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A chirothere footprint from the Otter Sandstone Formation (Middle Triassic, late Anisian) of Devon, UK
Proceedings of the Geologists’ Association 124 (2013) 520–524
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A chirothere footprint from the Otter Sandstone Formation (Middle Triassic, late Anisian) of Devon, UK Robert A. Coram a,*, Jonathan D. Radley b a b
6 Battlemead, Swanage, Dorset BH19 1PH, UK School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 October 2012 Received in revised form 12 November 2012 Accepted 18 November 2012 Available online 11 December 2012
The Middle Triassic (Anisian) Otter Sandstone Formation of Devon is well known as a source of vertebrate skeletal remains, particularly of reptiles. Here we report the first definite vertebrate trace fossil, a well-preserved chirothere footprint of uncertain taxonomic identity. Chirothere prints are mostly attributed to rauisuchian archosaurs, probable fragmentary remains of which have previously been recovered from the Otter Sandstone Formation. ß 2012 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Chirothere footprint Isochirotherium Otter Sandstone Formation Triassic Anisian Devon
1. Introduction Permian and Triassic continental ‘red beds’ are extensively exposed along the east Devon coast of south-west England between Exmouth and Axmouth, representing the western portion of the Dorset and East Devon Coast World Heritage site. Although these deposits are generally sparsely fossiliferous, remains of organisms including terrestrial plants and reptiles have been recovered from some horizons. Definite vertebrate trace fossils have hitherto not been recorded from the continental Triassic of south-west England. Here we report on the discovery of a wellpreserved chirothere footprint from the Middle Triassic (late Anisian) Otter Sandstone Formation. 2. Geological setting The Otter Sandstone Formation is exposed along approximately 10 km of dramatic sea cliffs and foreshore ledges between Budleigh Salterton and Sidmouth, Devon, south-west England. It comprises up to about 210 m of reddish-hued fine-grained
* Corresponding author. Tel.: +44 01929422501. E-mail address: [email protected] (R.A. Coram).
sandstones associated with conglomerates and mudstones (Edwards and Gallois, 2004). The most basal sandstones have been interpreted as aeolian in origin, but the remainder of the formation is represented by fluvial deposits produced by braided rivers under a seasonally semi-arid climate (Benton et al., 2002). At the top of the formation, the alternating sandstones and mudstones of the Pennington Point Member mark the gradual breakdown of the fluvial system leading to arid, desert environments (Sidmouth Mudstone Formation of Mercia Mudstone Group; Gallois, 2004). Deposition took place within the western part of the Wessex Basin, close to the eastern margin of the Cornubian massif (Chadwick, 1986; Warrington and Ivimey-Cook, 1992). These higher strata (Pennington Point Member) are most accessible in cliff and foreshore sections either side of Sidmouth and are the most fossiliferous rocks in the Permian-Triassic redbed succession of Devon. They have yielded a relatively diverse, although usually fragmentary, vertebrate fauna dominated by rhynchosaur remains, but also including fish, capitosaurid amphibians, procolophonids and archosaurs (Milner et al., 1990; Benton et al., 1994; Benton and Gower, 1997). The Otter Sandstone biota indicates a broadly Anisian (Middle Triassic) age (Benton et al., 2002) and magnetostratigraphic data indicate a late Anisian age for the fossiliferous Pennington Point Member (Hounslow and McIntosh, 2003).
0016-7878/$ – see front matter ß 2012 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pgeola.2012.11.002
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underwater when the print was generated, perhaps lining a shallow pond or lake. Elongate, branching green-grey patches within the mudstone may indicate the former presence of roots or invertebrate burrows.
3. Description 3.1. Introduction The new footprint was recovered from a small outcrop (several sq m) of mottled red and green-grey sandy mudstone temporarily exposed by storms in February 2012 on the foreshore below Peak Hill (NGR SY 1058 8637), approximately 1.4 km west-south-west of the beach access at Jacob’s Ladder, Sidmouth (Fig. 1). Stratigraphically, the footprint horizon is located approximately 8 m below the top of the Otter Sandstone, and lies between sandstone beds 2 and 3 within the Pennington Point Member of Gallois (2004; also see Fig. 2). The footprint is unusual in that it is preserved in convex epirelief, that is, it projects upwards from the sediment surface, rather than being impressed into it (or preserved as a cast on a bed base) like the vast majority of fossil footprints. Examination of the exposure and the presence of other poorly preserved footprints with similar relief at the same level (see below) confirm that the print was in situ rather than on the base of an overturned block. This atypical and counter-intuitive preservation has been occasionally recorded before (e.g. dinosaurs: Thulborn, 1990; pp. 30–32), and is likely to have resulted from the original concave print being infilled by sand which subsequently differentially resisted compaction and/or foreshore erosion of softer surrounding mudstone. The absence of preserved desiccation cracks suggests that the sediment surface was
3.2. Footprint The footprint is of obvious Chirotherium type (King et al., 2005). It represents the left pes and measures 232 mm from the proximal end of digit V to the tip of digit III (Fig. 3). It is deposited in the Royal Albert Memorial Museum and Art Gallery, Exeter (reg. no. EXEMS 11/2012). There were no identifiable associated manus prints, which in chirotheres are typically smaller and less deeply impressed (King et al., 2005). There were, however, indications of other, incomplete, pedal prints, similarly preserved in convex epirelief, including a probable example situated posteriorly to the main print (Fig. 4). These prints appear to have no preferred orientation and are of similar size to the recovered print, although much more poorly preserved and offering no further morphological information. They will be studied and mapped as fresh surfaces of the bed are intermittently exposed. We consider it unwise to attempt a definitive ichnospecies determination based on a single intact print because of possible preservational idiosyncracies. For example, the sand infilling the print may in places have overspilled and subtly altered its outline; and this atypical mode of preservation additionally prevents
Fig. 1. Geological sketch map of the coast around Sidmouth, Devon, south-west England, showing location of footprint site.
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Fig. 2. Approximate level of footprint horizon within the Pennington Point Member of the Otter Sandstone Formation. Stratigraphic notation from Gallois (2004).
elucidation of any features of the underside of the foot to compare with other chirothere prints. Furthermore, if the foot had slipped slightly in the substrate, this, too, could have affected the preserved shape and dimensions. Bearing these uncertainties in mind, it is nevertheless possible to make some taxonomic observations. The print can be definitively assigned to the ichnofamily Chirotheriidae Abel, 1935, being pentadactyl, semi-plantigrade, and with a postero-laterally positioned outwardly projecting fifth digit. Features that link it to the ichnogenus Chirotherium include the third digit being the longest, and the relatively reduced, posteriorly situated first digit. Within Chirotherium, the print most resembles C. barthii Kaup, 1835, known from numerous examples from the Anisian (Middle Triassic) of Britain and elsewhere, in that digits I to IV taper sharply to form distal points (King et al., 2005). However, with a preserved length of 232 mm, it falls just outside the known size range of c. 190–220 mm length. An apparent difference from Chirotherium (and indeed other chirotheriid ichnogenera) is the relatively reduced pad at the proximal end of the fifth digit. This may simply be a preservational effect; if, as suggested above, the print was made under fairly shallow water, this may have affected the gait of the animal
and the amount of downward pressure exerted by the back of the foot. The fifth digit is not as recurved as in many figured examples of Chirotherium prints, although this was no doubt a flexible structure, and, ignoring the pad, is similar to that seen in the holotype material of C. barthii. More troublesome as far as taxonomic identity is concerned is the length of the second digit. A key diagnostic feature of Chirotherium is that this digit is always shorter than the fourth digit (King et al., 2005), whereas in the Sidmouth print it is noticeably longer, closely approaching the length of the third digit (which is always the longest in Chirotherium). King et al. (2005) observed that in the larger Chirotherium ichnospecies (and possibly individuals within a biological species), the second digit becomes relatively longer, but not to the extent seen in the Sidmouth print. The possibility remains, nonetheless, that in particularly large examples of C. barthii, comparable in size to the new specimen, further elongation of the second digit occurred. Second digits of comparable relative length to that seen in the Devon print are, however, present in the ichnogenera Brachychirotherium and Isochirotherium. They also tend to have rather less recurved fifth digits (apparently lacking terminal claws, as in Chirotherium and the Sidmouth print), which in Isochirotherium can be rather narrower than in Chirotherium; again more like that seen in the Sidmouth specimen. Brachychirotherium ichnospecies, however, have broad digits with blunt distal ends, unlike those seen in the Sidmouth print, suggesting that it does not belong to this ichnogenus. Additionally, Brachychirotherium records from before the Late Triassic are considered dubious by Klein and Lucas (2010). Virtually all Isochirotherium ichnospecies are considerably smaller than the Sidmouth print (pes length c. 142–174 mm; King et al., 2005), excluding them as candidates. Additionally, the first and fourth pedal digits tend to be more similar in length. The exception is I. herculis (Egerton, 1838), which has a pes length typically in the region of 300 mm. This taxon does, however, have a rather poorly defined and chimaeric nature. The holotype, from Anisian rocks in Cheshire, is damaged and its outline possibly also partly ‘sculpted’ in the process of extracting it from the rock. King et al. (2005) considered I. herculis to be an invalid taxon and that prints ascribed to it are better considered as large Chirotherium. Clark and Corrance (2009), on the other hand, provisionally identified chirotheriid tracks from the Isle of Arran, Scotland, as I. herculis, having previously allocated them to Chirotherium barthii (Clark et al., 2002). The length of the I. herculis holotype pes (excluding the tarsus, which is not present in the Sidmouth specimen) is around 290– 300 mm (King et al., 2005). The Sidmouth print is considerably smaller than this but falls within the stated size range of the Arran pes prints (160–380 mm; Clark and Corrance, 2009). The best preserved Arran print, with a length of 240 mm, is similar in size to the Sidmouth print, and according to the interpretive drawing (Clark and Corrance, 2009; fig. 3) digits I–IV have similar relative lengths (i.e. digit II approaching III in length; digit I shorter than IV and also posteriorly offset). Whilst we recognise the similarities between the Sidmouth and Arran prints, we are reluctant to assign the former to I. herculis due to the currently dubious status of this taxon and the fact that the Devon print evidently differs from the holotype of I. herculis in its much smaller size and more posteriorly situated first digit. Pending the discovery of further well-preserved prints in the Otter Sandstone Formation, and clarification of the diagnostic features and taxonomic validity of Isochirotherium herculis, we suspect the new Sidmouth print to be either a large variant of C. barthii or a large Isochirotherium ichnospecies. More precise determination of the taxonomic identity of this and similar prints
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Fig. 3. Photograph (A) and interpretive drawing (B) of print EXEMS 11/2012.
may also require a re-evaluation of the significance of second digit length in characterising chirothere ichnogenera. 4. Discussion Vertebrate footprints have been recorded rarely from the Permian red beds of south-west England (Sarjeant, 1983; Edwards and Scrivener, 1999), but to our knowledge this is the first record from the continental Triassic. Chirothere prints were first described in 1835 by Johann Jakob Kaup, the name Chirotherium being derived from the Greek for ‘hand beast’, with reference to the unusual thumb-like fifth digit (which is actually most noticeable on the ‘foot’ rather than ‘hand’ and also lies on the outside, as opposed to the inside, of the foot). Subsequently, similar prints were noted in Triassic deposits in many other parts of the world, including Britain (particularly north-west England; Tresise and Sarjeant, 1997) and numerous ichnotaxa erected. The identity of the Chirotherium printmaker was long the subject of dispute. Kaup (1835) conjectured that it could have been an enormous marsupial. Richard Owen, in 1842, argued in favour of a large labyrinthodont amphibian, remains of which, unlike marsupials, were at least known at that time from British Triassic rocks. Current consensus is that the chirothere printmakers were archosaurian reptiles: probably herbivorous aetosaurs in the case of Late Triassic Brachychirotherium, and rauisuchians, relatively poorly known crocodile-like carnivores with a somewhat more
erect gait, for many or all of the remainder (Lucas and Heckert, 2011). The taxonomic status and relationships of the clade Rauisuchia is subject to debate, and they may not form a monophyletic group (Gower, 2000). Fragmentary archosaur remains with rauisuchian affinities, mostly teeth, have been recovered from the Otter Sandstone Formation of the Sidmouth area (Benton, 2011), including from a loose sandstone block a few metres from, and probably originally stratigraphically close to, the footprint horizon (pers. obs. RAC). Based on rauisuchian skeletal reconstructions in the literature (e.g. Carroll, 1988, figs. 13–17; Benton, 2000, fig. 6.4), the Sidmouth print was likely to have been made by an animal in the region of 3.0–3.5 m in length, depending on body proportions. The fossil teeth, with preserved lengths of between seven and 60 mm (Benton and Gower, 1997), indicate a range of body sizes that would have embraced that of the track-maker. Within the context of increasing aridity (Gallois, 2004), decreasing fluvial energy can be linked to the development of interbedded sandstone-mudstone lithofacies that evidently facilitated footprint preservation. King et al. (2005) noted that for lithological reasons vertebrate footprints are uncommon and poorly preserved in British Triassic rocks younger than the mid Anisian, and can only rarely be identified to ichnogenus level. The Pennington Point Member of the Otter Sandstone Formation, being late Anisian in age, therefore valuably augments the British chirothere record temporally, as well as geographically.
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fossil material. Dr. Ramues Gallois (Exeter) and Richard Edmonds (Earth Science Manager, Dorset and East Devon Coast World Heritage site) are also acknowledged for general advice.
References
Fig. 4. (A) Print EXEMS 11/2012 as exposed on foreshore, with probable partial print situated posteriorly. (B) Further poorly preserved pes prints (arrowed) at same level several metres away. Hammer length 280 mm.
Acknowledgements We wish to thank Dr. Geoff Tresise (National Museums Liverpool) for critical comments on an early draft and Prof. Mike Benton (University of Bristol) for commenting on Otter Sandstone
Benton, M.J., 2000. Vertebrate Palaeontology. Blackwell Science Ltd., Oxford, 452 pp. Benton, M.J., 2011. Archosaur remains from the Otter Sandstone Formation (Middle Triassic, late Anisian) of Devon, southern UK. Proceedings of the Geologists’ Association 122, 25–33. Benton, M.J., Cook, E., Turner, P., 2002. Permian and Triassic Red Beds and the Penarth Group of Great Britain. Geological Conservation Review Series, No. 24. Joint Nature Conservation Committee, Peterborough, xvi + 357 pp. Benton, M.J., Gower, D.J., 1997. Richard Owen’s giant Triassic frogs: archosaurs from the Middle Triassic of England. Journal of Vertebrate Palaeontology 17, 74–88. Benton, M.J., Warrington, G., Newell, A.J., Spencer, P.S., 1994. A review of the British Middle Triassic tetrapod assemblages. In: Fraser, N., Sues, H.-D. (Eds.), In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods. Cambridge University Press, pp. 131–160. Carroll, R.L., 1988. Vertebrate Paleontology and Evolution. W.H. Freeman and Company, New York, 698 pp. Chadwick, R.A., 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society, London 143, 465–488. Clark, N.D.L., Aspen, P., Corrance, H., 2002. Chirotherium barthii Kaup 1835 from the Triassic of the Isle of Arran, Scotland. Scottish Journal of Geology 38 (2), 83–92. Clark, N.D.L., Corrance, H., 2009. New discoveries of Isochirotherium herculis (Egerton 1838) and a reassessment of chirotheriid footprints from the Triassic of the Isle of Arran, Scotland. Scottish Journal of Geology 45 (1), 69–82. Edwards, R.A., Gallois, R.W., 2004. Geology of the Sidmouth District: A Brief Explanation of the Geological Map: Sheets 326 and 340 Sidmouth. British Geological Survey, Keyworth, 30 pp. Edwards, R.A., Scrivener, R.C., 1999. Geology of the country around Exeter. In: Memoir of the British Geological Survey, Sheet 325 (England and Wales), HMSO, London, 183 pp. Gallois, R.W., 2004. The type section of the junction of the Otter Sandstone Formation and the Mercia Mudstone Group (mid Triassic) at Pennington Point, Sidmouth. Geoscience in South-west England 11, 51–58. Gower, D.J., 2000. Rauisuchian archosaurs (Reptilia, Diapsida): an overview. Neues Jahrbuch fu¨r Geologie und Pala¨ontologie Abhandlungen 218 (3), 447–488. Hounslow, M.W., McIntosh, G., 2003. Magnetostratigraphy of the Sherwood Sandstone Group (Lower and Middle Triassic), south Devon, UK; detailed correlation of the marine and non-marine Anisian. Palaeogeography, Palaeoclimatology, Palaeoecology 193, 325–348. Kaup, J.J., 1835. Mitteilung u¨ber Tierfa¨rhrten von Hildburghausen. Neues Jahrbuch fu¨r Mineralogy, Geognosie. Geologie und Petrefaktenkunde 327–328. King, M.J., Sarjeant, W.A.S., Thompson, D.B., Tresise, G., 2005. A revised systematic ichnotaxonomy and review of the vertebrate footprint ichnofamily Chirotheriidae from the British Triassic. Ichnos 12, 241–299. Klein, H., Lucas, S.G., 2010. Tetrapod footprints – their use in biostratigraphy and biochronology of the Triassic. Geological Society, London, Special Publications 334, 419–446. Lucas, S.G., Heckert, A.B., 2011. Late Triassic Aetosaurs as the Trackmaker of the Tetrapod Footprint Ichnotaxon Brachychirotherium. Ichnos 18, 197–208. Milner, A.R., Gardiner, B.G., Fraser, N.C., Taylor, M.A., 1990. Vertebrates from the Middle Triassic Otter Sandstone Formation of Devon. Palaeontology 33 (4), 873–892. Owen, R., 1842. Description of parts of the skeleton and teeth of five species of the genus Labyrinthodon with remarks on the probable identity of the Cheirotherium with this genus of extinct Batrachians. Transactions of the Geological Society, London 6, 515–543. Thulborn, T., 1990. Dinosaur Tracks. Chapman and Hall, London, 410 pp. Tresise, G., Sarjeant, W.A.S., 1997. The Tracks of Triassic Vertebrates. Fossil Evidence from North-West England. The Stationery Office, London, xii + 204 pp. Sarjeant, W.A.S., 1983. British fossil footprints in the collections of some principal British Museums. The Geological Curator 3 (9), 541–561. Warrington, G., Ivimey-Cook, H.C., 1992. Triassic. In: Cope, J.C.W., Ingham, J.K., Rawson, P.F. (Eds.), Atlas of Palaeogeography and Lithofacies, vol. 13. Geological Society, London, Memoir, pp. 97–106.
Contents lists available at SciVerse ScienceDirect
Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola
A chirothere footprint from the Otter Sandstone Formation (Middle Triassic, late Anisian) of Devon, UK Robert A. Coram a,*, Jonathan D. Radley b a b
6 Battlemead, Swanage, Dorset BH19 1PH, UK School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 October 2012 Received in revised form 12 November 2012 Accepted 18 November 2012 Available online 11 December 2012
The Middle Triassic (Anisian) Otter Sandstone Formation of Devon is well known as a source of vertebrate skeletal remains, particularly of reptiles. Here we report the first definite vertebrate trace fossil, a well-preserved chirothere footprint of uncertain taxonomic identity. Chirothere prints are mostly attributed to rauisuchian archosaurs, probable fragmentary remains of which have previously been recovered from the Otter Sandstone Formation. ß 2012 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.
Keywords: Chirothere footprint Isochirotherium Otter Sandstone Formation Triassic Anisian Devon
1. Introduction Permian and Triassic continental ‘red beds’ are extensively exposed along the east Devon coast of south-west England between Exmouth and Axmouth, representing the western portion of the Dorset and East Devon Coast World Heritage site. Although these deposits are generally sparsely fossiliferous, remains of organisms including terrestrial plants and reptiles have been recovered from some horizons. Definite vertebrate trace fossils have hitherto not been recorded from the continental Triassic of south-west England. Here we report on the discovery of a wellpreserved chirothere footprint from the Middle Triassic (late Anisian) Otter Sandstone Formation. 2. Geological setting The Otter Sandstone Formation is exposed along approximately 10 km of dramatic sea cliffs and foreshore ledges between Budleigh Salterton and Sidmouth, Devon, south-west England. It comprises up to about 210 m of reddish-hued fine-grained
* Corresponding author. Tel.: +44 01929422501. E-mail address: [email protected] (R.A. Coram).
sandstones associated with conglomerates and mudstones (Edwards and Gallois, 2004). The most basal sandstones have been interpreted as aeolian in origin, but the remainder of the formation is represented by fluvial deposits produced by braided rivers under a seasonally semi-arid climate (Benton et al., 2002). At the top of the formation, the alternating sandstones and mudstones of the Pennington Point Member mark the gradual breakdown of the fluvial system leading to arid, desert environments (Sidmouth Mudstone Formation of Mercia Mudstone Group; Gallois, 2004). Deposition took place within the western part of the Wessex Basin, close to the eastern margin of the Cornubian massif (Chadwick, 1986; Warrington and Ivimey-Cook, 1992). These higher strata (Pennington Point Member) are most accessible in cliff and foreshore sections either side of Sidmouth and are the most fossiliferous rocks in the Permian-Triassic redbed succession of Devon. They have yielded a relatively diverse, although usually fragmentary, vertebrate fauna dominated by rhynchosaur remains, but also including fish, capitosaurid amphibians, procolophonids and archosaurs (Milner et al., 1990; Benton et al., 1994; Benton and Gower, 1997). The Otter Sandstone biota indicates a broadly Anisian (Middle Triassic) age (Benton et al., 2002) and magnetostratigraphic data indicate a late Anisian age for the fossiliferous Pennington Point Member (Hounslow and McIntosh, 2003).
0016-7878/$ – see front matter ß 2012 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pgeola.2012.11.002
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underwater when the print was generated, perhaps lining a shallow pond or lake. Elongate, branching green-grey patches within the mudstone may indicate the former presence of roots or invertebrate burrows.
3. Description 3.1. Introduction The new footprint was recovered from a small outcrop (several sq m) of mottled red and green-grey sandy mudstone temporarily exposed by storms in February 2012 on the foreshore below Peak Hill (NGR SY 1058 8637), approximately 1.4 km west-south-west of the beach access at Jacob’s Ladder, Sidmouth (Fig. 1). Stratigraphically, the footprint horizon is located approximately 8 m below the top of the Otter Sandstone, and lies between sandstone beds 2 and 3 within the Pennington Point Member of Gallois (2004; also see Fig. 2). The footprint is unusual in that it is preserved in convex epirelief, that is, it projects upwards from the sediment surface, rather than being impressed into it (or preserved as a cast on a bed base) like the vast majority of fossil footprints. Examination of the exposure and the presence of other poorly preserved footprints with similar relief at the same level (see below) confirm that the print was in situ rather than on the base of an overturned block. This atypical and counter-intuitive preservation has been occasionally recorded before (e.g. dinosaurs: Thulborn, 1990; pp. 30–32), and is likely to have resulted from the original concave print being infilled by sand which subsequently differentially resisted compaction and/or foreshore erosion of softer surrounding mudstone. The absence of preserved desiccation cracks suggests that the sediment surface was
3.2. Footprint The footprint is of obvious Chirotherium type (King et al., 2005). It represents the left pes and measures 232 mm from the proximal end of digit V to the tip of digit III (Fig. 3). It is deposited in the Royal Albert Memorial Museum and Art Gallery, Exeter (reg. no. EXEMS 11/2012). There were no identifiable associated manus prints, which in chirotheres are typically smaller and less deeply impressed (King et al., 2005). There were, however, indications of other, incomplete, pedal prints, similarly preserved in convex epirelief, including a probable example situated posteriorly to the main print (Fig. 4). These prints appear to have no preferred orientation and are of similar size to the recovered print, although much more poorly preserved and offering no further morphological information. They will be studied and mapped as fresh surfaces of the bed are intermittently exposed. We consider it unwise to attempt a definitive ichnospecies determination based on a single intact print because of possible preservational idiosyncracies. For example, the sand infilling the print may in places have overspilled and subtly altered its outline; and this atypical mode of preservation additionally prevents
Fig. 1. Geological sketch map of the coast around Sidmouth, Devon, south-west England, showing location of footprint site.
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Fig. 2. Approximate level of footprint horizon within the Pennington Point Member of the Otter Sandstone Formation. Stratigraphic notation from Gallois (2004).
elucidation of any features of the underside of the foot to compare with other chirothere prints. Furthermore, if the foot had slipped slightly in the substrate, this, too, could have affected the preserved shape and dimensions. Bearing these uncertainties in mind, it is nevertheless possible to make some taxonomic observations. The print can be definitively assigned to the ichnofamily Chirotheriidae Abel, 1935, being pentadactyl, semi-plantigrade, and with a postero-laterally positioned outwardly projecting fifth digit. Features that link it to the ichnogenus Chirotherium include the third digit being the longest, and the relatively reduced, posteriorly situated first digit. Within Chirotherium, the print most resembles C. barthii Kaup, 1835, known from numerous examples from the Anisian (Middle Triassic) of Britain and elsewhere, in that digits I to IV taper sharply to form distal points (King et al., 2005). However, with a preserved length of 232 mm, it falls just outside the known size range of c. 190–220 mm length. An apparent difference from Chirotherium (and indeed other chirotheriid ichnogenera) is the relatively reduced pad at the proximal end of the fifth digit. This may simply be a preservational effect; if, as suggested above, the print was made under fairly shallow water, this may have affected the gait of the animal
and the amount of downward pressure exerted by the back of the foot. The fifth digit is not as recurved as in many figured examples of Chirotherium prints, although this was no doubt a flexible structure, and, ignoring the pad, is similar to that seen in the holotype material of C. barthii. More troublesome as far as taxonomic identity is concerned is the length of the second digit. A key diagnostic feature of Chirotherium is that this digit is always shorter than the fourth digit (King et al., 2005), whereas in the Sidmouth print it is noticeably longer, closely approaching the length of the third digit (which is always the longest in Chirotherium). King et al. (2005) observed that in the larger Chirotherium ichnospecies (and possibly individuals within a biological species), the second digit becomes relatively longer, but not to the extent seen in the Sidmouth print. The possibility remains, nonetheless, that in particularly large examples of C. barthii, comparable in size to the new specimen, further elongation of the second digit occurred. Second digits of comparable relative length to that seen in the Devon print are, however, present in the ichnogenera Brachychirotherium and Isochirotherium. They also tend to have rather less recurved fifth digits (apparently lacking terminal claws, as in Chirotherium and the Sidmouth print), which in Isochirotherium can be rather narrower than in Chirotherium; again more like that seen in the Sidmouth specimen. Brachychirotherium ichnospecies, however, have broad digits with blunt distal ends, unlike those seen in the Sidmouth print, suggesting that it does not belong to this ichnogenus. Additionally, Brachychirotherium records from before the Late Triassic are considered dubious by Klein and Lucas (2010). Virtually all Isochirotherium ichnospecies are considerably smaller than the Sidmouth print (pes length c. 142–174 mm; King et al., 2005), excluding them as candidates. Additionally, the first and fourth pedal digits tend to be more similar in length. The exception is I. herculis (Egerton, 1838), which has a pes length typically in the region of 300 mm. This taxon does, however, have a rather poorly defined and chimaeric nature. The holotype, from Anisian rocks in Cheshire, is damaged and its outline possibly also partly ‘sculpted’ in the process of extracting it from the rock. King et al. (2005) considered I. herculis to be an invalid taxon and that prints ascribed to it are better considered as large Chirotherium. Clark and Corrance (2009), on the other hand, provisionally identified chirotheriid tracks from the Isle of Arran, Scotland, as I. herculis, having previously allocated them to Chirotherium barthii (Clark et al., 2002). The length of the I. herculis holotype pes (excluding the tarsus, which is not present in the Sidmouth specimen) is around 290– 300 mm (King et al., 2005). The Sidmouth print is considerably smaller than this but falls within the stated size range of the Arran pes prints (160–380 mm; Clark and Corrance, 2009). The best preserved Arran print, with a length of 240 mm, is similar in size to the Sidmouth print, and according to the interpretive drawing (Clark and Corrance, 2009; fig. 3) digits I–IV have similar relative lengths (i.e. digit II approaching III in length; digit I shorter than IV and also posteriorly offset). Whilst we recognise the similarities between the Sidmouth and Arran prints, we are reluctant to assign the former to I. herculis due to the currently dubious status of this taxon and the fact that the Devon print evidently differs from the holotype of I. herculis in its much smaller size and more posteriorly situated first digit. Pending the discovery of further well-preserved prints in the Otter Sandstone Formation, and clarification of the diagnostic features and taxonomic validity of Isochirotherium herculis, we suspect the new Sidmouth print to be either a large variant of C. barthii or a large Isochirotherium ichnospecies. More precise determination of the taxonomic identity of this and similar prints
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Fig. 3. Photograph (A) and interpretive drawing (B) of print EXEMS 11/2012.
may also require a re-evaluation of the significance of second digit length in characterising chirothere ichnogenera. 4. Discussion Vertebrate footprints have been recorded rarely from the Permian red beds of south-west England (Sarjeant, 1983; Edwards and Scrivener, 1999), but to our knowledge this is the first record from the continental Triassic. Chirothere prints were first described in 1835 by Johann Jakob Kaup, the name Chirotherium being derived from the Greek for ‘hand beast’, with reference to the unusual thumb-like fifth digit (which is actually most noticeable on the ‘foot’ rather than ‘hand’ and also lies on the outside, as opposed to the inside, of the foot). Subsequently, similar prints were noted in Triassic deposits in many other parts of the world, including Britain (particularly north-west England; Tresise and Sarjeant, 1997) and numerous ichnotaxa erected. The identity of the Chirotherium printmaker was long the subject of dispute. Kaup (1835) conjectured that it could have been an enormous marsupial. Richard Owen, in 1842, argued in favour of a large labyrinthodont amphibian, remains of which, unlike marsupials, were at least known at that time from British Triassic rocks. Current consensus is that the chirothere printmakers were archosaurian reptiles: probably herbivorous aetosaurs in the case of Late Triassic Brachychirotherium, and rauisuchians, relatively poorly known crocodile-like carnivores with a somewhat more
erect gait, for many or all of the remainder (Lucas and Heckert, 2011). The taxonomic status and relationships of the clade Rauisuchia is subject to debate, and they may not form a monophyletic group (Gower, 2000). Fragmentary archosaur remains with rauisuchian affinities, mostly teeth, have been recovered from the Otter Sandstone Formation of the Sidmouth area (Benton, 2011), including from a loose sandstone block a few metres from, and probably originally stratigraphically close to, the footprint horizon (pers. obs. RAC). Based on rauisuchian skeletal reconstructions in the literature (e.g. Carroll, 1988, figs. 13–17; Benton, 2000, fig. 6.4), the Sidmouth print was likely to have been made by an animal in the region of 3.0–3.5 m in length, depending on body proportions. The fossil teeth, with preserved lengths of between seven and 60 mm (Benton and Gower, 1997), indicate a range of body sizes that would have embraced that of the track-maker. Within the context of increasing aridity (Gallois, 2004), decreasing fluvial energy can be linked to the development of interbedded sandstone-mudstone lithofacies that evidently facilitated footprint preservation. King et al. (2005) noted that for lithological reasons vertebrate footprints are uncommon and poorly preserved in British Triassic rocks younger than the mid Anisian, and can only rarely be identified to ichnogenus level. The Pennington Point Member of the Otter Sandstone Formation, being late Anisian in age, therefore valuably augments the British chirothere record temporally, as well as geographically.
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fossil material. Dr. Ramues Gallois (Exeter) and Richard Edmonds (Earth Science Manager, Dorset and East Devon Coast World Heritage site) are also acknowledged for general advice.
References
Fig. 4. (A) Print EXEMS 11/2012 as exposed on foreshore, with probable partial print situated posteriorly. (B) Further poorly preserved pes prints (arrowed) at same level several metres away. Hammer length 280 mm.
Acknowledgements We wish to thank Dr. Geoff Tresise (National Museums Liverpool) for critical comments on an early draft and Prof. Mike Benton (University of Bristol) for commenting on Otter Sandstone
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