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a Department of Biological Sciences, Kent State University, Kent, Ohio 44242
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSLOCALITIES, AGE, AND...COMPOSITIONDISCUSSIONREFERENCES |
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Key Words: Chiapas • early Miocene • La Quinta Formation • Mexico • palynoflora
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSLOCALITIES, AGE, AND...COMPOSITIONDISCUSSIONREFERENCES |
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Megafossils reported from the region are leaflets and possible sepals of Hymenaea, a fossil flower of Tapirira, and a leaf of Acacia from Oligo-Miocene amber at Simojovel (Miranda, 1963). The amber has been shown by infrared spectrophotometry to be derived from Hymenaea (Langenheim, 1966). Plant microfossils have been described from the lower to middle Miocene Méndez Formation near Pichucalco/Raudales de Malpaso (Palacios Chávez and Rzedowski, 1993; Fig. 1) from the middle to upper Miocene Ixtapa Formation along Highway 195 near Ixtapa (Martínez-Hernández, 1992) and from a series of exposures of Eocene to latest Oligocene age from the region of Simojovel (Tomasini-Ortiz and Martínez-Hernández, 1984).
Pollen and spore assemblages have also been reported from other outcrops of Oligo-Miocene age near Simojovel by Langenheim, Hackner, and Bartlett (1967). That study was concerned primarily with establishing the depositional environment for amber inclusions within the Simojovel sediments. Pollen of the mangrove Rhizophora was studied in detail, while other palynomorphs were mentioned only briefly. Some additions to the list of palynomorphs also were made by Graham and Palacios Chávez (1996). In this study a more complete inventory is presented (Table 1), along with an estimate of the paleocommunities and the Oligo-Miocene climate and paleophysiography.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSLOCALITIES, AGE, AND...COMPOSITIONDISCUSSIONREFERENCES |
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LOCALITIES, AGE, AND STRATIGRAPHY |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSLOCALITIES, AGE, AND...COMPOSITIONDISCUSSIONREFERENCES |
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The area around Simojovel is in the Front Ranges and High Plateau physiographic province of Frost and Langenheim (1974; Fig. 1). In the diagram presented by Breedlove (1981), based on Müllerried (1957), this province is divided into the Central Plateau (adjacent to the Central Depression), Eastern Highlands, and Northern Highlands. The locality is ~90 km inland from the present Gulf Coast, but at the time the sediments were being deposited, the coast was farther west as indicated by the presence of marine invertebrates and Rhizophora-containing lignites in the Simojovel sediments.
The general geology of Mexico has been summarized by de Cserna (1989) and by Ferrusquía-Villafranca (1993), and the regional geology around Simojovel is described by Frost and Langenheim (1974; Fig. 4). In these summaries, the strata at the sites studied here, and others mentioned in Langenheim, Hackner, and Bartlett (1967), are assigned to the transition between the late Oligocene and early Miocene. They belong to the La Quinta Formation, which is divided by Allison (1967) into the Camino Carretero, Florida Limestone, and Finca Carmitto members. In the stratigraphic charts of Frost and Langenheim (1974, p. 29), lignites are described only from the Florida Limestone Member. They note (p. 31) that "the Florida Limestone Member contains a coral fauna which is unequivocally Early Miocene" and "thus the Florida Limestone Member and the uppermost part of the Camino Carretero Member are Early Miocene." These subdivisions of the La Quinta Formation, however, are not evident at all sites, and there have been no subsequent detailed stratigraphic studies published for the region. The palynomorphs described here are probably part of the Florida Limestone Member of the La Quinta Formation and are considered early Miocene in age. The composition of the Simojovel flora is given in Table 1, and the age, location, and references to other palynofloras mentioned in the text are given in Table 2.
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COMPOSITION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSLOCALITIES, AGE, AND...COMPOSITIONDISCUSSIONREFERENCES |
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spore length, inner margin entire; verrucate, verrucae low, irregular, ~4–6 µm in diameter; size 65–67 x 50–52 µm. Loc. Pl-7, slide 1, ESF G-43,1–3. Spores of this generalized type occur in virtually all Cenozoic palynofloras in northern Latin America, although these specimens are distinctive by their relatively large size. Similar spores are produced by members of the Blechnaceae, Polypodiaceae, Pteridaceae, and the parent plants grow in a wide range of habitats (Tryon and Tryon, 1982).
Monolete fern spore type 2 (Fig. 7). This spore type (52–54 x 36–38 µm) is similar to type 1, and it is distinguished by the more prominent verrucae. It is also produced by several members of the Blechnaceae, Polypodiaceae, and Pteridaceae, which grow in a variety of habitats, and the spore type is widespread in Cenozoic deposits of northern Latin America. Loc. Pl-7, slide 1, ESF T-40,4.
Sphaeropteris/Trichipteris (Cyathaceae; Fig. 8). Oblate, oval-triangular, apices rounded, outer margin lobate due to pits; trilete, laesurae straight, ~18 µm long, extending ~ distance to spore margin; wall conspicuously pitted, pits circular, 1.5–4 µm in diameter, inner margin entire, raised border ~1.5 µm wide, irregularly distributed; size 45 µm. Loc. Pl-7, slide 1, ESF T-30,1. Previous record: Paraje Solo Formation.
Sphaeropteris is a genus of ~23 species of tree ferns in the neotropics. It grows from Veracruz, Mexico, to Andean South America, Peru, Bolivia, and Amazonian and southeastern Brazil. It is primarily a plant of the lowland tropics, but it ranges up to 2000 m in elevation (Tryon and Tryon, 1982). In Chiapas, S. horrida grows on the edges of pine-oak-Liquidambar forests, and S. myosuroides grows in the montane rain forest (Smith, 1981). Trichipteris is a genus of ~55 species of tree ferns distributed from Veracruz, Mexico, to Brazil and Argentina. It grows in lowland rain forest, wet montane and cloud forests, and can extend into pine-oak-Liquidambar forests, savannas, grasslands, swamps, and brackish-water habitats. It is mostly found up to elevations of ~2000 m, but it can extend to 3500 m (Tryon and Tryon, 1982).
Ceratopteris (Pteridaceae; Fig. 9). Oblate, oval-triangular, apices rounded; trilete, laesura straight, 30–32 µm, extending ½ to distance to spore margin; striate, striae in numerous, broad, smooth bands 4–5 µm wide, area between bands 1–2 µm wide; wall 2–3 µm thick; size ~140 µm (folded). Loc. A-4 (= HU D-621, Simojovel landslide area), slide 1, ESF S-22 (also D-621, slide 1, ESF F-39). Prevous records: Cucaracha, Gatun, Gatuncillo, Paraje Solo formations.
Ceratopteris consists of two closely related species in the New World (two others occur in the Old World tropics). They typically grow as floating ferns in freshwater, but can range into brackish water habitats, and are most commonly found at elevations up to ~300 m. The genus is unreported for Chiapas, but probably occurs there (Smith, 1981).
Pteris (Pteridaceae; Fig. 10). Oblate, triangular, apices rounded; trilete, laesura obscure, ~26–28 µm long, bordered by lip ~4–5 µm wide; proximal surface laevigate (obscure), distal surface coarsely verrucate, verrucae low, irregular, plate-like, ~6–8 µm in diameter; flange 6–8 µm wide bordering outer spore margin, flange smooth, outer margin entire; wall ~2 µm thick; size 46–48 µm. Loc. Pl-7, slide 1, ESF P-34,3. Previous records: Artibonite, Cucaracha, Culebra, Gatun, Gatuncillo, Ixtapa, La Boca, Rio Banano, Uscari formation/group/sequence.
Pteris (~280 species) is a tree fern with ~60 species in the neotropics typically growing at low-to-mid-elevations in tropical moist and tropical wet forests.
cf. Antrophyum (Vittariaceae; Fig. 11). Oblate, amb triangular, apices rounded; trilete, laesura straight, 25–27 µm long, extending ~¾ distance to spore margin, inner margin entire; laevigate; wall ~2 µm thick; size 76–78 µm. Loc. Pl-7, slide 1, ESF M-39. Previous records: Artibonite, Cucaracha, Culebra, Gatuncillo, Herrería, La Boca formations/group.
Antrophyum is a genus of ~35 species, with ~10 in the neotropics. It is a member of rain forest vegetation, but it extends up to ~1500 m in elevation into cloud forests. The spore is larger than those in other assemblages from northern Latin America (e.g., Herrería Formation of Guatemala), and this may be due to swelling from storage of the HU specimens in glycerine jelly since ~1965.
Picea (Pinaceae; Fig. 12). Monocolpate, colpus extending between air sacs on lower side of body; vesiculate, air sacs 2, approximately spherical, reticulate and grading into sculpture pattern of body, 28 x 30 µm, body scabrate, 42–45 x 40–42 µm; no reentrant angle, no marginal flange; wall ~2 µm thick; size 61–63 x 40–42 µm. Loc. Pl-7, slide 1, ESF Q-41,1–2. Previous records: Paraje Solo, Padre Miguel formation/group.
These specimens are slightly corroded and possibly enlarged through long-time storage in glycerine jelly. In other respects they are similar to pollen of Picea. The closest stands of spruce are presently in the mountains of northern Mexico (P. mexicana, Coahuila, Nuevo Leon; P. chihuahuana, Chihuahua, Durango).
cf. Crysophila (Palmae; Fig. 13). Prolate-spheroidal; monocolpate, colpus straight, 20–22 µm long, extending entire length of grain, inner margin entire; reticulate, reticulum regular, width of muri (~1 µm) approximately equal to diameter of lumen, surface of muri smooth; tectate-perforate, wall ~2 µm thick, columellae evident (400x magnification); size 25–27 x 22–24 µm. Loc. Pl-7, slide 1, ESF U-30. Previous records: Cucaracha, Culebra formations.
Palmae type 1 (Fig. 14). Prolate; monocolpate, colpus straight, 19–20 µm long, extending nearly entire length of grain, inner margin entire; scabrate; tectate, wall ~2 µm thick, columellae evident (400x magnification); size 25–27 x 17–19 µm (widest part just off equator of grain). Loc. Pl-7, slide 1, ESF 29,2.
This generalized type of palm pollen is widespread in Cenozoic deposits of northern Latin America. The fossil pollen is not of the Sabal-Scheelea type, which are the two prominent genera in the modern palm forest of Chiapas (see Discussion: Modern vegetation, below). It probably represents a palm of the lowland to premontane wet/moist forests.
Bombacaceae (cf. Aguiaria type; Fig. 15). Peroblate, amb oval-triangular to nearly circular; tricolpate, colpi equatorially arranged, meridionally elongated, equidistant, situated in interapex area, short (4–5 µm, apex to equator), bordered by costae colpi ~3 µm wide; reticulate, reticulum regular, width of muri slightly less than diameter of lumen (~2 µm), surface of muri smooth; tectate-perforate, wall ~2 µm thick, columellae evident (400x magnification); size 25–27 µm. Loc. Pl-7, slide 1, ESF U-38,4. Previous records: Gatun, La Boca, Uscari formations/sequence.
These specimens are of the general Aguiaria type, but the wall is thinner; the Gatun, La Boca, and Uscari grains are more similar. Aguiaria is presently a monotypic genus of Brazil.
Ericaceae (Fig. 16). Tetrahedral tetrads, individual grains roughly spherical (compressed in tetrads); tricolpate, colpi equatorially arranged, meridionally elongated, equidistant, straight, inner margin entire, bordered by narrow margo ~2 µm wide, colpi shared (continuous across contact between grains); psilate; tectate, wall 2–3 µm thick, homogeneous (400x magnification); size—individual grains ~15 µm, tetrads 25–27 µm. Loc. Pl-7, slide 1, ESF P-39, 3–4. Previous records: Gatun, Uscari formation/sequence.
Pollen of the Ericaceae cannot be identified to genus. The family is represented in Central America by ~20 genera, which are most common in upland habitats.
Alfaroa/Oreomunnea (Juglandaceae; Fig. 17). Oblate, amb oval-triangular; triporate, pores equatorially arranged, meridionally elongated, equidistant, circular, 3–4 µm in diameter, inner margin entire; psilate to finely scabrate; tectate, wall 2 µm thick, homogeneous (400x magnification); size 24–25 µm. Loc. Pl-7, slide 1, ESF M-40, 1–3. Previous records: Artibonite, Cucaracha, Gatun, Gatuncillo, Herrería, La Boca, Paraje Solo, San Sebastian formations/group.
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Crudia (Leguminosae, Caesalpinioideae; Fig. 18). Prolate; tricolporoidate, colpi equatorially arranged, meridionally elongated, equidistant, straight, 40–42 µm long, extending nearly entire length of grain, pore area faint, circular, situated at midpoint of colpus; distinctly and coarsely striate, striae oriented parallel to long axis of grain, surface of striae smooth; tectate, wall ~2 µm thick, homogeneous (400x magnification); size 48–50 x 32–34 µm. Loc D-621, slide 1, ESF K-30. Previous records: Cucaracha, Gatun, Gatuncillo, La Boca formations.
Crudia is primarily an Amazonian genus of approximately ten species growing in riverine habitats. In addition to the microfossil record, pods of Crudia have been reported from the Eocene of western Tennessee (Herendeen and Dilcher, 1990).
Cedrela (Meliaceae; Figs. 19–21). Prolate-spheroidal, amb circular; tetracolporate, colpi equatorially arranged, meridionally elongated, equidistant, straight, 25–27 µm long, pores situated at midpoint of colpus, slightly elongated equatorially, 4–6 x 3–5 µm, costae pori ~4 µm wide; psilate; tectate, wall ~3 µm thick, homogeneous (400x magnification); size 46–48 x 37–39 µm. Loc. Pl-7, slide 1, ESF X-31, S-45, S-38,3. Previous records: Gatun, Paraje Solo formations.
Cedrela includes approximately six species of trees in Mexico and Central America. The two most common species are C. odorata, distributed from northern Mexico to South America, and C. angustifolia found along the Pacific slopes. Both grow at low-to-moderate elevations, and mostly in tropical moist and premontane wet forests, although C. angustifolia can extend into the drier habitats of the tropical deciduous forest.
Guarea (Meliaceae; Fig. 22). Oblate-spheroidal, amb circular; tetracolporate, colpi equatorially arranged, meridionally elongated, equidistant, straight, relatively short (8–9 µm), inner margin entire, pores situated at midpoint of colpus, elongated equatorially, 2 x 5 µm, inner margin entire; psilate; tectate, wall ~3 µm thick, homogeneous (400x magnification); size 22–24 µm. Loc. Pl-7, slide 1, ESF T-32,2. Previous records: Paraje Solo, San Sebastian, Gatun formations.
Guarea is a genus of ~150 species, with G. glabra and G. multiflora being two of the common Central American species. Both are distributed from Mexico to South America and grow as understory trees at low-to-moderate elevations in tropical wet and tropical moist forests.
Eugenia/Myrcia (Myrtaceae; Fig. 23). Oblate, amb triangular; tricolporate to tricolporoidate, syncolpate, colpi equatorially arranged, meridionally elongated, equidistant, straight, 7–9 µm long, inner margin entire, pore obscure, situated at mid-point of colpus, ~1 µm in diameter; psilate to scabrate; tectate, wall 2 µm thick, homogeneous (400x magnification); size 19–21 µm. Loc. Pl-7, slide 1, ESF Q-30,2–4. Previous records: Cucaracha, Culebra, Gatun, Gatuncillo, Paraje Solo, San Sebastian, Uscari formations/sequence.
The pollen of these genera cannot be distinguished at the level of light microscopy (Graham, 1980), and the family is widespread in a wide variety of habitats.
Pelliciera (Pelliceriaceae; Fig. 24). Oblate, amb circular; tricolporate, colpi equatorially arranged, meridionally elongated, equidistant, 22–26 µm long, inner margin entire, pore circular, 3–4 µm in diameter, situated at midpoint of colpus, inner margin entire; sculpture variable from finely to coarsely verrucate; tectate, wall 4 µm thick, columellae evident (400x magnification); size variable (45–65 µm). Loc. A-4 (= HU D-621, Simojovel landslide area), slide 1, ESF A-49. Previous records: Gatuncillo, La Boca, Rio Banano, San Sebastian, Chapelton formations.
Pelliciera (= Pelliceria) is a small mangrove tree presently distributed from Costa Rica to northwest Colombia and Ecuador. Its more widespread distribution in the Tertiary of the Caribbean region has been summarized by Graham (1977, 1995).
Rhizophora (Rhizophoraceae; Figs. 25–26). Prolate to prolate-spheroidal, amb circular; tricolporate, colpi equatorially arranged, meridionally elongated, equidistant, narrow, straight, 13–16 µm long, inner margin entire, costae colpi ~2 µm wide, pores elongated equatorially (colpi transversalis), ~1 x 4 µm, situated at midpoint of colpus, inner margin entire; finely reticulate; tectate-perforate, wall 2–3 µm thick, columellae evident (400x magnification); size 18–22 x 23–25 µm. Loc. Pl-7, slide 1, ESF G-39,2, U-37,1–3. Previous records: Cucaracha, Culebra, Gatun, Gatuncillo, Herrería, La Boca, Paraje Solo, Rio Banano, San Sebastian, Uscari formations/sequence.
Rhizophora is a small tree or shrub growing in brackish water along the coasts of warm-temperate to tropical regions. Its geologic history in the Caribbean region, along with that of other mangroves, has been summarized by Graham (1977, 1995).
Unknown 1 (Fig. 27). Prolate, amb circular; tricolporate, colpi equatorially arranged, meridionally elongated, equidistant, 32–34 µm long, inner margin entire, bordered by costae colpi 3–4 µm wide, pore circular, ~4 µm in diameter, situated at midpoint of colpus, inner margin entire; reticulate, reticulum regular, becoming finer toward colpus to form narrow margo, width of muri approximately equal to diameter of lumen, surface of muri smooth; tectate-perforate, wall 3 µm thick, columellae evident (400x magnification); size 40 x 30 µm. Loc. Pl-7, slide 1, ESF O-31.
Unknown 2 (Fig. 28). Prolate, amb circular; tricolporate, colpi equatorially arranged, meridionally elongated, equidistant, 20 µm long, inner margin entire, bordered by costae colpi 3–4 µm wide, pore circular, ~4 µm in diameter, situated at midpoint of colpus, inner margin entire; scabrate; tectate, wall 3 µm thick, homogeneous to columellae just evident (400x magnification); size 28 x 18 µm. Loc. Pl-7, slide 1, ESF Q-35, 1–3.
Unknown 3 (Fig. 29). Prolate to prolate-spheroidal, amb circular, tricolporate, colpi equatorially arranged, meridionally elongated, equidistant, 12 µm long, inner margin entire to minutely dentate, bordered by costae colpi ~3 µm wide, pores slit-like, ~0.5 x 3 µm, situated at midpoint of colpus; finely reticulate, reticulum regular, width of muri approximately equal to diameter of lumen (~1 µm), surface of muri smooth; tectate-perforate, wall ~3 µm thick, columellae evident (400x magnification); size 21 x 16 µm. Loc. Pl-7, slide 1, ESF M-32.
Unknown 4 (Fig. 30). Oblate to oblate-spheroidal, amb circular; tricolpate (pores, if present, obscure), colpi equatorially arranged, meridionally elongated, equidistant, 10–12 µm long, inner margin entire to minutely dentate; finely reticulate, reticulum regular, width of muri approximately equal to diameter of lumen (~1 µm), surface of muri smooth; tectate-perforate, wall ~2 µm thick, columellae evident (400x magnification), size 30 µm. Loc. Pl-7, slide 1, ESF N-39,1.
Unknown 5 (Fig. 31). Oblate, amb circular; tricolporate, colpi equatorially arranged, meridionally elongated, equidistant, 6–8 µm long, inner margin entire; pores circular (obscure), ~4–5 µm in diameter, situated at midpoint of colpus; finely reticulate, reticulum regular, width of muri approximately equal to diameter of lumen (~1 µm), becoming finer toward poles and colpi, surface of muri smooth; tectate-perforate, wall ~2 µm thick, columellae evident (400x magnification); size 27 µm. Loc. Pl-7, slide 1, ESF U-39,2–4.
In addition to the 19 identified and five unknown palynomorphs described here from the Simojovel assemblage, Palacios Chávez (in Graham and Palacios Chávez, 1996) provided photographs of Ilex and Asteraceae pollen from samples processed at the Instituto Politecnico Nacional, Mexico City. Also, Langenheim, Hackner, and Bartlett (1967, p. 319) found "a few poorly preserved grains probably of Pinus," Podocarpus, and Pachira, which were not seen during scans from two (D-621, Pl-7) of their 13 localities in the HU collections. The glycerine jelly in many of the HU slides has partly or completely deteriorated and some specimens are no longer recognizable. Ilex is widespread in northern Latin America and grows in a variety of habitats, Pinus and Podocarpus are members of the upland evergreen cloud forests, and Pachira is a lowland tropical, riverine plant occupying habitats similar to those of Crudia in South America.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSLOCALITIES, AGE, AND...COMPOSITIONDISCUSSIONREFERENCES |
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Breedlove (1981) classifies the vegetation of Chiapas into 19 formations [18 in Breedlove, (1973)]:
Tropical rain forest (= selva alta siempre, in part; selva alta perennifolia, in part; bosque tropical perennifolio, in part). This forest is mostly restricted to valleys bordering the Rio Usumacinta on the low eastern slopes of Front Ranges and High Plateaus and on Gulf Coastal Plain. It is surrounded by lower montane rain forest, and includes as the prominent genera Aspidosperma, Brosimum, Dialium, Erblichia, Ficus, Guatteria, Manilkara, Poulsenia, Protium, Swietenia, Talauma, Terminalia, and Vatairea, with an understory of Alchornea, Alibertia, Belotia, Blepharidium, Blumea, Bursera, Cassia, Cephaelis, Dracaena, Forchhammeria, Guarea, Hasseltia, Hirtella, Lacistemma, Licaria, Orthion, Ouratea, Piper, Pithecellobium, Quararibea, Sickingia, Wimmeria, and Zuelania.
Lower montane rain forest (selva alta siempre verde, in part; selva alta perennifolia, in part; bosque tropical perennifolio, in part; tropical premontane moist, wet, or rain forest). Remnants of this forest suggest it potentially would cover much of the Front Ranges and High Plateaus, and it also occurs as a narrow band along the western lowlands of the Sierra Madre de Chiapas. It is similar to the tropical rain forest, but it lacks the uppermost canopy layer of tallest trees, and lianas and epiphytes are more common. Most of the tropical rain forest genera are present, in addition to Alseis, Belotia, Calophyllum, Chaetoptelea (Ulmus), Licania, Nectandra, Ocotea, Quercus, Sebastiana, Talauma, and Vochysia in the canopy, and Chrysophyllum, Cleidion, Cymbopetalum, Faramea, Piper, Rinorea, Sloanea, Stemmadenia, and Trophis in the understory.
Montane rain forest (= selva mediana y baja siempre verde, in part; selva mediana o baja perennifolia, in part; bosque tropical perennifolio, in part; tropical montane moist, wet, rain forest). This forest is found at elevations of 900–2200 m in the Front Ranges and High Plateaus, and on the eastern and western slopes of the Sierra Madre de Chiapas. It is characterized by numerous epiphytic ferns, aroids, bromeliads, and orchids. The prominent genera include Alfaroa, Ardista, Brunellia, Calatola, Hedyosmum, Hyperbaena, Lunania, Matudea, Melisoma, Mirandaceltis, Oecopetalum, Oreopanax, Quercus, Platanus, Synardisia, Trichilia, and Turpinia. Understory trees and shrubs are Acalypha, Billia, Centropogon, Cephaelis, Chamaedorea, Eugenia, Hampea, Miconia, Mollinedia, Oreopanax, Palicourea, Parathesis, Psychotria, Siparuna, Trophis, and Urera.
Evergreen cloud forest (= cloud forest, in part; selva mediana y baja siempre verde, in part; selva mediana o baja perennifolia, in part; subtropical montane wet or rain forest). This forest is found near the crests of mountains at elevations of 1900–3200 m in the Front Ranges and High Plateaus and in the Sierra Madre de Chiapas. Hanging mosses and epiphytic ferns are common, fog and clouds are usually present, and minimum January temperatures may approach 0°C, although frost is rare. The common trees are Abies, Pinus, Podocarpus, Acer, Chiranthodendron, Clethra, Drimys, Magnolia, Meliosma, Microtropis, Olmediella, Oreopanax, Persea, Photinia, Quercus, Weinmannia, Wimmeria, and Zinowiewia. Understory plants include Cavendishia, Cleyera, Deppea, Eugenia, Fuchsia, Gentlea, Greigia, Mahonia, Miconia, Rapanea, Saurauia, Symplocos, Vaccinium, and Viburnum.
Evergreen seasonal forest (= selva alta subdecidua; selva alta or mediana subperennifolia; selva alta or mediana subcauducifolia; bosque tropical perennifolio, in part; tropical or subtropical premontane, moist forest). There are several transitional communities between the tropical rain forest and other vegetation types as rainfall decreases and becomes more seasonal. In the evergreen seasonal forest there is a marked dry season, and this vegetation covers much of the slopes of the Sierra Madre de Chiapas up to an elevation of ~1200 m, the southern and western part of the northern Front Ranges and High Plateaus, and the Gulf Coast Plain. Important trees include Astronium, Bernoullia, Brosimum, Brumelia, Calycophyllum, Ceiba, Coccoloba, Cordia, Cupania, Elaeodendron, Enterolobium, Ficus, Guettarda, Hymenaea, Lafoensia, Licania, Platymiscium, Rheedia, Sapium, Sterculia, Styrax, Tabebuia, and Vatairea. Associated shrubs and small trees are Amyris, Ardisia, Bourreria, Calyptranthes, Clusia, Eugenia, Gentlea, Karwinskia, and Psychotria.
Tropical deciduous forest (= deciduous seasonal forest, selva baja decidua, selva baja cauducifolia, bosque tropical deciduo, tropical or subtropical dry and very dry forest). In regions covered by this forest there is a pronounced dry season of 4–6 mo. It occurs along the Gulf Coastal Plain, on the lower hills of the Sierra Madre de Chiapas, and in the Central Depression. The common trees are Annona, Bucida, Bursera, Calycophyllum, Cecropia, Cedrela, Ceiba, Cochlospermum, Cordia, Eysenhardtia, Gliricidia, Godmania, Hauya, Heliocarpus, Hura, Ipomoea, Leucaena, Luehea, Lysiloma, Plumeria, Pseudobombax, Spondias, Stemmadenia, Swietenia, Tabebuia, and Triplaris.
Short-tree savanna (= sabana, selva baja subperennifolia, vegetacion sabanoide, savanna). This community grows on gradual slopes or flat bottomlands on shallow, poorly drained soils where it is usually associated with various other vegetation types. The common trees are Byrsonima, Crescentia, and Curatella, with Acacia, Alvaradoa, Cordia, Piscidia, Quercus, and Tetracera.
Thorn woodland (= selva baja espinosa cauducifolia, bosque espinoso, tropical and subtropical thorn woodland). This community is prominent in the Isthmus of Tehuantepec and it extends into Chiapas only in the northern part of the Central Depression and along the Pacific Coastal Plain. Important members include Acacia, Bauhinia, Buettneria, Bursera, Casearia, Cordia, Croton, Diphysa, Erythroxylon, Jacquinia, Karwinskia, Piptadenia, Pisonia, Pithecellobium, Randia, Sageretia, Stenocereus, and Zizyphus.
Pine-oak-Liquidambar forest (= cloud forest, in part; deciduous forest; bosque cauducifolio; bosque deciduo; bosque mesofilo de montãna; subtropical montane moist forest). This forest occurs on moist north and east-facing upland slopes in the Front Ranges and High Plateaus and in the Sierra Madre de Chiapas. Many of the genera also grow in the deciduous forest of eastern United States and include Pinus, Carpinus, Cassia, Citharexylum, Clethra, Cornus, Cupania, Erythrina, Fraxinus, Liquidambar, Meliosma, Montanoa, Nyssa, Ostrya, Perrottetia, Quercus, Rhus, Saurauia, Styrax, and Turpinia. Associates include Aphelandra, Baccharis, Bocconia, Cestrum, Fuchsia, Gaultheria, Guamatela, Hibiscus, Liabum, Parathesis, Phenax, Rapanea, Solanum, Symplocos, Triumfetta, Verbesina, and Viburnum.
Pine-oak forest (= pinares and encinares, bosque de pino y encino). This forest is best developed on dry south and west-facing slopes of the central Front Ranges and High Plateaus and on the east side of the Sierra Madre de Chiapas, mostly at elevations between 1300 and 2500 m. It includes Pinus, Arbutus, Buddleia, Crataegus, and Quercus. Common shrubs are Ceanothus, Chiococca, Garrya, Holodiscus, Lippia, Litsea, Mahonia, Monnina, Myrica, Rhus, Senecio, Solanum, and Viburnum.
Swamp and lowland riparian forest (= manglar, in part; canacoital). This forest is well developed along lowland rivers and in swamps directly behind the mangroves. The prominent trees are Taxodium, Andira, Bravaisia, Bucida, Calophyllum, Haematoxylum, Pachira, and Salix.
Mangrove swamp (= manglar). Mangrove vegetation extends along the entire Pacific Coast of Chiapas. The dominants are Avicennia, Conocarpus, Laguncularia, and Rhizophora. Batis and Philoxerus are common associated herbs.
Palm forest (= palmar). Vegetation dominated by palms is found along rivers in the Central Depression, along the Rio Usumacinta and tributaries in the eastern portion of the Front Ranges and High Plateaus, and in the southern portion of the Pacific Coastal Plain. The common palms are Sabal and Scheelea. Palm forests benefit and may result from human actitivies
Temperate riparian forest. This forest occurs along streams at elevations above 1500 m in the eastern Front Ranges and High Plateaus. Members include Acer, Alnus, Baccharis, Berchemia, Cornus, Crataegus, Cuphea, Platanus, and Salix.
Bunch grassland (= zacatonal). Communities characterized by tall bunch grasses are found in the eastern Front Ranges and High Plateaus, and include Briza, Bromus, Festuca, Muhlenbergia, Stipa, and Trisetum.
Evergreen cloud scrub (= paramo; paramos de altura; tropical or subtropical montane or subalpine moist, wet, or rain paramo). This vegetation occurs on the highest peaks of the Front Ranges and High Plateaus and the Sierra Madre de Chiapas. The common members are Lycopodium, Gleichenia, Alchemilla, Arenaria, Beshorneria, Calamagrostis, Comarostaphylis, Draba, Festuca, Gaultheria, Gnaphalium, Haplopappus, Holodiscus, Lupinus, Luzula, Muhlenbeckia, Myrrhidendron, Pernettya, Potentilla, Ternstroemia, Trisetum, Vaccinium, Weldenia, and Werneria.
Herbaceous marsh (= tulare, popal). Marshes occur scattered through Chiapas in areas of low elevation. The composition is variable, but frequently includes Calathea, Caperonia, Carex, Caladium, Cyperus, Heliconia, Hymenachne, Juncus, Leersia, Ludwigia, Lythrum, Neptunia, Nymphaea, Paspalum, Phragmites, Rhynchospora, Scirpus, Sesbania, Thalia, and Typha.
Coastal strand vegetation (= cordon littoral). This community is found on sandy beaches of the Pacific Coast and is characterized by scandent shrubs and herbs. The common members are Acanthocereus, Alibertia, Canavalia, Capparis, Coccoloba, Distichlis, Ipomoea, Monanthocloë, Okenia, Pectis, Prosopis, Salpianthus, Sesuvium, Stegnosperma, Uniola, and Ximenia.
Second-growth and successional forest and shrub associations. This is a disturbance and recovery vegetation of variable composition, but frequently includes Abutilon, Acacia, Arctostaphylos, Baccharis, Calliandra, Ceanothus, Clibadium, Coaxana, Lantana, Lippia, Malvaviscus, Muhlenbeckia, Pavonia, Smallanthus, Rhamnus, Rubus, Salvia, Tithonia, Vernonia, and Viburnum.
Paleocommunities and physical setting
The distribution of taxa identified from the La Quinta flora is arranged according to the extant community types in Chiapas in Table 3. The occurrence of fossil representatives of the modern vegetation elements in other Tertiary floras of the Gulf/Caribbean region is given in Table 4. One of the principal paleocommunities present during the early Miocene in eastern Chiapas was mangrove. This community now grows on the Pacific Coast Plain, and on the Gulf Coast Plain of Veracruz and Tabasco beyond the political boundaries of Chiapas. In the Tertiary, however, it was part of the shallow, brackish-water vegetation in the vicinity of Simojovel. It was different from the modern mangrove vegetation of Mexico and northern Central America because in addition to Rhizophora it included Pelliciera, which is now distributed from Costa Rica to northwestern South America.
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At higher elevations there was a montane rain forest as suggested by Alfaroa and possibly Eugenia, which are mostly lower montane/montane transition plants, and an evergreen cloud forest of Picea, Pinus, Podocarpus, and possibly Cavendishia/Vaccinium. In somewhat drier habitats there was a version of the evergreen seasonal forest (Hymenaea, Ilex, and possibly Eugenia), and the tropical deciduous forest (Cedrela).
Absent or poorly represented in the La Quinta flora are defining members of distinctly dry to arid commuities and high-elevation vegetation. The absence of pollen from the latter community may be a result of distance from the lowland basin of deposition, but no dominant plants of high-montane or paramo habitats were being blown or washed into the coastal sediments. Pollen of one of the highest-elevation trees in Mexico (Abies) does occur in other Neogene floras (e.g., Paraje Solo). There is also meager representation of pollen of northern temperate elements (Picea, possibly Pinus) in the La Quinta flora. This is consistent with the pattern based on other Tertiary assemblages in northern Latin America (Graham, 1973, 1997). These elements are mostly absent to poorly represented in early Miocene and older floras, prior to the middle Miocene temperature decline (Miller, Fairbanks, and Mountain, 1987), and appear in greater numbers in late Miocene and younger floras. There is also a trend from the greatest number of northern temperate taxa in northern Latin American Tertiary palynofloras (e.g., 12 in the Paraje Solo flora of Veracruz, Mexico), to fewer numbers to the south (e.g., only Quercus in the Gatun flora of central Panama). This places the principal time of introduction for these elements in the late Miocene and later, with the early and middle Miocene being the transitional time during which they first begin to appear.
Thus, the paleovegetation in the vicinity of Simojovel in the early Miocene included versions of mangrove swamp, swamp and lowland riparian forest, tropical rain forest, lower montane/montane rain forest transition vegetation, evergreen cloud forest, evergreen seasonal forest, and tropical deciduous forest.
Average elevation in the Sierra de Chiapas is presently ~1000 m, and the highest peak is in the south near the Guatemalan border (~4000 m). In the Front Ranges and High Plateaus province, which includes the Simojovel locality, the average elevation is presently ~2000 m and the highest peak is San Cristobal de Las Casas (3004 m). The geologic history of the region provides some constraints on estimates of the paleoelevation. The highlands arose by compressive deformation from the northeast to the southwest primarily in the earliest Miocene. Because Plio-Pleistocene strata show little deformation, this brackets the principal time of folding and uplift between the early Miocene and the Plio-Pleistocene. The La Quinta flora was being deposited near the beginning of highland formation, while the younger Pichucalco and Ixtapa floras were deposited after somewhat greater elevations had developed. The paleoelevation in the vicinity of Simojovel in the early Miocene is estimated at 1000–1200 m, which would allow for the presence of a midelevation forest of Picea, Pinus, Podocarpus, and Alfaroa, but not a high-elevation forest of Abies or paramo.
Paleoclimate
The present annual rainfall in the vicinity of Simojovel is ~2500 mm and summer wet. With the more inland position of the sea, and slightly greater and more evenly distributed precipitation in the late Miocene, the region would likely support a rain forest in the lowlands without converting the midelevation evergreen cloud forest into a more tropical selva, or the drier evergreen seasonal/tropical deciduous forests into more mesic formations. The present MAT at midelevations in the region is a relatively uniform 24°C, and it was at least at that value during the early Miocene. At 24°C, temperature is not a limiting factor for the growth of most low- to midelevation tropical forests if there is no great seasonal variation. Thus, it is difficult to detect warmer temperatures on the basis of plant microfossils from lowland basins of deposition. The absence of upper montane cool-temperate, and possibly paramo vegetation, is more likely a reflection of the absence of highlands in the immediate vicinity of Simojovel than of substantially higher MAT. By this scenario, Tertiary floras in the region younger than the Simojovel should have better representation of cool-temperate and higher elevation communities, and the Ixtapa (Martínez-Hernández, 1992) and Méndez floras (Palacios Chávez and Rzedowski, 1993) reflect this trend. Further modernization of the flora occurred with additional immigrations from the north concomitant with continued late Cenozoic cooling (Miller, Fairbanks, and Mountain, 1987), and tropical elements from the south with closure of the isthmian land bridge between ~3.5 and 2.5 Ma (Coates et al., 1992; Graham, 1992; Jackson, Budd, and Coates, 1996; Webb and Rancy, 1996).
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