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Paleobotany |
Department of Botany, University of Florida, and Florida Museum of Natural History, Gainesville, Florida 32611-7800 USA
Received for publication May 19, 2000. Accepted for publication November 30, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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Key Words: Aceraceae • Aceroideae • Asia • Bohlenia • Dipteronia • North America • Sapindaceae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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is an extant genus endemic to southern and central China with two living species, D. sinensis Oliver and D. dyerana Henry (1903)
. The genus is distributed in broadleaved deciduous forests, along stream margins at altitudes from 1450 to 2400 m, sometimes as low as 1000 m, in the Chinese provinces Gansu, Guizhou, Henan, Hubei, Hunan, Shaanxi, Sichuan, and Yunnan (Ying, Zhang, and Boufford, 1993
). Dipteronia and Acer have long been considered sister taxa, as indicated by their traditional placement in the bigeneric family Aceraceae (Pax, 1902
; Pojarkova, 1949
). Both share the characters of opposite, exstipulate leaves, flowers with five sepals, five nonappendaged petals, a nectariferous disc, eight stamens, tricolporate pollen, two carpels and two ovules per carpel, with only one seed developing per carpel, and schizocarpic winged fruits borne in pairs. Dipteronia is readily distinguished from Acer, however, by distinctive fruits in which the wing fully encircles the seed, lack of bud scales, and much larger inflorescences of >400 flowers (Ogata, 1967
; de Jong, 1976
; van Gelderen, de Jong, and Oterdoom, 1994
). The pinnately compound leaves of Dipteronia differ from the leaves of most Acer species, which are typically simple and palmately lobed. Acer may also be palmately compound, trifoliolate or, as in the case of Acer negundo, pinnately compound with only 5–7 leaflets instead of the 7–15 common in Dipteronia.
Recent phylogenetic investigations continue to support the close phylogenetic relationship between Dipteronia and Acer (Judd, Sanders, and Donoghue, 1994
; Gadek et al., 1996
). However, these analyses also indicate that these taxa are nested within the Sapindaceae; therefore the traditional recognition of Aceraceae at the family level renders the Sapindaceae paraphyletic. It is more appropriate to treat the Dipteronia-Acer clade as a subfamily (Aceroideae) or lower rank within the Sapindaceae. As an outgroup to Acer, Dipteronia may help in establishing character polarity for cladistic analyses of the maples.
Although the fossil record of Acer is well documented in the Tertiary of North America (Brown, 1935, 1937
; Wolfe and Tanai, 1987
), Europe (Walther, 1972
), and Asia (Akhmetiev, 1971
; Tanai and Ozaki, 1977
; Manchester, 1999
), that of Dipteronia has received relatively little attention and some of the previously published reports refer to isolated leaflet impressions of equivocal diagnostic value. The fossil record of Dipteronia may complement that of Acer, providing a better understanding of the geographic origins and timing of the radiation of the Aceroideae.
In this paper, we review the diagnostic characters of the fruits of the two extant Dipteronia species as a basis for recognizing fossil remains from western North America. We attempt to unravel the complicated nomenclature of North American fossil Dipteronia and Bohlenia. Although fossil fruits of Dipteronia have been illustrated from the Tertiary of North America, they were always attributed to species that were based on detached leaves of uncertain affinity to Dipteronia. We recognize a new species, Dipteronia brownii sp. nov., based on characteristic fruit remains, review its stratigraphic and geographic distribution, and compare its morphology with extant species. The significance of fossil remains as a basis to understanding the evolutionary and biogeographic history of Aceroideae is discussed.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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) in search of Dipteronia fruits.
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as the basis for establishing a palynological zonation that is now widely used in biostratigraphic investigations of Paleocene sediments in Wyoming and adjacent states. The Dipteronia fruits were found at the level of pollen zone P-4 sensu Nichols and Ott (1978)
, which is considered to be middle Paleocene (Nichols and Flores, 1993
), or 
60–63 million years old.
Localities of Middle Eocene age, 43–48 million years old, include West Branch Creek and White Cliffs Sr., Oregon; Republic, Washington; Driftwood Creek, Horsefly, Joseph Creek, McAbee, and One Mile Creek, British Columbia (localities 6, 9, and 11–16 in Table 1 and Fig. 1). There are many Late Eocene localities, including Sumner Spring, Teater Road, White Rock Gulch, and White Cap Knoll in Oregon; Ruby River Basin, Montana; and Florissant, Colorado (localities 2–5, 7, and 8 in Table 1 and Fig. 1). The youngest specimen is from the early Oligocene shales of Bridge Creek, Oregon (Brown, 1959
; Meyer and Manchester, 1997
). We rejected the identification of Dipteronia from the Beaverhead Basins of Montana based on our reexamination of the fruit reported by Becker (1969)
.
Manchester (1999)
illustrated a fruit of Dipteronia attributed to the Eocene Fushun locality of northeastern China. However, the provenance of this specimen has come into question. Although he found the specimen in a drawer at UCMP that contained fossil leaves labeled from the Fushun locality, the fruit specimen was unlabeled as to locality. The Dipteronia specimen was in a matrix of similar gray color to the Fushun leaves, which, in the absence of other dark-shale Eocene localities in the UCMP collection, led Manchester to conclude that the fruit was indeed from the Chinese locality. However, UCMP Museum Scientists Diane Erwin and Howard Schorn (personal communication to Manchester) informed us that the Eocene locality of Joseph Creek, British Columbia, contained a similar dark shale. The Joseph Creek collections of fossil leaves, formerly housed at UCMP, were transferred to the Geological Survey of Canada in the early 1970s, and it is possible that the fruit was left behind in that transfer. In October 2000, a sample of the shale from the specimen in question was processed for palynological comparison with known samples from Fushun and Joseph Creek. The palynoflora contains many elements shared with both Fushun and Joseph Creek, such as Pinus, Taxodiaceae, Alnus, Corylus, Ulmus, and occasional Tiliaeopollenites, and Pistillipollenites. However, Liquidambarpollenites, and Ephedripites that occur in samples from Fushun (Song and Cao, 1980
) were not recovered. Study of all specimens from the Fushun shales in the collections at Academy of Science Institutes in Nanjing and Beijing and of all specimens of Joseph Creek shales in the collections in Ottawa failed to produce any additional fruits of Dipteronia. However, as Dipteronia fruits have been recovered from other localities in British Columbia including Horsefly, McAbee, and Princeton, and the color of the shale exactly matches samples from Joseph Creek, it seems most plausible that UCMP 168335 came from Joseph Creek, rather than Fushun. Based on this interpretation, we can no longer claim any Tertiary fossil record for Dipteronia in Asia.
Modern comparative material was examined from herbaria at A, FLAS, and PE. Fruit length is the longest line parallel to the primary vein leading to the seed. Fruit width is the longest line perpendicular to the length (Fig. 2). Measurements cited in the text are for the largest measurement of each fruit, as the fruits are usually very similar in each dimension.
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MORPHOLOGY |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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4.5–6.0 cm in diameter, significantly larger than D. sinensis fruits (2.0–2.5 cm). The general appearance of the fruit is different, because D. dyerana fruits have a significantly thicker pericarp layer around the seed body and the wing is thicker. This results in a less distinct boundary between the pericarp and wing. In the few D. dyerana herbarium specimens studied, we observed an increased tendency for the fruits to develop only one mature mericarp per pair, resulting in a more circular fruit (Fig. 5). Also, the attachment scar between the two fruits is proportionally smaller than in D. sinensis.
Fossil
All of the fossil Dipteronia fruits that we examined appear to fall within a single species, below named D. brownii sp. nov. Compared to the two living species, the fossil fruits more closely resemble D. sinensis. The most obvious difference is their smaller size. The fossil mericarps studied range from 0.8 to 2.4 cm in diameter, with the majority of specimens between 1 and 2 cm, compared to the larger ranges of the modern species as described above. Dipteronia brownii fruits have approximately the same range of variation in general shape as modern D. sinensis fruits. Fossil fruits are readily distinguished from D. dyerana, because they are significantly smaller, the attachment scar is larger in proportion to the body of the wing, and the pericarp does not appear to be as thick.
Mericarps of the modern species almost always occur in pairs, although tricarpellate fruits have been reported (Hall, 1961
). We also know the fruits of the fossil species were borne both in twos (Figs. 10, 14, 20) and in threes (Figs. 13, 15–17). Complete undetached schizocarps of Dipteronia are rarely preserved as fossils, with only eight schizocarps currently known from the Eocene of western North America. Four of them clearly show the attachment of three mericarps to a shared pedicel (Figs. 13, 15–17), three show only two mericarps (Figs. 10, 14, 20), and one is equivocal (Figs. 18, 19). We are uncertain whether the two mericarp specimens were original pairs, or were triplets like the other two fossils, that had shed one mericarp prior to deposition. We favor the interpretation that D. brownii bore fruits both in triplets and in pairs.
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). Based on its large size, it falls within the range of the extant D. sinensis. With only one specimen, we cannot evaluate the population variability for the Bridge Creek occurrence, and our placement of the specimen with the older fossils of D. brownii is provisional. |
PREVIOUS NOMENCLATURE |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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was the first to recognize Dipteronia from fruits in the Tertiary of western North America. Unfortunately, the specific names that he applied to the fruits have priority for unrelated leaf fossils and the nomenclature has become confused. In 1935, he described a new species Dipteronia americana Brown from Republic, Washington and included both leaves and fruits in his concept of the species, although neither has ever been found attached. Brown figured one fruit and one unattached leaflet and cited a previously described leaflet of Berry (1929
, pl. 50, Fig. 5 [USNM 38094]) in synonymy, but he did not designate a holotype. Wolfe and Wehr (1987)
subsequently designated Berry's leaflet specimen as the lectotype for Brown's species. At the same time they transferred this species into their new extinct genus Bohlenia. Wolfe and Wehr (1987)
included both leaves and fruits in their concept of Bohlenia americana (Brown) Wolfe and Wehr, although the fruits have never been found attached to the Bohlenia foliage. The fruits cannot be distinguished morphologically from modern Dipteronia, except they are usually smaller and might be more commonly in threes, as discussed above. Because fossil Dipteronia fruits have never been found attached to any type of foliage, we restrict the name Bohlenia americana to the foliage upon which it is based.
Brown (1937)
also recognized a second fossil species of Dipteronia, D. insignis (Lesq.) Brown, when he transferred an isolated leaf impression previously called Myrica insignis by Lesquereux from Florissant, Colorado into the genus Dipteronia. Although Brown implied that the name should be applied both to fruits and foliage, the epithet insignis is based on leaf remains of equivocal relationship to Dipteronia. Hence, the name is inappropriate for accommodating the fossil fruits. Wolfe and Wehr (1987)
established the new combination Bohlenia insignis (Lesquereux) Wolfe & Wehr for this species. In our opinion, the name Bohlenia should be applied exclusively to foliage. The true systematic affinities of this foliage remain elusive because similar kinds of leaflets are produced by more than one extant genus of sapindales. Accordingly, all fossil fruits previously associated with the names Dipteronia and Bohlenia are orphaned and need to be placed into a new species established for fruits. Although Brown considered the fossil fruits to represent two species, we regard them all as variants of the single species described below.
Dipteronia brownii, McClain and Manchester, sp. nov. (Figs. 10–34)
Fruits schizocarpic, composed of 3 or 2 mericarps. Pedicel slender, broadened with disk and perianth at junction with fruit. Each mericarp with a central flattened elliptical to pyriform pericarp 3–8 mm diameter surrounded by a flat, subelliptical wing 8–24 mm in diameter. Wing margin entire, rounded, except for a flat proximal edge representing the attachment scar. Primary vein extending from the pedicel, 1.6–5.6 mm to the apical end of the attachment scar, then deflecting sharply at 90–135° and extending straight 2–8 mm to the centrally positioned seed of each mericarp (Fig. 11). Secondary veins forming a reticulum over the pyriform pericarp (Figs. 13, 22). Tertiary veins radiating from the seed body, dichotomizing and anastamosing enroute to the wing margin. Quaternary veins forming a fine reticulum of polygonal areoles throughout the wing with few or no free-ending veinlets (Fig. 12).
Holotype
UWBM 39729, Fig. 13, from middle Eocene of Republic, Washington (UWBM locality A0307).
Other specimens
Hell's Half Acre, WY: UF 15740D-2085, UF 15740E-23086 (Fig. 21); Republic, WA: UF 18048-30283 (Fig. 11), UF 18152-32028, UWBM 71380 (Fig. 16), UWBM 26006 (Fig. 17), UWBM 55044 (Fig. 22), UWBM 55045 (Fig. 31), SR 92-20-3 (Fig. 15), SR 96-08-07 (Fig. 14); White Cliffs, OR: UF 262-30285, UF 262-30303 (Fig. 26); West Branch Creek, OR: USNM 509800 (Fig. 32), UF 230-32131; White Rock Gulch, OR: UF 237-30301 (Fig. 24); Sumner Spring, OR: UF 75-21691 (Fig. 34), UF 283-31375 (Fig. 20), UF 283-31376 (Fig. 33), UF 283-21837, UF 283-32132, UF 283-32133; Whitecap Knoll, Oregon: UF 272-26428; Teater Road, OR: UF 256-20862 (Fig. 25); Horsefly, BC: GSC 7580B (Fig. 10); Florissant, CO: YPM 37270; Bridge Creek, OR: USNM 42351; McAbee, BC: USASK 38-9734 (Fig. 30), CMN-PB 099, CMN-PB 105, CMN-PB 107, CMN-PB 108; One Mile Creek, Princeton BC: CMN-PB 1827B (Fig. 23), UWBM 56575A (Fig. 18), 57496A (Fig. 27).
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Discussion of Dipteronia brownii sp. nov
Dipteronia brownii mericarps have a ratio of length to width that ranges from 0.68 to 1.45, but commonly are more or less orbicular. If only a few of the specimens were studied, one might interpret the morphological differences to indicate that more than one species is represented. However, at localities where numerous specimens are available, e.g., Republic, Washington (25 specimens), One Mile Creek, British Columbia (eight specimens), and Sumner Spring (25 specimens), it becomes clear that there is a wide and continuous range in size and shape of the wing and even the proportion of pericarp size to wing size. Some specimens have a shallow concavity in the wing on the basal side adjacent to the attachment scar (Figs. 24, 27, 30, 33), whereas others from the same localities are only convex along the basal wing margin.
In some specimens the proximal margin of the schizocarp continues straight well beyond the attachment scar before rounding (Figs. 25, 27); in others the corresponding margin becomes convex immediately beyond the attachment scar (Figs. 24, 30). Some specimens show a small circular area within the pericarp (Figs. 27, 31). This could be interpreted as the immature seed, or possibly the second aborted ovule, although aborted ovules in Acer are much smaller. It is possible that, similar to some Acer species, the pericarp may be fully developed although the seed is still growing.
The reticulum of quaternary veins forming a mesh between the radiating tertiary veins is well preserved in only a few specimens (Figs. 11, 12). It corresponds to that observed in the modern species (Figs. 8, 9).
No consistent morphological differences were noted in North American fossils across time or between localities. There are some differences in mean fruit size between localities, but the ranges overlap. Smaller fruits tend to occur in the populations at One Mile Creek (9.5–15.0, average 11.8 mm, N = 8), and at Sumner Spring (8.0–13.5, average 11.0 mm, N = 25), but at Republic there is both a greater range of size and greater mean size (10–18, average 16.0 mm, N = 25). From most localities only one or a few specimens are available, making it difficult to determine the "typical" fruits of those populations. For example, at McAbee, the five specimens range from 11.1 to 14.2 mm (average 12.8 mm).
In this paper we have used a broad circumscription of Dipteronia brownii, which accommodates the full range of morphological variability seen at the type locality (Republic) and other sites ranging from Paleocene to Oligocene. It is possible that if the corresponding foliage were available, we could distinguish more than one species, but the associated leaflets remain speculative. Fruits in twos vs. threes could possibly have been used as a basis to recognize two species, but because the fruits are very rarely preserved together, most fossil fruits would be unassignable to a species. In our view, such a distinction would be artificial, because carpel number was probably a variable feature in the population; even today fruits of D. sinensis occasionally develop from tricarpellate ovaries (Hall, 1961
). Wolfe and Wehr (1987)
cited the occurrence of three carpels as a basis for distinguishing a separate genus and argued that the taxon be moved from Aceraceae (two carpels) to Sapindaceae (usually three carpels). However, the clade containing Acer and Dipteronia likely originated within the Sapindaceae, from an ancestor with the primitive sapindalean trait of three carpels. Therefore, the three-fruited specimens merely call into question the level of universality of the bicarpellate fruit character. A reduction from three to two carpels may not have occurred at the base of the Aceroideae clade, but higher up. If this hypothesis is true, combining the two- and three-fruited fossils may possibly result in a paraphyletic species. However, because this is mostly conjecture, and number of carpels is not usually known in the fossils, we adopt a broad circumscription for Dipteronia brownii.
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BIOGEOGRAPHY |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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60–63 million years old. Surprisingly, the distinctive fruits have not been recovered from other Paleocene localities, which are widely distributed and well collected in Wyoming, Montana, and North Dakota (Brown, 1962
; Hickey, 1977
; Crane, Manchester, and Dilcher, 1990
; Wing, 1994
), and they have not yet been observed in early Eocene sites. However, they are well represented at middle and late Eocene localities. Middle Eocene localities are distributed in Washington, Oregon, and British Columbia. Late Eocene occurrences are known from Colorado, western Montana, and Oregon. The only locality of certain Oligocene age is from the Bridge Creek flora of Oregon, 32 million years old. The Asian fossil record is undoubtedly incomplete, and more collections need to be made; the single previous report of a fruit from the Chinese Tertiary has been discredited (see MATERIAL AND METHODS). In Europe, however, where paleobotanical study has been more intense, Dipteronia fruits have not been described in the literature and are not present in major museum collections (personal communication from H. Walther, Staatliches Museum für Minerologie und Geologie, Dresden and Z. Kvacek, Charles University, Prague to Manchester). Therefore, our current hypothesis assumes the genus was limited to Asia and North America throughout its history.
This leads to the supposition that the genus evolved in one of these continents and emigrated to the other. If we take the fossil record as it is currently known, we might conclude that the genus first appeared in North America in the Paleocene and migrated to Asia some time later. During the Paleocene and Eocene, both continents were at higher latitudes, but the climate was warmer and a land bridge was present (Tiffney, 1994
). Although the literature suggests there are records of Dipteronia in the Miocene of North America (LaMotte, 1952
), this information is based on either erroneous taxonomic assignments or inaccurate age dates. We are not aware of any valid occurrences in North America subsequent to the lower Oligocene. From this we infer that Dipteronia went extinct in North America by the end of the Oligocene.
The fossil record of Dipteronia is nearly continuous from the Paleocene through the Oligocene in North America, but because of the lack of fossil fruits in Asia, it is uncertain whether the genus inhabited the continent prior to the Recent. Our interpretation is that we have not found Chinese fossil fruits because they are rare. Dipteronia fruits are never very common fossils, except perhaps at Republic, Washington, and Sumner Spring, Oregon.
Phylogeny
Judd, Sanders, and Donoghue (1994)
suggest that Acer and Dipteronia diverged from a "winged samaroid clade" of the Sapindaceae including members of the tribes Thouinineae (Athayana, Bridgesia, Diatenopterys, and Thouinia) and Paulinieae (Serjania and Thinouia). The divergence of Aceroids from other Sapindaceae follows a pattern of temperate taxa diverging from large tropical families (Judd, Sanders, and Donoghue, 1994
). Judging from the Paleocene occurrences of Acer and Dipteronia, the divergence of Aceroideae probably occurred during the Late Cretaceous or Early Paleocene.
It is interesting that Dipteronia fruits have remained essentially the same from the Paleocene to the Recent, perhaps only just recently forming two distinct species. Although we do not know if there has been similar stasis in other parts of the plant, clearly the genus has not diversified as much as its sister taxon, Acer. Although the two genera have existed for the same amount of time, there are now two living species of Dipteronia and >120 species of Acer (van Gelderen, de Jong, and Oterdoom, 1994
).
It is essential that the fossil representatives of Dipteronia be understood from a "whole plant" perspective so that they can be used in a cladistic analysis of the Aceroideae. Once Dipteronia is well understood, the modern and fossil representatives can be included in phylogenetic analyses of the Aceroideae and possibly be used as an outgroup for the genus Acer.
To gain a better understanding of the phylogeny of Dipteronia, it is necessary to obtain more characters from other organs. It is likely that leaves of Dipteronia are also present at some of the localities where the fruits occur, but detailed comparative studies to show how to distinguish leaves of Dipteronia from those of other similar sapindalean genera have not been published. A better understanding of foliage variability within the genus is also needed. Once Dipteronia is well understood from both vegetative and reproductive features, the modern and fossil representatives can be effectively included in phylogenetic analyses of the Acer-Dipteronia clade.
Bohlenia foliage, which co-occurs with the Dipteronia fruits at Republic, Washington, appears to be sapindalean, but does not closely resemble modern Dipteronia leaflets. The leaflets of Bohlenia are much smaller, have rounder teeth, and the secondary veins sometimes terminate at a sinus in addition to those that terminate at a tooth (referred to as bohleneoid venation by Wolfe and Tanai, 1987
). Indeed, we believe the Bohlenia leaflets figured by Wolfe and Wehr (1987
, Plate 13, Figs. 1, 3) are more similar to the modern Koelreuteria elegans (Seem.) A. C. Sm. Koelreuteria is also represented at the same site by fruit remains (Wolfe and Wehr, 1987
).
In this paper, we have clarified the fossil record of Dipteronia in North America. Because Acer leaf and fruit fossils are known from Tertiary localities in North America, Europe, and Asia, we focused our biogeographic study of Dipteronia in these areas. However, we did not find any valid Dipteronia specimens from Europe or Asia. We believe that the lack of European Dipteronia fossils is significant, because Acer fossils are known from many European localities. However, the Asian plant fossil record is not as well understood as that of North America and Europe for many plant taxa. Because Dipteronia lives in China today, it is likely that fossils will be found if localities are collected further.
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FOOTNOTES |
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2 Author for reprint requests (mcclainamy{at}hotmail.com
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSMORPHOLOGYPREVIOUS NOMENCLATUREBIOGEOGRAPHYLITERATURE CITED |
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Becker H. F. 1960 Additions to the Tertiary Ruby Paper Shale flora of Southwestern Montana. Bulletin of the Torrey Botanical Club 87: 386-396[CrossRef]
———. 1961 Oligocene plants from the Upper Ruby River Basin, Southwestern Montana. Geological Society of America Memoir 82: 1-127
———. 1969 Fossil plants of the Tertiary Beaverhead Basins in Southwestern Montana. Palaeontographica 127: 1-142
Berry E. W. 1929 A revision of the flora of the Latah Formation. U.S. Geological Survey Professional Paper 154-H: 225-265
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———. 1937 Additions to some fossil floras of the Western United States. U.S. Geological Survey Professional Paper 186-J: 163-206
———. 1959 A bat and some plants from the Upper Oligocene of Oregon. Journal of Paleontology 33: 125-129[Abstract]
———. 1962 Paleocene Flora of the Rocky Mountains and Great Plains. U.S. Geological Survey Professional Paper 375: 1-119
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