PALEOBIOLOGY OF ANGIOSPERMIZATION

Abstract

Krassilov wrote in the beginning of his book (1989), that nearly all eminent botanists considered it their duty to speak on the problem of angiosperm origin. Conse- quently, as an entomologist, because of the close asso- ciations between plants and insects, I have been prompted to contribute to the discussion on this "abom- inable mystery" as Charles Darwin called it. This work began as a response to Rasnitsyn's (1988) observation that the beginning of the evolutionary transformations of insects occurred earlier than the distribution of angiosperms. The wide distribution of angiosperm plants caused the Middle Cretaceous biocoenosis crisis, according to Zherikhin (1978). Studying changes of insect composition during the extinction at the Cretaceous-Paleogene boundary, Zherikhin (1978) found, that the major change of insect composition occurred at the middle, rather than at the end of the Cretaceous. Lyell's reverse curve, showing a time change of extinct family percentage, served as a formal base. It showed a sharp change in the middle Cretaceous, with the number of extinct families being slightly increased in the Aptian-Albian. The revolu- tionary changes in the insect composition coincided in time with the range of angoisperms, and Zherikhin for- mulated the concept of ecological crisis—angiosperms broke the ecological succession characteristic of the Mesozoic, including the pioneer stages. The disappear- ance of the characteristic Mesozoic ecosystems was associated with the extinction of Mesozoic insects; at the culmination of this process, coenophobes, pre- served from older times as relicts, became common among insects, resulting in the development of fami- lies, that did not survive to the present. It should be noted, however, that the oryctocoenoses, on which the concept was based, had no fossil angiosperms nor pol- len, while in typically Mesophytic oryctocoenoses of the second half of the Lower Cretaceous, fossil angiosperms, particularly pollen, are rather common. The question of how angiosperms managed to occupy the pioneer stages of succession without subsequent burial remained unsolved. At present, most investiga- tors consider that these localities belong to the late Jurassic, rather than to the middle Cretaceous, while the general curve remains mainly the same. In 1988, Rasnitsyn attempted a detailed study of this process and found, that the bending of Lyell's reverse curve indicated that the crisis beginning is much earlier and falls in the Late Jurassic, while angiosperms, according to the then accepted concepts, appeared in the middle of the Early Cretaceous. Angiosperms occurs in considerable numbers in the near-shore ter- restrial landscapes in the middle of the Albian, a late stage of the Early Cretaceous. Rasnitsyn concluded that the cause of the Middle Cretaceous ecological crisis was other than that of the appearance of angiosperms. A similar result is shown by the recent analysis of data on the geological occurrence of insect families (based on the data of the Paleoentomological Laboratory, Pale- ontological Institute) in the "Treatise of Invertebrate Paleontology" (Carpenter, 1992). Abstract —Angiospermization, i.e., the parallel appearance of features, quite variable in plant origins, typical of angiosperms, is characteristic of the Late Mesozoic continental biota. All the most important taxa seem to be derived in a similar way (eucaryotization, metasoization, arthropodization, spermatophytization, tetrapodiza- tion, reptilization, ornithization, angiospermization, etc.), the environmental changes, occurring in this process resulting in the major evolutionary changes.