Did tropical rainforest vegetation exist during the Late Cretaceous? New data from the late Campanian to early Maastrichtian Olmos Formation, Coahuila, Mexico.
Abstract
A major problem in paleobotany and paleoclimatology is the origin of modern tropical and paratropical rainforests. Studies of leaf macrofossils, beginning with those of Wolfe and Upchurch, have suggested that tropical and paratropical (i.e., megathermal) rainforests with dominant angiosperms are of Cenozoic origin, and that comparable vegetation was either absent or greatly restricted during the Late Cretaceous. Earth System modeling studies, in contrast, predict the existence of megathermal rainforest vegetation during the mid- and Late Cretaceous, though with less areal extent than during the Late Cenozoic and Recent. Megathermal climate with year-round precipitation is simulated along the paleoequator and along the northern margin of the Tethys Ocean, and tends to occur in highly focused regions, in contrast to the more latitudinally zoned pattern of the Recent. Low-resolution climatic indicators, such as the distribution of coals and tree fern spores, are consistent with evidence from climate modeling for megathermal wet climates during the Late Cretaceous, and by extension megathermal rainforest vegetation. However, corroborative data from plant macrofossil assemblages is needed, because the physiognomy of leaves and woods directly reflects plant adaptation to the environment and can estimate climate independently of the generic and familial affinities of the paleoflora. Newly collected plant macrofossil assemblages from the late Campian to early Maastrichtian Olmos Formation of Coahuila, Mexico, provide evidence for megathermal rainforest vegetation on the northern margin of the Tethys Ocean at approximately 35 degrees paleolatitude. The newly collected leaf flora is 72 percent entire- margined and has abundant palms, features typical of modern megathermal rainforests. Thirty percent of the species have large leaves, and 50 percent of the species have drip tips, features indicative of wet conditions. Simple and multiple regression functions based on the physiognomy of modern vegetation indicate the presence of paratropical rainforest, with a mean annual temperature of 20-23°C and mean annual precipitation of 1.5-3 m. The wood flora contains 5 species of palms and at least 9 species of dicots, none of which have growth rings, and is most similar in hydraulic architecture to woods from wet tropical forests. The Olmos macrofossil assemblages occur in stratigraphic sequences that contain thick (>1m) coal beds and in a region where Earth System models predict high biomass vegetation that grew under paratropical to warm subtropical wet conditions. Our results imply that earlier plant macrofossil studies are in error and that better sampling of low-latitude floras will provide additional macrofossil evidence for megathermal rainforest vegetation during the mid- and Late Cretaceous.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMPP11D..07U
- Keywords:
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- 4914 Continental climate records;
- 4950 Paleoecology