A systematic account of the ammonite faunas of the Obtusum Zone (Sinemurian Stage, Lower Jurassic) from Marston Magna, Somerset, UK
Introduction
This work describes ammonite faunas from temporary exposures in farmland to the west of the Somerset village of Marston Magna that exposed rocks of the Sinemurian Stage, Obtusum Zone (Fig. 1). This interval is named after the index ammonite species, Asteroceras obtusum (Sowerby).
The division and correlation of strata by fossil content (biostratigraphy) was a skill that can be originally credited to William Smith (1816), and a standard zonation, based on ammonite species, for the predominantly marine Lower Jurassic rocks of Northwest Europe was presented by Dean et al. (1961). This comprises a series of zones and subzones that have remained in use with few modifications since. Subzonal division is not the limit of biostratigraphical resolution, however, and further division of the Lower Jurassic chronostratigraphical column into a series of biohorizons has been developed by Page, 1992, Page, 1995, Page, 2003 and Dommergues et al. (1994). The precise definitions of these terms and their use has been discussed previously (Page, 2003), but collectively they provide a framework with which to discuss the relative chronological sequence of fossil assemblages, such as that within the Obtusum Zone (Fig. 2).
Rocks of the Obtusum Zone in Britain are best known as forming a substantial part of the Black Ven Marl Member of the Charmouth Mudstone Formation exposed in the sea cliffs of Black Ven and Stonebarrow to the east of Lyme Regis in Dorset. Here, the total thickness of rocks attributable to the Obtusum Zone reaches about 15 m, although this varies laterally with some beds being present at Black Ven, but not at Stonebarrow. The Obtusum Subzone is particularly well developed here, and its ammonite sequence allowed Page (1992) to recognise five biohorizons within it, and a further five in the Stellare Subzone (Fig. 2). However, the Obtusum Zone is incompletely represented in the Dorset coastal exposures since the youngest strata, at about the level of Lang and Spath's (1926) Bed 89 within the Stellare Subzone, are followed by a non-sequence that cuts out much of this Subzone, the entire Denotatus Subzone, the Oxynotum Zone and the basal part of the Raricostatum Zone. Therefore, our knowledge of the upper parts of the Obtusum Zone has been derived from sites elsewhere.
In the British Isles, the best documented exposures of higher intervals of the Obtusum Zone are on the foreshore at Robin Hood's Bay on the north Yorkshire coast, and the Frodingham ironstone deposits at Scunthorpe, South Humberside. In Robin Hood's Bay, the Obtusum Zone is represented by approximately 15 m of the Siliceous Shale and Calcareous Shale members of the Redcar Mudstone Formation (Howarth, 2002, Simms et al., 2004). It is more complete than the Dorset coast exposure in that all three subzones are represented, but fossils are less conspicuous and large ammonites are uncommon. Nevertheless, based on this section, Page (in Simms et al., 2004, p. 245) recognised further biohorizons in the Stellare and Denotatus subzones (Fig. 2).
The upper 2 m of the Frodingham Ironstone Member (of the Scunthorpe Mudstone Formation) has been a very rich source of ammonites from the Stellare and Denotatus subzones (Page in Simms et al., 2004) but stratigraphical data are lacking for most of these specimens and the deposit may, in any case, be condensed with some mixing of elements from different biohorizons. Consequently, the Scunthorpe fauna has added little to our understanding of Obtusum Zone biostratigraphy, and has been used primarily to confirm the broad scheme derived from Robin Hood's Bay.
The Charmouth Mudstone Formation, and within it the Obtusum Zone, forms a broad outcrop extending northwards across the Wessex Basin towards the Bristol Channel. Records from temporary sections show that some of the intervals missing on the Dorset coast are represented inland in SW England (Cope et al., 1980, Page, 2009). One of the most celebrated inland deposits is the ammonite-rich Marston Magna Ammonite Marble, also known as the ‘Marston Marble’ or ‘Ammonite Marble’. The village of Marston Magna is situated 7 km northeast of Yeovil in Somerset in the low-lying Vale of Ilchester (Fig. 1).
The so-called Ammonite Marble occurs as large cementstone concretions crowded with small ammonites, most notably of the genera Promicroceras Spath, Xipheroceras Buckman and Asteroceras Hyatt. Specimens were found in the collection of Sir Hans Sloane who died in 1753, but the first written record appears to be that of William Maton in his ‘Observations relative chiefly to the Natural History and picturesque scenery and antiquities of the Western Counties of England’ (Maton, 1797) in which he notes:
‘In the year 1778, at the opening of a marl pit a stratum of very uncommon and beautiful species of stone was discovered, containing a congeries of ammonite, scarcely deprived of the animal part of their composition (the nacre of the shells being still visible).’
Maton (1797) noted that the ‘marble’ was found about eight feet (2.4 m) below the surface and was eight or nine inches thick (20–23 cm). Woodward (1893) referred to a large mass of the stone found in 1815 during the sinking of a well in Marston Magna. Sowerby, in his description of Ammonites planicosta (Sowerby, 1812) observed that the Ammonite Marble had been found in a number of locations sometimes in pieces big enough to form tolerable sized sideboards. Kellaway and Wilson (1941) found cementstones in situ in 1938 in the bed of the brook a short distance west of the village. Wilson et al. (1958) state that specimens of Marston Marble were found in situ in the bed of the mill stream “1/2 mile” west of Marston Magna church.
A general assumption about the Ammonite Marble, perhaps fostered by its name, seems to be that these cementstone nodules all derive from the same horizon. Small fragments of this material abound in museums and private collections, and large polished pieces may be seen in various public buildings in Dorset and Somerset. They have a very characteristic appearance and many may indeed have come from the same stratigraphical horizon exposed over the years in temporary excavations around the village and possibly more widely. Page (2009) in his survey of the Lower Lias, Charmouth Mudstone Member of the West of England considered the then-known Marston Magna fauna to be broadly equivalent to the Asteroceras blakei Biohorizon (SN41) of Page (1992). However, the existence of occasional specimens of Ammonite Marble in which Asteroceras, rather than Promicroceras, is the dominant ammonite has hinted at the possibility that lithologically similar cementstones are developed at more than one horizon. This is also suggested by the observation that occasional specimens of Ammonite Marble in museum collections have an atypically septarian structure.
Over the last few years, excavations adjacent to Marston Magna, the exact location of which are not given at the request of the landowner, have exposed sections that collectively span ∼4 m of argillaceous sediment of the Obtusum Zone and have enabled representative fossils to be obtained from measured horizons in the Charmouth Mudstone Member. These faunas allow for a greater understanding of the Obtusum Zone at this location and raise some important issues concerning the evolution and taxonomy of the ammonites that are discussed here.
Section snippets
Description of sections
Excavations were studied in fields to the west of the village made in three areas, two of which were in very close proximity to each other. Thus, the first area was approximately 35 m north of the second, and 220 m northeast of the third. Although these sections were close, and the beds appeared undisturbed and horizontal, there are some difficulties in correlating between them since a consistent marker bed could not be identified across all three areas to serve as a constant datum horizon (Fig. 3
Biostratigraphical interpretation of the section
Based on the correlation of the Marston Marble with the Ast. blakei biohorizon (Page, 2009), we expected to find ammonites proving the Stellare Subzone and this was the case. We consider that the occurrence of Ast. blakei deep in the shale layers (‘dsn’ nodules) succeeded by Aeg. sagittarium, along with rare early Eparietites Spath, in the overlying near-surface weathered layers can be roughly correlated, respectively, with the Ast. blakei and Aeg. sagittarium biohorizons of Page (1992) at
Systematic description of the ammonite fauna and taxonomical issues arising
Class Cephalopoda Cuvier, 1795
Subclass Ammonoidea Zittel, 1884
Order Ammonitida Fischer, 1882
Suborder Ammonitina Fischer, 1882
Superfamily Psiloceratoidea Hyatt, 1867
Family Schlotheimiidae Spath, 1923
Genus Angulaticeras Quenstedt, 1883
Type species: Ammonites lacunatus J. Buckman, 1844
Angulaticeras boucaultianum (d’Orbigny, 1844)
(Fig. 4a.)1844 Ammonites Boucaultianus d’Orbigny, pl. 90, figs. 1–3 1879 Ammonites boucaulti (d’Orbigny) Reynés, pl. 42, figs. 3 and 4 (only) 1879 Aegoceras boucaultianum
Evolution within the Asteroceratidae and the origins of Asteroceras grp. blakei and Aegasteroceras
One of the most important findings of our work in Marston Magna was to confirm the presence of Aegasteroceras in discrete beds and not associated with Promicroceras/Xipheroceras or Ast. grp. blakei. The presence of Aegasteroceras in horizons below the Ast. blakei biohorizon (and in the Vale of Ilchester) had been reported previously, but these records were held in some doubt (e.g. Page, 2009). Our observations cannot confirm this, but do raise the possibility of Aegasteroceras occurring at
Acknowledgements
The authors thank Mr. Andy Cowap for preparation of the specimens, the landowner for permission to make exploratory excavations and Mr. T. Smith for his assistance. We are also indebted to Professor J.C.W. Cope for his generous advice during the writing of this manuscript and for providing reference material from the National Museum of Wales. Similarly, we are grateful to Dr. Martin Munt and Miss Zoe Hughes at the Natural History Museum for access to study type and historical reference
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