Hadrosaurs. David A. Eberth
heavily rooted histic gleysol with numerous calcareous concretions (Main, 2009; Fig. 5.2). The uppermost part of the AAS section is distinguished by a lag deposit overlain by heterolithic mudstones and, subsequently, finely laminated and rippled sandstones.
We interpret the AAS section as recording fine-grained, organic-rich sediments that were deposited successively on a flood-inundated coastal plain. Within the peat, claystones, and mudstones, numerous coprolites and fossil wood have been recovered that can be used to study the ecosystem of the region. The large amount of fossil plant and wood at the site indicates a plant-rich, coastal wetland, such as a fen or mangrove forest. Several large (>3 m) and numerous partial (<2 m) logs were excavated from the peat bed immediately underlying the primary dinosaur bonebed. The ornithopod fossil-bearing horizon occurs within the paleosol, in association with calcareous concretions. Within the concretions are charcoal fragments and several large (28–36 cm long) charcoal tree stumps and roots (Fig. 5.2). We propose that concretion formation indicates seasonal dryness, and that the charcoal tree stumps and roots are possible evidence of wildfires. The charcoal (wildfire) horizons occur below, within, and above the vertebrate fossil horizons. The charcoal-rich conglomerates contain numerous charcoal and fossil wood fragments (1–4 cm long) bound in an iron-rich sandstone. We speculate that the charcoal and other clasts were transported and deposited during distributary-channel flooding.
Taphonomy
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.
SYSTEMATIC PALEONTOLOGY
DINOSAURIA Owen, 1842
ORNITHISCHIA Seeley, 1888
ORNITHOPODA Marsh, 1882
HADROSAUROIDEA Cope, 1869
HADROSAUROIDEA indet.
Material Hadrosauroid represented by at least two individuals. UTA-AASO-2003 appears to represent an adult individual, and includes a scapula, coracoid, ilium, ischium, pubis, cervical and dorsal ribs, an axis, and cervical, dorsal, and caudal vertebrae. UTA-AASO-125 is a partial femur from a subadult or juvenile individual.
Locality and Horizon All specimens were recovered from UTA Location 50 (the Arlington Archosaur Site; AAS), northern Arlington, Tarrant County, TX. GPS coordinates for the location, maps of the AAS excavation, and collections database with all relevant site data are on file with the Perot Museum of Nature and Science, Dallas, Texas. The specimens are from a single bone-bearing horizon in a clay-rich paleosol within the Lewisville Member of the Woodbine Formation (Cenomanian). UTA-AASO-125 is likely from the same horizon, but the exact location is unknown.
Comments Hadrosauroid remains designated UTA-AASO-2003 were recovered as disarticulated but associated elements from an ~6 m2 area through ~0.6 m of section. Most of the material was collected by UT Arlington students Phil Kirchhoff and Bill Walker (who also discovered the site) in collaboration with Art Sahlstein. Initial excavation of the site occurred prior to the authors’ involvement with the project, and before large-scale excavation and study began. Maps of the initially recovered material were generated by interviewing Kirchhoff and Sahlstein, and by reviewing photos of the initial dig. All known AAS hadrosauroid material was prepared at the Earth and Environmental Sciences Department, UT Arlington. The material was prepared by DJM and paid student interns under his supervision. All prepared fossils were later donated to the Perot Museum of Nature and Science, Dallas, Texas.
5.4. Axis of UTA-AASO-2003. (A) anterior view; (B) left lateral view; (C) posterior view.
DESCRIPTION
Cervical Vertebrae The axis is relatively complete, with minor deformation to the neural spine.