Bear Gulch Limestone

Coordinates: 46°57′N 109°00′W / 46.95°N 109°W / 46.95; -109
Source: Wikipedia, the free encyclopedia.
Bear Gulch Limestone
Ma
↓
Heath Formation
Thickness30-40 meters
Lithology
Primarylimestone, shale
Othersiltstone
Location
Coordinates46°57′N 109°00′W / 46.95°N 109°W / 46.95; -109
Country United States
Extent Montana
Type section
Named forBear Gulch, Montana
Bear Gulch Limestone is located in Montana
Bear Gulch Limestone

The Bear Gulch Limestone is a

Heath Formation. The Bear Gulch Limestone reconstructs a diverse, though isolated, marine ecosystem which developed near the end of the Serpukhovian age. It is a lagerstätte, a particular type of rock unit with exceptional fossil preservation of both articulated skeletons and soft tissues. Bear Gulch fossils include a variety of fish, invertebrates, and algae occupying a number of different habitats within a preserved shallow bay.[1][2]

coleoid cephalopods.[7][8] Fossils of typical benthic (seabed-attached) organisms are rare, and those that do occur are restricted to reef-like sponge patches in eastern exposures. This indicates that the conditions of the bay responsible for the Bear Gulch Limestone were at odds with other marine areas nearby.[2]

Some Bear Gulch fossils were preserved so rapidly and efficiently that mating behaviors,[9][10] internal organs,[11] coloration patterns,[12] gut content,[9][13] and even the pattern of blood vessels[11] could be observed in fossils. This exceptional preservation may be a consequence of the prevailing warm monsoonal climate, as storms could frequently and rapidly cover the seabed with oxygen-poor organic runoff from shallower areas.[2] Although up to 40 meters of sediment are found in the Bear Gulch Limestone, biostratigraphic data suggests that the lens was emplaced in only 1000 years, a geological instant.[6]

Geology

Geological setting

The Bear Gulch Limestone is commonly considered to be part of the

Laurussia,[1][14][15] but this connection may have been broken by the time of the Bear Gulch Limestone's deposition.[3][4]

Many distinct limestone lenses (localized sediment packages) are developed in the Heath Formation. They overlap each other in an east-to-west sequence which extends over a distance of 160 km in the Central Montana Trough. The only exposed portions of the sequence are found at Potter Creek Dome, a small uplifted area northeast of the Big Snowy Mountains.[1][2] The last few limestone lenses form a large portion of the Upper Heath Formation,[14] which is sometimes termed the Bear Gulch Member in recognition of the most well-exposed and fossiliferous lens in the sequence.[1][18][4] This lens, the Bear Gulch Limestone, was also one of the last in the sequence, only succeeded by the Surenough Beds immediately west of it.[1][2] The Bear Gulch Limestone can be observed in numerous outcrops, spread out over an area of more than 50 km2 in Fergus County, Montana.[2][6][4]

The creation of limestone lenses in the Heath Formation has been linked to tectonic activity extending the seaway by excavating bays out of the surrounding land. As old bays are filled in and buried by sediment, faulting and seismic events form new bays in a long eastward to westward succession.[1][14][2] It may have taken a mere 1000 years for the bay responsible for the Bear Gulch Limestone lens to fill in completely, after only 25,000 years for the entire bay formation sequence to run its course across Montana.[6][4] The final limestone deposits in the area were succeeded by freshwater lake sediments of the Cameron Creek Formation, the oldest unit of the early Pennsylvanian-age Amsden Group.[1][2]

Age

A wide variety of

Chesterian stage, near the end of the Mississippian subperiod (early Carboniferous period). The upper Chesterian is the North American regional equivalent to the global Serpukhovian stage, as well as the lower part of the European Namurian stage. The Bear Gulch Limestone in particular is correlated with ammonoid zone E2b (middle Arnsbergian) of the Namurian,[18] not far from the top of the Serpukhovian. Strata from zone E2b in England and the Czech Republic have been dated to around 324 million years ago.[19][20]

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.[2]

Sedimentology and ecosystems

Fine-grained lithographic limestone (plattenkalk) is predominant in the Bear Gulch lens, though clay to silt-sized siliclastic material also forms a significant portion of rock in some areas. Several facies (sediment associations) are developed in different parts of the bay.[1][2]

The thick and fossiliferous central basin facies develop along the main northwest-to-southeast axis of the bay, which is delimited by small

homogenous) beds followed by laminae with graded bedding, which may be bioturbated in the upper part of the layer. These small sequences, sometimes termed microturbidites, draw comparisons to the Bouma sequences of deep marine turbidite deposits. The pale shale layers of the central basin were likely slowly accumulated during the dry season, while the darker layers correspond to settling bay margin sediments washed into the basin during the rainy season.[2] Sedimentary slumps along the central basin axis have been used to reconstruct flow within the basin, indicating that it deepened and drained to the northeast.[1][14][15][2] The Bear Gulch Limestone reached a thickness of 30 meters at its eastern outlet,[14][2] and total basin depth was approximately 40 meters.[1][15][6]

At its eastern outlet and nearby sheltered alcoves, the central basin transitions into the lightly colored laminated sediments of the Arborispongia-productid facies. They are characterized by reefs and patches of the arborescent (branching) sponge Arborispongia, which is commonly covered with spiny productid brachiopods. Marine benthic organisms, such as algae, bivalves, bryozoans, and crinoids, make up a significant portion of the ecosystem in these few areas.[15][2] At the calm and shallow northwest rim of the basin, Arborispongia reefs, large stromatolites, and plant debris can be observed. Salt crystal casts are abundant, indicative of hypersalinity and high rates of evaporation in this area.[6] As the bay was filled in by sediments, similar conditions extended into the center of the basin. This results in deposition of the "shallow" or "marly" facies, a collection of very pale micrites with very little organic or siliciclastic material. They preserve various algal laminations, diverse Arborispongia reefs, and local concentrations of chert and gypsum nodules.[1][14][2] The end of the bay's lifespan was indicated by a sequence of layers with leaf fragments, limestone conglomerates,[17] marls, and finally fully freshwater sediments.[2]

When the bay was in its heyday, the south edge of the central basin axis saw the development of the filamentous algal facies. These are mostly dark, organic-rich limestones and shales, similar to the central basin facies though with more siliciclastic silt and fewer microturbidites. As the name indicates, strands of filamentous algae are abundant.[15][2] Further beyond the main basin axis, the rocks are even more silty, not clearly bedded, and have a very dark coloration. These sediments, the marginal facies, have a high content of peloids, plant debris, and other organic material. They likely correspond to shallow, brackish areas with an influx of freshwater. Fossils of fully saltwater taxa are rare and poorly preserved relative to other environments, though organisms with a wider range of salinity tolerance (Acanthodes, gastropods, filamentous algae) are fairly common.[1][15][2]

Preservation of fossils

The Bear Gulch Limestone is a konservat lagerstätte, meaning that its fossils are uniquely well-preserved, including soft tissue details which offer rare insights into the biology of Carboniferous organisms.[2] The fine-grained sediments common in the formation allow for fossilized structures to retain fine resolution, as seen in equivalent plattenkalk-based lagerstätten throughout geological history. Most fossils are isolated and flattened into very thin films between sheet-like layers. Arborispongia assemblages, cephalopod shells, and large vertebrate bones occasionally project through several thin layers, approaching three-dimensional preservation.[14][15]

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.[1][15]

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.[14][2] 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).[1]

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.

asphyxiation as a cause of death.[11][15][2] 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.[2]

Paleobiota

The ecosystem represented by the Bear Gulch Limestone was vibrant and well-represented by fossils, with many described and undescribed species of vertebrates and invertebrates among their ranks. Animals show a wide range of body types and show preferences to certain areas of the bay, indicating that the bay offered a wide range of habitats and niches to be filled.[4] Many indicators of paleoecology and paleobiology have been preserved, from associations between encrusting shelled organisms and algae,[23] to gut contents and other feeding traces,[13] and even some animals fossilized while mating.[9][10] Fish are diverse and abundant, with thousands of specimens representing approximately 150 species (as of 2015).[4] Many of these species are undescribed and unnamed, only mentioned through "code names"; unnamed species are not included in paleobiota lists here.

Chondrichthyans

The Bear Gulch Limestone is renowned for its unusual and ecologically diverse

Thrinacoselache"), deep-bodied durophages (petalodontiforms), small active swimmers (most holocephalans), ray-like benthic forms (Squatinactis) and more typical shark-like predators (symmoriiforms and elasmobranchs).[4] Chondrichthyan diversity and abundance patterns are correlated, with the highest number of species and fossils found in the central basin facies and the lowest in the marginal facies. Large predatory chondrichthyans such as Listracanthus and symmoriiforms (except Falcatus) make up a higher portion of fossils in the hypersaline upper bay, while Falcatus and holocephalans are the most common fossils in the marginal and central basin facies. A sheltered reef habitat at the eastern edge of exposures has produced a high number of Heteropetalus fossils. Regardless, each habitat observes significant species overlap with other habitats, and "common" chondrichthyans (>3 specimens) are not abundant to the point that they outnumber more rare species (<3 specimens), in terms of total specimen count.[6]

Chondrichthyans of the Bear Gulch Limestone
Taxon Species Notes Images
cf. Agassizodus A
euchondrocephalan[9][6][4]
Belantsea montana
Debeerius ellefseni
Echinochimaera meltoni
Falcatus falcatus
Harpacanthus fimbriatus
Heteropetalus elegantulus
Stethacanthus altonensis and Stethacanthus productus
"Thrinacoselache" (Thrinacodus) gracia
Bealbonn[24] B. rogaire A gregoriid euchondrocephalan[25]
Belantsea[26] B. montana A belantseid petalodontiform euchondrocephalan[27]
Carcharopsis A putative
euselachian shark[4]
Cladodus A "cladodont"[9][6][4]
Cochliodus C. contortus A "cochliodont" holocephalan[28]
Damocles[29] D. serratus A large Falcatus-like symoriiform[9]
Debeerius[12] D. ellefensi A debeeriid euchondrocephalan
Delphyodontos[30] D. dacriformes An unusual holocephalan known from sharp-toothed fetuses[9]
Echinochimaera E. meltoni[31] An
echinochimaerid holocephalan[9]
E. snyderi[32] An echinochimaerid holocephalan[9]
cf. Erismacanthus E. maccoyanus A putative "cochliodont"[9][4]
Falcatus[33] F. falcatus A small and common symoriiform[33][29]
Fissodopsis[27] F. robustus An obruchevodid petalodontiform
Gregorius G. rexi[24] A gregoriid[25]
Harpacanthus H. fimbriatus A euchondrocephalan with a tentaculated snout[9][34]
Harpagofututor[35] H. volsellorhinus A long-bodied chondrenchelyiform holocephalan[9][11]
Heteropetalus[36] H. elegantulus A slender debeeriid[9][12]
Janassa J. clarki[26] A janassid petalodontiform[9][27]
Listracanthus A spiny-backed chondrichthyan[9][3][6][4]
Obruchevodus[37] O. griffithi An obruchevodid petalodontiform[27]
Orestiacanthus[38] O. fergusi A stethacanthid symoriiform[9]
Orodus An orodontiform[9]
Netsepoye[26] N. hawesi An obruchevodid petalodontiform[27]
Papilionichthys[39] P. stahlae An
iniopterygian
Petalorhynchus P. beargulchensis[26] A petalorhynchid petalodontiform[27]
Polyrhizodus P. digitatus A petalodontid petalodontiform[40][26][9][27]
Psephodus A "cochliodont"[9][28][3][6][4]
Rainerichthys[39] R. zangerli An iniopterygian
Siksika[26] S. ottae A basal petalodontiform[27]
Squatinactis[41] S. montanus An unusual "cladodont" with broad
Squatina)[42]
Srianta[24] S. dawsoni A gregoriid[25]
S. iarlis A gregoriid[25]
S. srianta A gregoriid[25]
Stethacanthus S. altonensis A stethacanthid[43][44][38][33][29]
S. productus A stethacanthid[44][38][33][29]
Thrinacodus T. gracia[25] An eel-like
Thrinacoselache"[25][45][46]
Traquairius T. agkistrocephalus A "cochliodont"[28]
T. nudus A "cochliodont"[28][34]
T. spinosus A "cochliodont"[28]
Tristychius A tristychiid elasmobranch[9][4]
Venustodus V. argutus A broad-toothed euchondrocephalan[9]

Actinopterygians

larvae have been found in isolated reef habitats near the central basin.[6]

Ray-finned fish of the Bear Gulch Limestone
Taxon Species Notes Images
Aesopichthys[47] A. erinaceus An
palaeoniscoid
"
Discoserra pectinodon
Paratarrasius hibbardi
Beagiascus[48] B. pulcherrimus A "palaeoniscoid"
Cyranorhis[49] C. bergeraci A rhadinychthyid "palaeoniscoid"
Discoserra[50] D. pectinodon A
guildayichthyiform
Guildayichthys[50] G. carnegiei A guildayichthyiform
Kalops[51] K. diophrys A "palaeoniscoid"
K. monophrys A "palaeoniscoid"
Lineagruan[48] L. judithi A "palaeoniscoid"
L. snowyi A "palaeoniscoid"
Paphosiscus[52] P. circulocaudus A paphosisciform "palaeoniscoid"
P. scalmocristus A paphosisciform
Paratarrasius[53]
 P. hibbardi A tarrasiiform[54]
Proceramala[47] P. montanensis An aesopichthyid
Wendyichthys[49] W. dicksoni A rhadinychthyid
W. lautreci A rhadinychthyid

Other fish

Non-actinopterygian and non-chondrichthyan fish make up a fairly small component of the overall diversity, though their fossils are still common in some habitats. Preserved

actinistians (coelacanths) and the rhizodont Strepsodus, which have very different habitat preferences. Coelacanths, particularly fossils of the genus Caridosuctor, are abundant in the central basin and its margins, but are practically absent in the upper bay. Strepsodus, on the other hand, is moderately common in the upper bay and very rare elsewhere. Neither are common in the sheltered eastern reef habitat. Acanthodes, the only acanthodian known from the Bear Gulch Limestone, has a similar distribution pattern to Strepsodus: common in the upper bay and nowhere else. Lampreys, represented by Hardistiella, are too rare to estimate their habitat preference.[6][4]

Fish of the Bear Gulch Limestone
Taxon Species Notes Images
Acanthodes A. lundi[55] An acanthodian[6][4]
Allenypterus montanus
Caridosuctor populosum
Allenypterus A. montanus An unusual
actinistian (early coelacanth)[56][57]
Caridosuctor[56] C. populosum A rhabdodermatid actinistian[57]
Hadronector[56] H. donbairdi A hadronectorid[57]
Hardistiella[5] H. montanensis One of the oldest known petromyzontiforms (lampreys)[58]
Lochmocercus[56] L. aciculodontus An indeterminate actinistian[57]
Polyosteorhynchus[56] P. beargulchensis A hadronectorid[57]
Strepsodus A rhizodont[6][4]

Arthropods

Arthropods are diverse, the most abundant and speciose being various early hoplocarids, relatives of modern stomatopods (mantis shrimps).[59][60] Other arthropods include an assortment of typical Carboniferous crustaceans,[61][59] rare xiphosurans (horseshoe crabs),[62] and a single "synziphosurine" (Anderella).[63] Trilobites, though common in adjacent formations, are practically absent in the Bear Gulch Limestone.

Arthropods of the Bear Gulch Limestone
Genus Species Notes Images
Aenigmacaris[61] A. cornigerum An aeschronectid hoplocarid[59]
Aenigmacaris cornigerum
Anderella parva
Daidal (Tyrannophontes) acanthocercus
Dithyrocaris rolfei
"Perimecturus" rapax
Anderella[63] A. parva The youngest known "synziphosurine" ( a group of xiphosuran-like chelicerates)
Bairdops B. beargulchensis[61] A possibly
junior synonym of Tyrannophontes[59]
Belotelson B. magister A belotelsonid shrimp[61][59]
Concavicarida
indet. 		An indeterminate concavicarid, possibly referable to Ankitokazocaris or Concavicaris[66]
Crangopsis C. eskdalensis A rare aeschronectid hoplocarid[59]
Daidal[65] D. acanthocercus An "archaeostomatopod" (hoplocarid closely related to modern mantis shrimps), previously referred to Tyrannophontes theridion[61][59] and sometimes considered its own species of Tyrannophontes.
Dithyrocaris D. rolfei[61] A large phyllocarid shrimp[59]
Euproops E. sp. A rare xiphosuran (early horseshoe crab)[62]
Ostracoda indet. Indeterminate and very rare ostracods[1]
"
Paleolimulus
" 		"P". longispinus[62] A xiphosuran with uncertain classification[67]
"Perimecturus" "P." rapax[61] A possibly
polyphyletic "palaeostomatopod" (primitive hoplocarid), often considered a relative of Bairdops[64][59][60][65]
Sairocaris S. centurion[61] A probable phyllocarid shrimp[59]
Schramine
 		S. montanaensis[68] One of the oldest of the crab-like
Halicyne[68]
Titanoprosoma[70] T. edgecombei A simplified arthropod tentatively identified as a
euchelicerate.[70]


Molluscs

Molluscs
of the Bear Gulch Limestone
Taxon Species Notes Images
Bivalvia sp. Several unnamed or undescribed species of bivalves, including scallop-like (Aviculopecten?) and mussel-like (Myalina?) forms.[61] Some encrusting pterioid bivalves (Caneyella, Ptychopteria) have been found attached to strands of sargassum-like brown algae.[23]
Reticycloceras sp.
Cephalopoda sp. Various cephalopods, which are locally abundant in the Bear Gulch Limestone. They include coiled nautiloids (Anthracoceras, Epistroboceras, Tylonautilus),[18][71] orthoconic (straight-shelled) nautiloids (Reticycloceras),[71] ammonoids (Fayettevillea, Eumorphoceras, Metadimorphoceras),[18][10] and coleoids (Gordoniconus, etc.).[7][8][72][73]
Gastropoda sp. Unnamed or undescribed species of gastropods (sea snails), which are only common in marginal facies[2]


Worms

Worms of the Bear Gulch Limestone
Genus Species Notes
Archisymplectes A. rhothon A nemertine (ribbon worm)[74]
Astreptoscolex A. anasillosus A nephtyid polychaete worm[74]
Brochosogenys B. reidiae A kielanoprionid polychaete[75]
Carbosesostris[74] C. megaliphagon A goniadid polychaete
Deuteronectanebos[74] D. papillorum An indeterminate worm
Nemavermes N. mackeei A nematode (roundworm)[74]
Phiops[74] P. aciculorum A lumbrinerid polychaete
Ramesses[74] R. magnus An indeterminate polychaete
Soris[74] S. labiosus An indeterminate polychaete
Symmetroprion S. sp. A symmetroprionid polychaete[75]

Other invertebrates

Invertebrates of the Bear Gulch Limestone
Genus / Taxon Species Notes Images
Brachiopoda sp. Several unnamed or undescribed species of brachiopods, including productids, spiriferids, Orbiculoidea, and Lingula.[61] Small and large spiny productids are common encrusters on other organisms.[2]
An unnamed sea urchin
Typhloesus wellsi
Bryozoa sp. Very rare and undescribed encrusting bryozoans, including Pycnopora regularis and Discretella robusta.[18]
Conodonta sp. Various conodonts, only recovered from coprolites or gut content of Typhloesus. May include Lochriea, Kladognathus, Gnathodus, and Cavusgnathus.[13][76]
Echinodermata sp. Various rare echinoderms, including unnamed or undescribed species of
asteroids (sea stars). The only moderately common echinoderm is Lepidasterella montanensis, a many-armed sea star.[77]
Paraconularia P. subulata A conulariid[78]
Porifera sp. Various rare to locally abundant sea sponges, including hexactinellids, Norfordia, and Belemnospongia.[75] By far the most abundant is Arborispongia delicatula, a ramose (branching) sponge which serves as the foundation for complex reef-like ecosystems.[2]
Sphenothallus A common but enigmatic organism with a strand-like tubular shape, possibly represented by multiple species. May be related to cnidarians or conulariids.[79]
"Square objects" Very common but enigmatic barrel-shaped planktonic organisms, possibly salps or cnidarians
Typhloesus[13] T. wellsi An unusual swimming invertebrate originally mistaken for large conodonts ("Lochriea wellsi", "Scottognathus elizabethi") with complex jaws. Conodont elements are now understood to be gut content.
heteropod-like gastropod.[80]

See also

Wikimedia Commons has media related to Bear Gulch Limestone.

External links

References

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