COMPLETENESS of the FOSSIL RECORD and the VALIDITY of SAMPLING PROXIES at OUTCROP LEVEL by ALEXANDER M
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[Palaeontology, Vol. 55, Part 6, 2012, pp. 1155–1175] COMPLETENESS OF THE FOSSIL RECORD AND THE VALIDITY OF SAMPLING PROXIES AT OUTCROP LEVEL by ALEXANDER M. DUNHILL1*, MICHAEL J. BENTON1, RICHARD J. TWITCHETT2 and ANDREW J. NEWELL3 1School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK; e-mails: [email protected]; [email protected] 2School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK; e-mail: [email protected] 3British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford OX10 8BB, UK; e-mail: [email protected] *Corresponding author. Typescript received 13 May 2011; accepted in revised form 16 December 2011 Abstract: Most studies of the adequacy of the fossil record do not correlate well with the other sampling proxies, nor have been carried out at a global or continental scale, and with apparent diversity, suggesting that the total amount of they have used sampling proxies that generally do not incor- sedimentary rock preserved does not influence apparent porate all aspects of sampling (i.e. rock volume, accessibility, diversity at a local scale, that is, the rock record at outcrop effort). Nonetheless, such studies have identified positive cor- has been sampled efficiently. However, we do find some cor- relations between apparent diversity and various sampling relations between apparent diversity and proxies for worker proxies. The covariation of fossil and rock record signals has effort. The fact that the proxies do not correlate significantly been interpreted as evidence for bias or for a common cause, with each other suggests that none can be regarded as an all- such as sea level change, or as evidence that the signals are encompassing sampling proxy that covers all aspects of bias. in some ways redundant with each other. Here, we compare Further, the presence of some correlations between sampling a number of proxies representing the three main aspects of proxies and diversity most probably indicates the bonanza sampling, (1) sedimentary rock volume, (2) rock accessibility effect, as palaeontologists have preferentially sampled the and (3) worker effort, with palaeodiversity in a geographi- richest rock units. cally and stratigraphically constrained data set, the marine Lower Jurassic outcrop of the Dorset and East Devon Coast. Key words: diversity, sampling, proxy, GIS, Dorset, Lower We find that the proxies for rock volume and accessibility Jurassic. Many recent publications have highlighted a significant Johnson 2001; Johnson 2003), (3) a common cause hav- positive correlation between palaeodiversity and various ing driven both the rock and fossil records simultaneously proxies for sampling (Raup 1972; Sheehan 1977; Peters (Sepkoski 1976; Peters and Foote 2002; Foote 2003; Peters and Foote 2001; Smith 2001; Smith et al. 2001; Peters 2005, 2006; Peters and Heim 2010) or (4) redundancy and Foote 2002; Crampton et al. 2003; Smith and McGo- between sampling proxies and apparent diversity (Benton wan 2005, 2007; Wang and Dodson 2006; Fro¨bisch 2008; 2010; Benton et al. 2011; Dunhill 2012). As accepted by Barrett et al. 2009; Butler et al. 2009, 2011; Wall et al. most palaeontologists, it is likely that any particular fossil 2009; Benson et al. 2010; Mannion and Upchurch 2010). record reflects a complex mix of rock bias, effort, com- The covariation between palaeodiversity and sampling mon cause and redundancy, and it may be hard to distin- proxies has led to four possible interpretations that the guish purely ‘geological’ and ‘biological’ signals, despite fossil record is significantly influenced by (1) bias driven suggestions to the contrary (e.g. Smith 2007; Smith and by the amount of sedimentary rock available for sampling McGowan 2007; Barrett et al. 2009). (Raup 1972, 1976; Peters and Foote 2001, Smith 2001, Sampling is a combination of geological and human fac- 2007; Peters and Foote 2002; Smith and McGowan 2005, tors (Raup 1972) and can be split into three aspects: (1) 2007; Fro¨bisch 2008; Barrett et al. 2009, Wall et al. 2009), sedimentary rock volume, (2) accessibility and (3) worker (2) variable human collecting effort expended both tem- effort (Raup 1972; Benton et al. 2011). The most com- porally (Sheehan 1977) and geographically (Jackson and monly used sampling proxies are effort by palaeontologists ª The Palaeontological Association doi: 10.1111/j.1475-4983.2012.01149.x 1155 1156 PALAEONTOLOGY, VOLUME 55 (Sheehan 1977; Purnell and Donoghue 2005; Bernard et al. the Hettangian to Toarcian stages, which correspond to 2010), estimates of sedimentary rock volume (Raup 1972, the lithostratigraphical Lias Group in the UK (Simms 1976; Kalmar and Currie 2010), rock outcrop (i.e. geologi- 2004) (Fig. 1). The Dorset and East Devon Coast provides cal map) area (Smith 2001; Crampton et al. 2003; Smith the most continuous section of Lower Jurassic exposure and McGowan 2005, 2007; Uhen and Pyenson 2007; Man- in the UK (Simms 2004) and stretches for around 15 km der and Twitchett 2008; McGowan and Smith 2008; Marx from Pinhay Bay in the west, to Burton Bradstock in the 2009; Wall et al. 2009; Mannion and Upchurch 2010; east (Fig. 2), forming part of the Dorset and East Devon Peters and Heim 2010), number of sedimentary rock Coast World Heritage Site (Lord et al. 2010). The Lias ‘packages’ bounded by hiatuses (Peters 2005, 2006; Peters Group consists of marine mudstones, sandstones and and Heim 2010), number or area of fossiliferous localities limestones, most recently revised by Cox et al. (1999) into per time interval (Fara 2002; Barnosky et al. 2005; Foun- five formations: Blue Lias, Charmouth Mudstone, Dyr- taine et al. 2005; Lloyd et al. 2008; Butler et al. 2011) or ham, Beacon Limestone and Bridport Sand (Fig. 1). Many number of sedimentary formations per time interval older names have been retained as members (e.g. Eype (Peters and Foote 2001, Peters and Foote 2002; Fountaine Clay, Down Cliff Sand, Thorncombe Sand, Marlstone et al. 2005; Wang and Dodson 2006; Fro¨bisch 2008; Bar- Rock), with the Lower Lias members having been revised rett et al. 2009; Butler et al. 2009, 2011; Benson et al. recently by Page (2009), with the Shales-with-beef and 2010; Marx and Uhen 2010). Black Ven Marls combined into the Black Ven Mudstone, The majority of palaeodiversity studies are carried out and the Belemnite Marls and Green Ammonite Beds on a global or continental scale, and the geological sam- renamed as the Stonebarrow Marl and Seatown Marl pling proxies used are, at best, vague approximations and respectively (Fig. 1). do not accurately represent the actual amount of rock avail- The Blue Lias Formation consists of 26–32 m of pale able for sampling (Twitchett et al. 2000; Crampton et al. grey limestone interbedded with dark, organic-rich lami- 2003; Benton 2010; Benton et al. 2011; Dunhill 2011, nated shale and marl (Hallam 1960; Simms 2004; Paul 2012). Very few palaeodiversity studies have been under- et al. 2008). The Charmouth Mudstone Formation is taken at small temporal and ⁄ or geographical scales. The divided into three members (Fig. 1): the Black Ven Mud- few that exist have discovered little evidence of sedimentary stone Member (71–79 m) comprises mostly dark mud- rock or human sampling bias (e.g. Wignall and Benton stones with numerous bands of fibrous calcite (‘beef’), 1999; Benton et al. 2004; Mander and Twitchett 2008). together with a few limestone bands; the Stonebarrow The Lower Jurassic of the Dorset Coast has been a Marl Member (23–29 m) consists of blue-grey mudstone mecca for fossil collecting over more than two centuries alternating with paler marl; the Seatown Marl Member (Lord et al. 2010) and represents the most intensively (15–37 m) comprises blue-grey mudstone, with occasional sampled marine section of this age in the UK (Simms thin limestone beds, becoming increasingly sandy towards 2004), if not the world. The diverse fossil assemblages are the top, except for the top 2 m that are a dark, highly well documented, including 14 type specimens of reptiles pyritic, mudstone. The Dyrham Formation comprises (Benton and Spencer 1995), many fishes (Dineley and three members: the Eype Clay Member (67–78 m) con- Metcalf 1999) and a substantial number of invertebrate sists largely of grey, silty mudstone with a few sandstone species. The section also benefits from exceptionally layers; the Down Cliff Sand Member (22–28 m) consists detailed stratigraphic accounts, especially by Lang (1924, mainly of grey-brown sandy mudstone, becomes sandier 1936; Lang et al. 1923, 1928; Lang and Spath 1926) and towards the top and grades into the Thorncombe Sand Buckman (1910, 1917, 1922), who devised high-resolution Member; Thorncombe Sand Member (26–28 m) consists bed-by-bed stratigraphical frameworks for the Lias a unit of yellow-weathering sandstones with frequently (Simms 2004; Lord et al. 2010), which have provided the cemented ‘doggers’. The Beacon Limestone Formation basis for modern lithostratigraphical interpretation. (3.2–4.6 m) is divided into the pink conglomeratic lime- Given the long history and intensity of study, diverse stone of the Marlstone Rock Member and the pale con- fossil fauna and detailed bed-by-bed stratigraphic context, glomeratic limestone of the Eype Mouth Limestone the Lower Jurassic of the Dorset Coast represents a per- Member. The Bridport Sand Formation (51–64 m) has in fect test case for investigating sampling bias at a strati- its basal 21 m the Down Cliff Clay Member consisting of graphically and temporally constrained scale. silty sandstone, which become sandier upwards. GEOLOGICAL SETTING FOSSIL FAUNA The Lower Jurassic Series spans about 22 million years The diverse fossil assemblages of the Dorset and East from 200 to 178 million years ago (Ma) and comprises Devon Coast Lias are well recorded and consist of a larger DUNHILL ET AL.: COMPLETENESS OF THE FOSSIL RECORD 1157 FIG.