ORIGIN OF ANGIOSPERMS

Speculation on Pre-Cretaceous Angiosperms has been made by several authors, and many authors have hypothesized the possible mechanisms of the evolutionary change. Many of the purported pre-angiosperm ancestors have "angiosperm" leaf characters (net-like venation pattern) which has arisen independently in several clades.

• Sanmiguelia is an Late Triassic herbaceous plant whose leaves appear "palm-like," but lack the midvein and petiole features of true palms. This enigmatic plant is considered by some to by allied with cycads, whereas others have allied this with pteridosperms.
• Marcouia is an Late Triassic pinnate leaf that is palmately compound with four-five basally united pinnae. Secondary veins that dichotomize and anastomose originate from a well-defined midrib. This genus is known from limited material and probably does not have any relation to angiosperms.
• Furcula is an Late Triassic genus that combines angiosperm-leaf characteristics and other groups. These features include a forked midrib (primary vein) from which secondary veins arise; intercostal veins between these anastomose to form a reticulate pattern that delimits poorly defined areoles. Stomates are syndetocheilic, also an angiosperm character.
• Pannaulika is a Late Triassic leaf (only one known specimen) described as having brochidodromous secondary veins with tertiary veins forming loops outside the secondary loops, and this architecture has been ascribed to angiosperms.
• Monoculpate pollen grains have been described from the Late Triassic-Early Jurassic in which an outer exine is differentiated into a well-defined tectum supported by columellae (angiosperm features).

The apparent burst of monocot and dicot angiosperms into the fossil record without "transitional" forms has led workers to a quandary and a search for pre-Cretaceous angiosperms. There is little doubt that angiosperms display the most advanced seed plant state, and that the features that characterize them originated from a gymnosperm ancestor.

The Gnetales, a small relict group of gymnosperms, may hold the key to angiospermy. Recent investigations have shown that one of the primary characters of angiospermy - double fertilization, anastomosing leaf venation, reduction of the male gametophyte, tetrasporic megagametophyte with free nuclei serving as eggs, and a feeder in the embryo - occurs sporadically in Ephedra, and a welwitschian-type reproductive structure (Archaestrobilus) found in the Late Triassic of Texas has been interpreted to have been a precursor to angiosperms. There is growing agreement that these plants may be the closest living relative to flowering plants.

A recent discovery in China of the world's oldest flower, Archaefructus liaoningensis, has pushed the earliest evidence of this group to the Berriasian (earliest Cretaceous) at 142 MY BP. The fossils were recovered from a lake deposit in which limestones and volcanic ashes accumulated.

The most abundant angiosperm remains are known from the Potomac Group in Virginia. Doyle and Hickey (1976) have demonstrated that both pollen (monocolpate) and leaves (laminate, entire) of angiospermous origin exist there, and recently Friis et al. (1994) have recovered remains of flowers. These assemblages are poorly diversified and morphologically possess "disorganized" patterns. Their arrival into depositional sites has spawned postulation that the group originated in a variety of settings:

• Stebbins (1965, 1974) suggests that the first angiosperms were weedy xerophytic shrubs that possibly evolved in higher altitudes;
• Axelrod (1970) suggests that the angiosperms gradually entered the lowland record possibly due to climatic shifts;
• Retallack & Dilcher (1981) suggest that the Early Cretaceous angiosperms were pioneering plants in coastal environments allowing for their rapid rise along tidal flat, etc.
• Krasilov (1977) suggests that the evolution of the angiosperms represents addition to the character pool until angiosperms are recognized.

Recent discoveries of a herbaceous angiosperm fossil record in the Early Cretaceous has led to two competing hypotheses - that early angiosperms were paleoherbs, rather than trees or shrubs.

• The Paleoherb Hypothesis has been promoted by results of cladistic analyses conducted by Taylor and Hickey in which the basal angiosperms are a group of tropical flowering plants with uncomplicated flowers and a mix of monocot and dicot features.
• The implication of their proposal is that the key innovations of flowers and a rapid life cycle were present in the earliest angiosperms. It has been suggested that changes in climate or geography provided opportunities for these early angiosperms to diversify.
• The Woody Magnoliid Hypothesis is the result of cladistic analyses conducted by Doyle and Donoghue in which an early angiosperm with morphology similar to living members of the Magnoliales and Laurales is en visioned as the basal angiosperm stock. Magnolids and Laurales are comprised of plants that are small to medium-sized trees with long broad leaves and large flowers with indeterminate numbers perianth parts. The carpels are imperfectly fused, and make a physical intermediate between a folded leaf and fused pistil.
• Evolutionary studies based upon DNA sequencing (plastid gene rbcL) appear to support this hypothesis. This relatively long-held paradigm would indicate that the earliest angiosperms were understory trees and shrubs, and that the flower was NOT the key innovation for the rapid diversification of angiosperms.