Fig 1.
Geological time scale showing tetrapod taxa distribution and fossil lineages (modified from [3,7,8]).
Colors indicate fossil occurrences and phylogenetic ghost lineages: red represents tetrapod fossils from the East Kirkton Quarry; black denotes tetrapod fossils from other localities; and gray indicates phylogenetic tree-modeled ghost lineages. Taxa abbreviations: A, Anthracosauria; B, Temnospondyli; C, Microsauria; D, Seymouriamorpha; E, Diadectidae; F, Nectridea; G, Aistopoda; H, Amniota; I, Baphetidae; J, Colosteidae; K, Gephyrostegidae; L, Casineria; M, Crassigyrinus; N, Whatcheeriidae; O, Adelogyrinidae; P, Ventastega; Q, Ichthyostega; R, Acanthostega; S, Tulerpeton. The approximate extent of Romer’s Gap is highlighted by the dark orange box. The blue dashed box indicates the current biostratigraphic age range assigned to fossils from the East Kirkton Limestone (based on [3,9–12]).
Fig 2.
(A) Map of the United Kingdom highlighting the East Kirkton Quarry study area (red) in the Bathgate Hills, Midland Valley of Scotland (modified from [41]). (B) Generalized environmental reconstruction of the East Kirkton setting, based on previous schematic diagrams and interpretations (modified from [34,42–44]). The reconstruction includes Westlothiana lizziae (not to scale) depicted resting on a rock.
Fig 3.
Generalized geological formations and chronostratigraphic divisions of the West and East Lothian localities in Scotland, modified from [49,67].
The East Kirkton Limestone at the East Kirkton Quarry is highlighted with a red star.
Fig 4.
(A) Paleoenvironmental reconstruction of the East Kirkton Quarry locality and surrounding areas, illustrating a unique restricted hydrothermal lacustrine setting influenced by sporadic marine incursions. The diagram highlights various volcanic and clastic deposits near Lake Cadell, modified from [49,68]. Reprinted from [49] under a CC BY license, with permission from Elsevier, original copyright 2017. (B) Geologic map of the East Kirkton Quarry showing sample collection locality by [46] and modified from [50,68]. Reprinted from [50] under a CC BY license, with permission from Elsevier, original copyright 2017.
Fig 5.
Samples EK82 and EK83 kernel density estimation (KDE) distribution plots.
LA-ICP-MS 238U-206Pb and 207Pb-206Pb zircon dates with (±2σ) uncertainties from 10% discordance filter dataset. Maximum depositional age (MDA) methods (detrital youngest single grain [Dz YSG]; most concordant youngest single grain [YSG]; youngest cluster of three or more grains overlapping within 2σ uncertainty [YC2σ+3]; youngest mode weighted mean [YMWM]). Colors represent the period (pink: Precambrian; light green: Cambrian; teal: Ordovician; aquamarine: Silurian; orange: Devonian; light blue: Mississippian; blue: Pennsylvanian).
Fig 6.
Cathodoluminescence (CL) images and concordia diagrams of representative zircon grains from samples EK82 and EK83.
CL images display 238U-206Pb LA-ICP-MS dates, while concordia diagrams show 238U-206Pb data, filtered to include only grains with ≤10% discordance for zircons younger than 1000 Ma. Zircon grain distributions illustrate 238U-206Pb Paleozoic dates for sample EK82 and combined 238U-206Pb and 207Pb-206Pb Precambrian–Paleozoic dates for sample EK83. Maximum depositional age (MDA) approaches are color-coded as follows: green for the youngest detrital single grain (Dz YSG), light blue for the most concordant youngest single grain (YSG), black box for the youngest cluster of three or more grains overlapping within 2σ uncertainty (YC2σ+3), and red box for the youngest mode weighted mean (YMWM). All uncertainties are reported as 2σ.
Table 1. X.
ray diffraction (XRD) mineral composition (Wt.%) from the East Kirkton Limestone.
Table 2.
XRF major-element oxide concentrations (Wt.%) from the East Kirkton Limestone.
Table 3.
XRF trace-element concentrations (ppm) from the East Kirkton Limestone.
Fig 7.
Trace element enrichment factor (EF) of samples in this study from the East Kirkton Quarry Limestone after [85].
An EF < 1 indicates depletion relative to the upper continental crust, an EF ≈ 1 suggests similar concentrations, and an EF > 1 indicates enrichment.
Fig 8.
Stratigraphic, chronostratigraphic, XRD, and XRF chemostratigraphic plots for Units 66–88 of the East Kirkton Limestone at the East Kirkton Quarry locality, incorporating LA-ICP-MS YMWM dates obtained from this study’s samples EK82 and EK83 alongside the stratigraphic column adapted from [46].
The chemostratigraphic data include proxies such as %SiO₂, representing quartz, clastic sediment input, and chert; %CaO, indicating carbonate content; %Al₂O₃, reflecting clay minerals; %Fe₂O₃, associated with clays and carbonates; %K₂O, indicative of K-feldspar, mica, and illite/clay content; %TiO₂, representing continentally derived sediment; and [ppm Ni, Cr, Co, V], proxies for basaltic and mafic sediment input. Additionally, ppm Zr serves as a proxy for continentally derived sediment and volcanic phases, while ppm Rb indicates contributions from K-feldspar, mica, and illite/clays. Quantitative XRD data provide whole-rock mineralogy percentages, offering a detailed geochemical and mineralogical characterization of the East Kirkton Limestone.
Fig 9.
Mississippian geological time scale illustrating the previously assessed biostratigraphic age of the East Kirkton Quarry tetrapods (green box) and the updated U-Pb interpreted age from this study (red dashed box) [9,28,46,56,57,62].
The approximate temporal extent of Romer’s Gap is highlighted in the orange box. Regional substages and age references are based on the British Geological Survey geological time chart.
Fig 10.
East Kirkton Limestone clastic sediment provenance discrimination diagram based on XRF major oxides elemental composition (after [86]).
Function 1= -1.773TiO2+0.607Al2O3+0.76TFe2O3-1.5MgO+0.616CaO+0.509Na2O-1.22K2O-9.09. Function 2= 0.445TiO2+0.07Al2O3-0.25TFe2O3-1.142MgO+0.432Na2O+1.426K2O-6.861.