Discalis is a genus of extinct of the Early Devonian (Pragian or Siegenian stage, around ). The name is derived from the Greek δίσκος, referring to the disc-shaped Sporangium (spore-forming organs). The genus was first described by Hao in 1989 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China.
Description
The leafless
sporophyte of
D. longistipa consisted of creeping stems (axes) up to in diameter with many K- or H-shaped branches as well as upturned or trailing stems, slightly smaller in diameter, which also branched. All stems had irregularly arranged multicellular spines up to long with expanded tips. The stems which did not bear sporangia had coiled (circinnate) tips. Fertile stems bore disc-shaped sporangia laterally on stalks up to 5 mm long, forming open spikes. The sporangia, which were about in diameter, had spines like the stems, and split (dehisced) along their margin to release the trilete
, which were in diameter.
Vascular tissue was present in the stems, with
having annular, spiral thickenings.
[, p. 254]
D. longistipa somewhat resembles Sawdonia, but differs in branching pattern and in the arrangement of the sporangia, including their long stalks.
Phylogeny
The evolutionary relationship between spines in early plants and the small leaves ('
Microphyll') of later
Lycopodiopsida (club-mosses and allies) has been much discussed. Hao considered that the spines of
Discalis could be precursors of lycophylls.
Boyce regarded the occurrence of the spines of
Discalis on sporangia as evidence that they were not related to lycophylls in this way.
[, p. 491]
A cladogram published in 2004 by Crane et al. places Discalis in a paraphyletic stem group of broadly defined "", basal to the Lycopodiophyta (living and extinct clubmosses and relatives).
Hao and Xue in 2013 used the absence of terminal Sporangium to place the genus in the Paraphyly order Gosslingiales, in the family Discaliaceae. The Gosslingiales are considered to be zosterophylls with indeterminate growth and fertile branches generally showing circinate vernation (initially curled up).[
]
External links
