Haplomitriopsida

Haplomitriopsida is a newly recognized class of liverworts comprising fifteen species in three genera. Recent cladistic analyses of nuclear, mitochondrial, and plastid gene sequences place this monophyletic group as the basal sister group to all other liverworts. The group thus provides a unique insight into the early evolution of liverworts in particular and of land plants in general.
Description.
Plants of "Treubia" grow as a prostrate leafy thallus. The bifid leaves extend like wings on either side of the midrib, or may be folded upwards and pressed close together, giving the plants a ruffled appearance. By contrast, "Haplomitrium" grows as a subterranean rhizome with erect leafy stems. The thin, rounded leaves are arranged around the upright stems, giving the appearance of a soft moss. The species "Haplomitrium ovalifolium" of Australia often has bifid leaves that are asymmetrical, somewhat like those in "Treubia".
"Haplomitrium" has a number of unique characters that distinguish it from other liverworts, such as lacking rhizoids. The vegetative stems possess a central water-conducting strand with large perforations derived from plasmodesmata. This central strand is surrounded by a cylinder of cells that conduct food throughout the plant. Such an arrangement is evocative of the xylem and phloem found in vascular plants. Although some thalloid liverwort species in the Pallaviciniaceae also possess a central conducting strand, "Haplomitrium" differs in having a food-conducting layer and in producing no callose.
"Treubia" also has features that differ from those found in other bryophytes, such as the differentiation of five identifiable zones in the stem midrib. Unlike other leafy species, the oil bodies in its cells are restricted to certain clusters of cells, as they are in the Marchantiopsida. These oil body clusters appear as dark spots in the leaves when the plant is held up to the light.
Diversity.
Living representatives of the group exhibit an essentially Gondwanan distribution with its center of diversity in Australasia. Such a distribution implies that the modern genera radiated prior to the beginning of the Cretaceous when Gondwana broke apart. Schuster proposes that species distributed in the northern hemisphere "rafted" on the Indian subcontinent to Asia, then spread across the Bering Strait into North America.
Most species in the Haplomitriopsida are found in south of the equator, though there are northern ones. The genus "Treubia" is restricted to the southern hemisphere, while "Apotreubia" has one species in New Guinea and another disjunct between eastern Asia and British Columbia. The genus "Haplomitrium" exhibits a wider distribution, with species in both North and South America, northern and central Europe, the Himalayas, Japan, and Australasia.
Classification.
Class Haplomitriopsida includes two orders, each with one family. The group as a whole comprises fifteen species in three genera. A fourth genus, "Gessella", is known only from Permian fossils. The orders, families, and genera are as follows:
An additional fossil "Treubiites kidstonii" previously has been compared to the extant genus "Treubia". However, upon re-examination of the material, specimens were determined to be more like "Blasia" and not at all to resemble "Treubia" as previously thought. Accordingly, "Treubiites" is now assigned to the Blasiales rather than the Haplomitriopsida.