Fossil Record 9(2) (2006), 198–212 / DOI 10.1002/mmng.200600008 The ammonoids from the Three Forks Shale (Late Devonian) of Montana Dieter Korn*,1 & Alan Titus**,2 1 Museum fu¨ r Naturkunde der Humboldt-Universita¨t zu Berlin, Invalidenstraße 43, D-10115 Berlin, Germany 2 Grand Staircase-Escalante National Monument, 190 East Center Street, Kanab, UT 84741, U.S.A. Received 19 December 2005, accepted 10 March, 2006 Published online 17 July 2006 With 7 figures Key words: Ammonoidea, biostratigraphy, Three Forks Shale, Late Devonian, Montana. Abstract The ammonoid fauna from the Late Devonian Three Forks Shale of Montana is revised. Six taxa were recognised, which belong to the genera Tornoceras, Pernoceras, Raymondiceras, Platyclymenia, Pleuroclymenia, and Carinoclymenia. The ammo- noid assemblage suggests a stratigraphic position within the middle Famennian, most probably the Platyclymenia annulata Zone. The ammonoids display extreme septal crowding in intermediate as well as adult growth stages, which can be regarded as evidence for instable palaeoecological conditions during lifetime of the animals. Schlu¨ sselwo¨ rter: Ammonoidea, Biostratigraphie, Three Forks Shale, Oberdevon, Montana. Zusammenfassung Die Ammonoideenfauna aus dem oberdevonischen Three Forks Shale von Montana wird revidiert. Sechs Taxa werden un- terschieden; sie geho¨ ren zu den Gattungen Tornoceras, Pernoceras, Raymondiceras, Platyclymenia, Pleuroclymenia und Carino- clymenia. Die Ammonoideen-Vergesellschaftung spricht fu¨ r eine stratigraphische Position im mittleren Famennium, wahrscheinlich in der Platyclymenia annulata Zone. Die Ammonoideen zeigen auffa¨llige Dra¨ngung der Septen in intermedia¨- ren und adulten Wachstumsstadien, die als Hinweis auf instabile Lebensbedingungen fu¨ r die Tiere gewertet werden kann. # 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Introduction attributed the rocks with this fauna to the ‘latest Devonian’. The ammonoid fauna from the Three Forks The material, stored in the Carnegie Museum, Shale of Madison County, Montana, was for a Pittsburgh was then studied by Schindewolf in rather long period the only rather rich Famen- 1933. In the following year, he published a dis- nian assemblage from the New World. It was cussion of the assemblage and reached the con- discovered by Percy E. Raymond, who in clusion that the fauna has to be attributed in three articles (Raymond 1907, 1909, 1912) pro- the ‘Prolobites-Platyclymenia-Stufe’ (Schindewolf vided descriptions of his five new species Tor- 1934). His revision included the confirmation of noceras crebriseptum, Tornoceras douglassi, the four species erected by Raymond and the Prolobites simplex, Platyclymenia americana, establishment of the new genus Raymondiceras and Platyclymenia polypleura. He already for ‘Prolobites simplex’, but he was not able to * Corresponding author: e-mail: [email protected] ** e-mail: [email protected] # 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Fossil Record 9(2) (2006) 199 assign this remarkably simple-sutured ammonoid Devonian (Fammenian) age, however the upper to a specific ammonoid group. A relationship few meters of the Sappington Member belong with the superficially similar Middle Devonian unequivocally to the Early Carboniferous (Tour- genus Subanarcestes Schindewolf, 1933 was taken naisian). into consideration, but also the possibility that The ammonoids described herein all come Raymondiceras derived from cheiloceratids, in from the Trident Member, which consists almost possessing a secondarily simplified morphology. entirely of greenish or reddish weathering grey Miller (1938) later revised and re-figured the shale with concretions of limonite and siderite. fauna and discussed its stratigraphy once again. Fossils, including cephalopods are locally abun- He was also puzzled by the composition of the dant. The type section of the Trident Member is fauna and not sure about the age of the assem- only a few kilometers northeast of Milligan Can- blage, but stated that it “... is almost certainly yon section discussed herein, so correlation of younger than that of the Canadaway with Manti- the ammonoid bearing unit there, as well as that coceras” (Miller 1938, p. 9). of the other locality, is straightforward. Although The Three Forks fauna lost its exclusivity, caused the lithology of the Trident Member is fairly mainly by the peculiarly simple morphology of monotonous, there is a distinct succession of fos- Raymondiceras, with the description of the species- sils. Small limonitic internal molds of ammo- rich Famennian ammonoid assemblages from the noids, especially tornoceratids, seem more com- South Urals by Bogoslovsky (1960, 1971). Bogo- mon in the lower part of the studied section. slovsky found another species that he treated as Larger ammonoids and nautiloids, along with a Raymondiceras, R. aktubense (now attributed to sparse benthic fauna of bivalves and gastropods, Roinghites Korn, 2002) from middle Famennian become more common upsection, but then dis- rocks of Kazakhstan. Petersen (1975) added a third appear, being replaced by a rich brachiopod- species to the genus, R. inceptum, from the Platy- bryozoan dominated benthic fauna. clymenia horizon of Western Australia. Although the Three Forks Shale is mapped House (1962), who figured some of the species widely in southwestern Montana, good exposures described earlier, contributed the additional new of the ammonoid bearing Trident Shale Member species Rectoclymenia haynesi from the Three are actually quite uncommon. Forks Shale. In a subsequent article (House 1965), The junior author examined two localities dur- he described Tornoceras crebriseptum in detail. ing a limited survey of Three Forks Shale out- A rich collection of new material from several crops done in the summers of 1990 and 1995, exposures at Three Forks (Fig. 1) was assembled both of which had been reported previously by by A.T. between 1993 and 1995 and serves as the Sandberg (1965). The first locality is within Milli- basis for the revision of the ammonoid fauna. It gan Canyon (Fig. 1), just over 8 kilometers west was surprising that, despite of the repeated re- of the town of Three Forks. The Milligan Can- ports on the ammonoid assemblage from the yon drainage is classically viewed as the type sec- Three Forks, new data could be gained that allow tion for the Three Forks Shale, however, good an update of the knowledge of this occurrence. outcrops of the Trident Member with ammonoid fossils were found to be limited to a rather small area. Exposures are best in the east-center of Stratigraphy Sect. 36, T 2 N, R 1 W (Montana Principal Mer- idian), along a small north trending drainage. The Three Forks Formation was named by Peale The lower contact of the Trident Member with (1893). Although he did not designate a type the underlying Logan Gulch Member is not ex- section, the Milligan Canyon area west of the posed, and the lowest exposed bedrock in the town of Three Forks has been used as such by immediate succession is a prominent andesite sill default. The formation as originally defined in- that is pervasive in the area. Just above the cluded all of the clastic and mixed carbonate hornfels caused by the sill are excellent, gener- clastic less resistant strata between the underly- ally north-dipping outcrops of the upper portion ing Jefferson Dolomite (Devonian) and the over- of the Trident Member and the overlying Sap- lying Lodgepole Limestone (Mississippian). La- pington-Lodgepole sequence. A measured sec- ter, the Three Forks Formation was divided by tion was recorded and all Milligan Canyon Sandberg (1965) in the type area into, in ascend- specimens came from this locality (Fig. 1). Am- ing order, the Logan Gulch, Trident, and Sap- monoids generally decrease in abundance upsec- pington Members. Most of the unit has a Late tion, coincident with the appearance of orange # 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1museum-fossilrecord.wiley-vch.de 200 Korn, D. & A. Titus, Ammonoids from the Three Forks Shale of Montana Lodgepole Limestone MONTANA buff to yellow sandstone Jefferson County Milligan Canyon grey to buff shale/siltstone Red Hill tan sandstone 100 km na Sappington Member grey to tan monotonous siltstone ptum sp. montana sp. crebrise 5m sp. grey to olive shale ras without concretions ymenia ymenia zone of ironstone concretions Tornoceras Pernoce Raymondiceras simplex Platycl Platycl Pleuroclymenia america in light grey shale Carinoclymenia sample TF-4 TF-2 dark grey to black shale sample TF-5 with locally abundant cone-in-cone concretions TF-3 goethitic nodules sample sample Trident Member sample TF-1 igneous sill Fig. 1. Geographic position of the Three Forks localities, columnar section of the Milligan Canyon section, and stratigraphic range of the ammonoid species within the section. and yellow weathering sideritic carbonate con- Palaeontological descriptions cretions and a much more abundant and diverse benthic fauna. Material The second locality on Red Hill, is located 240 specimens are available for study. The majority of these spe- about 22 kilometers west and slightly north of cimens, however, are fragments, and distinct specific interpreta- the Milligan Canyon section in the center of tion is not always possible. Except for a few individuals, the en- tire material is preserved as limonite (previously markasite) Sect. 22, T.2.N, R.3.W (Fig. 1). Exposures of the steinkern specimens that only rarely bear remains of the shell. middle and upper portions of the Trident Mem- ber are better at Red Hill, but the remainder of Suborder Tornoceratina Wedekind, 1918 the section is not as well exposed, and the base Family Tornoceratidae von Arthaber, 1911 of the Trident was not seen. In overall aspect the section is similar to that of the Milligan Can- Tornoceras Hyatt, 1884 yon area, but is somewhat thicker. Ammonoids were especially common at this locality, indicat- Tornoceras sp. ing the collecting had probably not occurred Figs 2A, 3A there for many years. Unfortunately, the nature of the outcrop made collecting with a high de- M a t e r i a l. One specimen (MB.C.9126) from Red Hill. gree of stratigraphic control exceedingly diffi- D e s c r i p t i o n.