Microvertebrates from multiple bone beds in the Rhaetian of the M4–M5 motorway junction, South Gloucestershire, U.K.

Abstract

11The Rhaetian (latest Triassic) is best known for its basal bone bed, but there are numerous 12 other bone-rich horizons in the succession. Boreholes taken around the M4-M5 motorway 13 junction in SW England provide measured sections with multiple Rhaetian bone beds. The 14 microvertebrate samples in the various bone beds differ through time in their composition and 15 in mean specimen size. The onset of the Rhaetian transgression accumulated organic debris to 16 form a fossiliferous layer high in biodiversity at the base of the Westbury Formation. The 17 bone bed at the top of the Westbury Formation represents a community with lower 18 biodiversity. The bone beds differ in their faunas: chondrichthyan teeth are dominant in the 19 basal bone bed, but actinopterygians dominate the higher bone bed. These differences could 20 be taphonomic, but are more likely evidence for ecological-evolutionary changes. Further, a 21 change from larger to smaller specimen sizes up-sequence allows rejection of an earlier idea 22 that the successive bone beds represented multiple reworkings of older bone beds. 23 24

Note on the Rhaetian fish fauna from a subrosion pipe in Winterswijk (the Netherlands), with a discussion on the validity of the genus Severnichthys Storrs, 1994

“…We therefore based our identifications on comparison with material described in the above-mentioned literature. Comparison of our fauna with that from other European Rhaetian localities, especially those of the British Penarth Group (Korneisel et al, 2015;Lakin et al 2016;Slater et al, 2016;Cavicchini et al, 2018;Cross et al, 2018) shows an overall resemblance in faunal composition, with the shark genera Lissodus and Rhomphaiodon, and the actinopterygians Gyrolepis, Saurichthys and Birgeria (the latter two often jointly referred to as Severnichthys) as the most abundant elements. See Table 1 for an overview.…”

A Rhaetian microvertebrate fauna from Stowey Quarry, Somerset, U.K.

“…The occurrence of multiple bone beds in Rhaetian strata is not unusual and has been documented before: Sykes (1977), for example, listed some 40 Rhaetian outcrops and the bone beds they contain, and he proposed criteria for the characterization of supposed "primary" and "secondary" bone beds, based on elements that indicate reworking of fossils. Such multiple bone beds in the British Rhaetian have been confirmed many times since, by Martill (1999), Allard et al (2015), Mears et al (2016), andSlater et al (2016), among others. However, Sykes' (1977) model, in which he argued that all Rhaetian bone beds have been reworked from elsewhere, and that the higher bone beds in particular show evidence that they were reworked from the basal bone bed, has been rejected on the basis of several lines of evidence, by Mears et al (2016), Slater et al (2016), and others: (1) the higher Rhaetian bone beds do not show any difference in abrasion levels from the basal bone bed; (2) basal Westbury Formation bone beds range from largely allochthonous (e.g.…”

“…Such multiple bone beds in the British Rhaetian have been confirmed many times since, by Martill (1999), Allard et al (2015), Mears et al (2016), andSlater et al (2016), among others. However, Sykes' (1977) model, in which he argued that all Rhaetian bone beds have been reworked from elsewhere, and that the higher bone beds in particular show evidence that they were reworked from the basal bone bed, has been rejected on the basis of several lines of evidence, by Mears et al (2016), Slater et al (2016), and others: (1) the higher Rhaetian bone beds do not show any difference in abrasion levels from the basal bone bed; (2) basal Westbury Formation bone beds range from largely allochthonous (e.g. Westbury on Severn) to mixed or largely allochthonous (e.g.…”

“…Nonetheless, the bone-rich horizon termed informally the "basal bone bed" is dense with chondrichthyan and osteichthyan fish remains, as well as fossils of marine reptiles such as ichthyosaurs and plesiosaurs, and the enigmatic Pachystropheus. Most palaeontological studies of Rhaetian microvertebrates have focused on the basal bone bed, but there are as many as four or five further bone-rich units higher in the Westbury Formation and low in the Cotham Beds (Duffin, 1980;Macquaker, 1999;Storrs, 1994;Allard et al, 2015;Mears et al, 2016;Slater et al, 2016). Most of these locations are in south Gloucestershire, close to the shores of the present-day Severn Estuary.…”

Fish remains from the Rhaetian (Late Triassic) of Winterswijk, the Netherlands (Pisces: Chondrichthyes and Actinopterygii)

“…The presence of the lateral cusplets indicate that these teeth are anterolateral teeth (Duffin, 1998a, text fig. 20B;Korneisel et al, 2015;Slater et al, 2016;Cross et al, 2018;Moreau et al, 2021;Williams et al, 2022). The size ranges between about 2 and 3 mm.…”