NEWLY DISCOVERED MICROVERTEBRATE LOCAL FAUNA IN THE AGUJA FORMATION, TEXAS

A thin, isolated, silty mudstone lens in the lower part of the upper shale member of the Aguja Formation is approximately 1 m² in area and < 1 cm thick. Located southeast of Terlingua and designated as KB-D, the fully excavated lens yielded approximately 2,500 microvertebrate elements. Also present are eggshell fragments and a few palm seeds. KB-D contains abundant amber globules and profuse leaf material in comminuted form. The lens rests on patches of well-preserved whole leaves in exceptional condition that clearly suggest deposition in a wet environment with brief residence time. Below the leaves are thin, isolated, sandstone lenses associated with a layer of small logs bored by Teredolites. Borings are on the top surfaces of the logs, a condition that suggests boring in place. Rare but well-preserved elements of Chirostenotes were collected as float from approximately the same level as KB-D. More than 100 dinosaur bones have been recovered from a thin, discrete layer 5 m above the KB-D lens. Large, plant-filled, dinosaur coprolites are present at the top of the section. The coprolites are in discrete piles that are distributed several meters apart.

KB-D and the Terlingua local fauna share population similarities, not the least of which is greater relative abundance and diversity when compared to the Tally Mt. microfauna in Big Bend National Park. Examples of taxa that provide paleoecological controls at both Terlingua sites include Hybodus, Lissodus, and Lepisosteus as well as amphibians and crocodilians. Both sites yielded Judithian mammals. The Terlingua sites differ from Talley Mt. in exhibiting an estuarine character rather than having originated in a fluvial setting more inland. Aside from abundant and sometimes large gar scales, 90% of the KB-D elements are smaller than 2 mm. This preferential concentration of small elements may partially explain the absence of even rare dinosaur bones in the lens. Light microscopy revealed no weathering or abrasion suggesting minimal exposure before burial. Most long bones are broken in a transverse fashion although longitudinal and spiral fractures are present. It is possible that the spiral fractures are evidence of predation.