Sprecher
Beschreibung
Emsian fossil assemblages from Laurentia have yielded evidence for anti-herbivore defenses: secretory capitate glands and other glandular emergences (trichomes), spinescent emergences (spines, hereafter) with sclerified cortex and specialized morphology and wound response periderm for piercing-sucking and putative surface feeding wounds. Circumstantial evidence from the Emsian includes epidermis containing secondary metabolites and putative secretory cells in cortical tissues. Together, these demonstrate that by 400 Ma ago land plants had evolved diverse and sophisticated defenses – both mechanical and chemical – against herbivory and mechanisms for healing herbivore-induced wounds. This implies that herbivory had reached significant intensity and ubiquity before the Middle Devonian. In turn, this suggests that plant-animal interactions involving herbivory were initiated and diversified well before 400 myr ago. However, pre-Emsian direct evidence of herbivory is sparse. Rare coprolites with plant material and coeval animal mouthparts cannot be assigned unequivocally to herbivores vs detritivores. Thus, the pre-Emsian evolution of herbivory can only be inferred from indirect evidence. While spines may have served functions other than defense, their occurrence nevertheless provides the best approximation for the intensity of herbivory, in the absence of direct evidence. An exhaustive survey of spines and other putative defenses shows a steady rise of spine-bearing taxa after the Silurian and throughout the Early Devonian, as well as a rise in plants with chemical defenses and with stouter spines, suggesting that the incidence and intensity of herbivory rose quickly starting at the beginning of the Devonian, much earlier than previously thought or implied by direct evidence.