New Early Devonian pteraspidids (Agnatha: Heterostraci) from Nevada and California.

摘要

The main objectives of this research were to describe the pteraspididae (Agnatha, Heterostraci) from new vertebrate faunas of the northern Egan Range, east-central Nevada, and Death Valley National Monument, southeastern California, and to revise the hypothesized phylogeny of the pteraspidids from the western United States using cladistic methods.; I describe three new species and genera of pteraspidids from Nevada: Tuberculaspis elyensis, Pirumaspis lancasteri, and Lamiaspis longiripa. In addition, I describe one new species and genus, Panamintaspis snowii, and redescribe two previously known species, Lamiaspis longiripa and Blieckaspis priscillae n. gen. from California.; Phylogenetic study suggests that all the pteraspidids from the western United States may have to be placed in a new subfamily separate from the subfamily Pteraspidinae, and that the Pteraspidinae should be divided into separate subfamilies due to regional endemism within the group. It is, however, not possible to characterize any of these hypothetical subfamilies sufficiently.; An important feature of this research has been the use of the new pteraspidids in dating and biostratigraphically correlating the vertebrate faunas of California and Nevada, and correlating these faunas with those of the Water Canyon and the Beartooth Butte Formations of Utah and Wyoming. Although Early Devonian pteraspidids have previously been reported from localities in the western United States, their biostratigraphic utility has been limited because they were known only from parts of Utah and Wyoming. The occurrence of similar vertebrates in different Early Devonian vertebrate faunas across the western United States makes it possible to correlate these faunas, and to suggest a late Emsian (inversus-serotinus Zones) age for all these faunas. This is based on stratigraphic relationships in the southern localities, and on spore data from the Beartooth Butte Formation.; Studies of the paleoecology and depositional environment of the new, vertebrate-bearing, terrigenous clastic unit in Death Valley National Monument have also been important aspects of this study. This has resulted in the hypothesis that the vertebrate-bearing unit is a channel-fill deposit with limited lateral extent and a thickness of at least 100 m, deposited in an estuarine environment with low or fluctuating salinity.