Origins of microfossil bonebeds: insights from the Upper Cretaceous Judith River Formation of north-central Montana

Abstract

Microfossil bonebeds are multi-individual accumulations of disarticulated and dissociated vertebrate hardparts dominated by elements in the millimeter to centimeter size range (≤75% of bioclasts ≤5 cm maximum dimension). Modes of accumulation are often difficult to decipher from reports in the literature, although predatory (scatological) and fluvial/hydraulic origins are typically proposed. We studied the sedimentology and taphonomy of 27 microfossil bonebeds in the Campanian Judith River Formation of Montana in order to reconstruct formative histories. Sixteen of the bonebeds examined are hosted by fine-grained facies that accumulated in low-energy aquatic settings (pond/lake microfossil bonebeds). Eleven of the bonebeds are embedded in sandstones that accumulated in ancient fluvial settings (channel-hosted microfossil bonebeds). In lieu of invoking separate pathways to accumulation based on facies distinctions, we present a model that links the accumulation of bioclasts in the two facies. We propose that vertebrate material initially accumulates to fossiliferous levels in ponds/lakes and is later reworked and redeposited as channel-hosted assemblages. This interpretation is grounded in reasonable expectations of lacustrine and fluvial depositional systems and supported by taphonomic data. Moreover, it is consistent with faunal data that indicate that channel-hosted assemblages and pond/lake assemblages are similar with regard to presence/absence and rank-order abundance of taxa.This revised model of bonebed formation has significant implications for studies of vertebrate paleoecology that hinge on analyses of faunal data recovered from vertebrate microfossil assemblages. Pond/lake microfossil bonebeds in the Judith River record are preserved in situ at the scale of the local paleoenvironment, with no indication of postmortem transport into or out of the life habitat. Moreover, they are time-averaged samples of their source communities, which increases the likelihood of capturing both ecologically abundant species and more rare or transient members of the paleocommunity. These attributes make pond/lake microfossil bonebeds excellent targets for paleoecological studies that seek to reconstruct overall community membership and structure. In contrast, channel-hosted microfossil bonebeds in the Judith River record are out of place from a paleoenvironmental perspective because they are reworked from preexisting pond/lake assemblages and redeposited in younger channel facies. However, despite a history of exhumation and redeposition, channel-hosted microfossil bonebeds are preserved in relatively close spatial proximity to original source beds. This taphonomic reconstruction is counter to the commonly held view that microfossil bonebeds are biased samples that have experienced long-distance transport and significant hydrodynamic sorting.

Stable isotopic insights into paleoclimatic conditions and alluvial depositional processes in the Kaiparowits Formation (Campanian, south-central Utah, U.S.A.)

“…We attribute these disparate isotopic patterns to differences in floodplain depositional patterns related to sedimentation rates, overbank flooding, and avulsion frequency, which are consistent with broader taphonomic and sedimentologic differences in the two areas (Rogers and Kidwell, 2000;Rogers and Brady, 2010;Roberts, 2007;Roberts et al, 2013).…”

“…2D). However, this taphonomic condition is likely due to in situ bioturbation, possibly trampling, and compaction rather than transport based on sedimentologic features, and thus this type of fossil accumulation is considered autochthonous (Rogers and Kidwell, 2000;Roberts, 2007;Rogers and Brady, 2010;Roberts et al, 2013).…”

“…Specifically, fluvial sandbodies are interpreted as meandering in the JRF rather than anastomosed in the KAF (see Rogers, 1998;Roberts, 2007), implying a greater degree of mobility in the former. There are numerous examples of fluvial sand-bodies scouring into adjacent and subjacent pond deposits in the JRF (Rogers and Kidwell, 2000;Rogers and Brady, 2010;. Moreover, the abundant microfossil vertebrate sites hosted within fluvial deposits of the JRF represent reworked pond-hosted vertebrate sites based on nearly identical taxonomic composition, early fossilization geochemical and mineralogic characteristics, and field examples of channels eroding into pond deposits and incorporating microfossils into the channel base (Rogers and Kidwell, 2000;Rogers and Brady, 2010;.…”

Enigmatic Early Cretaceous ootaxa from Western Europe with signals of extrinsic eggshell degradation

“…Of especial interest are some microfossil bonebeds (Buscalioni et al 2008;Canudo et al 2010;Gasca et al 2012) from the Iberian Chain outcrops that have been regarded as time-averaged samples of their source communities (sensu Rogers and Brady, 2010) ei.e. they record the faunal and floral association of the ecosystem during periods of hundreds or thousands of years, instead of recording single punctual events.…”

Growth mechanisms in dinosaur eggshells: an insight from electron backscatter diffraction

“…The faunal assemblage of La Cantalera consists almost exclusively of terrestrial and amphibious taxa that lived nearby (Canudo et al 2010). Because there is no indication of significant transport, the faunal as- semblage can be regarded as a time-averaged sample of the source community (sensu Rogers and Brady 2010), specifically the La Cantalera wetland ecosystem.…”