Семейство папоротников Osmundaceae: древняя группа и ископаемые находки.

Family of ferns

Osmundaceae (royal fern family) is a family of ferns containing four to six extant genera and 18–25 known species. It is the only living family of the order Osmundales in the class Polypodiopsida (ferns) or in some classifications the only order in the class Osmundopsida. This is an ancient (known from the Upper Permian) and fairly isolated group that is often known as the "flowering ferns" because of the striking aspect of the ripe sporangia in Claytosmunda, Osmunda, Osmundastrum, and Plensium (subtribe Osmundinae). In these genera the sporangia are borne naked on non laminar pinnules, while Todea and Leptopteris (subtribe Todinae) bear sporangia naked on laminar pinnules. Ferns in this family are larger than most other ferns.

The stems of Osmundaceae contain vascular tissue arranged as an ectophloic siphonostele; that is, a ring of phloem occurs on the outside only of a ring of xylem, which surrounds pith (and no other vascular tissue). Stipules can be discerned at the leaf bases of these ferns. The hardened leaf bases are persistent and overlap to form a hardened layer surrounding the stem. The mantle of sclerenchymatous leaf bases and intermixed roots can form a woody trunk when the stem emerges above ground, up to in Todea barbara. Extinct members of the family, which flourished during the Mesozoic, could reach the stature of trees and be termed tree ferns. The leaves are either holodimorphic, with separate fertile and sterile fronds assuming an entirely different structure, or have fertile and sterile portions of the frond very distinct in structure. Sporangia in the Osmundaceae are large, and open at a slit on the top; the annulus that drives the sporangium opening is on the side. 128 to 512 spores are typically present. The spores are green, nearly round, and trilete. The spores germinate into gametophytes, which are green (photosynthetic) and grow at the surface. They are large and heart shaped. The base chromosome number for members of the order is 22.

Smith et al. (2006) carried out the first higher level pteridophyte classification published in the molecular phylogenetic era, creating four classes of ferns (Polypodiopsida). At that time they used the term Polypodiopsida sensu stricto to apply to the largest of these. Later the term Polypodiopsida sensu lato was used to refer to all four subclasses, and the large subclass renamed Polypodiidae. This is also referred to informally as the leptosporangiate ferns. The Polypodiidae contain seven orders whose phylogenic relationship is shown in the following cladogram, where Osmundales is seen as a sister to all other members of the subclass.

Bomfleur et al. state that “Osmundales has arguably the richest and most informative fossil record of any extant group of ferns”. The order is well represented in the fossil record from the Permian onwards. Osmundalean or potentially osmundalean fern foliage including Anomopteris Brongn., Todites Seward, Cladotheca T. Halle, Osmundopsis T. M. Harris, Cacumen Cantrill & J. A. Webb, Osmunda, Damudopteris D. D. Pant & P. K. Khare, Dichotomopteris Maithy, and Cladophlebis Brongn., 1849) is commonly found from the Permian onwards together with dispersed spores such as Osmundacidites Couper and Todisporites Couper. But their affinitity to the Osmundales, and Osmundaceae in particular, is difficult to judge. In the Triassic foliage becomes frequent which is similar to identical to the fronds of modern day Todea, Osmundastrum and Claytosmunda. The better understood rhizome fossil record indicates that the group was most diverse in the Permian, and already much reduced in the Triassic regarding the number of substantially different forms (see Bomfleur et al. and literature cited therein). The first fossil representatives of the modern Osmundaceae (= tribus Osmundeae) include rhizome fossils with Claytosmunda anatomy or with structural features characteristic for Osmundastrum cinnomomeum and its precursors. The same holds for the equally old leaf fossil record of the group. The main diagnostic feature of Osmundeae (modern Osmundaceae) is a heterogenous sclerenchymatic ring in the stipe basis. This potential synapomorphy is the only character differentiating between Osmundeae and the paraphyletic genus collecting their potential early ancestors and sister lineages, Millerocaulis. The basic Bauplan shared with 'Millerocaulis' was generally kept within the Osmundeae lineage and only slightly to moderately modified in the last 200 million years. Triassic Jurassic rhizome and frond morphologies remained essentially unaltered in the lineage leading to Claytosmunda claytoniana. This makes it impossible to discern direct ancestors of C. claytoniana from the ancestors of its sister lineages Osmunda and Plenasium), or their shared ancestors. Molecular dating placed the split between Osmunda and Plenasium, and their divergence from Claytosmunda in the Early Cretaceous, co eval with the divergence between the sister genera Todea and Leptopteris. The Osmundastrum lineage diverged much earlier (probably Middle Triassic), which fits with the new classification of re evaluated rhizome fossils originally included in Millercaulis. A comprehensive list of rhizome and leaf fossils associated with modern Osmundaceae (Osmundeae in the classification of Bomfleur et al.) can be found at datadryad. org. The only explicit reconstruction regarding the evolution of morphological traits in Osmundaceae can be found in Miller's groundbreaking work. Notable is that Permian rhizomes of both families in the Osmundales, the extinct Guaireaceae and the Osmundaceae including the extant species, show already relatively complex stele anatomies in comparison to the surviving members of the group. The rhizome fossil record also indicates several independent radiations of likely arborescent lineages, the Guaireaceae and Thamnopterioideae in the Permian, Osmundacaulis in the Triassic, and Plenasium (subgenus Aurealcaulis) in the late Cretaceous to Paleogene. As already noted by Miller, highly derived forms not directly related to the extant species and genera, can be found in the Jurassic and Cretaceous, such as Millerocaulis (Osmundacaulis) kolbii. Another general trend is that the Permian Osmundales were much larger than their modern counterparts. Today, the widespread species of Osmundaceae are rhizomatous with small, low dissected steles.