Abstract Detail

Paleobotany

Toledo, Selin [1], Bippus, Alexander C. [2], Tomescu, Alexandru [3].

Early hints of structural complexity: a new euphyllophyte from the Lower Devonian of Quebec.

The Battery Point Formation of the Gaspé Peninsula (Quebec, Canada) hosts a rich Emsian permineralized flora, one of the rare occurrences of anatomically-preserved Early Devonian plants. One of these plants is a new type of structurally complex euphyllophyte represented by >10 axis segments up to 8mm in diameter, bearing alternate decussate lateral appendages. Lateral appendages, at least 1.5cm long, 3mm wide x 2mm thick, bear small, dichotomous, alternate ultimate appendages. The axes have actinosteles with 4 xylem ribs and protoxylem strands at the center of the stele, along rib midplanes (2-3 per rib), and at rib tips; central and rib tip protoxylem strands occasionally form lacunae. Metaxylem tracheids have distinctive P-type secondary thickenings with spongy structure. Primary phloem forms an incompletely preserved thin layer of narrow cells around the xylem. Secondary xylem forms a 5-6 cell-thick layer of radially aligned tracheids that is thicker around the tips of primary xylem ribs and thinner along their sides. The inner cortex, incompletely preserved, hosts a discontinuous layer of longitudinally elongated sclereids with thick secondary walls. The outer cortex has alternating zones of sclerenchyma and parenchyma reminiscent of Sparganum-type anatomy. Areas of sclerenchyma cells extend from the outer cortex into the bays between primary xylem ribs. Axes and lateral appendages bear capitate trichomes. Traces to lateral appendages consist of pairs of Y-shaped bundles emerging tangentially from the tip of primary xylem ribs, lateral appendages, like the traces supplying them, have bilateral polarity. This Emsian plant occupies an intermediate position between basal euphyllophytes (trimerophytes) and the diverse younger Devonian lineages of structurally complex euphyllophytes. Consistent with this position, this plant exhibits a unique combination of characters that can illuminate the evolution of euphyllophyte structural complexity: (i) P-type tracheids (seen only in trimerophytes), (ii) features of select younger Devonian euphyllophytes – tangential trace divergence (Stenokoleales, Triloboxylon, Tristichia), protoxylem lacunae (Cairoa, Stenokoleos) –, and (iii) characters more broadly represented in the same groups – mesarch protoxylem, secondary growth, heterogeneous outer cortex. Possessing P-type tracheids, the Gaspe plant is potentially basal to two other structurally complex Emsian euphyllophytes (Gensel 1984 and Gothanophyton). Together, these plants indicate that several euphyllophyte lineages were already exploring structural complexity prior to the Middle Devonian. These plants also illustrate a snapshot of the mosaic pattern of morphological evolution that characterizes the radiation of vascular plants during the Devonian Explosion, which appears to have become canalised into distinct lineages by the Middle Devonian.

1 - Humboldt State University, Department of Biological Sciences, 1, Harpst Street , Arcata, CA, 95521, USA
2 - Humboldt State University, Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA
3 - Humboldt State University, Department Of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA

Keywords:
Progymnosperms
Euphyllophytes 
Stenokoleales
Devonian
Early Devonian
Emsian
Plant Anatomy
Trimerophytes
Seed plants
fossil
Paleobotany.

Presentation Type: Oral Paper
Session: 16, Cookson/Moseley and Paleozoic paleobotany
Location: Sundance 4/Omni Hotel
Date: Monday, June 26th, 2017
Time: 3:45 PM
Number: 16001
Abstract ID:119
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award