DOI: 10.5061/dryad.zcrjdfnb2
title: Data from: Palynology of a short sequence of the Lower Devonian Beartooth Butte Formation at Cottonwood Canyon (Wyoming): Age, depositional environments and plant diversity

description: The Beartooth Butte Formation hosts the most extensive Early Devonian
 macroflora of western North America.  The age of the flora at
 Cottonwood Canyon (Wyoming) has been constrained to the Lochkovian-Pragian
 interval, based on fish biostratigraphy and unpublished palynological
 data.  We present a detailed palynological analysis of the
 plant-bearing layers at Cottonwood Canyon.  The palynomorphs
 comprise 32 spore, five cryptospore, two prasinophycean algae and an
 acritarch species.  The stratigraphic ranges of these
 palynomorphs indicate a late Lochkovian - Pragian age, confirming previous
 age assignments.  Analyses on samples from three different
 depositional environments of the plant-bearing sequence – layers with in
 situ lycophyte populations, flood layers that buried those populations and
 an organic matter accumulation zone within a flood layer – demonstrate
 distinct palynofacies. Comparisons between palynomorph and plant
 macrofossil diversity reveal some discrepancies.  Whereas
 zosterophylls and lycophytes, most diverse and abundant among the
 macrofossils, have only one known corresponding spore type (assignable to
 zosterophylls) in the palynomorph assemblage, the trimerophytes, rare in
 the macrofossil assemblage, are represented by three spore
 types.  Some of these discrepancies reflect taphonomic
 differences between macrofossils and palynomorphs, others could be due to
 the fact that the parent plants of most palynomorph types in the
 Cottonwood Canyon assemblage are unknown.  These observations
 emphasize the need for concerted efforts to bring together the knowledge
 of macro- and microfloras within Early Devonian localities. 
 Nevertheless, given the palaeophytogeographic significance of the
 Beartooth Butte Formation flora, its palyno- and macrofossil assemblages,
 taken together, provide new data relevant to future discussions of Early
 Devonian biogeography.
description: Data collection, generation, and processing.
 Geology<br> The Beartooth Butte Formation is a
 discontinuous Lower Devonian unit bounded by the Ordovician Bighorn
 Dolomite and the Upper Devonian Jefferson Dolomite (or Jefferson
 Dolomite).  The unit consists primarily of dolomitized siltstone and shale
 with dolomitized sandstone interbeds.  The large-scale geometry of the
 Beartooth Butte Formation, its sedimentology and fossil content, along
 with isotopic data, suggest fresh- to brackish-water depositional
 environments of estuarine to fluvial nature.  Aside from the type locality
 at Beartooth Butte (Park County, Wyoming), the unit is exposed at a few
 other known locations throughout the rugged landscape of northern Wyoming
 and southern Montana.  One of the best studied of these is the Cottonwood
 Canyon locality, in the Bighorn Mountains of northern Wyoming, Big Horn
 County.  Analyses of the palynoflora at the two localities by D.C.
 McGregor proposed a late Lochkovian to early Pragian age for the Beartooth
 Butte Formation at Cottonwood Canyon, and a mid-late Emsian age at the
 Beartooth Butte locality.  These age assignments are broadly consistent
 with fish biostratigraphy data. Depositional
 environments and sampling<br> A total of 28 palynological samples
 were collected from different horizons of a c. 1-meter thick sequence of
 the Beartooth Butte Formation at Cottonwood Canyon.  The sampled horizons,
 rich in plant macrofossils, are part of a heterolithic sequence consisting
 of an alternation of two types of deposits.  One type is represented by
 dark, finely laminated shales that preserve dense in situ populations of
 the early lycopsid Sengelia radicans, which formed dense mats of
 interwoven stems with rhizomatous growth.  The Sengelia shales alternate
 with massive beds of hard-cemented siltstones rich in transported plant
 material and other organic detritus, often highly fragmented. The
 siltstone beds are penetrated by vertical in situ root-bearing axes from
 the Sengelia populations of the overlying dark shale layer.  The
 heterolithic sequence has been interpreted as reflecting cycles of
 colonization by Sengelia of flood deposits at the water’s edge, and
 subsequent burial of Sengelia populations by periodic flood
 events.<br> Samples were processed at the Laboratory of
 Palaeopalynology (Palaeontology Section, Museo Argentino de Ciencias
 Naturales “Bernardino Rivadavia” – MACN) following conventional methods
 with HCl and HF acid maceration and they were not oxidized.  Extracted
 organic residues were sieved through a 10 µm mesh and the &gt;10 µm
 fraction was mounted on standard microscope slides with acrylate. 
 Productive samples were analysed and imaged for identification of
 palynomorphs and quantification of palynofacies,  The material (slides,
 residues, rock samples) is deposited in the MACN collections under BA Pal
 accession numbers 6615-6641. Palynofacies
 analysis<br> To explore the correlation between depositional
 environments and palynofacies, a subset of 11 samples differentiated
 between the two types of sediment – Sengelia shales and flood deposits,
 referred to as “Sengelia mat” (3 samples) and “flood layer” (8 samples),
 respectively – were studied.  Aside from these two types of sediment, an
 especially thick flood layer hosted a thin (&lt;1 cm thick) but
 conspicuous lens of dark material covering a c. 2 m2 surface area.  This
 dark lens consisted of a concentrated accumulation of mostly fragmentary
 and amorphous organic matter, with a significant plant fraction.  This
 lens rich in organics was sampled separately and is referred to as
 “organic lens” (2 samples).<br> In each of the samples analysed for
 palynofacies two tallies were performed under transmitted light
 microscopy.  To ensure consistency of tallies between samples, we mounted
 equal amounts of organic aliquot extracted from each sample on slides,
 diluting it with the same amount of mounting medium.  To account for
 inherent variability in the distribution of palynofacies elements in the
 slides, we tallied all palynofacies elements (main axis &gt;5 µm)
 observed in the field of view at 200x total magnification in several
 randomly selected areas in each slide, until reaching a target minimum of
 100 observations per sample and, where possible (in most cases, i.e., 10
 out of 13 samples), more than 300 observations.<br> Preliminary
 test-tallies led us to distinguish the following categories of
 palynofacies elements for the analysis (Fig. 3): amorphous organic matter
 (AOM); wefts of fibrous structures (WFS); structured phytoclasts,
 translucent, sheet-like, showing cell outlines (SPCO); structured
 phytoclasts, mostly brown to dark brown, often with opaque areas and
 usually with sharp edges (SPSE); phytodebris, usually brown, with less
 well defined surface structures or edges (PLD); phytodebris with
 well-defined round outline (PDRO); opaque organic debris, black, mostly of
 small size (OOD); and spores (SPO). Parent plants of
 dispersed spores<br> The Beartooth Butte Formation hosts a
 relatively diverse, albeit incompletely characterized, macroflora at
 Cottonwood Canyon.  To explore correlations between this macroflora and
 the palynoflora, we queried the literature to assemble a list of parent
 plant species for those spores and cryptospores in our samples for which
 this type of data have been published.