A New Lagerstätte from the Late Ordovician Big Hill Formation, Upper Peninsula, Michigan

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Research article Journal of the Geological Society Published online September 9, 2016 doi:10.1144/jgs2016-059 | Vol. 174 | 2017 | pp. 18–22

A new Lagerstätte from the Late Ordovician Big Hill Formation, Upper Peninsula, Michigan

James C. Lamsdell1,2*, Steven T. LoDuca3, Gerald O. Gunderson4, Ronald C. Meyer5 & Derek E. G. Briggs1,6 1 Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA 2 Present address: Division of Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA 3 Department of Geography and Geology, Eastern Michigan University, Ypsilanti, MI 48197, USA 4 6413 Elmwood Avenue, Middleton, WI 53562, USA 5 352e Raintree Ct., Louiseville, CO 80027, USA 6 Yale Peabody Museum of Natural History, Yale University, New Haven, CT 06511, USA * Correspondence: [email protected]

Abstract: A new exceptionally preserved marginal marine biota is reported from the Late Ordovician Big Hill Formation of Stonington Peninsula in Michigan’s Upper Peninsula. The new Lagerstätte hosts a moderately diverse fauna of medusae, linguloid brachiopods, non-mineralized arthropods and orthocone nautiloids, alongside dasycladalean green algae. The biota is similar to those of Lagerstätten from the Late Ordovician of Canada, revealing an extensive distribution of a distinctive marginal marine palaeocommunity in Laurentia at this time. The Big Hill biota extends the geographical range of exceptionally preserved Late Ordovician faunas in Laurentia and indicates that further examples remain to be discovered. Received 26 May 2016; revised 5 July 2016; accepted 10 July 2016

Konservat Lagerstätten, which preserve organisms that do not Geological setting otherwise survive normal decay processes, provide unique windows on ancient ecosystems. Such exceptionally preserved fossils provide The Big Hill Formation (Fig. 2) is part of the Richmond Group, detailed information on the biology, relationships and ecology of overlying the Stonington Formation and disconformably overlain – – extinct groups, permitting a more complete view of the structure and by the Late Ordovician Silurian (Hirnantian Llandovery) development of ecosystems in deep time. A number of Konservat Manitoulin Dolomite (Bergström et al. 2011). A core from Lagerstätten are known from the Ordovician (Van Roy et al. 2015), Schoolcraft County shows a total thickness for the Big Hill – including the Tremadocian–Floian (485 – 470 Ma) Fezouata biota Formation of 39 m (Ehlers et al. 1967), but only the lowermost 3 (Van Roy et al. 2010) and the latest Hirnantian–earliest Silurian 4 m are exposed on the Stonington Peninsula (Votaw 1980). The (443 – 442 Ma) Soom Shale (Aldridge et al. 1994), both from Big Hill Formation is dominated by fine-grained dolostones with Africa. Middle and Late Ordovician Lagerstätten are known from occasional bands of shale (Kesling 1975); grey dolomitic North America, including the Darriwilian (467 – 458 Ma) mudstones occur towards the base. The underlying Stonington Winneshiek Lagerstätte (Liu et al. 2006) and mid-Katian (450 – Formation comprises mudstone and argillaceous limestone (the Bay 449 Ma) Beecher’s Trilobite Bed (Farrell et al. 2009) in the USA de Noc Member) overlain by limestone with layers of chert nodules and the Katian Cat Head (‘early Maysvillian’; 450 – 449 Ma), (the Ogontz Member), suggesting a shallowing upward trend into William Lake (‘late Richmondian’; 446 – 445 Ma), and Airport the Big Hill Formation. This trend is supported by the evidence of Cove (‘late Richmondian’; 446 – 445 Ma) biotas from Manitoba, conodonts (the Cincinnati region biofacies model of Sweet 1988), Canada (Young et al. 2007, 2012). Collectively, these Lagerstätten which show a transition from a deeper water Phragmodus undatus chronicle the transition from a mixed Cambrian–Palaeozoic fauna in fauna in the Bills Creek and Stonington formations to a shallower the Early Ordovician (e.g. Van Roy et al. 2010, 2015; Botting et al. Rhipodognathus symmetricus fauna in the Big Hill Formation 2015) to a predominantly Palaeozoic fauna in the Late Ordovician (LaRowe 2000). The soft-bodied biota is located in a 24 cm thick (e.g. Farrell et al. 2011; Botting et al. 2011; Young et al. 2012). As interval within the Big Hill Formation characterized by buff thinly – such, they provide information on the Great Ordovician bedded fine-grained dolostone beds and green brown dolomitic Biodiversification Event (Harper 2006; Servais et al. 2008), shales (Fig. 2). The fossils occur in the upper few millimetres of the which included major restructuring of ecosystems into modern dolostone beds where they are overlain by shale. frameworks (Finnegan & Droser 2008; Servais et al. 2010) alongside dramatic radiation of new taxa. Fossil biota Here, we report a new late Katian (‘Richmondian’; 449 – 445 Ma) Lagerstätte from the Big Hill Formation of the Stonington The Big Hill biota, comprising some 31 taxa, includes benthic and Peninsula in Michigan’s Upper Peninsula, USA (Fig. 1). This soft- demersal components (Fig. 3): medusae, lingulid brachiopods and bodied biota comprises algae, medusae, unidentified chitinous chasmataspidid arthropods are the most common elements. tubes, bryozoans, brachiopods, cephalopods, bivalves and gastro- Macroalgae (Fig. 3a and b) are moderately abundant and include pods, together with a chasmataspidid arthropod and eurypterids. a tuft-like form and two species of dasycladalean green algae, The biota is similar to the approximately coeval William Lake and Chaetocladus sp. and Archaeobatophora typa Nitecki (1976), the Airport Cove biotas from Manitoba (Young et al. 2007, 2012). latter previously described on the basis of material from the Big Hill

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remainder of the sessile benthos. The mobile benthos is sparse, represented by a few decalcified bivalved molluscs (Fig. 3i), which may represent Vanuxemia, and rare gastropod specimens including species of Lophospira (Fig. 3s). The biota is characterized by non-mineralized arthropods, of which a new, large species of chasmataspidid is the most abundant (Fig. 3e); it is often preserved in accumulations of exuviae. At least two new species of eurypterid (Fig. 3g and h) are known from fragmentary material, which preserves the organic cuticle as a red– brown carbonaceous film as in some other Ordovician eurypterids (Lamsdell et al. 2013, 2015). Numerous accumulations of small leperditiid valves (Fig. 3j) are found in close proximity to the chelicerate exuvia, an association observed in other Silurian eurypterid-bearing Lagerstätten (e.g. Vrazo et al. 2014). The only other arthropods currently known from the formation are a rare non- mineralized enigmatic form exhibiting diplotergites (Fig. 3q) and a single articulated isoteline trilobite specimen, which represents the sole mineralized component of the arthropod fauna. Medusae (Fig. 3l–n) form an important component of the biota. Specimens range from examples preserving evidence of internal structures to those showing fraying of the margin or just a poorly defined outline, presumably reflecting different stages of decay. However, their outline and style of preservation (ranging from Fig. 1. Geographical location of Late Ordovician Laurentian Lagerstätten. impressions to carbonized films), closely spaced radiating lineations Palaeogeographical reconstruction © Ron Blakey, Colorado Plateau and hints of concentric ridges near the margin indicate that they Geosystems. represent Conchopeltis, which is otherwise known from the Katian of New York State (Oliver 1984; Babcock 2011). The remainder of Formation. Archaeobatophora is known only from this occurrence, the biota consists of at least two moderately common species of but Chaetocladus is associated with many Ordovician and Silurian actinocerid and pseudoorthocerid orthocone nautiloids (Fig. 3k) Lagerstätten (LoDuca 1997; LoDuca et al. 2011). Alongside preserved as decalcified moulds, with some preserving dark macroalgae, microbial mats (Fig. 3c) are also found colonizing the periostracal films. Orthocones from the Big Hill Formation were sediment surface. Lingulid brachiopods (Fig. 3o and p) are the most previously assigned to Actinoceras and Spyroceras (Hussey 1926), common component of the biota and are tentatively identified as but these genera are now considered wastebasket taxa (Kröger Lingula changi Hussey (1926), which occurs in dense clusters 2013) and the material needs further study. intermixed with larger indeterminate linguloids and rare discinoids assigned to Trematis rugosa Hussey (1926). Rare halloporid Palaeoenvironment bryozoans, possibly belonging to the genus Hallopora (Fig. 3d), and large, chitinous annulated tubes of uncertain affinity with a The delicate algal specimens are relatively complete, suggesting narrow, possibly agglutinated base or stalk (Fig. 3f) make up the little transport before burial. This in turn indicates that the environment of deposition was within the photic zone in a sheltered setting. Trace fossil-bearing evaporation surfaces are located 60 m to the NE of the excavation that yielded the exceptionally preserved fossils, from a horizon c. 2.5 m lower in the section, suggesting that the biota was deposited in relatively shallow water. The lack of desiccation or adhesion structures in association with the fossils indicates that there was no subaerial exposure and the absence of current or wave ripples on bedding surfaces suggests a depth of at least several metres. The unusual and limited taxonomic composition of the biota together with minimal bioturbation points to a dysoxic setting, and a lack of rhynchonelliform brachiopods and echinoderms combined with an abundance of lingulids may indicate a salinity other than normal marine. However, sedimentological evidence of hypersalinity, such as evaporite moulds, is lacking. Further evidence of a stressed environment is provided by microbial mats, but the occurrence of chelicerates and large numbers of medusae indicates at least some degree of connection to open marine conditions. The different stages of decay represented among the medusae indicate that they were not deposited during a single event but were carried into nearshore settings by surface winds or currents and accumulated relatively undisturbed on the sediment surface. The orthocones may also represent empty conchs that Fig. 2. Stratigraphic position of the Big Hill Lagerstätte soft-bodied biota, drifted into the environment (Reyment 2008). with the general geology of the Late Ordovician of the Michigan Basin The chasmataspidid specimens are interpreted as exuviae based shown on the left and the Big Hill Formation section at Stonington on the curvature of the opisthosoma and the outstretched position of Peninsula on the right. The top of the section forms the bedrock surface the prosomal limbs (characteristics of modern scorpion moults: and is overlain by glacial till. McCoy & Brandt 2009), as well as the frequent dorso-ventral Downloaded from http://jgs.lyellcollection.org/ by guest on January 6, 2017 20 J. C. Lamsdell et al.

Fig. 3. Representative fossils from the Late Ordovician Big Hill Lagerstätte. (a) Dasycladalean green alga, Chaetocladus sp. (UWGM 2310). (b) Tuft-like alga (UWGM 2311). (c) Microbial mat (UWGM 2304). (d) Halloporid bryozoan, Hallopora? sp. (UWGM 1964). (e) Chasmataspidid arthropods (UWGM 1873). (f) Chitinous tube of uncertain affinity with agglutinated stalk (UWGM 1870). (g) Eurypterid appendage preserving organic cuticle (UWGM 1951). (h) Eurypterid swimming paddle preserving organic cuticle (UWGM 1953). (i) Pelecypod bivalve, Vanuxemia? sp., exhibiting dissolution of calcium carbonate shell (UWGM 2056). ( j) Leperditiid (UWGM 1871). (k) Actinocerid (left) and pseudoorthocerid (right) orthocone nautiloids (UWGM 1841). (l– n) Conchopeltis medusae (UWGM 1958, 1859 and 1837 respectively). (o) Linguloid brachiopod (UWGM 1846). (p) Linguloid brachiopods, Lingula changi (UWGM 1850). (q) Enigmatic arthropod exhibiting diplotergites, shown in dorsal view with the anterior to the right (UWGM 2306). (r) Gastropod, Lophospira sp. (UWGM 2047). Scale bars in (a)–(i), (k)–(n) and (q) represent 5 mm; those in ( j), (o), (p) and (r) represent 1 mm. Specimens are accessioned at the University of Wisconsin Geology Museum, Madison, WI (UWGM). Downloaded from http://jgs.lyellcollection.org/ by guest on January 6, 2017 New Ordovician Lagerstätte from Michigan 21 disarticulation of the bucklers and lack of organic staining around Acknowledgements and Funding the specimens (Lamsdell et al. 2009). Their articulated nature We thank C. Eaton (University of Wisconsin Geology Museum) for specimen suggests that the moults were buried soon after ecdysis in a low- curation and facilitating access to the material, L. Babcock for insights on energy environment (Tetlie et al. 2008). This assemblage is notable Conchopeltis, and S. Butts (Yale Peabody Museum) and J. Bazeley-Utrup (Yale Peabody Museum) for discussion on the affinities of the brachiopods and for providing the first evidence that chasmataspidids gathered in bryozoans. We thank C. Brett (University of Cincinnati) and an anonymous sheltered regions to moult en masse in the manner hypothesized for reviewer for helpful comments that improved the paper during review. S.T.L. was eurypterids (Vrazo & Braddy 2011). supported in part by National Science Foundation grant EAR 1250756. Scientific editing by Philip Donoghue Discussion and conclusions The new Lagerstätte is similar to the latest Katian William Lake and References Airport Cove biotas of Manitoba, Canada, which represent shallow, Aldridge, R.J., Theron, J.N. & Gabbott, S.E. 1994. The Soom Shale: a unique marginal marine environments deposited under restricted conditions Ordovician fossil horizon in South Africa. Geology Today, 10, 218–221. (Young et al. 2012). Big Hill shares a number of taxa with these Babcock, L.E. 2011. 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