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Paleobotany |
Posgrado de Ciencias Biológicas; Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F. C.P. 04510 Mexico; Departamento de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F. C.P. 04510 Mexico
Received for publication February 8, 2007. Accepted for publication September 17, 2007.
ABSTRACT
The amber of Simojovel de Allende, Chiapas, Mexico, of Late Oligocene–Early Miocene age, has yielded a new flower representing the Meliaceae. The flower of Swietenia miocenica Castañeda-Posadas & Cevallos-Ferriz sp. nov. is characterized by small size; free calyx composed of five glabrous lobes, ciliolated along the margin lobes; corolla composed of five free, contortedly inserted petals with ciliolated margins; cylindrical staminal tube ending in 10 acuminate or toothed accessories and 10 sessile anthers; and a discoid stigma divided in five lobular stigmatic glands. The morphology of S. miocenica is well represented among Meliaceae. Although the new species shares many characters with Swietenia microphylla, small differences in the length and width of petals and the length of staminal tube support its recognition as a new species. The presence of this genus demonstrates the establishment of tropical communities in southern Mexico by the early Miocene and highlights the influence of the northern hemisphere flora on the extant neotropical flora of the area.
Key Words: fossil flower • Mexico • Miocene • Simojovel de Allende • Swietenia
Besides its involvement in the Zapatista political movement, which started in 1994, Simojovel de Allende, Chiapas, Mexico, is known mainly for two related activities: the mining of amber (with the goal of recovering beautifully preserved, fossilized invertebrates, vertebrates, and plants) and the production of amber jewelry by the local artisans. The amber is collected from strata containing huge quantities of organic matter, including coal, and forming part of a sedimentary sequence that alternates with sandstones of different grain size and shale of La Quinta Formation of Oligocene–Miocene age (Licari, 1960 in Langenheim, 1966
); however, recent paleomagnetic data suggest a Late Oligocene–Early Miocene age for the rocks (M. Benami, Instituto de Geofísica, UNAM, personal communication). The amber from Chiapas was produced by the polymerization of the resin secreted by Hymenaea mexicana Poinar & Brown, a legume (Poinar, 2002a
, b
; Ragazzi et al., 2003
). This origin is supported by the geochemical comparison of resins derived from extant and fossil plants, in addition to the identification of flowers and leaves related to this plant (Miranda, 1963
; Langenheim, 1966
).
Though the flower is a delicate and ephemeral structure whose preservation in the fossil record is relatively rare, the documentation of fossil plants through their flowers has become more common in recent years. Most paleobotanical work reporting flower morphology (and occasionally anatomy) is based on charcoalified remains (e.g., Crepet et al., 1992
; Gandolfo et al., 1998
; Eklund, 2000
; Friis et al., 2001
; Schönenberger et al., 2001
; Kva
ek et al., 2002
; Krassilov and Golovneva, 2004
), permineralized specimens (e.g., Cevallos-Ferriz et al., 1993
; Erwin and Stockey, 1994
; Hernández-Castillo and Cevallos-Ferriz, 1999
), or compression/impression material (e.g., Manchester, 1992
; Manchester and Hermsen, 2000
; Yun-Fei et al., 2001
; Quiang et al., 2004
). Only a few reports have been based on amber material (e.g., Poinar, 2002a
, b
).
Amber inclusions represent a special type of fossilization in which delicate structures are preserved in three dimensions with relatively little distortion. In Mexico, as elsewhere, scientific reports on organisms preserved in amber are few, and worldwide 85% of the reports concern invertebrates, 10% concern vertebrates, and less than 5% concern plants (Poinar and Poinar, 1994
, 1999
; Poinar, 2002a
, b
; Frahm, 2004
; Martínez-Delclos et al., 2004
). In this paper we expand the understanding of past biodiversity in the Mexican tropics, complementing the neotropical flora concept through the description of a flower included in the Late Oligocene–Early Miocene amber from Simojovel de Allende, Chiapas, Mexico.
The fossil record of Swietenia (Meliaceae) is scarce, represented by a single fossil flower and a fruit. The flower was illustrated informally by Poinar and Poinar (1999)
from amber material of the Dominican Republic, and the fruit of Toona sulcata (Bowerbank) Reid & Chandler, obtained from the Early Eocene London Clay Formation of England, was recently reported to have similarities with Swietenia (Pigg et al., 2006
). Compared to the fossil record of Swietenia, the fossil record of the family Meliaceae is more complete. In addition to the record of Toona, Cedrela is well represented since the late Eocene and through the Pliocene in the USA (Brown, 1935
; Berry, 1937
; MacGinitie, 1953
; Becker, 1972
; Millar, 1996
; Manchester, 2001
; Alroy et al., 2004
); although Cedrela has also been reported from late Eocene sediments from Argentina and Venezuela (Berry, 1939
), these reports have been considered dubious (Graham, 1999
). Cedrela has also been documented from Miocene/Pliocene strata of Chiapas, Oaxaca, Tlaxcala, and Veracruz in central southern Mexico (Berry, 1923
; Graham, 1999
; Castañeda-Posadas, 2004
) and in Japanese Miocene localities (Ozaki, 1991
).
In contrast, another genus in the family Meliaceae, Trichilia, is only known from Tertiary (a more precise age is not known) outcrops in Puerto Rico, Venezuela, and the USA (Berry, 1937
; MacGinitie, 1953
; Graham, 1996
); from Eocene–Miocene sediments in Argentina (Berry, 1939
, though this report has to be reconsidered), and from Miocene strata in the Dominican Republic (Poinar, 2002b
).
MATERIALS AND METHODS
The fossil material was collected in the Simojovel de Allende Municipality, located in the northern zone of the state of Chiapas, at 17°08'19''N latitude and 92°43'00''W longitude (Fig. 1), at an altitude of 600 m a.s.l. The area is bordered to the north by the Huitiupán, Sabanilla, and Tila municipalities; to the east by the Chilón municipality; to the south by the Pantheló, Chalchihuitán, and El Bosque municipalities; and to the west by the Jitotol and Pueblo Nuevo Solistahuacán municipalities (CEEM, 1988
).
|
). However, Grimaldi (1996)
mentioned that most of the amber deposits are associated with lignites, friable shale, and deltaic clays included in the overlying Balumtun Sandstone of Early Miocene age. Nevertheless, all these sequences correspond with zones N3 and N4, suggesting a numerical age of 22.5–26.0 Ma (Early Miocene–Late Oligocene) (M. Benami, personal communication; Gómez-Bravo, 2005
) (Fig. 2).
|
After the morphology of the fossil flower was described, the literature was searched to find families and genera with similar characteristics. This search was further supported by the Meka (Duncan and Meacham, 1987
) and Delta databases (Watson and Dallwitz, 1992
). After families related to the fossil flower were identified, material from the National Herbarium (MEXU) at the Biology Institute, UNAM (Table 1) was reviewed and compared, until a final determination was attained. The search for literature on other fossil records of Meliaceae was aided by reference to the Paleobiology Database (Alroy et al., 2004
).
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Class—Magnoliopsida
Subclass—Rosidae
Order—Sapindales
Family—Meliaceae
Genus—Swietenia Jacquin
Species—Swietenia miocenica Castañeda-Posadas & Cevallos-Ferriz sp. nov. (Fig. 3).
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Age—Late Oligocene-Early Miocene.
Locality—Simojovel de Allende, Chiapas, Mexico.
Etymology—The specific epithet refers to the age of the geological formation that contains the fossil material.
Diagnosis—Small flower; free calyx composed of five glabrous lobes, ciliolated along the lobe margins; corolla composed of five free, contortedly inserted petals with ciliolated margins; cylindrical staminal tube ending in 10 acuminate or toothed accessories and 10 sessile anthers; discoidal stigma divided in five lobular stigmatic glands.
Description—Small flower 8.0 mm long by 7.5 mm wide, with glabrous pedicel, 2.7 mm long. Free calyx composed of five glabrous lobes, rounded lobes with ciliolated margin (Fig. 3D, E), 1.0 mm long. Corolla composed of five oval to oblong, contorted, free petals with ciliolated margin (Fig. 3F, G), 3.0–4.0 mm long by 1.7–2.0 mm wide (Fig. 3A). The ovary is superior and is encircled by a cylindrical, glabrous, staminal tube with a slightly narrow neck, 2.3 mm long by 3.0 mm in diameter (Fig. 3B, C). At the top of this staminal tube 10 acuminate accessories alternate with 10 sessile anthers (Fig. 3H). Five fused styles have a distal discoid head divided into five lobes that correspond with a globular stigma (Fig. 3I, J).
DISCUSSION
The observed characteristics of the flower preserved in amber from Simojovel de Allende, Chiapas, have certain peculiarities such as the presence of a staminal tube formed by the fusion of the filaments. This floral structure is present in families such as Bombacaceae, Clusiaceae, Malvaceae, Meliacea, and Sterculiaceae, though in these families the floral tube has a large variety of forms and sizes (e.g., Cronquist, 1981
; Judd et al., 1999
). However, characters such as the complete fusion of the staminal tube, the floral arrangement based on five parts, the insertion of floral segments in a contorted pattern, and the presence of ciliolated petals and sepals clearly make the fossil flower a member of Meliaceae. Further, a comparison of flowers from the fossil and living members of Meliaceae substantiates important similarities among them, such as the five-lobed calyx with free sepals, five free petals, ciliolated perianth parts, fused filaments to form a staminal tube with toothed margin and/or appendices alternating with dithecal anthers, interstaminal disc, and superior ovary (e.g., Pennington et al., 1981
; Duncan and Meachan, 1987
; Watson and Dallwitz, 1992
; Judd et al., 1999
).
Floral variation and diversity among Meliaceae genera and species is great, making the taxonomic assignation of this new plant based on its flower difficult. However, one of the most important characters differentiating genera of Meliaceae is the staminal tube: the form, degree of fusion, and terminal morphology of the stamens (Pennington et al., 1981
). For example, Cedrela has free filaments (Fig. 4v, w) and lacks a staminal tube. In contrast, one of the most diverse genera in the family, Guarea, is characterized by its cylindrical, completely fused staminal tube with the distal end holding inclusive or slightly extrorse anthers (Fig. 4a–q). Trichilia, considered the most diverse genus in the family, has flowers with distinguishing characteristics, though the morphological pattern has important variations. The presence of free filaments with terminal anthers characterizes T. elegans Jussieu (Fig. 4h, i) and T. lepidota Martius (Fig. 4t); partially fused (ca. three-quarters of the filament length) filaments are typical in T. micrantha Benth (Fig. 4k), T. stellato Apan & Aquidabain (Fig. 4r), and T. pallida Swartz (Fig. 4s, u); and completely fused filaments or filaments that are fused from their middle zone to their tip distinguish T. elegans Jussieu (Fig. 4h, l), T. tomentosa Kunth (Fig. 4j), T. trifolia Linnaeus (Fig. 4n, o), and T. rubra de Candolle (Fig. 4p, q). The shape of the anthers is an alternative characteristic that is useful for recognizing species of Trichillia, as are the ciliolate or glabrous condition of the filaments, the form of the accessory terminals (e.g., spiny, glabrous), and the urceolated shape of the staminal tube. Carapa, on the other hand, can be distinguished by its cupuliform to urculeated staminal tube with rounded ends (Fig. 4x, y). Another genus, Ruagea, has completely fused filaments that form a cylindrical staminal tube with the anthers located towards the interior of the tube, being little exposed (Fig. 4z–ab). A similar pattern is found in Cabrela (Fig. 4ac). The genus Swietenia is most similar to the fossil flower; both have a cylindrical to urceolated staminal tube with a spiny or teethed tip and exposed anthers alternating with the spines (Fig. 4ad–af, redrawn from Pennington et al., 1981
).
|
; Helgason et al., 1996
; Brown et al., 2003
; Lemes et al., 2003
). The relative taxonomic importance of the ciliolate condition in carpels or petals or the number of carpels, free sepals, and terminal construction of the staminal tube among other characteristics have been widely debated, and a consensus has not been reached.
A qualitative and quantitative comparison of the fossil flower from Simojovel de Allende, Chiapas, with the three accepted extant species of Swietenia, supports the recognition of a new species, at least until more plant organs are considered (Table 2). In Table 2, a comparison of the flowers of the extant and fossil plants demonstrates great similarity among them, even among flowers of different sexes. Although a longitudinal section of the fossil flowers is needed to determine the sex, the well-developed and exposed anthers suggest that it was either male or hermaphroditic. Its comparison with the flower of Swietenia humilis highlights differences such as flower size and form of petals and calyx lobes, which at the specific level are important discriminators (Table 2). In contrast, the presence of ciliolated margins in the petals and calyx lobes differentiates S. miocenica from S. mahagoni (Table 2) (Helgason, 1996
). Swietenia miocenica is most similar to S. microphylla because only small variations in the length and width of the petals or the length of the staminal tube distinguish them (Table 2). However, given the criteria used by Brown et al. (2003)
, Lemes et al. (2003)
, and Roth Novic et al. (2003)
for identifying and classifying extant species within Swietenia, it seems reasonable to recognize the fossil plant as a new species.
|
. Judging from the single published photograph, the flowers are quite similar, but the staminal tube of Poinar's flower is more urceolate than cylindrical and the petals are ligulate rather than oblong. Further comparison among these fossil flowers will clarify their taxonomic and biogeographic relationships. Unfortunately, the flower from Dominican Republic has not been described properly, and its taxonomic affinity to Swietenia was suggested by Poinar and Poinar (1999)
based on a single photograph.
Recognition of Swietenia based on macrofossils corroborates other evidence such as the pollen grains from Chiapas and Dominican Republic for its presence in North America, especially in low latitude regions (Palacios Chaves and Rzedowski, 1993
; Poinar, 2002a
, b
). The current report certainly contributes to our understanding of the biodiversity in southern Mexico some 20 Ma. Because this genus is presently limited to evergreen or semi-evergreen wet forest (tropical forest), we can infer that such conditions may have existed here 20 Ma. Furthermore, the presence of this genus adds a new plant to the growing list of neotropical elements that grew in low tropical North America during the Neogene, long before the closure of the Panama Isthmus and the supposed great biotic exchange of the Plio-Pleistocene. This supports the need to reevaluate ideas about the origin of biodiversity in this area and reinforces the idea that some neotropical plants originated from northern hemisphere relatives (e.g., Calvillo-Canadell and Cevallos-Ferriz, 2005
; Cevallos-Ferriz and González-Torres, 2005
). Further work, including a detailed comparison of other plants preserved in the amber from Simojovel de Allende, Chiapas, will reinforce the historical perspective of low-latitude North American neotropical forests.
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1 The authors thank J. Avendaño Gil, Museo de Palentología Eliseo Palacios Aguilera, Tuxtla Gutiérrez, Chiapas, for the loan of the sample and support through the study; L. Calvillo-Canadell for comments that enhanced our identification and discussion, and G. Tolson and M. Alcayde, all from Instituto de Geología, UNAM, for comments that improved the English of a previous version of this manuscript; and members of the Paleobotany Laboratory, Instituto de Geología, UNAM, for help in improving observations and photography. This paper was supported by COCyTECH-CHIAPAS through a FOMIX project (CHIS-2002-C01–6835) and CONACYT 45065 and PAPIIT-UNAM 228307 projects to S.R.S.C.-F., and a CONACYT student scholarship to C.C.-P. 
4 E-mail: ccpaleo{at}correo.unam.mx
5 E-mail: scrscfpb{at}servidor.unam.mx
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