Abstract Detail

Paleobotany

Lalica, Madison [1], Tomescu, Alexandru [1].

Wound-response periderm in an Early Devonian euphyllophyte from the Battery Point Formation (Gaspé Peninsula, Quebec, Canada).

The Battery Point Formation, exposed on the Gaspé Peninsula, hosts a rich Early Devonian (Emsian, c. 400 Ma) flora. Fossils in this unit represent one of the rare occurrences of anatomically-preserved Early Devonian plants and rival in diversity the classic permineralized flora of the Rhynie chert. Among the anatomically-preserved plants, a previously unrecognized type of euphyllophyte is represented by a single, thin axis 2mm thick. The central xylem strand, 240µm thick, has tracheids up to 32µm wide displaying Psilophyton-type or irregular scalariform wall thickenings. Protoxylem or consistent patterns of xylem maturation cannot be identified unequivocally. Surrounding the xylem, a layer of phloem 50µm across has long narrow cells with tapered ends. Outside the phloem, the incompletely preserved inner cortex up to 1mm wide contains aggregates that may represent specialized contents of decayed cells, forming concentric zones: globose aggregates in the innermost zone; larger elongated aggregates around the inner zone; and wedge-shaped structures in a zone beneath the outer cortex. The outer cortex, up to 315µm thick, consists of large elongated sclerenchyma and forms a regular pattern of alternating longitudinal ridges and grooves on the axis surface. Stomata are present in the grooves in high numbers. A branch, truncated at the base, diverges from the axis at a wide angle. This new euphyllophyte is unique in its xylem that possesses Psilophyton-type pitting but lacks a clear maturation pattern, and the outer sclerenchymatous cortex with large cells forming ridges and grooves. A striking feature is the presence of three deeply concave wounds up to 540µm wide suggestive of surface feeding herbivory and located in close proximity to each other. The necrotic layer at the surface of wound areas is underlain by periderm, which is recognized by regular cell files and is developed to variable extents in different areas of the wounds. This fossil ranks among the oldest occurrences of wound-response periderm (and of periderm, in general), along with a few other cases reported in the 1980s-1990s from the same unit (Battery Point Formation) and also attributed to herbivory by Harlan Banks. Taken together, these occurrences allow us to reconstruct the developmental sequence of periderm produced by the earliest euphyllophytes in response to wounding. In this sequence, initiation of the phellogen by recruitment of parenchyma at the wound margin is followed by production of phellem layers while cell walls of the parenchyma beneath the periderm accumulate resilient compounds (possibly suberin). In a second phase, the phellogen switches to producing primarily phelloderm, whose cells may be co-opted in differentiation pathways corresponding to the identity of the primary tissues they replace. In a broader perspective, this new specimen adds to the diversity of the Early Devonian Gaspé flora and contributes to the sparse body of knowledge of periderm evolution. The multiple occurrences of periderm and vascular cambial growth in this flora illuminate the timeline of the evolution of secondary growth and raise questions about potential shared origins of these two types of secondary growth.

1 - California State Polytechnic University - Humboldt, Department of Biological Sciences, Arcata, California, 95521, USA

Keywords:
fossil
paleobotany
Devonian
periderm
wound response
secondary growth
Anatomy
Canada
euphyllophyte
herbivory
plant-animal interaction
Development.

Presentation Type: Oral Paper
Number: PB1002
Abstract ID:793
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award