Isotaphonomy in concept and practice: an exploration of vertebrate microfossil bonebeds in the Upper Cretaceous (Campanian) Judith River Formation, north-central Montana

Abstract

Vertebrate microfossil bonebeds (VMBs)—localized concentrations of small resilient vertebrate hard parts—are commonly studied to recover otherwise rarely found small-bodied taxa, and to document relative taxonomic abundance and species richness in ancient vertebrate communities. Analyses of taphonomic comparability among VMBs have often found significant differences in size and shape distributions, and thus considered them to be non-isotaphonomic. Such outcomes of “strict” statistical tests of isotaphonomy suggest discouraging limits on the potential for broad, comparative paleoecological reconstruction using VMBs. Yet it is not surprising that sensitive statistical tests highlight variations among VMB sites, especially given the general lack of clarity with regard to the definition of “strict” isotaphonomic comparability. We rigorously sampled and compared six VMB localities representing two distinct paleoenvironments (channel and pond/lake) of the Upper Cretaceous Judith River Formation to evaluate biases related to sampling strategies and depositional context. Few defining distinctions in bioclast size and shape are evident in surface collections, and most site-to-site comparisons of sieved collections are indistinguishable (p≤0.003). These results provide a strong case for taphonomic equivalence among the majority of Judith River VMBs, and bode well for future studies of paleoecology, particularly in relation to investigations of faunal membership and community structure in Late Cretaceous wetland ecosystems. The taphonomic comparability of pond/lake and channel-hosted VMBs in the Judith River Formation is also consistent with a formative model that contends that channel-hosted VMBs were reworked from pre-existing pond/lake assemblages, and thus share taphonomic history.

A new microvertebrate assemblage from the Mussentuchit Member, Cedar Mountain Formation: insights into the paleobiodiversity and paleobiogeography of early Late Cretaceous ecosystems in western North America

“…Taphonomic processes such as transport, sorting, winnowing, reworking, time averaging, and other hydrodynamic factors have been suggested to affect the comparability between sites and perhaps introduce significant bias ( Wilson, 2008 ). However, Rogers et al (2017) , compared six intraformational vertebrate microfossil bonebeds in the Upper Cretaceous Judith River Formation, represented by three lacustrine settings and three channel-hosted settings, and found them to be largely comparable with regards to taphonomic bias.…”

The Early Miocene Critical Zone at Karungu, Western Kenya: An Equatorial, Open Habitat with Few Primate Remains

“…Lagomorphs (Kenyalagomys) and elephant-shrews form an important proportion of the small mammal fauna at the NG15 site, but the highly specialized Myohyrax (Macroscelidea) is less abundant than in the overall Ngira assemblage. The small proportion of microfossils in surface collections can be attributed to multiple factors, including their relatively higher erodibility from outcrop and the inherent bias toward sampling larger fossils in surface collection (Rogers et al, 2017).…”

Paleocommunity mixing increases with marine transgression in Dinosaur Park Formation (Upper Cretaceous) vertebrate microfossil assemblages

“…Jablonski and Sepkoski 1996); however, multiple biases confound direct site comparisons (Blob and Fiorillo 1996; Cutler et al 1999; Gates et al 2010). Vertebrate microfossil bonebeds (VMBs or “microsites”) record genuine paleocommunity signals, including species composition and relative abundance differences possibly related to paleoenvironmental, stratigraphic, and/or geographic differences among sites, but VMB assemblages also reflect different taphonomic biases (Eberth 1990; Rogers et al 2017). Assemblage specimen compositions may differ, in part, because different species have different transport and preservation potentials and/or because different amounts of spatial and temporal averaging result in variable specimen mixing at sites (Miller 1988; Blob and Fiorillo 1996; Tomasovych and Kidwell 2009a, 2010; Bürkli and Wilson 2017; Rogers et al 2017).…”

“…Vertebrate microfossil bonebeds (VMBs or “microsites”) record genuine paleocommunity signals, including species composition and relative abundance differences possibly related to paleoenvironmental, stratigraphic, and/or geographic differences among sites, but VMB assemblages also reflect different taphonomic biases (Eberth 1990; Rogers et al 2017). Assemblage specimen compositions may differ, in part, because different species have different transport and preservation potentials and/or because different amounts of spatial and temporal averaging result in variable specimen mixing at sites (Miller 1988; Blob and Fiorillo 1996; Tomasovych and Kidwell 2009a, 2010; Bürkli and Wilson 2017; Rogers et al 2017). Fossil specimen concentrations in terrestrial formations can result from attritional, lag, and allochthonous depositions (Behrensmeyer et al 2000; Brinkman et al 2005b; Rogers and Brady 2010; Rogers et al 2017).…”

“…Assemblage specimen compositions may differ, in part, because different species have different transport and preservation potentials and/or because different amounts of spatial and temporal averaging result in variable specimen mixing at sites (Miller 1988; Blob and Fiorillo 1996; Tomasovych and Kidwell 2009a, 2010; Bürkli and Wilson 2017; Rogers et al 2017). Fossil specimen concentrations in terrestrial formations can result from attritional, lag, and allochthonous depositions (Behrensmeyer et al 2000; Brinkman et al 2005b; Rogers and Brady 2010; Rogers et al 2017). These processes facilitate rare species accumulation but often homogenize deposits, obscuring formation beta diversity (Kidwell and Flessa 1995; Tomasovych and Kidwell 2009a; Kidwell and Tomosovych 2013).…”