not commonly found

Leaves

  • Palmately compound (Sagenopteris)

    • Four to six leaflets

  • Lanceolate with entire margin

  • Reticulate venation, but lacking orders of venation found in angiosperm leaves

  • Stomata are haplocheilic and abaxial only

Reproductive Structures

  • Fertile branch system with seed-bearing cupules (Caytonia)

    • Planated, bilaterally symmetrical branch system bearing lateral opposite cupules

      • Interpreted as a megasporophyll by Thomas (1925)

    • Cupules round and recurved, with lip-like projection near the point of attachment

    • Several, orthrotropous ovules per cupule

      • Ovules possess a single integument (unitegmic); they are flattened and bilateral (=platyspermic)

      • Micropyle faces the cupule opening

  • Pollen organ (Caytonanthus)

    • Slender axis bears flattened pinnate lateral branches

      • Each branch bears one to three elongate synangia

    • Pollen is Vitreisporites-type

Classification

└Embryophytes

└Polysporangiophytes

└Tracheophytes

└Euphyllophyte

└Lignophytes

└Spermatophytes

└Caytoniales †

Diversity

  • Caytonanthus

  • Caytonia

  • Sagenopteris

Caytoniales may be the sister group to the angiosperms (Hilton and 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 carpal is derived from a leaf (not stem).