Abstract Detail

Paleobotany

McQueen, Camryn R. [1], McCuistion, Remy V. [2], Tomescu, Alexandru [3].

New data on the Early Devonian (Emsian) euphyllophyte Leptocentroxyla tetrarcha.

The Battery Point Formation of the Gaspé Peninsula (Quebec, Canada) contains a rich Emsian (Early Devonian, c. 400 Ma) tracheophyte flora. The flora includes compression fossils and permineralizations that rival the Rhynie Chert in number of anatomically-preserved plant types. A euphyllophyte described recently from the Battery Point Formation presents a unique combination of characters. The plant, Leptocentroxyla tetrarcha, has slender axes (2 mm diameter) characterized by a central primary xylem strand 1.1 mm across, consisting of a thick central area from which four narrow lobes, with mesarch protoxylem strands at the tips, extend c. 380 μm. Groups of sclerenchyma form a discontinuous layer in the parenchymatous outer cortex. Adding to the initial description, we investigated the entire length of a 5.5 cm axis to document the branching pattern and additional anatomical details for the species. At the center of the xylem strand, an area of variable diameter (40-200 μm) consists of metaxylem tracheids with significantly thinner walls that approach a scalariform thickening pattern, in contrast with the Psilophyton-type pitting of the rest of the metaxylem. Branches are not preserved, but the divergence pattern of xylem traces to laterals indicates opposite decussate branching. Traces diverge radially from the xylem lobes and are radially elongated, c. 20 x 45 μm in size. Protoxylem strands at the lobe tips are 20-100 μm in diameter proximal to the point where they branch radially. The protoxylem strand that continues along the axis becomes only 1-2 cells thick, whereas the strand that enters the lateral trace expands to 25-60 μm. Compared to coeval plants with lobed xylem – Gothanophyton and a euphyllophyte reported previously from the Battery Point Formation –, Leptocentroxyla is considerably smaller and exhibits different protoxylem architecture and lateral trace anatomy. These two features also distinguish it from three other euphyllophytes identified recently by us in the Battery Point Formation. Leptocentroxyla is similar to aneurophyte progymnosperms (e.g. Tetraxylopteris) in its opposite decussate branching, and to moniliformopsids in its protoxylem architecture devoid of a central strand. However, its P-type tracheids place it outside these lineages and in a recently emerging group of Emsian euphyllophytes that combine plesiomorphic tracheid pitting and anatomical features consistent with structural complexity. In this context, Leptocentroxyla is unique among coeval plants in possessing a conspicuous central area of thinner-walled metaxylem tracheids whose significance is intriguing and which could represent an early step in the evolution of a medullated protostele.

1 - Humboldt State University, Department of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA
2 - Humboldt State University, 1 Harpst Street, Arcata, CA, 95521, USA
3 - Humboldt State University, 1 Harpst St., Arcata, CA, 95521, United States

Keywords:
Devonian
Canada
euphyllophyte
xylem
protoxylem
fossil
anatomy.

Presentation Type: Oral Paper
Session: CK1, Cookson Award Session I
Location: Tucson G/Starr Pass
Date: Monday, July 29th, 2019
Time: 9:00 AM
Number: CK1005
Abstract ID:795
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award