A Late Triassic flora and associated invertebrate fossils from the basal beds of the Chinle Formation in Dinnebito Wash, east-central Arizona, USA
published: Dec 30, 2009
The distinctive flora described in this article occurs in the Shinarump Member of the Chinle Formation of Late Triassic age and is particularly noteworthy because it offers insight into the recovery of the land flora in western North America after the end-Permian life crisis. Most of the fossils in this so-called basal flora are preserved as impressions but a few of the leaves have well-preserved cuticles. The flora includes the stems and leaf sheaths of Equisetites bradyi Daugherty emend. Ash 2005, the remains of the fern leaves Marattia cooleyi sp. nov. and Phlebopteris smithii (Daugherty) Arnold 1947, abundant seeds of the seed fern Chilbinia lichi Ash 2006, the bennettitalean leaves Zamites powellii Fontaine (in Fontaine & Knowlton 1890), Z. macombii comb. nov. Ash, and Nilssoniopteris ciniza Ash 1978, the bennettitalean pollen organ Weltrichia minutis sp. nov., the small leaf-like pollen structure Bradyanthus ivesii gen. et sp. nov. Ash of unknown affinity, and a leafy branch of Brachyphyllum sp. Five of the species (M. cooleyi, C. lichi, Z. macombii, W. minutis, and B. ivesii), are known only from the Shinarump and equivalent members; the other four species (E. bradyi, P. smithii, Z. powellii, N. ciniza) are also known from the overlying parts of the Chinle Formation, primarily in Petrified Forest National Park, about 130 km to the east and other localities. The invertebrate fossils associated with the plant remains are the shells of a small unionoidean bivalve, tubes of Spirorbis sp., large endophytic eggs of odonate insects (damselflies?) embedded in the stems of E. bradyi, masses of small eggs, and insect feeding traces on the pinnae of the fern M. cooleyi, and on the leaf N. ciniza. Not unexpectedly, several of the plant and invertebrate fossils were recently recognized and described from the contemporaneous Cameron locality about 60 km to the northwest of this locality. Analysis of the evidence suggests that the plant fossils described here represent a riparian flora that grew along and near a northwestward flowing stream during the Norian stage of the Late Triassic. A review of the basal flora at all localities where it has been found, shows that it includes representatives of all major plant groups except the angiosperms and is dominated by the ferns and the cycadophytes and only a few conifers occur in it. The basal flora is larger and much more diverse than the impoverished flora in the non-marine parts of the underlying Moenkopi Formation of Early and Middle Triassic age and is smaller (26 species) and not quite as diverse as the flora in the more productive overlying parts of the Chinle Formation which includes some 60 species. It is evident from the review that the Late Carnian land flora in western North America had reached approximately the same stage of recovery from the end-Permian life crisis as had the contemporary floras in Europe and China.