Beacon Limestone Formation

The Beacon Limestone Formation, historically known as the Junction Bed, is a formation of early Jurassic age (PliensbachianToarcian). It lies above the Dyrham Formation and below the Bridport Sand Formation. It forms part of the Lias Group. It is found within the Wessex Basin and parts of Somerset,[1] in England. It is well known for the Strawberry Bank Lagerstätte, which contains the 3-dimensionally preserved remains of vertebrates, including marine crocodyliformes, ichthyosaurs and fish, as well as insect compression fossils.[2]

Beacon Limestone Formation
Stratigraphic range: Upper PliensbachianToarcian Spinatum-lowermost Thouarsense
~185–175 Ma
Cliff exposure at Thorncombe Beacon
TypeGeological formation
Unit ofLias Group
Sub-unitsMarlstone Rock, Barrington & Eype Mouth Limestone Members
UnderliesBridport Sand Formation
OverliesDyrham Formation
ThicknessUp to 5 metres (16 ft)
Averaging <1.5 metres (4.9 ft)
Lithology
PrimaryLimestone
Location
Coordinates50.9°N 2.8°W / 50.9; -2.8
Approximate paleocoordinates38.4°N 7.4°E / 38.4; 7.4
RegionSomerset, Dorset
Country England
ExtentWessex Basin
Type section
Named forThorncombe Beacon
LocationCliff exposure beneath Thorncombe Beacon
Beacon Limestone Formation (England)

Fossil content

Among others, the following fossils have been reported from the formation:

Vertebrates

Vertebrates of the Beacon Limestone
GenusSpeciesLocationStratigraphic positionAbundanceNotesImages
Hauffiopteryx H. typicus Strawberry Bank Falciferum zone Lectotype: GPIT 1491/4, referred: BRLSI M1399, BRLSI M1400,

BRLSI M1401, BRLSI M1403, BRLSI M1404 and BRLSI M1406[3] partial skeletons

Ichthyosaur subsequently suggested to belong to Leptonectes[4]
Stenopterygius S. triscissus Holotype: GPIT 12/0224‐2 referred: BRLSI M1405, BRLSI M1407, BRLSI M1408 and BRLSI M1409[3] partial skeletons Ichthyosaur
Pelagosaurus P. typus "four isolated skulls and associated postcranial skeletal elements along with two articulated specimens, one of which is a small juvenile"[5] Thalattosuchian
Pachycormus P. macropterus BRLSI M1297, BRLSI M1299, BRLSI M1320, BRLSI M1332, BRLSI M1359, BRLSI M1389, BRLSI M1395, BRLSI M1341, BRLSI M1351, BRLSI M1366, BRLSI M1393[6] Pachycormiform fish
Leptolepis Indeterminate BRLSI M1259 complete specimen.[2] 4–8 cm (1.6–3.1 in) in length GB3D
Lepidotes Indeterminate 40–45 cm (16–18 in) in length
Caturus Indeterminate "two specimens, including an isolated neurocranium (BRLSI M1288)"[2] Amiiform fish BRLSI

Invertebrates

Invertebrates of the Beacon Limestone
GenusSpeciesLocationStratigraphic positionAbundanceNotesImages
Trivenapteron[7] T. moorei Strawberry Bank Falciferum zone Dermapterid earwig
Fulgoridulum Indeterminate Fulgoridiid planthopper
Necrotaulius[8] N. parvulus Necrotauliid Trichopteran
Liassogomphidae Indeterminate Odonata
Elcanidae Orthoptera
Archegocimicidae Hemiptera

References

  1. "Beacon Limestone Formation". BGS Lexicon of Named Rock Units. Retrieved 2018-03-08.
  2. Williams, Matt; Benton, Michael J.; Ross, Andrew (November 2015). "The Strawberry Bank Lagerstätte reveals insights into Early Jurassic life". Journal of the Geological Society. 172 (6): 683–692. doi:10.1144/jgs2014-144. ISSN 0016-7649.
  3. Caine, Hannah; Benton, Michael J. (September 2011). "Ichthyosauria from the Upper Lias of Strawberry Bank, England". Palaeontology. 54 (5): 1069–1093. doi:10.1111/j.1475-4983.2011.01093.x.
  4. Marek, Ryan D.; Moon, Benjamin C.; Williams, Matt; Benton, Michael J. (2015-06-15). "The skull and endocranium of a Lower Jurassic ichthyosaur based on digital reconstructions". Palaeontology. 58 (4): 723–742. doi:10.1111/pala.12174. ISSN 0031-0239.
  5. Pierce, Stephanie E.; Benton, Michael J. (2006-09-11). "Pelagosaurus typus Bronn, 1841 (Mesoeucrocodylia: Thalattosuchia) from the Upper Lias (Toarcian, Lower Jurassic) of Somerset, England". Journal of Vertebrate Paleontology. 26 (3): 621–635. doi:10.1671/0272-4634(2006)26[621:PTBMTF]2.0.CO;2. ISSN 0272-4634.
  6. Cawley, John J.; Kriwet, Jürgen; Klug, Stefanie; Benton, Michael J. (2018-09-20). "The stem group teleost Pachycormus (Pachycormiformes: Pachycormidae) from the Upper Lias (Lower Jurassic) of Strawberry Bank, UK". PalZ. 93 (2): 285–302. doi:10.1007/s12542-018-0431-7. ISSN 0031-0220.
  7. Kelly, Richard S.; Ross, Andrew J.; Jarzembowski, Edmund A. (June 2016). "Earwigs (Dermaptera) from the Mesozoic of England and Australia, described from isolated tegmina, including the first species to be named from the Triassic". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 107 (2–3): 129–143. doi:10.1017/s1755691017000329. ISSN 1755-6910.
  8. Kelly, Richard S.; Ross, Andrew J.; Coram, Robert A. (2018). "A Review of Necrotauliids from the Triassic/Jurassic of England (Trichoptera: Necrotauliidae)". Psyche: A Journal of Entomology. 2018: 1–12. doi:10.1155/2018/6706120. ISSN 0033-2615.
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