Rhacophyton ceratangium

Период: Девон

Rhacophyton ceratangium (early fern)
Rhacophyton ceratangium is abundant at Red Hill and accounts for nearly 40% of the identifiable plant fossils collected from the floodplain pond facies. It's co-dominant with Archaeopteris spp. in the floodplain, probably occurring as monotypic stands in more open areas. Its biomass was apparently sufficient to fuel the low-intensity fires reported by Walt Cressler, which are among the earliest records of wildfires in the fossil record. Rhacophyton ceratangium is dominant in other North American Famennian (Late Devonian) localities. In Virginia and West Virginia, Rhacophyton occurrs in nearly monotypic stands in deltaic marshes and peat-accumulating backswamps, while it's co-dominant with Archaeopteris in the somewhat drier floodplain forests. Rhacophyton is also co-dominant with a species of Archaeopteris in drier portion of a floodplain at a latest Devonian locality in Arctic Norway. Elsewhere, species of Rhacophyton have been reported from New York, Maine, Belgium, Germany and western Siberia.

Cornet et. al. (1976) concluded that Rhacophyton ceratagium is a tall shrub with a central main stem 1-1.5 m in height. The laterals exhibit a quadriseriate branching pattern in which they form two vertical rows of alternating sets of paired branches. These laterals extended as much as 50 cm and exhibited a variety of morphologies. Vegetative laterals typically exhibited a biseriate arrangement (two lateral rows forming a plane) of side branches (pinnae) reminiscent of fern fronds. (Rhacophyton laterals were true branches containing secondary xylem, whereas fern fronds are compound leaves.) They can also exhibit a 3-dimensional quadriseriate side branching, often at the distal ends of otherwise biseriate laterals. The pinnae- and pinnule-like ultimate branches vary in their degree of flattening, but none are webbed (laminar tissues extending between veins). Fertile lateral segments have a 3-dimensional quadriseriate arrangement, typically with two elongate sterile pinnae and two highly branched fertile pinnae. Fertile segments have been found on the distal ends of otherwise sterile laterals, but laterals bearing fertile structures have yet to be found attached to the main stem. Rhacophyton is probably homosprous. In other words, it produces only one type of spore.

There are some indications that Rhacophyton employed clonal propagation to occupy and monopolize preferred habitats. Dense mats of adventitious roots are commonly found along the main stem and on the laterals. Instead of a cluster of separate and erect shrubs, Rhacophyton may have adopted a sprawling habit with the main axis and some laterals functioning like rhizomes.

The systematic position of Rhacophyton is uncertain. Andrews and Phillips (1968) concluded that it was related to progymnosperms, while Cornet et. al. (1976) regarded it as neither a progymnosperm nor a fern. Unlike ferns, it possesses secondary xylem in its laterals branches. On the other hand, it has the distinctive vascular tissue morphology found only in ferns. Most authorities now place Rhacophyton within the ferns, but outside the living fern lineages. They are most commonly placed either within the fern order Zygopteridales (Devonian to Permian) or in the Rhacophytales, a separate order that contains Rhacophyton, Protocephalopteris and Protopteridopyton (Middle to Late Devonian).

Another early "fern", Gillespiea, has also been found at Red Hill. You can also learn more about ferns.