Palmlike plants from the Dolores formation (Triassic), southwestern Colorado

Abstract

Plant remains from the Dolores formation in southwestern Colorado include a few small twigs of the conifer Brachyphyllum and large, many-ribbed leaves described as those of an angiosperm, tentatively regarded as a primitive palm and named Sanmiguelia lewisi. This is the earliest known angiospermous flowering plant. Its occurrence in the Middle to Late Triassic, however, indicates that evidence for the origin of the angiosperms should be looked for in the earlier Triassic or late Paleozoic. GEOLOGIC OCCURRENCE OF THE PLANTS In September 1953, G. Edward Lewis, of the U. S. Geological Survey, searching for fossil remains in the upper part of the San Miguel Kiver valley in southwestern Colorado, found a palmlike leaf impression in the red Dolores formation near Placerville. This specimen, 38 cm long and 20 cm wide, was fragmentary, lacking the apex and base. Therefore, further search was made at the same locality early in 1954, but Lewis found only a few more fragments, none of which gave a clue to the nature of the basal or petiolar portion of the leaves. However, in September 1954, Lewis and the writer, at an outcrop one-half mile north of the original site, succeeded in finding fairly good specimens that illustrate the salient features of these leaves, some of which were still attached to part of a structureless cast of a stem. Further exploration of the fossilbearing bed along its outcrop for 10 miles on both sides of the San Miguel Kiver yielded equally good material to the writer in June, 1955. The principal locality is in SE% sec. 12, T. 43 N., R. 11 W., in a low cliff of reddish calcareous sandstone, on the hillside of the right bank, 420 feet above the San Miguel Kiver and 145 feet above the base of the Dolores formation, near the old Fall Creek post office, about 2% miles southeast of Placerville. Exposed there is the following pertinent sequence of strata, omitting the overlying 1000 feet of the Jurassic Wanakah and Morrison formations, and 250 feet of the Cretaceous Dakota sandstone: 364884—56 Partial section at old Fall Creek post office, Colorado Jurassic Entrada sandstone. 50 ft. Whitish massive sandstones with few dark, shaly or limy thin-bedded strata. Triassic Dolores formation. 485 ft. Bright red, calcareous, massive and thin-bedded cliff-forming sandstones, few shales, and some conglomerates. It yields remains of metoposaurs, phytosaurs, and plants. The lowermost whitish conglomeratic sandstone rests on an erosion surface of the Cutler formation. Permian Cutler formation. 275 ft. Dark red shales and sandstones, with fragmentary plants. The Dolores formation, named and described by Whitman Cross (1899) from exposures in the valley of Dolores River in southwestern Colorado, at first included the red beds that are now known as the Cutler formation. However, the discovery in 1904 near Ouray of an angular unconformity below the fossiliferous horizon in the Dolores caused Cross and his associates (Cross, Howe, Ransome, 1905; Cross, Ransome, 1905; Cross, Howe, Irving, 1907) to restrict the name Dolores to the Triassic strata and to name the more reddish underlying beds the Cutler formation, provisionally of Permian age. Cross reported that R. C. Hills in 1880 and 1882 had announced the discovery of fossils in the Dolores, including reptilian teeth, ganoid fishes, mollusks, ana plants. Before these could be described, however, they were lost. Cross said that in the course of his survey of the area more reptilian teeth and a plant, identified by David White as Pachyphyllum [an error for Brachyphyllum] munsteri, were found. All of these fossils reputedly indicated the Triassic age of the Dolores. At the Fall Creek post office locality in the Dolores the writer and Lewis in 1954 found phytosaurian teeth and the plant impressions described in the following pages, including the object of unknown identity (pi. 33, fig. 4). The nonmarine sediments of the Triassic in the four corners area of the southwestern States are believed 205 206 SHORTER CONTRIBUTIONS TO GENERAL ASSEMBLY FIGURE 29. Tentative reconstruction of Sanmiguelia lewisi Brown. In life the leaves were probably somewhat stiff, like the lower right hand leaf, rather than undulant. No sheaths or crests are shown because discernible evidence for them is lacking. X 1/5 to have accumulated on floodplains and in scattered pools and lakes at not many hundreds of feet above sea level. Such plants, therefore, as were entombed in the sediments, most likely originated in the lowlands of the basin of deposition, whereas remains of more distant upland plants did not get into the fossil record. Whatever the reason, whether land vegetation was sparse on account of climatic conditions, or whether the circumstances attending the accumulation of the sediments were unfavorable, plant remains in the Dolores apparently are relatively scarce. DESCRIPTION OF THE SPECIMENS