Monocotyledon - Evolution

Evolution

For a very long time, fossils of palm trees were believed to be the oldest monocots, first appearing 90 million years ago, but this estimate may not be entirely true (reviewed in Herendeen and Crane, 1995). At least some putative monocot fossils have been found in strata as old as the eudicots (reviewed in Herendeen et al., 1995). The oldest fossils that are unequivocally monocots are pollen from the Late Barremian-Aptian - Early Cretaceous period, about 120-110 million years ago, and are assignable to clade-Pothoideae-Monstereae Araceae; being Araceae, sister to other Alismatales (Friis et al., 2004:) for fossil monocots, see Gandolfo et al. 2000 and Friis et al. 2006b ). They have also found flower fossils of Triuridaceae (Pandanales) in Upper Cretaceous rocks in New Jersey (Gandolfo et al. 2002 ), becoming the oldest known sighting of saprophytic /mycotrophic habits in angiosperm plants and among the oldest known fossils of monocotyledons.

Topology of the angiosperm phylogenetic tree could infer that the monocots would be among the oldest lineages of angiosperms, which would support the theory that they are just as old as the eudicots. The pollen of the eudicots dates back 125 million years, so the lineage of monocots should be that old too.

The molecular age estimates also hold the view that monocots are as old as eudicots. Bremer (2000 2002 ), using the rbcL sequence data and the method of the middle way ("mean-path lengths method") to estimate divergence times, dates back the origin of the crown group of the monocots (the time Acorus genus divides the rest of the group) to some 134 million years, which would mean that estimates of the main group of monocots are even older. However Wikström et al. (2001 ), using Sanderson’s (1997 ) non-parametric approach (nonparametric rate smoothing approach "), produced ages for the crown group of monocots between 158 and 141 million years up until now (see Sanderson et al. 2004 ), ages markedly older than Bremer’s, so that the trunk group of the monocots would also be older than Bremer’s estimations. The discrepancy between these two estimates is probably due to the highly conservative calibration point used in the study of Wikström et al. 2001 (the split between Fagales and Cucurbitales were considered to be in the late Santonian period).

In fact the age of the monocot crown has been variously estimated, besides the two mentioned, it has been estimated around 200 ± 20 million years BC (Savard et al. 1994 ), 160 ± 16 million years BC (Goremykin et al. 1997 ), 135-131 million (Leebens-Mack et al. 2005 ), 133.8 to 124 million (Moore et al. 2007 ), etc.

Assuming Triuridaceae is a member of Pandanales, their fossils would give support to a crown group age closer to Bremer’s estimations (2000 ).

Bremer’s estimation (2000 ) was used in a more recent analysis that formed the basis for dating the age of the monocots in general (Janssen and Bremer 2004 ).

The age of the core group of so-called ‘nuclear monocot’ or ‘core monocots’ by the APW ("core monocots" in English), which correspond to all orders except Acorales and Alismatales, is about 131 million years to present, and crown group age is about 126 million years to the present. The subsequent branching in this part of the tree (i.e., Petrosaviaceae, Dioscoreales + Pandanales and Liliales clades appeared), including the crown Petrosaviaceae group may be in the period around 125-120 million years BC (about 111 million years so far in Bremer 2000 ), and stem groups of all other orders, including Commelinadae would have diverged about or shortly after 115 million years (Janssen and Bremer 2004 ). These and many clades within these orders may have originated in southern Gondwana, i.e., Antarctica, Australasia, and southern South America (Bremer and Jansen 2006 ).

The aquatic monocot Alismatales have commonly been regarded as "primitive" (Hallier, 1905, Arber 1925, Hutchinson, 1934, Cronquist 1968, 1981, Takhtajan 1969, 1991 Stebbins 1974, Thorne 1976 ). They have also been considered to have the most primitive foliage, which were cross-linked as Dioscoreales (Dahlgren et al). 1985 and Melanthiales (Thorne 1992a, 1992b ). Keep in mind that, as stressed by Soltis et al. 2005, the "most primitive" monocot is not necessarily "the sister of everyone else." This is because the ancestral or primitive characters are inferred by means of the reconstruction of characteristic states, with the help of the phylogenetic tree. So primitive characters of monocots may be present in some derived groups. On the other hand, the basal taxa may exhibit many morphological autapomorphies. So although Acoraceae is the sister group to the remaining monocotyledons, the result does not imply that Acoraceae is "the most primitive monocot" in terms of its characteristics. In fact, Acoraceae is highly derived in most morphological characteristics, which is precisely why so many Alismatales Acoraceae occupied relatively imitative positions in trees produced by Chase et al. 1995b and Stevenson and Loconte 1995. (see section phylogeny).

Some authors support the idea of an aquatic phase as the origin of monocots (Henslow 1893, and also cited and argued in the phylogeny section that Alismatales are the most primitive). The phylogenetic position of Alismatales (many water), which occupy a relationship with the rest except the Acoraceae, do not rule out the idea, because it could be ‘the most primitive monocots’ but not ‘the most basal’. The Atactostele stem, the long and linear leaves, the absence of secondary growth (see the biomechanics of living in the water), roots in groups instead of a single root branching (related to the nature of the substrate), including sympodial use, are consistent with a water source. However, while monocots were sisters of the aquatic Ceratophyllales, or their origin is related to the adoption of some form of aquatic habit, it would not help much to the understanding of how it evolved to develop their distinctive anatomical features: the monocots seem so different from the rest of angiosperms and it’s difficult to relate their morphology, anatomy and development and those of broad-leaved angiosperms (e.g. Zimmermann and Tomlinson 1972; Tomlinson 1995 ).

In the past, taxa which had petiolate leaves with reticulate venation were considered "primitive" within the monocots, because of its superficial resemblance to the leaves of dicotyledons. Recent work suggests that these taxa are sparse in the phylogenetic tree of monocots, such as fleshy fruited taxa (excluding taxa with aril seeds dispersed by ants), the two features would be adapted to conditions that evolved together regardless (Dahlgren and Clifford 1982; Patterson and Givnish 2002, Givnish et al. 2005b, 2006b ). Among the taxa involved were Smilax, Trillium (Liliales), Dioscorea (Dioscoreales), etc. A number of these plants are vines that tend to live in shaded habitats for at least part of their lives, and may also have a relationship with their shapeless stomata (see Cameron and Dickison 1998 for references on this last characteristic). Reticulate venation seems to have appeared at least 26 times in monocots, in fleshy fruits 21 times (sometimes lost later), and the two characteristics, though different, showed strong signs of a tendency to be good or bad in tandem, a phenomenon Givnish et al. (2005b, 2006b ) described as “concerted convergence” (“coordinated convergence”).