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The Princeton chert: evidence for in situ aquatic plants

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The Princeton chert: evidence for in situ aquatic plants
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  Review of Palaeobotany and Palynology, 70 (1991): 173-185 I73 Elsevier Science Publishers B.V., Amsterdam The Princeton chert Evidence for in situ aquatic plants Sergio R.S. Cevallos-Ferriz a, Ruth A. Stockey b and Kathleen B. Pigg c alnstituto de Geologia, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Apartado Postal 70-296, 04510 Mexico D.E, Mexico bDepartment of Botany, University of Alberta, Edmonton, Alta T6G 2E9, Canada CDepartment of Botany, Arizona State University, Tempe, AZ 85287-1601, U.S.A. (Received February 15, 1990; revised and accepted January 14, 1991). ABSTRACT Cevallos-Ferriz, S.R,S., Stockey, R.A. and Pigg, K.B., 1991. The Princeton chert: Evidence for in situ aquatic plants. Rev. Palaeobot. Palynol., 70: 173-185. The Middle Eocene Princeton chert from southern British Columbia represents one of the richest known assemblages of permineralized Tertiary plants. Affinities with modern aquatic angiosperms, anatomical modifications for the aquatic habit and associated fresh water faunal elements support the interpretation of some components of the Princeton chert as in situ aquatic plants. Among these are fossil plants with affinities to the extant Nymphaeaceae (Allenbya), Araceae (KeratospermaL Alismataceae (Heleophyton), Cyperaeeae/Juncaceae (Ethela) and Lythraceae (Decodon). Anatomical modifications include aerenchyma n vegetative issues (Eorhiza, Dennstaedtiopsis, Heleophyton and Uhlia), he thin-walled tracheary elements without prominent secondary wall thickenings and the presence of protoxylem lacunae surrounded by a ring of cells with thickened inner walls (Heleophyton). Seeds that share morphological features with extant aquatics are characterized by a palisade layer, operculum, external mucilage, small amounts of endosperm and abundant perisperm. Associated faunal elements include turtle bones in the peat matrix and freshwater fish at the top of the section. In situ preservation of these aquatic forms is supported by the presence of rooted axes, the large number of plant organs of the same type and preservation of complete flowers, delicate tissues and organic connections allowing for whole plant reconstruction. Introduction The first descriptions of plants preserved in chert permineralizations from the Middle Eocene Al- lenby Formation were of ovulate cones of Pinus arnoldii, leaves and twigs of P. similkameenensis (Miller, 1973) and axes of Eorhiza arnoldii (Robi- son and Person, 1973; Stockey and Cevallos-Ferriz, 1987), a dicotyledonous rhizome of uncertain affinities. Shortly thereafter, vegetative remains and pollen and seed cones of the taxodiaceous conifer Metasequoia milleri (Basinger, 1976a, 1981, 1984; Basinger and Rothwell, 1977; Rothwell and Basinger, 1979) and a rosaceous flower Paleorosa similkameenensis (Basinger, 1976b; Cevallos-Ferriz et al., 1991) were described. Fungal remains, bryo- phytes and ferns assignable to Dennstaedtiopsis were also described in early reports (Basinger, 1976a; Basinger and Rothwell, 1977). Subse- quently, research has centered on the description of Princeton chert angiosperm fossils. The suite of angiospermous remains that have been described to date (Table I) include fruits and seeds assignable to Nymphaeaceae (Cevallos-Fer- riz and Stockey, 1989), Vitaceae (Cevallos-Ferriz and Stockey, 1990a), Araceae (Cevallos-Ferriz and Stockey, 1988a), Sapindaceae (Erwin and Stockey, 1990), Rosaceae (Cevallos-Ferriz and Stockey, 1991) and Lythraceae (Cevallos-Ferriz and Stockey, 1988b). Vegetative angiosperm remains include woody twigs of Magnoliaceae (Cevallos- Ferriz and Stockey, 1990b) and Rosaceae (Ceval- los-Ferriz and Stockey, 1990c), vegetative remains of Alismataceae (Erwin and Stockey, 1989), Are- caceae (Erwin and Stockey, 1987, 1991c), Cypera- ceae/Juncaceae (Erwin and Stockey, 1991b) and Liliaceae (Erwin and Stockey, 1991a). Addition- ally, flowers and fruits of unresolved affinities have 0034-6667/91/$03.50 c~?:, 1991 Elsevier Science Publishers B.V. All rights reserved  174 S R S. CEVALLOS-FERRIZZ ET AL. been described (e.g., Princetonia, Stockey, 1987; Stockey and Pigg, 1991). Although this list is not exhaustive, and a number of new forms remain to be described, it is clear that enough plants have been described from the Princeton chert to begin to assess their ecologi- cal setting. Strong evidence suggests that while a number of infrequently preserved forms are thought to represent outlying plants whose remains were transported or dropped into the basin (e.g., Vitaceae, Magnoliaceae), a suite of Princeton chert plants were probably preserved in situ. The pres- ence of rooted axes in the chert, large numbers of plant organs of the same type, and preservation of delicate tissues such as inflorescences and phloem sieve plates and sieve areas support this in situ interpretation. Depositional setting, affini- ties with modern aquatic groups, association with fresh water fauna and anatomical modifications suggest that many of these in situ plants were aquatic. Materials and methods The Princeton chert outcrops approximately 8.4 km south of Princeton, British Columbia. Plant specimens occur on the east side of the Similka- meen River in a section consisting of an interbed- ded sequence of chert and coal with an occasional thin ash bed replacing a chert layer. Forty-nine exposed layers of chert have been recorded and systematically sampled (Stockey, 1987). The local- ity has been referred to as locality I (Boneham, 1968) and the Princeton chert locality (Basinger, 1976a; Stockey, 1984, 1987). The outcrop at the Princeton chert locality con- tains black to dark grey thick-bedded siliceous shales alternating with coal seams which are re- placed by plant-rich chert layers (Wilson, 1980). The chert is part of the Allenby Formation of the Princeton Group which has been dated as Middle Eocene based on palynology (Rouse and Srivas- tava, 1970), mammals (Russell, 1935; Gazin, 1953), fish (Wilson, 1977, 1982) and potassium-argon dating (Hills and Baadsgaard, 1967). Further cor- relation of the Princeton Group with other Tertiary sedimentary and volcanic sequences in south central British Columbia is based on fish, insect and plant records (Rouse and Matthews, 1961; Wilson, 1977). Fossils are preserved as silica permineralizations. All chert blocks were cut into slabs and studied using the cellulose acetate peel technique as modi- fied for silicifications with hydrofluoric acid (Joy et al., 1956; Basinger and Rothwell, 1977). Peel sections were mounted in Coverbond xylene- soluble mounting medium for microscopic examination. Plant specimens are housed in the University of Alberta Paleobotanical Collection (UAPC-ALTA). Animal specimens are housed in the University of Alberta Laboratory of Vertebrate Paleontology (UALVP). Results Depositional setting: The Allenby Formation, which contains the Princeton chert member and correlated strata in British Columbia, including the Quilchena Beds, Tranquille Beds, Horsefly River Beds and Driftwood Creek Beds, are inter- preted as soft-bottom lake deposits that show no indication of fluvial sedimentation (Wilson, 1988). Among some of these lake deposits Wilson (1980, 1988) has distinguished three lateral biofacies on the basis of fish, insect and compression plant remains. These include an off-shore/deep water association, an intermediate to near-shore associa- tion, and a shallow/near-shore association. The shallow/near-shore environment is repre- sented by varvate lacustrine sediments in the Horsefly River Beds and several other Eocene lake deposits in British Columbia (Wilson, 1988). Near- shore deposits typically contain abundant disartic- ulated fish assignable to Amia L. and Libotonius Wilson, incompletely preserved insects (Bibioni- dae) and taxodiaceous leaves and show high diver- sity in all fossil groups (Wilson, 1980, 1988). These varvate lacustrine sediments are composed of cou- plets of laminae of different composition that suggest seasonal variation (Olsen et al., 1978; Wilson, 1988). At the Horsefly locality, summer sediments are characterized by a large number of leaves and by coprolite material, while winter sediments have abundant fish scales (Wilson, 1988). A similar pattern occurs in the deposits of  PRINCETON CHERT 175 the Blakeburn mine in southern British Columbia (Wilson, 1988). The Princeton chert locality is also thought to represent a series of lacustrine deposits, and in particular, is indicative of a shallow/near shore environment. Wilson (1982) reported a partially articulated skeleton and several isolated bones of the freshwater fish Amia and disarticulated remains of Amyzon Cope and Libotonius from the shale overlying the plant beds. This shale also contained remains of a soft-shelled turtle (Plate I, 1; Wilson, 1982). Fragments of turtle bones also have been identified in the chert itself (R.C. Fox, 1989, pers. commun.). (It should be noted that in Wilson's (1988) analysis the plant fossils mentioned are compression forms that Wilson interprets as al- lochthonous remains. The permineralized remains in the Princeton chert were not included in his analysis). Affinities to extant aquatic plants: To date, plants described from the Princeton chert include a number of aquatic or partly aquatic angiosperm groups such as the Nymphaeaceae, Lythraceae, Alismataceae, and Araceae (Sculthorpe, 1967; Cronquist, 1981). The Nymphaeaceae are repre- sented by the perrnineralized seeds of Allenbya Cevallos-Ferriz and Stockey (1989). These seeds have a characteristic nymphaeaceous morphology and anatomy. They are 7 mm long, ovoid and anatropous, with an adjacent hilum and micropyle, a longitudinal raphal ridge, an apical operculum, abundant perisperm and small embryos. The integ- ument is composed of an outer columnar palisade, one cell thick, comprised of pitted cells and digitate anticlinal walls and an inner thin-walled zone one to two cells thick (Plate III, 3). As is typical of the Nymphaeaceae, seeds have relatively little endo- sperm and abundant perisperm. In contrast to some seeds of extant Nymphaeaceae, Allenbya seeds lack an aril (Cevallos-Ferriz and Stockey, 1989). In living water lilies this modification can trap air, allowing the seed to float for a short period (e.g., Nymphaea caerulea Savigny, Sculth- orpe, 1967). Seeds of Allenbya lack this structure. However, they do have a prominent integumentary palisade. In extant seeds, e.g., Melilotus alba Medi- cus, such palisades have been shown to be water tight and contain lipids (Bevilacqua et al., 1989). Thus, seeds of Allenbya, like many aquatics, may have had a dormant period (Sculthorpe, 1967) and germinated after the loss of the apical cap. Other extant dicot families represented in the Princeton chert contain both aquatic and non- aquatic members today (Sculthorpe, 1967). For example, in the primarily terrestrial family Lythra- ceae some species of Ammannia L., Lythrum L., Rotala L. and Decodon Gruel. are hydrophytic or occupy a marginal habitat. Decodon verticillatus (L.) Ell. is known to grow along riverbanks and lake shores with its stem bending toward the water. This plant forms a floating rhizome-like structure that produces new roots and vertical shoots (Gra- ham and Graham, 1964; Sculthorpe, 1967). Deco- don allenbyensis from the Princeton chert is known from capsular fruits and seeds that are anatomi- cally very similar to extant D. verticillatus (Cevall- os-Ferriz and Stockey, 1988a). Large numbers of these pyramidal seeds were shed from fruits and, like those extant D.verticillatus, also germinated by means of a germination valve. An additional lythraceous seed type from the Princeton Chert (Cevallos-Ferriz and Stockey, 1988a) is larger (2mm long) than that of D. allenbyensis and exhibits what looks to be an outer mucilaginous integumentary layer (Plate III, 1) resembling that of Lythrum. This layer may enhance buoyancy and/or adhesion to animals for dispersal. A similar floating mechanism has been noted in some aquatic plants whose seeds and small fruits adhere by means of mucilage or muci- laginous hairs to aquatic birds or tetrapods (Sculthorpe, 1967). The Alismataceae, a monocot family of aquatic and semi-aquatic herbs (Cronquist, 1981: Tomlin- son, 1982), is represented by vegetative organs of Heleophyton Erwin and Stockey (1989). Petiolar anatomy in Heleophyton, including vascular bundle arrangement in five distinct series, supports its assignment to this family (Plate I, 5; Erwin and Stockey, 1989). The Araceae is represented in the Princeton chert by fruits and seeds (Plate III, 2) of Kera- tosperma allenbyensis Cevallos-Ferriz and Stockey (1988a). Seed characters that support assignment to the Araceae include features of the integument, micropyle, endosperm and embryo. Seeds of this  N  PRINCETON CHERT 177 type have a spiny integument composed of isodia- meteric sclereids that increase in diameter toward the periphery of the seed, three rows of dorsal spines and idioblasts between spines and ridges. Keratosperma probably produced little endosperm and has been demonstrated to have a monocotyle- donous embryo (Cevallos-Ferriz and Stockey, 1988a). Keratosperma can be most closely com- pared to Cyrtosperma Griffith, a subtropical to tropical genus that lives along the shoreline of lakes. Ethela sargantiana Erwin and Stockey (1991b) stems with attached roots and leaves, show affini- ties to the aquatic taxa of the Cyperaceae (sedges) and Juncaceae (rushes). Leaf anatomy is most similar to genera of the Cyperaceae, however, no silica bodies have been demonstrated. Anatomical modifications: A suite of anatomical features that characterize aquatic plants can be found among many of the Princeton chert plants. These include development of arenchymatous tis- sues, reduced vascular systems and the presence of protoxylem lacunae. Aquatic plants quite fre- quently develop aerenchymatous tissues that pro- vide for aeration and buoyancy (Sculthorpe, 1967). Several Princeton chert plants have aerenchyma produced either schizogenously (Plate I, 2, 5, 7) or lysigenously (Plate II, 4; Esau, 1965). The rhizoma- tous plant Eorhiza has well developed aerenchyma in stems (Plate I, 2) and mature leaves (Plate I, 2, 4: Stockey and Cevallos-Ferriz, 1987). Air spaces in the leaves are more or less rectangular and larger than those in the rhizome which are angular to slightly oval (Robison and Person, 1973; Stockey and Cevallos-Ferriz, 1987). Although im- mature leaves have a mesophyll composed of dense parenchyma (Plate I, 3), as leaves mature they become aerenchymatous (Plate I, 2). Petioles of emergent and submerged plants and peduncles of nymphaeaceous taxa are charac- terized by large air spaces (Sculthorpe, 1967; Tom- linson, 1982). Such aerenchyma is thought to maximize resistance to strain from bending and pulling and to enhance aeration in submerged organs (Sculthorpe, 1967). The alismataceous genus Heleophyton illustrates this anatomy (Erwin and Stockey, 1989; Plate I, 5). The coryphoid palm, Uhlia also has well devel- oped aerenchyma in all its vegetative parts with the exception of its leaf laminae (Basinger, 1976a: Erwin and Stockey, 1987, 1991c). The area occu- pied by aerenchyma varies from organ to organ. Roots have a comparatively larger zone of aeren- chyma in contrast to stems and petioles. Root cortical tissues are composed of parenchymatous cells interspersed with smaller, polyhedral cells with dark contents, and radially elongate lacunae (Plate I, 7). In the stem and petiole, lacunae are less regularly arranged and are scattered through- out the cortex (Plate I, 6). This variation in tissue structure may suggest that stems and petioles were emergent. Several types of fern rhizomes and petioles from the chert are presently being investigated (H. Nis- hida, pers. commun., 1990). These forms resemble Dennstaedtiopsis and like this taxon represent fili- calean ferns rather than true heterosporous water ferns (Arnold and Daugherty, 1964). However, unlike the living Dennstaedtia (Arnold and Daugh- erty, 1964), their vegetative body was at least partially aerenchymatous (Plate II, 1, 6; Basinger, 1976a). Aerenchyma is also found in a polypodia- ceous fern referred to as "Fern B" by Basinger (1976a; Plate II, 2). Another organ with lacunar tissue is a fruit whose affinities remain uncertain (Cevallos-Ferriz, PLATEI 1. Carapace and part of the skeleton of a soft-shelled turtle from the shale above the chert. UAVLP 13392, x 0.75. 2. Transverse section of a rhizome of Eorhiza with air spaces in leaf mesophyll (above) and rhizome cortex (below). P2733 H bot No.10, x33. 3. Transverse section of a bud of Eorhiza. Note absence of aerenchyma in young leaves. P2674 E bot No.2, × 16. 4. Transverse section of Eorhiza vegetative leaf. P4268 B top No.0, x 11. 5. Transverse section of Heleophyton petiole. P2313 B top No.0, × 190. 6. Transverse section of Uhlia petiole. P1274 A2 no.12, x 38. 7. Transverse section of coryphoid palm root with radially elongate lacunae. P1124 E bot No.0, × 35.
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