Zosterophylls †

Ecology & Form

Sporophyte (spore-bearing) phase

Stem

• Anisotomous branching in proximal axes

• Isotomous branching in the distal axes

• Elliptical exarch haplostele for anatomy

Leaves

• Leafless 

• Enations in some; smooth axes in others

Roots

• Advanced members of this group exhibit root-like axes (e.g., Bathurstia)

• These represent some of the earliest known root-like structures in the fossil record

Reproduction

• Aggregated sporangia with clam-like dehiscence

• Sporangia are lateral on the stems, not terminal like the "rhyniophytes" of the same time 

• The sporangia can appear cone-like in some members

Craswallia haegensis † 

• Morris & Edwards, 2014

• Early Devonian (Lochkovian) of the Welsh Borderland, UK

• Possible zosterophyll, comprising bivalved elliptical sporangia, borne distally on stout unbranched stalks

• Smooth stems in parallel alignment with rare lateral sporangia, all at approximately the same level in adjacent stems

• The stems show very little change in diameter along their lengths, measuring 1.3 to 1.8 mm diameter, and no branching

Discalis longistipa †

• Hao, 1989

• Early Devonian (Pragian), Posongchong Formation of Yunnan, China

• Leafless sporophyte consisted of creeping axes up to 5 mm in diameter 

  • 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 vegetative stems exhibit circinnate growth. 

• 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 dehisced along their margin to release the trilete spores. 

• Vascular tissue was present in the stems, with tracheids having annular, spiral thickenings.

Guangnania †

• Wang & Hao, 2002

G. cuneata  

• Wang & Hao, 2002

• Lower Devonian (Pragian) Posongchong Formation of southeast Yunnan and the (Pragian-early Emsian) Xujiachong Formation of east Yunnan, China

• Pseudomonopodial axes; anatomy is unknown

• Fertile region consists of lateral sporangia attached helically

  • Each sporangium is elongate cuneate and long-stalked dehiscing into two unequal valves, a large abaxial valve and a small adaxial one, which in transverse section are curved towards the axis

  • The dehiscence line is bordered by the thickening along the entire sporangial valve margin

G. minor

• Edwards et al., 2016

• Distal and basal branching systems preserved separated by naked parallel-sided axes of a diameter that is remarkably uniform along their length (1.0–1.6 mm wide, n = 21, x = 1.3; majority are 1.2 and 1.4 m) 

• Infrequent isotomous branching with some overtopping; bifurcation may occur close to the fertile region 

• Bifurcations more common in the basal regions, where it is accompanied by K configurations and occasional, apparently short, unbranched laterals with rounded 

• Sporangia are lateral and arranged helically, although their exact disposition is unknown

  • Tips of sporangia are at approximately the same level, but sporangial insertion shows very small vertical spacing (up to three consecutive stalks observed) such that they are not opposite or verticillate

  • Slightly vertically extended mounds or depressions probably represent superficial insertion of sporangia which are not preserved

  • Strobili on the larger specimen show considerable relief, extending through some depth of sediment such that sporangia are fractured in different planes. 

  • There is no evidence of pronounced curvature: the stalks are inserted at an acute angle, are decurrent and superimposed on the strobilar axis, appear as broad ridges or depressions 

  • The abaxial part is continuous with most of the rest of the sporangium which is interpreted as the abaxial valve

  • Some specimens preserved dorsiventrally show a very narrow line around the distal margin, but this is not comparable to the marginal fractures noted in zosterophylls nor does it equate to the marginal split recorded above

Ventarura lyonii † 

• Powell et al., 2000

• Pragian from Rhynie Chert

• Stems of two kinds without clear connections between them. 

• Underground stems had single-celled hairs, presumably rhizoids, on all sides. They branched irregularly and more often compared to aerial stems

• Aerial stems were leafless, smooth, and isotomously-branched, reaching 12 cm in length (7.2 mm max diameter)

• They contained an exarch xylem strand containing G-type tracheids. 

• The cortex of the aerial axes displays an  inner and outer cortical layer is separated by a dark middle cortex zone of sclerenchymatous cells 

  • The outer cortex comprises closely-packed, thin-walled cells. 

  • Evidence of sclerenchyma (found in the middle zone) is rare in Rhynie Chert plants

  • The inner cortex again comprises thin-walled cells, but with a well-developed inter-cellular air space network.

• Sporangia were born on the sides of the stems, attached without clear stalks. 

  • They consisted of two circular- to pear-shaped 'valves', one slightly narrower facing the stem, one away from it. 

  • Sporangia may have formed a two-rowed spike or strobilus. 

  • Spores were shed via a slit at the top of the sporangium between thickened valve borders