Читать книгу Hadrosaurs - David A. Eberth - Страница 91

The ornithopod fossil remains were recovered from an approximately 6 m² surface, through ~0.6 m of section within the paleosol. Most of the specimens were originally recovered from the hillside by amateur fossil collectors and then, later, donated to the collection once formal excavation and mapping began. Thus, the precise positions of all the remains are uncertain, limiting our taphonomic interpretations. A simple map of the largest specimens has been reconstructed using anecdotal information provided by the collectors, and their photographs (Fig. 5.3). Elements were associated but disarticulated, thus appearing to belong to a single individual. An important exception is the femur (UTA-AASO-125), which, in addition to its smaller size (discussed below), also lacks many of the surface weathering features seen on the other specimens. This suggests that it was collected from the same bed, but from a different location along the hillside.

Evidence suggests that the remains experienced a complex biostratinomic and fossil-diagenetic history following the death of the individual. All specimens vary in color between dark brown to light orange. The predominance of relatively dense bones from the axial column and limbs, and disarticulation suggest some degree of hydraulic sorting and, possibly, transport of the remains, although some elements may be missing due to the activity of amateur fossil collectors and earth-moving equipment prior to formal excavation.

5.3. Reconstructed map of Arlington Archosaur Site ornithopod quarry based on discussions with original excavators. Bone locations are approximate, as is the scale.

All the bones discussed here, except the femur, demonstrate weathering stages 2–4 (Behrensmeyer, 1978), including flaking, cortical exfoliation, small mosaic cracking, and deeper longitudinal cracking, indicating prolonged subaerial exposure prior to final burial. In contrast, the femur exhibits weathering stages 1–2.

The remains underwent postburial fossil-diagenetic alteration in the form of plastic and brittle deformation. Several bones, most notably a cervical vertebra (described below) and caudal vertebrae, show plastic deformation of the neural spine and/or centrum. Plastic deformation likely occurred early in diagenesis due to clay shrink-swell cycles in the paleosol. Slickensides are present in this horizon, with one preserved in situ along the posterior surface of a dorsal vertebra. Brittle deformation due to lithostatic compression is present in the dorsal margin of the ilium and the coracoid. In both cases these elements were flattened leaving large, cemented cracks. Several bones have undergone more recent post-fossilization weathering and now exhibit a simple few to a network of non-cemented cracks from the action of roots, moisture, and temperature extremes that characterize the modern seasonal climate of North Texas. This weathering is most intense close to the surface and lessens with depth. This overall taphonomic signature is similar to that described by González Riga and Astini (2007) in early to mid Late Cretaceous overbank deposits containing titanosaur sauropods in the Neuquén Basin of Argentina. There, sauropod remains apparently underwent prolonged subaerial exposure as well as disarticulation and sorting from sporadic, low-energy ancient hydraulic activity.

Despite lacking a knowledge of precise bone positions, the presence of these remains in an active delta-plain environment, their disarticulated and sorted nature, and the overall taphonomic similarity with material in an Argentinean sauropod bonebed strongly suggest that the AAS ornithopod material is parautochthonous. Upon death the individual likely experienced prolonged episodes of subaerial exposure and carcass rotting that alternated with episodes of short-distance transport and sorting due to sporadic, low-energy flooding from a nearby distributary channel. Upon final burial in muds, climatic seasonality and soil-forming processes resulted in extensive plastic deformation of the elements. Brittle deformation of elements resulted later from lithostatic compression by overlying strata. Finally, recent actions by humans and natural erosion exposed remaining bones to extensive post-fossilization weathering and wear.