Petriellales †

Form & Structure

Ecology & Form:

• Small, evergreen,  shade-adapted  perennial shrubs 

• Colonized the understory of the deciduous forest biome of polar Gondwana

• "This life form is very unlike that of any other known seed-plant group of that time. By contrast, it fits remarkably well into the “dark and disturbed” niche that some authors considered to have sheltered the rise of the flowering plants some 100 Myr later." (Bomfleur et al. 2014)

Stems:

• Small, less than 3mm wide

  • Thought to be less than 1m tall

• Eustele with bifacial cambium and growth rings & Pycnoxylic wood 

• Largest stems contain up to four growth rings, demonstrating that they were perennial and persisted over several growth seasons.

Leaves 

• Rochipteris: dichotomously-dissected leaves 

• Kannaskoppifolia: cuneate to flabellate foliage

  • Lack petiole and deeply divided into possibly dichotomous segments

• Evidence for leaves being evergreen

• Shade-adapted leaves with low photosynthetic capacities, including (1) an extremely thin lamina, (2) undifferentiated mesophyll (i.e., lacking a palisade layer), and (3) large intercellular air spaces (Givnish 1988; Smith et al. 1997)

• Petriellales were probably able to acclimate to unfavorable conditions by temporarily enrolling and shriveling the leaf laminae, also indicating a long leaf life span

Roots: Unknown

Reproductive Structures

• Possible similarities to angiosperm carpals

• Female cones (megastrobilus)

  • Petriellaea: bilateral, elongated cupules, which are on dichotomously branched axes

  • Cupules contain 2-6 small triangular ovules, which are on the adaxial surface of the cupule

  • Ovules are orthotropic; integument is thickened only at the edges

  • In Petriellaea triangulata, the nucellus and integument are fused and extended at the distal end to a short micropyle

• Male cones (microstrobilus): unknown

Possible connection to the evolution of angiosperms (Bomfleur et al. 2014)

• "Many authors have noted the similarity of petriellalean cupules to those of the Caytoniales, a group of gymnosperms that continues to figure prominently in theories about the mysterious origin of flowering plants (e.g., Thomas 1925; Crane 1985; Doyle 2006; Taylor and Taylor 2009). "

• "Recent hypotheses propose that the earliest angiosperms may have been small, woody shrubs that colonized disturbed sites in the damp understory of humid forests (Feild et al. 2004; Feild and Arens 2005, 2007; see Coiffard et al. 2012). "

• "The reconstructed physiology and ecology of the Petriellales matches this life form to such detail that we suggest these unusual gymnosperms may represent convergent ecological analogues of early flowering plants."