Sand pseudomorphs of dinosaur bones: Implications for (non-) preservation of tetrapod skeletal material in the Hartford Basin, USA

Abstract

Early Jurassic tetrapod tracks are common in the Hartford and adjacent Deerfield Basins (Newark Supergroup) of Connecticut and Massachusetts, USA, but skeletal material is rare. Among the few examples is a set of several bones preserved as natural casts on the base of a slab of arkosic sandstone that probably derived from the Portland Formation of Middletown, Connecticut. This mode of preservation is otherwise unknown in the Hartford Basin, and examination of this specimen suggests a complex taphonomic history. The bones were fluvially transported and arranged in a v-shaped pattern as they were deposited on a muddy substrate. Longitudinal cracks, gouging, breakage, rounding, and invertebrate boring indicate significant alteration prior to preservation as casts. Furthermore, the boring represents the first record of osteophagy in the Hartford Basin. Details of the cast preservation, such as sediment rims that wrapped up and over some of the bones, suggest that the bones were dissolved in the subsurface prior to lithification of the overlying sand bed. The decrease in pH that led to bone dissolution probably resulted from the infiltration of naturally acidic rainwater. Root respiration and the decomposition of plants under oxidative conditions may have contributed as well. The depauperate skeletal fossil record of the Hartford Basin is attributable to dissolution under acidic and oxidative sediment conditions. These natural casts offer a unique view of the actions of bone dissolution as a taphonomic process. The lack of body fossils in similar aged deposits elsewhere may also be due to groundwater acidity.

Introduction

Exceptionally preserved fossils such as feathered dinosaurs and Burgess Shale-type fossils attract considerable interest because of the unique information they provide about evolution and paleoecology. However, there is also increasing interest in the opposite phenomenon: the non-preservation of fossils. Organic and mineralized tissues alike are preserved only under specific conditions, and when these conditions are not met, biological remains can be lost to physical, chemical, or biological destruction (e.g., Koch and Sohl, 1983, Retallack, 1984, Retallack, 1988, Cherns and Wright, 2000, Seilacher et al., 2001, James et al., 2005). If exceptionally preserved fossils provide unique views of ancient life, episodes of non-preservation represent blind spots that can limit or bias our understanding of the history of life (Hendy, 2009, Sessa et al., 2009). Sediments can provide some informational redundancy in the fossil record (Bush and Bambach, 2004)—trace fossils, which are biologically-produced sedimentary structures (Bertling et al., 2006), record the activities of organisms that may not be preserved, whereas molds, casts, and impressions of body parts (e.g., the specimen discussed herein) can record morphology even when the original body fossils are destroyed (McAlester, 1962).

The Early Jurassic strata of the Hartford Basin of Connecticut and Massachusetts provide an interesting case study in the non-preservation of body fossils. Tetrapod skeletons are quite rare, but their footprints are plentiful and have attracted attention since the seminal works of Hitchcock, 1848, Hitchcock, 1858, Hitchcock, 1865, Deane, 1861. The identities of the tracemakers are known, at least at a high taxonomic level, with examples of theropods, prosauropods, ornithischians, and crocodylomorphs represented (Olsen and Padian, 1986, Olsen et al., 1998, Olsen and Rainforth, 2003, Rainforth, 2003). Thus, we know what should be found in the body fossil record, and, indeed, the trace fossil record is consistent with the few skeletons that have been found (see Section 5, below).

Numerous factors may contribute to the lack of tetrapod skeletal material in the fossil record. For example, unburied bones would have rapidly decomposed (Behrensmeyer, 1978), as would have bones buried in oxidative or acidic sediments (Rapp and Hill, 2006). Additionally, diagenetic alteration of sediments can lead to significant bone alteration (e.g., Holz and Schultz, 1998). The effect of the modern temperate climate on the outcrops also leads to the destruction of body fossils (e.g., Sues et al. 2000, see Section 5, below). Of these factors, the hypothesis that the sediments of the Hartford Basin were chemically inhospitable to the preservation of bone because they were highly oxidative has received the most consideration (e.g., McDonald, 1992, McDonald, 1995, McDonald, 1996). Here, we analyze the taphonomy of one of the few skeletons recovered from the Hartford Basin and offer support for the hypothesis that post-burial, early diagenetic bone dissolution occurred in the Early Jurassic of New England.