| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Brief Communications |
2Department of Biological Sciences, 3Department of Earth Sciences, University of South Alabama, Mobile, Alabama 36688 USA
Received for publication March 12, 2002. Accepted for publication June 4, 2002.
 |
ABSTRACT |
|---|
 TOP ABSTRACT LITERATURE CITED |
|---|
Key Words: Alabama • Betulaceae • bract • Carpinus • Citronelle Formation • Pliocene • Tertiary
Carpinus is a well-known genus of the family Betulaceae (subfamily Coryloideae) consisting of at least 25 species native to North America, Europe, and Asia. In many of these areas, species of Carpinus constitute an important component of the forest vegetation as understory or larger canopy trees. In North America, the genus is represented by Carpinus caroliniana Walter, a common understory tree sometimes reaching 
10.5 m in height (Vines, 1960
). The deciduous leaves of C. caroliniana are simple with doubly serrate margins. The fruit is a small, ribbed nutlet attached to a foliaceous bract. This bract is a distinctive compound structure consisting of a large central bract fused at the base with two small lateral bractlets.
Although Carpinus is a common element in the modern vegetation, it exhibits an unusual and somewhat perplexing fossil distribution (Manchester, 1999
). The earliest fossils that can confidently be attributed to the genus occur in the Eocene of North America, Europe, and Asia. The European and Asian Eocene records are convincing (e.g., Tanai, 1972
; Uemura and Tanai, 1993
; Wilde and Frankenhäuser, 1998
) and include the nutlet bracts in some cases. The North American Eocene record is based on wood (Wheeler, Scott, and Barghoorn, 1977
) and a nutlet (Wehr, 1995
). Interestingly, there is no convincing record of Carpinus based on reproductive structures in the entire subsequent North American Tertiary fossil record. This absence is particularly striking considering the fact that Carpinus is widely represented throughout the European and Asian Tertiary (e.g., Berger, 1953
). Previous reports from the post-Eocene Tertiary of North America, typically based on leaves, leaf fragments, and occasional reproductive structures, have not withstood more recent reinvestigations as unequivocal evidence for Carpinus (Taggart and Cross, 1980
; Crane, 1981
, 1989
; Fields, 1996
; Manchester, 1999
). Occasional pollen records are also uncertain, because the pollen of Carpinus cannot be distinguished from that of the closely related genera Ostrya and Ostryopsis (Chen, 1991
). The pollen records are more likely indicative of Ostrya, because this genus is known from an unambiguous Tertiary macrofossil record in North America (Meyer and Manchester, 1997
). It is in this context that we present unequivocal macrofossil evidence of Carpinus, based on a distinctive foliaceous nutlet bract, from Pliocene deposits in southern Alabama.
This research is part of an ongoing investigation of fossil plants from the Citronelle Formation, the only significant megaflora of probable Pliocene age in the entire southeastern United States. The localities investigated include the Lambert's Station site along the abandoned Mobile and Ohio Railroad bed south of Citronelle, Alabama, first described by Berry (1916)
; the "Red Bluff" exposures along Perdido Bay in Baldwin County, Alabama, also described by Berry (1916)
; and a new locality on private property within the Mobile, Alabama, city limits. The Mobile locality was once a sand pit and has unfortunately been reclaimed. Specimens from these localities are providing additional taxa to the macrofossil plant record of the Citronelle Formation, such as Acer, Castanea, Liquidambar, Liriodendron, Myrica, Platanus, and Populus. Additionally, we have recovered material of many taxa reported by Berry (1916)
, along with reproductive structures such as Carya male catkins with in situ pollen. Ultimate shoots of the conifer Taxodium distichum dominate all localities. Leaflets of Carya are the most common angiosperm leaf component. To date, Carpinus has only been found at the new Mobile locality.
Plant fossils occur as stained impressions, or more rarely as compressions, within relatively fine-grained intervals of the mostly sandy Citronelle Formation. To characterize the geology of the plant-bearing intervals, we examined a series of three sedimentary profiles each at the Lambert's Station and Mobile sites. We used a portable soil auger to penetrate up to 6 m below the base of the outcrops and collected sediment samples down the profiles approximately every 20 cm. We penetrated through the plant-bearing layer in at least one of the borings at each site and determined that none are consistent in thickness (e.g., 2 to >6 m). The plant-bearing interval is underlain in places at both sites by red-pink oxidized clay. The plant-bearing sediment is largely composed of silty-clay (or "mud" using the classification of Folk, 1980
; J. Hathorn and R. Sanders, University of South Alabama, unpublished data). Well-exposed profiles at the Mobile study site demonstrate that the plant-bearing interval has pronounced parallel lamination, weak color-banding (gray to brown), and isolated horizontal ichnofossils consistent with a low-energy, brackish water, coastal environment of deposition. These characteristics support the results of earlier studies that examined the clay mineralogy of fine-grained intervals in the Citronelle Formation (Isphording and Lamb, 1971
).
The claim that the Citronelle Formation Carpinus occurrence is of Tertiary age rests on the evidence that the formation is, in fact, Pliocene. The age of this formation has historically been a focal point for debate (Isphording and Lamb, 1971
) since the first detailed investigation (Matson, 1916
) indicated it was Pliocene. Subsequent investigators have proposed ages ranging from Miocene to Pleistocene (Roy, 1939
; Stringfield and LaMoreaux, 1957
; Doering, 1958
; Isphording and Lamb, 1971
). Disagreements over age arose primarily from differences of opinion regarding the presence of structural unconformities and from the general scarcity of age diagnostic fossils.
Sedimentological variation within the Citronelle Formation also contributes to the difficulty in resolving its age. The upper portion of the Citronelle across much of the Alabama Coastal Plain consists of oxidized gravelly sand that in places is in sharp erosive contact with older beds below. The gravelly sand unit is generally interpreted as a coarse-grained fluvial deposit (Isphording and Lamb, 1971
). At our Mobile study site, the unit contains a basal lag of mud clasts and petrified wood fragments that was doubtless collected from the plant-bearing units it was incising into. Unfortunately, we known of no means to constrain the age of the uppermost fluvial unit. Nevertheless, the growing consensus, based on recent geological and paleontological investigations reviewed by Otvos (1998)
, indicates a Pliocene age for most of the Citronelle Formation, including the plant-bearing clays, in the range of 3.4–2.7 million years old.
The new Carpinus bract specimen (USNM #519544) is housed in the Paleobiology Division of the National Museum of Natural History, in Washington, D.C., which is also the repository of Berry's (1916)
original Citronelle megafloral collection. The bract is a stained impression and is rather poorly preserved because of its coarse, siltstone matrix, but it is unequivocally a Carpinus nutlet bract (Fig. 1). The bract lamina is 13.0 mm long and 14.0 mm wide at the widest point. The petiole is not preserved. Only the primary venation and some of the secondary veins are visible. In addition to general morphology, the most convincing evidence for the Carpinus affinity of this bract is the presence of a clearly defined, characteristic nutlet attachment scar between the basal bractlets.
|
This specimen represents the first, unequivocal, post-Eocene record of Carpinus in the North American Tertiary based on the distinctive nutlet bract, and thus constrains speculation regarding the unusual paleobiogeography of the genus. Either Carpinus was present throughout the post-Eocene Tertiary in North America but left no clear megafossil record or it more recently migrated back to the continent sometime after becoming extinct there following the Eocene. The poor North American record of the genus, along with the current absence of a well-supported phylogeny, makes evaluation of these possibilities difficult. Nonetheless, the newly discovered Carpinus bract from Alabama clearly demonstrates that Carpinus was present in North America by the Pliocene, and any future research on the biogeography of the genus must consider this fact.
If Carpinus was a significant component of the Pliocene flora of Alabama, its remains should be common in the fossil record, because the putatively related modern species (C. caroliniana) prefers wet hammock and floodplain habitats. This habitat preference is the source of one of the many common names of this tree ("water beech") and may be expected to produce a taphonomic bias in favor of Carpinus. Because the only evidence for Carpinus discovered so far in the otherwise rich Citronelle Formation megaflora consists of the single bract described here, it seems that this plant may not yet have been an important component of the vegetation during the Pliocene.
Previous studies have indicated that the Gulf Coastal Plain was spared temperature extremes during the Late Tertiary (Berry, 1916
; Graham, 1993
) and Quaternary (Delcourt and Delcourt, 1993
). Interestingly, Berry (1907)
reported the occurrence of Carpinus in Pleistocene deposits in southern Alabama based on leaf fragments. It may be that this region served as a refugium and beginning in the Pliocene was home to small populations of Carpinus from which it established its current wide geographic range.
|
FOOTNOTES |
|---|
4 Author for reprint requests (baxsmith{at}jaguar1.usouthal.edu
)
|
LITERATURE CITED |
|---|
|
TOPABSTRACTLITERATURE CITED |
|---|
Berry E. W. 1907 Pleistocene plants from Alabama. American Naturalist 61: 689-697
Berry E. W. 1916 The flora of the Citronelle Formation. United States Geological Survey Professional Paper 98-L. Washington, D.C., USA
Chen Z.-D. 1991 Pollen morphology of the Betulaceae. Acta Phytotaxonomica Sinica 29: 494-503
Crane P. R. 1981 Betulaceous leaves and fruits from the British Upper Palaeocene. Botanical Journal of the Linnean Society 83: 103-136
Crane P. R. 1989 Paleobotanical evidence on the radiation of nonmagnoliid dicotyledons. Plant Systematics and Evolution 162: 165-191
Delcourt P. A. H. R. Delcourt 1993 Paleoclimates, paleovegetation, and paleofloras during the Late Quaternary. In Flora of North America Editorial Committee [eds.], Flora of North America, vol. 1, 71–94. Oxford University Press, New York, New York, USA
Doering J. A. 1958 Citronelle age problem. Bulletin of the American Association of Petroleum Geologists 42: 764-786[Abstract]
Fields P. F. 1996 The succor creek flora of the middle Miocene Sucker Creek Formation; southwestern Idaho and eastern Oregon: systematics and paleoecology. Ph.D. dissertation, Michigan State University, East Lansing, Michigan, USA
Folk R. L. 1980 Petrology of sedimentary rocks. Hemphill, Austin, Texas, USA
Graham A. 1993 History of the vegetation: Cretaceous (Maastrichtian)—Tertiary. In Flora of North America Editorial Committee [eds.], Flora of North America, vol. 1, 57–70. Oxford University Press, New York, New York, USA
Isphording W. C. G. M. Lamb 1971 Age and origin of the Citronelle Formation in Alabama. Geological Society of America Bulletin 82: 775-780[Abstract]
Manchester S. R. 1999 Biogeographical relationships of North American Tertiary floras. Annals of the Missouri Botanical Garden 86: 472-522[CrossRef][ISI]
Matson G. C. 1916 The Pliocene Citronelle Formation of the Gulf Plain. United States Geological Survey Professional Paper 98-1. Washington, D.C., USA
Meyer H. W. S. R. Manchester 1997 The Oligocene Bridge Creek flora of the John Day Formation, Oregon. University of California Publications in Geological Sciences 141: 1-195
Otvos E. G. 1998 Citronelle Formation, northeastern Gulf Coastal Plain: Pliocene stratigraphic framework and age issues. Gulf Coast Association of Geological Societies Transactions 68: 321-333
Roy C. J. 1939 Type locality of Citronelle Formation, Citronelle, Alabama. Bulletin of the American Association of Petroleum Geologists 23: 1553-1559[Abstract]
Stringfield V. T. P. E. LaMoreaux 1957 Age of Citronelle Formation in Gulf Coastal Plain. Bulletin of the American Association of Petroleum Geologists 41: 742-757[Abstract]
Taggart R. E. A. T. Cross 1980 Vegetation change in the Miocene Sucker Creek flora of Oregon and Idaho: a case study in paleosuccession. In D. Dilcher and T. Taylor [eds.], Biostratigraphy of fossil plants: successional and paleosuccessional analysis, 185–210. Dowden, Hutchinson and Ross, Stroudsburg, Pennsylvania, USA
Tanai T. 1972 Tertiary history of vegetation in Japan. In A. Graham [ed.], Vegetation and vegetational history of northern Latin America, 235–255. Elsevier, Amsterdam, The Netherlands
Uemure K. T. Tanai 1993 Betulaceous leaves and fruits from the Oligocene of Kitami, Kokkaido, Japan. Memoirs of the National Science Museum (Tokyo) 26: 21-29
Vines R. A. 1960 Trees, shrubs, and woody vines of the Southwest. University of Texas Press, Austin, Texas, USA
Wehr W. C. 1995 Early Tertiary flowers, fruits, and seeds of Washington State and adjacent areas. Washington Geology 23: 3-16
Wheeler E. F. R. F. Scott E. S. Barghoorn 1977 Fossil dicotyledonous woods from Yellowstone National Park. Journal of the Arnold Arboretum 58: 280-302[ISI]
Wilde V. H. Frankenhäuser 1998 The Middle Eocene plant taphocoenosis from Eckfeld (Eifel, Germany). Review of Palaeobotany and Palynology 101: 7-28
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
