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RAPID COMMUNICATION / COMMUNICATION RAPIDE
On the discovery of a unique terrestrial faunal assemblage in the classic Pennsylvanian section at Joggins, Nova Scotia Brian L. Hebert and John H. Calder
Abstract: A rich fossil assemblage discovered from the classic Pennsylvanian locality of Joggins, Nova Scotia, is here described for the first time. The 2 m-thick Hebert sandstone, within the lower Joggins Formation of early Langsettian age, is the most productive stratum at Joggins of terrestrial tetrapods and the pulmonate gastropod Dendropupa vetusta exclusive of the historic fossil forest locality of Dawson and Lyell, and the most productive Pennsylvanian locality in the world of the large unionoid bivalve genus Archanodon. Although the land snail–archanodontid bivalve–tetrapod assemblage of the Hebert Sandstone comprises a unique late Paleozoic terrestrial dryland biota, individual taxa were not endemic to drylands alone. Résumé : Un riche assemblage fossilifère découvert à Joggins, en Nouvelle-Écosse, une localité du Pennsylvanien classique, est décrit ici pour la première fois. Le grès Hebert, d’une épaisseur de deux mètres, dans le bas de la Formation de Joggins, d’âge Langsettien précoce, est la strate la plus productive de tétrapodes terrestres et du gastéropode pulmoné Dendropupa vetusta à Joggings, à l’exception de la localité forestière fossile historique de Dawson et Lyell, et la localité pennsylvanienne la plus productive au monde du genre Archanodon, un grand bivalve (Unionoida). Bien que l’assemblage terrestre escargot–bivalve archanodontidé–tétrapode du grès Hebert comprenne un biote terrestre de terre aride unique datant du Paléozoïque tardif, les taxons individuels n’étaient pas endémiques aux seules terres arides. [Traduit par la Rédaction]
Hebert and Calder
Introduction The Joggins section, with its rich fossil record, has long been held as the archetype for the terrestrial realm of the Pennsylvanian “Coal Age” wetlands (Dawson 1863; Carroll et al. 1972; Milner 1987; Carroll 1994). Less well known is the fact that dryland environments are well represented in the stratal succession of Joggins (Davies and Gibling 2003) and that they differ from wetland environments, at least in terms of relative floral composition (Falcon-Lang 2003). Important insight into this dryland realm and the level of its endemism is gained by an hitherto undescribed faunal assemblage, which we report here. The fossiliferous bed from whence this assemblage derives has been informally named the Hebert sandstone (Calder et al. 1999) in recognition of the discovery of the fossil biota by Brian L. Hebert of Lower Cove, co-author of this paper, and its utility as a distinctive biostratigraphic unit. Apart from contributing to the little-known fossil record of the
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terrestrial dryland environment of the Carboniferous, this singular locality within the classic Joggins section (Fig. 1) has contributed the greatest number of specimens known in the world of one invertebrate taxon and the most diverse tetrapod assemblage yet discovered at Joggins from a single bed, including osteological elements previously undescribed.
Stratigraphic and sedimentologic setting The Hebert sandstone lies within the Joggins Formation of the Cumberland Group (Ryan et al. 1991) at a stratigraphic horizon 273 m above the lowermost coal (45 of Logan 1845) that defines the base of Logan’s Division IV and at 292 m of the measured section of Davies and Gibling (2003) (Fig. 2). The age of the Joggins Formation, as deduced from palynology (Dolby 1991) and macroflora (Wagner 1999), is earliest Langsettian (Westphalian A), with late Namurian floral elements that record the proximity of the Namurian–Westphalian boundary (Calder 1998). The Hebert sandstone occurs within
Received 30 May 2003. Accepted 25 November 2003. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 26 February 2004. Paper handled by Associate Editor B. Chatterton. B.L. Hebert. RR 1, Joggins, NS B0L 1A0, Canada. J.H. Calder.1 Nova Scotia Department of Natural Resources, Box 698, Halifax, NS B3J 2T9, Canada. 1
Corresponding author (e-mail:
[email protected]).
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248 Fig. 1. Location of the Hebert Sandstone on the classic Joggins section, south of Lower Cove and Little River, Nova Scotia.
Can. J. Earth Sci. Vol. 41, 2004 Fig. 2. Stratigraphic position of the Hebert sandstone, Joggins Formation. Divisions are those of Logan (1845); cycle numbers and facies assemblages refer to those of Davies and Gibling (2003).
a red-bed-dominated section of the formation assigned by Davies and Gibling (2003) to their stratal cycle 4, which they interpret as an example of their well-drained floodplain facies (WDF) assemblage. This facies assemblage comprises small, fixed channel bodies of anastomosing type, with crevasse splay sandstones and immature red mudstone paleosols. The fossil-bearing bed (Fig. 3) is a sandstone body 1.8–3.5 m in thickness with crosscutting channel forms that were infilled with lateral accretion surfaces during active flow and rapidly abandoned (Rygel et al. 2002; Davies and Gibling 2003). The Hebert sandstone has been interpreted as a meandering channel deposit formed during late stage evolution of a fixed, anastomosing fluvial system (Rygel et al. 2002).
The fossil assemblage The known biota of the Hebert sandstone comprises three faunal components that contribute to a unique and hitherto undescribed fossil assemblage: (i) large, articulated shells of the rare bivalve Archanodon (Asthenodonta) westoni (Figs. 4a, 4b); (ii) the diminutive, early land snail Dendropupa vetusta (Fig. 4c); and (iii) numerous osteological elements of tetrapods, including cranial and post-cranial structures (Figs. 4d-4f). The floral record of the sandstone (Rygel et al. 2002; FalconLang et al. 2004), is dominated by cordaitean gymnosperms (often charcoalified) with subordinate calamites, decorticated lycopsids and (?derived) permineralized pteridosperm rhizoliths. A preliminary description of the fauna of the Hebert sandstone follows.
Invertebrates Phylum Mollusca Class Bivalvia Linné 1758 (Buonanni, 1681) © 2004 NRC Canada
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Order Family Genus
Unionoida Stoliczka 1871 Archanodontidae Weir 1969 Archanodon Howse, 1878
Archanodon westoni (Whiteaves 1893) Very large, thick, inequilateral, elongated, subelliptical, anodontiform shells (Fig. 4a), occurring both as disarticulated and articulated valves, measuring up to 230 mm in length, 95 mm in height. Shells calcareous, ranging in thickness from 5 to 15 mm (commonly 10 mm), being thicker anteriorally. Articulated pairs are chordate in vertical crosssection, with a width of 42–70 mm. Conspicuous growth rings and distinct palial line. Umbo evident but unpronounced (Fig. 4b), with equivocal, putative lunule. Specimens catalogued at the Fundy Geological Museum (FGM): FGM998GF5, FGM998GF6, FGM998GF70, FGM998GF71, FGM998GF72, FGM998GF73, FGM998GF74, all collected by B.L. Hebert; FGM998GF69, FGMOOOGF75, collected by K.G. Adams; NSM002GF031.002, collected by J.H. Calder. Specimen at Joggins Fossil Centre: DRC99.124 (Don Reid Collection of FGM), collected by Eleanor Reid.
DESCRIPTION:
MICROSTRUCTURE: Articulated valves of specimen NSM002GF031.002 exhibit a predominantly laminar shell microstructure in transverse thin section, with intervening thin organic walls parallel to the shell margin. Although recrystallization by calcite in places overprints or obliterates shell microstucture, simple prisms arranged perpendicular to the outer shell margin are consistent with an original, outer prismatic layer (Taylor et al. 1969). The interior surfaces of both valves are coated with biogenic, convex upward structures of minute, stromatolitic algal bodies, with a subsequent infill of micritic sand that bears detached algal layers, oncolites, and bioclastic material.
SHELL
DISCUSSION: The only previous occurrence of Archanodon known from the Joggins section derives from a bed 71 m stratigraphically higher than the Hebert sandstone, from which T.C. Weston of the Geological Survey of Canada collected two valves in 1892. Originally named Asthenodonta westoni
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by Whiteaves (1893), Weir (1969) later synonomized Whiteaves’ genus with Archanodon, largely on inconclusive evidence of key differences in adductor scars as cited by Whiteaves. These few valves constituted the only assemblage of the family from the Pennsylvanian and the last known appearance of the genus and family. A solitary shell (DRC 99.124) collected in 1995 by Eleanor Reid of Joggins and a specimen collected subsequently by one of the authors (BLH) derive from the same bed that is thought to have provided the original discovery by Weston, underlying a historically persistent fossil lycopsid forest succession between coals 29a and 32 of Logan (1845) (see Fig. 2 and Calder et al., In press). The collection reported herein, comprising eleven specimens, enlarges four-fold the known record of A. westoni and the Pennsylvanian record of the family Archanodontidae. The fossil record of the archanodontids is restricted areally to a circum-Atlantic provenance and temporally to the Middle Devonian to Late Carboniferous (Weir 1969; Friedman and Chamberlain 1995; Chamberlain et al. 2002; Chamberlain et al. 2004). In the northeastern United States of America, A catskillensis is known from the Bellvale and Gilboa (Givetian) formations and Oneonta (Frasnian–Famennian) Formation of New Jersey and New York and from the Catskill Formation of Pennsylvania. In western Europe, Archanodon is known from the Devonian Old Red Sandstone of Great Britain (A. jukesi) and from the Givetian of the North Rhineland, Germany (A. rhenana). Carboniferous occurrences are still more restricted, with occurrences reported from the Mississippian of England and the Pennsylvanian (Langsettian) Joggins Formation of Nova Scotia. The temporal and areal restriction of Archanodon to reflect the lack of host-mediated dispersal of parasitic larvae (glochidia) amongst the archanodontids, a dispersal strategy employed by the extant Unionacea (Weir 1969). Friedman and Chamberlain (1995) suggested, however, that host-mediated, rather than planktonic, larval dispersal may have facilitated the migration of the archanodontids into freshwater, riverine habitats via fishes. Archanodon has been interpreted as the first bivalve to have exploited these low-salinity, nutrient-rich environments and in so doing to have made the transition from marine to fresh water (Friedman and Chamberlain 1995; Chamberlain et al. 2002; Chamberlain et al. 2004). Class Gastropoda Subclass Pulmonata (Stylommatophora) Order Orthurethra Genus Dendropupa Owen, 1861 Dendropupa vetusta (Dawson), 1855 Pupaeform to cylindrical, thin multispiral shells, with fine axial lirae (ribbing); length 8.5–10 mm, width typically 4.5 mm (Fig. 4c). Aperture with slight dextral offset. Shells in places bear a thin pyritic coating. Specimens catalogued at the Nova Scotia Museum of Natural History (NSM), collectively assigned specimen no. NSM002GF031.189; collected by B.L. Hebert.
DESCRIPTION:
DISCUSSION: Dendropupa vetusta, the earliest land snail known, has three modes of occurrence at Joggins, its most widespread locality, all of which accord with the inference expressed by
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Fig. 4. Representative fauna of the Hebert sandstone. (a) Archanodon westoni (Whiteaves) (×1) FGM998GF70. (b) Anterior detail and beak of A. westoni (×1) FGM998GF74. (c) Dendropupa vetusta Dawson (×2) NSM002GF031.189. (d) Tooth (arrowed) with labyrinthine infolding (×1) FGM000GF104a, counterpart to FGM000GF104b (Fig. 4e). (e) Tetrapod mandible embedded in red-grey, poorly sorted sandstone (×1) FGM000GF104b; arrow indicates mold of tooth figured in (d). (f) Pelvic girdle of a large tetrapod, possibly of baphetid affinity (×0.5) FGM998GF7.
Solem and Yochelson (1979) and Yochelson (personal communication, 1994) that Dendropupa was an opportunistic detritivore: (1) These land snails contribute to the celebrated tree stump fauna within once-hollow standing lycopsid trees, as originally described by Lyell and Dawson (1853). (2) Dendropupa is found within red-bed intervals concentrated on reduced, greenish mudrock horizons associated with plant detritus, occurring either sparsely distributed or as dense agglomerations of -20 individuals per 5 cm2 (personal observation of the authors). (3) as loose agglomerations within metre-scale, poorly sorted sandstone beds containing intraformational mudstone clasts, typically within red-bed sequences (Calder et al. 1999; Rygel et al. 2002; Davies and Gibling 2003) and interpreted as deposits of flashy, seasonally reactivated dryland channels that served ecologically as “waterholes” (Calder et al. 1999; Rygel et al. 2002; Falcon-Lang et al. 2004). The Hebert sandstone serves as the type example for this mode of occurrence. Within the Hebert sandstone, Dendropupa shells are concentrated on at least two discrete horizons (type (3) occurrence, just mentioned); two blocks of very fine-grained red-grey sandstone with grey mud intraclasts, totalling -1 m2, have yielded over 30 specimens, and we estimate nominally 50 to be contained within these blocks alone, rendering this the single most productive stratum known in the fossil record for this taxon.
stem tetrapods at Joggins was betrayed only by their recently discovered footprint record (Calder 1998; Calder et al. 1999). These large trackmakers are inferred to have been the top predator of the Joggins ecosystem. Other skeletal elements of large tetrapods found in the Hebert sandstone include a femur of possible anthracosaur affinity, a scapula measuring - 80 × 120 mm, a robust rib (FGM000GF26) nominally 116 mm in length, and a large mandible (FGM000GF104, Fig. 4e). The mandible, embedded in red-grey, poorly sorted sandstone with grey mud intraclasts and cordaitalean gymnosperm compressions, greatly exceeds its 100 mm exposed length. An exposed robust, curved, conical tooth with labyrinthine infolding, set within the medial dentary is - 20 mm long (Fig. 4d). Candidates for these large skeletal components include stem tetrapods (cf. Baphetes, and a probable baphetid jaw of similar size to that embedded in the Hebert sandstone, described by Godfrey and Holmes (1989), from the coeval Parrsboro Formation of northern Nova Scotia) and anthracosaurs (cf. Callignethelon, described by Steen (1933) and by Godfrey et al. (1991) from Joggins). Two diminutive jaws (FGM000GF19, FGM000GF25) measuring 20 mm in length, at least one of which is of probable microsaur affinity (A.R. Milner, personal communication, 1999), fall at the other end of the size spectrum for Pennsylvanian tetrapods; all specimens were collected by B.L. Hebert and are catalogued at the Fundy Geological Museum.
Discussion Vertebrates The paleontological record of the Hebert sandstone includes osteological remains of terrestrial vertebrates (Figs. 4e, 4f), including cranial (jaw) and post-cranial (shoulder and pelvic girdles, vertebrae, rib and limb) elements of several tetrapod families. This one stratum thus far has yielded the most diverse tetrapod assemblage yet discovered at Joggins, exclusive of the tree stump fauna and “clam coals,” vernacular for NaiaditesCurvirimula-bearing organic-rich shale and limestone beds. An articulated pelvic girdle assembly (FGM998GF7, Fig. 4f) has been examined by Andrew Milner, Birkbeck College, University of London, and its full description and analysis are in preparation. Preliminary investigation suggests it to be of baphetid (loxommatid) affinity (A.R. Milner, personal communication, 1999). This primitive group of stem tetrapods (Milner and Lindsay 1998), comprising the genera Baphetes, Loxomma, Megacephalus, Spathicephalus and Kryinion (Clack 2003), is represented almost exclusively in the fossil record of paleotropical Euramerica by cranial elements (Beaumont 1977; Milner and Lindsay 1998). If subsequent study of the pelvic girdle confirms its baphetid affinity, it may represent the first osteological confirmation of this part of the baphetid skeleton. With the solitary exception of a jaw to which Dawson (1863) assigned the name Baphetes minor, the presence of
Calder et al. (1999) suggested that the Hebert sandstone and its unusual faunal assemblage represent a dryland “waterhole” analogous to those of the “Channel Country” of Australia (Gibling et al. 1998), which similarly host a fauna of large unionid bivalves, small gastropods, and tetrapods. Subsequent investigation of the fluvial sedimentology and paleobotany of the unit (Rygel et al. 2002) supports this preliminary interpretation, as does a fuller treatment of the sedimentology, paleoecology and floral elements of this unusual fossil horizon (Falcon-Lang et al. 2004). Although this faunal assemblage as yet remains the only one known from the Pennsylvanian and from this classic section, a metre-scale, erosionally based sandstone bed lying 71 m stratigraphically higher in the formation, between coals 32 and 31 of Logan (1845) shares some paleontological aspects of the Hebert sandstone. The two beds differ, however, in their broader sedimentological and environmental context. This sandstone caps a ca. 3-m shoaling up interdistributary bay fill underlying an historically persistent fossil lycopsid forest succession beneath the Fundy coal seam (Calder et al. in press). Two isolated Archanodon shells derive from this bed, which we believe to be the source bed of the holotype A. westoni, and heretofore the only Pennsylvanian specimens of Archanodon. It is noteworthy that this sandstone bed also yielded a trackway of the largest tetrapod known from the © 2004 NRC Canada
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Can. J. Earth Sci. Vol. 41, 2004 Table 1. Paleoenvironments of the Joggins Formation in which individual fauna of the Hebert sandstone occur, illustrating their cosmopolitan occurrence in both dryland and wetland environments. Hebert “waterhole” fauna
Dryland ecology
Wetland ecology
Archanodon westoni Dendropupa vetusta
Dryland waterhole1,2 Dryland stream beds1 (including waterholes1,2) Dryland litter horizons1 Dryland waterhole1 Dryland floodplains5 Dryland waterhole1 Dryland waterhole1
Shoaling open water of interdistributary bays3 Hollow lycopsid trees of forest swamps4
Baphetid stem tetrapods Anthracosaurs Microsaurs
Shoaling open water of interdistributary bays3 Estuaries6 Forest swamps3 Forest swamps, incl. hollow lycopsid trees7 Wetland floodplains5
Note: 1, This paper; 2, Falcon-Lang et al. 2004; 3, Calder et al. in press; 4, Lyell and Dawson 1853; 5, Inferred from unpublished trackway observations of the authors; 6, Dawson 1863; 7, Dawson (1882) and Milner (1987).
Joggins section, which may be of baphetid–loxommatid affinity, although as yet no land snails have been observed from this bed. Unlike the Hebert sandstone, however, this bed lies within the productive, past-mined, grey coal measures that reflect persistent wetlands (Calder et al. in press), which Davies and Gibling included in their stratal cycle 6 and assigned to their poorly drained floodplain facies (PDF) assemblage. For Archanodon, the areally limited waterholes of the dryland landscape accord with its sporadic distribution in the fossil record, but its rarity within the wetlands suggests that availability of flowing water and sandy substrate alone were not the only parameters controlling its distribution. Modern unionids require lotic (flowing or aerated) conditions (Paul Johnstone, written communication 2003), which would have prevailed when the ancestral Hebert streambed was active, perhaps seasonally. During dry seasons, Archanodon and possibly Dendropupa (see Falcon-Lang et al. 2004), may have aestivated, a strategy that may have been employed by other fauna at Joggins (Calder 2003). Within the peat-forming wetlands, unfavourably lentic (dysaerobic) conditions would have been commonplace, but so to was disturbance in the form of interdistributary flooding (Calder et al. in press), which should have provided aerated flow at least sporadically. Taphonomic bias, therefore, may play a role in the apparent paucity of the archanodontids within the wetlands given their calcareous shells, which tend not to be preserved in an environment rich in organic acids (Schultze 1996; Paul Johnston, personal communication, 2003). The taphonomy of the fossil fauna of the Hebert sandstone indicates that they are essentially in situ, if in part parauthochthonous, and insignificantly time averaged (Behrensmeyer et al. 2000), and so represent part of a hitherto undescribed ecological niche within the Paleozoic terrestrial landscape. Individually, however, the Hebert fauna were not endemic to drylands alone (Table 1). The implication is that terrestrial fauna — including land snails and tetrapods — and some aquatic fauna — archanodontid bivalves — may not have been ecologically partitioned by wetlands or drylands, but sought out similar ecological niches in suitable geomorphic settings within both of these broader landscapes.
Conclusions The faunal assemblage reported here from the Hebert sandstone is highly significant in several compelling aspects: (1) A land snail–archanodontid bivalve–tetrapod assemblage is here described for the first time from the fossil record. (2) The number of Archanodon specimens from the Hebert sandstone specimens provides a collection, enhanced by preservation of shell microstructure that, with further study, has potential to resolve the taxonomy of the family and evolutionary relationships with the extant Unionoidea. (3) The tetrapod assemblage is the largest and most diverse yet reported external to the standing trees and provides an important comparative counterpoint to the tree stump fauna, from whence the terrestrial faunal record at Joggins almost exclusively derives. (4) The Hebert sandstone provides the best example of a third mode of occurrence for the earliest land snail Dendropupa, that of sparse aggregations in seasonally flashy dryland stream beds or waterholes. (5) The land snail–archanodontid bivalve–tetrapod assemblage of the Hebert sandstone provides rare insight into the terrestrial dryland environment of the late Paleozoic and ecological patterns in communities of dryland and wetland landscapes, including dryland waterholes. Among these insights is that these individual fauna were not ecologically partitioned between dryland and wetland landscapes. Finally, the Hebert sandstone demonstrates that the fossil record of Joggins, even though long studied, contains assemblages yet to be described and that this classic site remains a virtual Rosetta Stone for the terrestrial environment of the Pennsylvanian “Coal Age.”
Acknowledgments The authors would like to thank numerous persons with whom we are currently engaged in further exploration of this unique faunal assemblage and its paleoecology, and who have helped to shed light on the fauna described herein through discussion and ongoing collaboration, in particular Andrew Milner for his insight into the tetrapod assemblage, © 2004 NRC Canada
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as well as Michael Rygel, Martin Gibling, Howard Falcon-Lang, Sarah Davies, Paul Johnston, John Chamberlain, Bob Ryan, Tim Fedak, and Deborah Skilliter. The manuscript benefitted greatly from the incisive review by Paul Johnstone and John Chamberlain, and the advice of Brian Chatterton, Associate Editor. We are indebted to Mike Rygel for his assistance “above and beyond,” and for producing the graphic illustrations. This research was supported in part by a Nova Scotia Museum Paleontology Research Grant (1999) to JHC.
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© 2004 NRC Canada
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