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Abstract Detail



Themes of land plant evolution, a celebration of the contributions of Leo J. Hickey

Upchurch, Garland [1].

Potomac Group angiosperm leaves: A new look at an old problem.

Since the seminal work of Hickey and Doyle, advances in molecular systematics and ecophysiology provide a fresh perspective for viewing the early angiosperm leaf record from the Potomac Group. The oldest Potomac Group leaves come from the Aptian to early Albian (pollen Zone I) and represent basal lineages. Representatives of the ANA grade include Ficophyllum and dispersed cuticle with features present in Austrobaileyales. Ficophyllum appears to nest above Amborellales and Nymphaeales on the basis of cuticular striations, which are present in Austrobaileyales and Chloranthales but absent from more basal taxa. At least two genera of serrate-margined leaves resemble extant Chloranthaceae in having pinnate venation with craspedodromous secondary veins and chloranthoid teeth. The teeth have darkened areas over the apex and accessory veins that are interpreted as sclerenchyma tissue, based on the stronger development of this tissue in non-Potomac Group relatives, and the occurrence of restricted areas of sclerenchyma on the teeth of certain Hedyosmum species. The exact phylogenetic position of Zone I chloranthoid leaves is unclear. Vitiphyllum has a ternate pattern of lobation that characterizes leaflets of Ranunculales, but at least one species has the remains of probable oil cells, indicating a more basal position. Acaciaephyllum has a suite of architectural features found in extant monocots. Younger Potomac Group leaves from the middle to late Albian (pollen Zone II) include more derived lineages. Proteales are the most abundant recognizable lineage in the leaf macrofossil record and include early Nelumbonaceae (Nelumbites) and genera with affinities to Platanaceae/Proteaceae (Sapindopsis and the platanoids). Also present are lobed leaves of Laurales, which are distinguishable from platanoids by their oil cells and venation.
Studies of ecophysiology and functional anatomy in extant angiosperms make predictions that can be tested against the early angiosperm record. Vein density in Zone I and younger leaves corroborates evidence from extant angiosperms for low photosynthetic rates in the first angiosperms, and higher photosynthetic rates in later groups. Some authors have suggested that early angiosperms were of low stature, perhaps shrubby to herbaceous, and that the first vessels arose in plants with short hydraulic path lengths. These suggestions are consistent with the occurrence, in Zone I, of multiple genera of fossil leaves attached to stems with well-developed cuticle and little or no secondary growth.

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1 - Texas State University, Department of Biology, 601 University Drive, San Marcos, TX, 78666, USA

Keywords:
Early Cretaceous
Systematics
Angiosperms.

Presentation Type: Symposium or Colloquium Presentation
Session: C6
Location: Belle-Chasse/Riverside Hilton
Date: Tuesday, July 30th, 2013
Time: 4:45 PM
Number: C6013
Abstract ID:423
Candidate for Awards:None


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