10.5061/DRYAD.ZCRJDFNB2
Tomescu, Alexandru
0000-0002-2351-5002
Humboldt State University
Noetinger, Sol
0000-0002-4027-2049
Bernardino Rivadavia Natural Sciences Museum
Bippus, Alexander
Oregon State University
Data from: Palynology of a short sequence of the Lower Devonian Beartooth
Butte Formation at Cottonwood Canyon (Wyoming): Age, depositional
environments and plant diversity
Dryad
dataset
2021
palynology, paleobotany, biostratigraphy
National Science Foundation
https://ror.org/021nxhr62
1546593,IIA‐1322504
Humboldt State University
https://ror.org/02qt0xs84
American Philosophical Society
https://ror.org/04egvf158
Consejo Nacional de Investigaciones Científicas y Técnicas
https://ror.org/03cqe8w59
PIP 11220120100182CO
2021-07-27T00:00:00Z
2021-07-27T00:00:00Z
en
https://doi.org/10.1002/spp2.1395
13637 bytes
3
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
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based on fish biostratigraphy and unpublished palynological data.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.
Data collection, generation, and processing. Geology 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 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.
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 >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 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
(<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). 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 >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. 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 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.