Geological setting The Bear Gulch Limestone is commonly considered to be part of the
Heath Formation, the youngest formation in the
Big Snowy Group of central
Montana. Some authors instead consider the Bear Gulch Limestone to be an early member of the
Tyler Formation, a patchy but widespread unit of Carboniferous
limestone and terrestrial sediments. Most of the Heath Formation is represented by
black shales and
marls, indicative of
brackish and salty
littoral environments. It developed along a
transgressive sequence in a narrow saltwater seaway, known as the
Central Montana Trough Climate During the time of deposition, the area was about 10-12 degrees north of the
equator, on the boundary between the arid
subtropics and tropical equatorial region. Cyclical deposition supports a climate model arguing that the overall climate was warm and
monsoonal, with pronounced rainy and dry seasons. During the quiet
dry season, sedimentation was low and the basin would have been influenced by northeasterly
trade winds,
tidal currents, and
evaporation. Early in the
rainy season, warmer temperatures and reduced trade winds would have isolated the basin and increased its overall
salinity. As the rainy season progressed, heavy rainfall enhances the bay's horizontal salinity gradient, from the freshwater-influenced upper bay to the marine-influenced lower bay. Storms would also produce a shallow layer of freshwater, washing sediment and organic material from the basin margins down into deeper areas. Uncommon gut contents indicate the diet of some Bear Gulch animals, and
phosphatized muscles have been found in very rare situations. Soft
invertebrates are indicated by a variety of
molds, casts, and organic discolorations on rocks. In the central basin, fossils of invertebrates with
calcareous shells are mostly dissolved, leaving only molds in the surrounding rock. On the other hand, invertebrates with
chitinous or
phosphate-rich shells become more common in the central basin. This may have been a consequence of acidity in the pore fluids of sediments, with less acidic fluids favoring the preservation of calcareous fossils and more acidic fluids favoring phosphatized fossils. There is some uncertainty over how this exceptional preservation was achieved. Most fossils are complete and undecomposed animals, with no signs of disturbance from
scavengers or strong currents. To prevent
decomposition of fragile soft tissue in a warm environment, death and burial had to have been very rapid for the vast majority of articulated skeletons. Rare disarticulated fragments may correspond to large or buoyant carcasses which rise to the water surface to gradually decay and fall apart in a "bloat and float"
taphonomic process. Fossils are dispersed throughout the Bear Gulch lens, rather than concentrated in specific fossil-rich beds (which would be expected if organisms were killed by
algal blooms). The cyclically deposited seabed of the central basin, though fossiliferous, is deprived of benthic invertebrates. Some authors have suggested that the deepest waters in the basin were too salty or oxygen-poor for most life. However, bottom-dwelling fish are common in the central basin, suggesting that
anoxia (low oxygen) was not a persistent quality of bottom waters. Nevertheless, many fish fossils are found with distended gills, favoring
asphyxiation as a cause of death. One possible explanation for rapid asphyxiation and burial places blame on freshwater runoff during storms in the wet season. Heavy rainfall would bring a cascade of organic-rich marginal sediments into the central basin. As the sediments sink, they quickly absorb oxygen from the seawater, killing and burying basin organisms in only a matter of hours. ==Paleobiota==