ANATOMICALLY PRESERVED PLANT REPRODUCTIVE STRUCTURES FROM THE UPPER TRIASSIC CHINLE FORMATION IN PETRIFIED FOREST NATIONAL PARK, ARIZONA
Kathleen B. Pigg and William C. Davis
Department of Botany
Arizona State University
Tempe, AZ 85287-1601
Department of Geology
Weber State University
Ogden, Utah 84408
Anatomically preserved plant reproductive structures including seeds, seed-bearing, and pollen-bearing organs occur in calcareous nodules embedded in a lens of cuticular shale in the
Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona. The fossil plant material in the nodules is significant because it represents the first nonstem material with internal anatomical preservation from the Chinle Formation, and promises to provide new information about Triassic seed plants and their affinities. Currently under study are small, isolated radiospermic seeds, four-chambered sac-like seed-bearing structures and synangiate pollen organs with nonstriate bisaccate Alisporites pollen. Affinities of these reproductive organs appear to be with the Mesozoic pteridosperms; this is of particular interest because so few confirmed Mesozoic pteridosperms have been described from North America.
The Upper Triassic Chinle Formation, which outcrops extensively in northeastern Arizona and adjacent areas of New Mexico, Colorado and Utah, contains one of the most floristically rich Upper Triassic fossil plant deposits in the world (Ash, 1980). Over 70 plant taxa are known from the Chinle Formation, including lycopods, ferns, cycads, conifers, bennettitaleans, and several forms that are currently unclassified. Most of these fossils occur as compression-impression remains which show the outer form of the plant and often provide well preserved cuticular remains of taxonomic utility (e.g., Ash, 1980). A considerably smaller proportion of the Chinle flora is known from petrified stems that can show internal anatomical structure. This latter type of material includes the famous logs of Araucarioxylon arizonicum, of the Petrified Forest National Park, Arizona (Daugherty, 1941), as well as the enigmatic stem Schilderia adamanica and several smaller plants such as the lycopod Chinlea (Miller, 1968) and the cycad Charmorgia (Ash, 1985). However, although currently known Chinle megafossils provide a good bit of information on the diversity of the Late Triassic flora of North America, relatively little is known about the internal anatomical structure of these plants.
A small lens of cuticular shale which outcrops in the lower part of the Petrified Forest member of the Upper Triassic Chinle Formation (Ash, 1980; Pigg, Davis and Ash, 1993) in Petrified Forest National Park, Arizona contains plant remains with internal anatomical preservation. The lens represents an overbank deposit of transported mud, silt and plant fragments. This unusual material offers the first opportunity to study anatomically preserved seeds, leaves and reproductive material in the Chinle Formation. The nodules contain a variety of fragmentary plant remains including leaves with preserved cuticle of the conifers Brachyphyllum and Pagiophyllum and of Dinophyton, a plant of uncertain taxonomic affinity, small woody twigs, seeds and seed- and pollen-bearing reproductive structures (Pigg, Davis and Ash, 1993).
Illustrated here are some of the most completely preserved gymnospermous reproductive structures found thus far in the Petrified Forest nodules. The radiospermic seeds are about 2.2 x 2 mm with a megaspore membrane, a typically prominent nucellus, and three-zoned integument (Fig. 1). The nucellus is free from the integument for most of its length. The innermost integumentary layer is composed of tangentially elongate, thin-walled cells, the middle layer of darker, radially elongate, barrel-shaped cells and the outermost layer which is typically poorly preserved, but appears to be composed of smaller, compact cells. Some of these seeds show a slightly elongate beak-like prolongation of the micropylar end. Four-chambered seed-bearing structures (not figured), also under study, may have borne these seeds.
The pollen organs are synangiate and contain bisaccate Alisporites-type pollen. These organs are up to 1.4 mm thick in transverse section and are attached to fragmentary laminar tissue (Fig. 2-6). Within the pollen organ, clusters of pollen are surrounded by a uniseriate sporangial wall composed of cuboidal cells and to the inside, a region of thin-walled tissue up to several cells thick (Fig. 3-5). Larger palisade-like cells with dark contents surround the pollen organ (Fig. 2-5), which is sometimes found adhering to laminar tissue (Fig. 5). Up to five pollen sacs have been counted in the incompletely preserved specimens that have been studied. Based on the six specimens presently under study, it is estimated that pollen organs were probably composed of about 10-12 pollen sacs, which are fairly completely fused to one another for at least part of their length. Dehiscence slits have not yet been observed in the pollen sacs. The pollen is of the Alisporites type, bisaccate, nonstriate, and up to 80 um (Fig. 6).
Based on the structural features of this material, we suggest that seeds, seed-bearing and pollen-bearing organs are most similar to those of the Mesozoic pteridosperms (seed ferns), and may represent a previously unknown plant in the Triassic Chinle Formation. The proposed seed fern affinities of this material are based on the following criteria: size, shape and morphology of seeds, organization of the integument of the seeds, fused, cupulate nature of four-armed, presumed seed-bearing structures, the pollen organ nature of pollen-bearing structure, and in situ occurrence of Alisporites pollen which is known to belong to the Corystospermales, a group of primarily Southern Hemisphere pteridosperms (Taylor and Taylor, 1993). Interestingly, however, the present material appears to include synangiate pollen organs, while typical corystosperm pollen organs are nonsynangiate (Taylor and Taylor, 1993). Earlier, Stone (1978) noted the presence of a corystosperm-like fructification containing the pollen Pityosporites in the Chinle.
Until recently, Triassic seed ferns have been known primarily from the southern continents, with related groups not becoming common in the northern hemisphere until the Jurassic. The presence of possible Mesozoic seed ferns in the Chinle Formation therefore is of considerable importance, in extending their paleogeographic range.
This research was supported in part by a grant from the Petrified Forest Museum Association to KBP.
Ash, S. R., 1980. Upper Triassic floral zones of North America; in Dilcher, D. L., and Taylor, T.N., eds., Biostratigraphy of fossil plants: Stroudsburg, PA, Dowden, Hutchinson and Ross, p.153-170.
Ash, S., 1985. A short thick cycad stem from the Upper Triassic of Petrified Forest National Park, Arizona, and vicinity: Museum of Northern Arizona Bulletin, v. 554, p.17-32.
Daugherty, L.H., 1941. The Upper Triassic flora of Arizona: Contributions to Paleontology, Carnegie Institution of Washington Publication 526, Washington, DC.
Miller, C. N., Jr., 1968. The lepidophytic affinities of the genus Chinlea and Osmundites walkeri: American Journal of Botany v.55,p.109-115.
Pigg, K. B., W. C. Davis, and S. Ash, 1993. A new permineralized Upper Triassic flora from Petrified Forest National Park, Arizona: a preliminary report: in Lucas, S. G., and Morales, M., eds. The Nonmarine Triassic, Albuquerque, New Mexico Museum of Natural History & Science Bulletin No. 3, p.411-413.
Stone, J.F., 1978. Pollen and spores: in Ash, S.R., ed., Geology, paleontology, and paleoecology of a Late Triassic Lake, Western New Mexico: Provo, UT, Brigham Young University Geology Studies, v.25, p.45-60.
Taylor, T.N., and E.L. Taylor, 1993. The biology and evolution of fossil plants; Englewood Cliffs, N.J., Prentice Hall.
Figure 1. Longitudinal section of radiospermic seed showing megaspore membrane (m), nucellus (n), and three-parted integument. Inner zone of integument (i) is composed of tangentially elongate, thin-walled cells, middle cone (mi) of radially elongate, barrel-shaped cells with dark walls, and outer zone (ou) is incompletely preserved but apparently of small, compact cells. 2F Top #40 x 36. Figure 2. Oblique longitudinal section of pollen organ showing two pollen sacs and thick outer sporangial wall (at top left). 2E Bot #24 x 90. Figure 3. Oblique transverse section through pollen organ showing three pollen sacs (a-c), sporangial wall (at lower right) and thin-walled tissue surrounding pollen within sacs, and outermost dark-celled tissue delimiting pollen organ. More proximal view of specimen in Fig. 5. 12E Top # 21 x 90. Figure 4. Oblique section of pollen-bearing structure with in situ pollen. 2E Top a #3 x 90. Figure 5. More distal view of specimen in Fig. 3, showing four pollen sacs (a-d). 12E Top #36 x 90. Figure 6. Detail of in situ bisaccate pollen in Fig. 4. 12 E Top #21 x 300.
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