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Journal of Paleontology
formation of nodular hematite. The siltstone-sandstone facies (Fig. S1.2) is interpreted to represent deposition immediately following flood events. The thick laminations of coarse silt and fine sands, and subrounding of the quartz grains in association with orthoclase and granite clasts, indicate flux from a fluvial source. The associated caliche nodules (Fig. S1.3) are almost always found just above or within the siltstone-sandstone facies discussed above and are directly associated with the vertebrate dig sites found on the property. Further characterization of the facies (e.g., paleo- sol classification) is confounded by the absence of pedo- genic structures. The early diagenesis required for caliche development, the presence of calcified root/burrow struc- tures, and frosted subrounded quartz grains in this facies indicate a prolonged period of subaerial exposure. The frosted quartz grains recovered from caliche nodules are indicative of the input of windblown quartz during the per- iod of exposure. Subaerial exposure is also supported by the surface characteristics of the bones. They are uniformly cracked, which is usually caused by prolonged exposure to the sun and strong desiccation (e.g., Behrensmeyer, 1978). Prolonged subaerial exposure of remains would also explain the prevalence of scavenging marks on many of the elements (Reisz and Tsuji, 2006; Flear et al., 2017). Any determination of the relation of the section at Mud Hill
with those of other localities in the Vale Formation, particularly those of a greater distance away, is limited at present without a high-resolution dataset of stratigraphic sections and without more detailed lithological and petrographic descriptions of those localities. As noted by Wilson (1953), most beds appear fairly localized and nontraceable over broad distances, and the fossil record is not sufficient to provide reliable correlation. Similar concerns about the possible scope of extrapolation from a limited localized dataset were noted by Olson (1958, p. 422, 423). The lack of any exposures of the underlying Arroyo Formation and the loss of the overlying Choza Formation at Mud Hill presents additional challenges. Furthermore, the Vale Formation is noted to be several hundred meters thick in some areas (Olson, 1958), whereas the exposure at Mud Hill (Figs. 2, 3) does not exceed 8 m. The general position can be determined based on the stratigraphic patterns listed by Olson (1958) and Olson and Mead (1982). Red shales are persistent throughout the Vale Formation, but there is a pronounced coarsening upward in the upper Vale that results in widespread conglomeratic deposits with clasts that are sourced from the incised strata (Olson and Mead, 1982). The dominance of finer- grained shales and sandstones at Mud Hill suggests that the locality is relatively low, and Mead (1971) suggested that calcareous clasts at the Sid McAdams locality (situated in the lower Vale) were caliche nodules and hematite concretions, both of which are found at Mud Hill. Previous paleoenvironmental interpretations of the Vale
Formation, both on local and regional scales, are consistent with those presented here. The conditions necessary to form caliche, as well as aspects of the facies at Mud Hill (e.g., frosting of the quartz), reflect subaerial exposure that was likely associated with periodic desiccation at the site (further discussed below). Pronounced seasonality, likely associated with the development of a monsoon system around the equatorial regions (e.g., Tabor
and Montañez, 2002; Tabor et al., 2002, 2008; Woodhead et al.,
2010), would have resulted in episodic inundation and over- flowing of stream channels that would have provided both the lithological material and the transport for many of the fossils that are preserved. Subsequent desiccation would result in subaerial weathering of the fossils and their encapsulating horizons and the formation of pedogenic carbonates. Because the section at Mud Hill does not capture any higher-energy depositions such as channel fills, it is unclear whether the localized environment contained perennial bodies of water.
Comparison of Mud Hill with other Vale Formation localities.—Mud Hill is characterized by a relatively abnor- mal faunal assemblage for the early Permian of Texas (Table 3). Some of the represented taxa (e.g., Diplocaulus, captorhinids) are commonly recovered constituents of other deposits, but others are significantly rarer (Diadectes, Var- anops) or previously unknown (the recumbirostran). Addi- tionally, many of the common constituents of other Vale Formation localities, such as xenacanthid sharks, temnos- pondyls (e.g., Trimerorhachis, Eryops), and Dimetrodon are absent at Mud Hill. The specimens of the sole aquatic con- stituent, Diplocaulus (and the other nectridean specimens), are small and likely represent early stages of ontogeny. The lack of a more precise context for the locality within the Vale Formation and relative to the other localities prevents any confident characterization of these faunal trends as faunal turnover, because taphonomic bias remains a likely con- tributor to some degree; the presence of only extremely small-bodied Diplocaulus is one line of evidence in this regard. The disarticulation but general association of some of the specimens at each of these localities is indicative of relatively little transport, and in the case of Mud Hill, in situ preservation with minimal disturbance of skeletal remains. The Sid McAdams locality features a significant skew
toward Dimetrodon; Mead (1971) estimated a minimum number of individuals (MNI) of 22, and even relatively common taxa such as Diplocaulus and Trimerorachis are represented only by a handful of fragmentary specimens at the site. Mead’s interpretation, as was the classic interpretation of many Paleozoic and Mesozoic mass death assemblages of a particular taxon, was that some sort of abnormal event produced an unusually dense concentration of fossilized individuals. Because Mud Hill has not been extensively quarried, more individuals, particularly of diadectids affinities, might be preserved but are presently unknown. Whether the general paucity of aquatic forms is associated with a small size and isolation or with ephemerality of the aquatic environment in which the localities formed is unclear. The hypothesis of an ephemeral body of water could explain the Mud Hill site, as it would account for the general paucity of aquatic forms and the relative immaturity of such forms when they occur; it could, for example, have been utilized as a habitat by small diplocaulids. The extreme abundance of Diplocaulus at the Stamford locality, which otherwise preserves only the similarly aquatic Trimer- orhachis and Xenacanthus Beyrich, 1848, suggests in situ preservation of an isolated, desiccating aquatic environment (Dalquest and Mamay, 1963). The Blackwood locality features a more even distribution of various taxa with significant
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