Conchostracans from the Late Triassic fissure deposits of Cromhall Quarry (Gloucestershire, UK) by Jacob D. Morton1, David I. Whiteside1, Manja Hethke2 and Michael J. Benton1 1School
of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK;
[email protected],
[email protected],
[email protected] 2Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74-100, D-12249 Berlin, Germany;
[email protected]
Overview The enigmatic fissure deposits of southeast England and southern Wales are famous for their unique assemblage of Late Triassic vertebrates, although their age is contentious. While recent studies of palynomorphs have dated some as Rhaetian, their conchostracan (Crustacea: Branchiopoda) assemblages are yet to be described in detail or used in biostratigraphy. We find that species determination of British Late Triassic conchostracans requires detailed observations of both shape and ornamentation. We also provide evidence that although Euestheria brodieana is generally smaller than Euestheria minuta, with some slight differences in carapace ornamentation, the traditional view that they are very similar, is upheld. The use of conchostracans as a biostratigraphic tool is here tested by application to the British Triassic fissures at Cromhall Quarry, where the usual stratigraphic evidence provided by superposition is absent. We find no distinction between conchostracans from bedded Rhaetian deposits of the UK and specimens collected from the fissure deposits of Cromhall Quarry, Gloucestershire, supporting a late Rhaetian age for these deposits.
Fig. 1 - Map of the United Kingdom, with all conchostracan-bearing localities included in this study indicated by black markers. Outcrops of Rhaetian strata are highlighted in grey, and nearby towns and cities are denoted by white markers for reference.
Methods Analysis of linear measurements. Comparison of carapace outline using Fourier Shape Analysis, using Hangle and Hmatch for Fourier Fast Transform, and PAST to create Principal Component Plots. Creation of synthetic outlines using Hcurve. Scanning Electron Microscopy and description of carapace ornamentation. Statistical tests including NPMANOVA and Canonical Variate Analysis. Fig. 2 - Geology and setting of the Cromhall Quarry fissure deposits. A, Palaeogeographical reconstruction of the Bristol Channel region during the Late Triassic (based on Whiteside and Marshall 2008). B, Geological map of the area surrounding Cromhall Quarry, with the approximate extent of the quarry indicated (based on BGS digimap © 2016). C, Map of Cromhall Quarry as it appeared in both 2010 and 1957 (Robinson 1957), with the positions of fissures S1-S7 indicated. The diagram to the left of this plot shows a cross-section through the western face of the quarry (based on Behan et al. 2012). D, Labelled photograph of fissure S2 (51° 62’25.6” N, 2° 42’94.6” W), with three of the major lithologies indicated.
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Key: Cromhall Quarry (n=18) Euestheria brodieana (n=40) Euestheria minuta (n=25) Carapacestheria (n=18) Unregistered Carapacestheria (n=23) Euestheria minuta from the United Kingdom (n=8) Fig. 3 - Bean plot showing the range of values for each of the eight linear measurements for each of the four clusters (Cromhall Quarry, Euestheria brodieana from bedded strata, Euestheria minuta and Carapacestheria). Large, horizontal bars indicate mean values.
Euestheria minuta from the Germanic basins (n=10) Euestheria brodieana (Gloucestershire) (n=16)
A
Euestheria brodieana (Warwickshire and Worcestershire) (n=6) Euestheria brodieana (Morayshire) (n=7) Euestheria brodieana (US deposits) (n=8)
B
Euestheria brodieana (White Lias) (n=6)
Fig. 4 – Plots of principal components 1 and 2 for Fourier coefficients. A – Comparison of Cromhall Quarry specimens, E. brodieana, E. minuta and Carapacestheria. Synthetic outlines show changes in carapace morphology across the plot. B - Comparison of Cromhall Quarry specimens, E. brodieana, E. minuta and Carapacestheria with removal of the dorsal margin. C – Comparison of E. brodieana from different parts of the UK. D – Comparison of E. brodieana from the UK and Germany. E – Comparison of E. brodieana from the White Lias (Langport Member) to other UK specimens. F – Comparison of E. brodieana from the UK and USA. Fig. 5 – SEM images of key specimens from Cromhall Quarry (7059[22]) and normally bedded Rhaetian deposits at Almondsbury, Gloucestershire (Cg2133[3]), showing fine-scale carapace ornamentation.
Key References HETHKE, M., FÜRSICH, F. T., SCHNEIDER, S. and JIANG, B. 2016. Sex determination of the Early Cretaceous clam shrimp Eosestheria middendorfii (Yixian Formation, China). Lethaia, doi: 10.1111/let.12180 KOZUR, H. W. and WEEMS, R. E . 2010. The biostratigraphic importance of conchostracans in the continental Triassic of the northern hemisphere. In LUCAS, S. G. (eds). The Triassic Timescale. Geological Society, London, Special Publications, 334, 315–417. JONES, T. R. 1862. A monograph of the fossil Estheriae. The Palaeontographical Society, London. ROBINSON, P. L. 1957. The Mesozoic fissures of the Bristol Channel area and their vertebrate faunas. Journal of the Linnean Society, Zoology, 43, 260–282. SCHOLZE, F. and SCHNEIDER, J. W. 2015. Improved methodology of ʻconchostracanʼ (Crustacea: Branchiopoda) classification for biostratigraphy. Newsletters on Stratigraphy, 48, 287–298. WHITESIDE, D. I. DUFFIN, C. J., GILL, P. G., MARSHALL, J. E. A., and BENTON, M. J. 2016. The Late Triassic and Early Jurassic fissure faunas from Bristol and South Wales: Stratigraphy and setting. Palaeontologia Polonica, 67, 257–287.
Conclusions At least some of the fissure deposits at Cromhall Quarry and their associated vertebrate faunas are late Rhaetian in age. Descriptions of E. brodieana and E. minuta in the monograph ‘A monograph of the fossil Estheriae’ (Jones, 1862) are accurate and representative of the real specimens housed in the NHM, London. Large amounts of variability within conchostracan taxa make species identification using Fourier Shape Analysis difficult, although size and carapace ornamentation are sufficient to distinguish between E. brodieana and E. minuta.
