Triasic Plant Fossil Structure

FIGURE 4.24 Lateral view of Perimheste horrida (Jurassic). Bar = 500pm. (Courtesy M. Feist.)

FIGURE 4.25 Lateral view ofFlabellocharagrovesi (Cretaceous). Bar = 500pm. (Courtesy M. Feist.)

coiled elements, with five (the common number in living species) established by the Pennsylvanian (Peck and Eyer, 1963). The evolution of modern forms has led to the elimination of the apical pore as a result of the tighter association of the spiral cells at the apex (Grambast, 1974) (FIG. 4.27).

The reproductive structures in the Paleozoic charophycean families Sycidiaceae (Silurian-Carboniferous) (FIG. 4.28), Trochiliscaceae (Devonian), Chovanellaceae (Devonian-Carboniferous), and Pinnoputamenaceae (Devonian) include a utricle, a calcified supplementary vegetative cover that is believed to protect the zygote against desiccation and which surrounds the gyrogonite. These fossil charophyte families

FIGURE 4.26 Lateral view of Atopochara triquetra (Cretaceous). Bar = 500pm. (Courtesy M. Feist.)

FIGURE 4.27 Louis Grambast. (Courtesy J. Galtier.)

have been placed within a single order, the Sycidiales (Feist et al., 2005a) (FIG. 4.29). In other Paleozoic families, including the Eocharaceae (Middle Devonian-?Triassic), fructifications are more similar to those seen in modern taxa, which do not produce utricles; these families are included in the sb sc sb sc

Fossil Plants Triassic

t pb PC

FIGURE 4.28 Suggested reconstruction of Sycidium xizangense utricle showing vesicle (v), thallus (t), primary (pb), secondary (sb) branches, and primary (pc) and secondary canals (sc) (Devonian). (Modified from Feist et al., 2005a.)

t pb PC

FIGURE 4.28 Suggested reconstruction of Sycidium xizangense utricle showing vesicle (v), thallus (t), primary (pb), secondary (sb) branches, and primary (pc) and secondary canals (sc) (Devonian). (Modified from Feist et al., 2005a.)

FIGURE 4.29 Suggested charophyte phylogeny. (From Feist et al., 2005a.)

Charales. The only Paleozoic family containing utricle-producing and utricle-free species is the Moellerinaceae (Silurian-Permian), which has been interpreted as occupying a central position in the early phylogeny of the group.

In addition to the large number of fossil gyrogonites that have been described and used as index fossils, the vegetative parts of many fossil charophytes are also known. Two genera have been reported from the Upper Devonian of South Africa (Gess and Hiller, 1995). In Hexachara (FIG. 4.30A) each node produces a whorl of six laterals, and oogonia are produced on each lateral, whereas in Octochara (FIG. 4.3 0B) a whorl of eight laterals are borne at each node; each lateral is branched and produces an oogonium.

Palaeonitella cranii (FIG. 4.31) is a relatively small, anatomically preserved charophyte initially described by Kidston and Lang (1921a) from the Early Devonian Rhyme chert of Aberdeenshire, Scotland. The thallus consists of branched, septate filaments that possess a nodal organization. Associated with some of the filaments are long tubular cells, separated from one another by an enlarged node of

FIGURE 4.29 Suggested charophyte phylogeny. (From Feist et al., 2005a.)

FIGURE 4.30 Octochara crassa A. and Hexachara setacea B. (Modified from Gess and Hiller, 1995.)

Palaeonitella
FIGURE 4.31 Palaeonitella cranii showing three nodes (Devonian). Bar = 200 pm. (Courtesy W. Remy and H. Hass.)

small cells. These tubular cells are similar to rhizoids in living Charales. Uncalcified oogonia found associated with P. cranii axes (but not in organic connection) are composed of six sinistrally spiraled cells with an equal number of coro-nula cells arranged in a single layer around an apical pore (Kelman et al., 2004). The shape of the oogonium is reminiscent of the extant Chareae, whereas the morphology of the thallus is similar to that of the Nitelleae. Another species, P. tarafiyensis, comes from the Upper Permian of Saudi Arabia (Hill and El-Khayal, 1983), and P. vermicularis has been reported from the Lower Cretaceous of Spain, along with three other charalean fossils (Martin-Closas and Dieguez, 1998). One of these, Charaxis spicatus, closely resembles members in the extant genus Chara, whereas the other two have been assigned to Clavatoraxis, a morphoge-nus created for sterile, verticillated clavatoracean (family Clavatoraceae) vegetative remains that cannot be attributed to any species of gyrogonite.

The Clavatoraceae is a large group of exclusively Mesozoic Charales that has frequently been used in biostratigraphy of continental facies (Martin-Closas, 1996). They are known from the Oxfordian (Upper Jurassic) through Cretaceous of all continents except Australia and Antarctica, based on both fructifications and vegetative parts, often in organic connection. Clavatoracean fructifications are composed of an oogo-nium surrounded by a calcified utricle. Utricles are important as characters in species identification as they underscore morphological variability. The Clavatoraceae shows development of utricles similar to those in Paleozoic sycidialean families, and this is interpreted as being a result of similar external constraints, rather than expressing true phylogenetic relationships (Feist et al., 2005a).

Fossils of the clavatoracean genus Clavator (Jurassic-Cretaceous) consist of strongly calcified stems with narrow internodes and six lateral branches in each whorl. Oogonia are located in a single vertical row on the adaxial side of a branch, one per node. The vegetative parts of another genus in the Clavatoraceae, Echinochara from the Morrison Formation (Jurassic) of North America, are known in some detail. The plant body consists of 12 dextrally spiraled, cortical tubes constructed of elongate cells arranged in a linear series. At the distal end of each cortical cell are five long spines. Oogonia were produced in whorls of six.

Zygnematales

Thin sections of chert from the Middle Devonian of New York revealed both marine and freshwater algae (Baschnagel, 1966), including a representative of the Zygnematales (or conjugate algae). Paleoclosterium leptum is formed of solitary, elongate, lunate cells <46 pm long and 5 pm wide that appear morphologically similar to species in the extant genus Closterium. Another fossil member of the conjugates is Palaeozygnema spiralis, which occurs in Cretaceous amber from southern Germany (Dörfelt and Schäfer, 2000). This fossil has unbranched chains of cells, each <20 pm long by 14 pm wide, in which chloroplasts and zygotes are exquisitely preserved. Palaeozygnema (FIGS. 4.32, 4.33) is similar to the modern genus Zygnema, although the process of game-togenesis is apparently different in the fossil. Zygnematacean zygospores reported in palynological samples have been useful in assessing changing depositional environments (Zavattieri and Pramparo, 2006).

Was this article helpful?

0 0

Responses

  • Stefan
    Does placenta protect zygote from desiccation in charophytes?
    2 years ago

Post a comment