Discalis
Encyclopedia
Discalis is a genus of extinct vascular plant
s of the Early Devonian (Pragian or Siegenian stage, around ). The name is derived from the Greek , referring to the disc-shaped sporangia
(spore-forming organs). The genus was first described by Hao in 1989 based on fossil specimens from the Posongchong Formation, Wenshan district
, Yunnan, China.
of D. longistipa consisted of creeping stems (axes) up to 5 mm 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 2.5 mm 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 3.7 mm in diameter, had spines like the stems, and split (dehisced) along their margin to release the trilete spore
s, which were 30-50 µm in diameter. Vascular tissue
was present in the stems, with tracheid
s having annular, spiral thickenings.
D. longistipa somewhat resembles Sawdonia, but differs in branching pattern and in the arrangement of the sporangia, including their long stalks.
') of later lycoposid
s (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.
A cladogram published in 2004 by Crane et al. places Discalis in a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids
(living and extinct clubmosses and relatives).
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s of the Early Devonian (Pragian or Siegenian stage, around ). The name is derived from the Greek , referring to the disc-shaped sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
(spore-forming organs). The genus was first described by Hao in 1989 based on fossil specimens from the Posongchong Formation, Wenshan district
Wenshan Zhuang and Miao Autonomous Prefecture
Wenshan Zhuang and Miao Autonomous Prefecture is an autonomous prefecture in Yunnan Province, China.-Subdivisions:-Ethnic groups:Wenshan is highly diverse...
, Yunnan, China.
Description
The leafless sporophyteSporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
of D. longistipa consisted of creeping stems (axes) up to 5 mm 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 2.5 mm 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 3.7 mm in diameter, had spines like the stems, and split (dehisced) along their margin to release the trilete spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s, which were 30-50 µm in diameter. Vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
was present in the stems, with tracheid
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
s having annular, spiral thickenings.
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 ('lycophyllsMicrophyll
The terminology of fossil plants is in places a little confusing. In the discipline's 200+ year history, certain concepts have become entrenched, even though improved understanding has threatened the foundations upon which they are based...
') of later lycoposid
Lycopodiophyta
The Division Lycopodiophyta is a tracheophyte subdivision of the Kingdom Plantae. It is the oldest extant vascular plant division at around 410 million years old, and includes some of the most "primitive" extant species...
s (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.
A cladogram published in 2004 by Crane et al. places Discalis in a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids
Lycopodiophyta
The Division Lycopodiophyta is a tracheophyte subdivision of the Kingdom Plantae. It is the oldest extant vascular plant division at around 410 million years old, and includes some of the most "primitive" extant species...
(living and extinct clubmosses and relatives).
External links
- Cladogram from
