Early history (1825 – 1983)

As early as the 1820s, East Kirkton was noted to be an enigmatic site, relevant to broad debates on the nature of geological processes.^[5] In the late 18th and early 19th centuries, petrologists (geologists who study the formation of rocks) were split into two camps. Neptunists argued that most rocks precipitated out of mineral-rich waters, while plutonists identified magma as the medium from which most rocks originate. Experiments on carbonate-based rocks (such as limestone) supported neptunist interpretations of petrology, while evidence from silica-based rocks (such as granite) favored plutonist views.^[6] East Kirkton presented a contradiction: thick layers of carbonate (limestone) intermingled alongside rarer siliceous (chert) beds, emphasizing how both rock types can occur in close succession.^[5][7]

The first geologist to study the site was John Fleming (1825), a Scottish neptunist who considered both the carbonate and chert to be derived from heated groundwater.^[5] During the 1830s and 1840s, the quarry yielded some interesting fossils of Carboniferous plants and eurypterids ("sea scorpions", a type of extinct arthropod), though this was not uncommon for quarries in the area.^{[5][8][9][10]} English geologist Samuel Hibbert (1836) discussed Fleming's interpretation, identifying the limestone as freshwater due to the prevalence of plant fossils and the absence of marine fossils. He drew attention to the occurrence of volcanic tuff at the site, and attributed the silica and carbonate to hot spring mineralization.^[8]

When the quarry ceased operations in 1844, the site was, for the most part, forgotten as a geological footnote.^[10] Scottish geologist Archibald Geikie (1861) determined that the limestone of "Kirkton" was not a single unit, but instead two distinct sequences, one at the nearby West Kirkton quarry and the other at East Kirkton. He supported Hibbert's interpretation, considering the Kirkton quarries to represent large lakes influenced by hot springs on an ancient volcanic plain.^[7] East Kirkton saw little attention in the following decades, as neptunism diminished in most applications while plutonism established itself as a robust scientific theory. The land south of the quarry was developed for housing while the quarry sat abandoned.^[11][12] The prevailing view was that, with the exception of rare freshwater eurypterids, East Kirkton's fossil content was comparatively unremarkable.^[1][10][13] One notable study in the century since Geikie's paper was by Muir and Walton (1957), who reviewed previous research and investigated the carbonate's microscopic texture and origin in more detail.^[9]

Stan Wood's fossil discoveries (1984 – present)

East Kirkton's decades of obscurity ended in 1984, when Scottish fossil collector Stan Wood discovered a fragmentary tetrapod skull among the limestone slabs of the quarry's spoil heap.^[11] According to popular accounts, he became aware of the spoil heap while refereeing a football match in a nearby field. Wood purchased the abandoned quarry from the West Lothian District Council the following summer.^[14] Wood and University of Cambridge paleontologist Timothy R. Smithson began systematic fossil collection from the quarry from 1985–1990. With news of the discovery, a team of National Museums of Scotland geologists (headed by W.D. Ian Rolfe) initiated a stratigraphic investigation in 1987–1992.^[15][1][10]

Fossils discovered by Wood include an expansive sample of both terrestrial and freshwater fossil arthropods and early amphibians from the East Kirkton Limestone.^[16] A smaller sample of fish and plant fossils were found in younger strata a few years later.^[10] Wood and his colleagues published on their initial finds in a 1985 Nature letter.^[16][10][13] Two notable taxa mentioned in the 1985 letter were the oldest known harvestman (a "daddy longleg", dubbed Brigantibunum in 2005)^[17] and the oldest known temnospondyl amphibian (described as Balanerpeton in 1993).^[18] East Kirkton is perhaps most famous for Westlothiana, a small tetrapod discovered in 1988 and initially reported to be the oldest known reptile in 1989.^[19][20] Wood's excavations at East Kirkton, and then-unnamed Westlothiana, were featured in the first episode of Lost Worlds, Vanished Lives, a 1989 BBC documentary hosted by Sir David Attenborough.^[14] Westlothiana has subsequently been reinterpreted as a stem-amniote reptiliomorph. In other words, it was an amphibian closely related to amniotes (the group containing dry-adapted tetrapods like reptiles and mammals, with reinforced eggs and thickened skin). Thanks to its degree of completeness, Westlothiana is still among the best paradigms of the amphibian-amniote transition.^[21][22]

East Kirkton was the main subject of a conference hosted by the Royal Society of Edinburgh in 1992. The conference produced a series of over 20 papers published in 1993–94 as "Volcanism and early terrestrial biotas" (volume 84, issues 3–4 of Earth and Environmental Science Transactions of the Royal Society of Edinburgh).^{[1][10][23][13]} New species have continued to be described from Wood's collections and other expeditions up to the present day.^[13][24][25] In 2011, Cambridge paleontologist Jennifer A. Clack named a new species of East Kirkton microsaur, Kirktonecta milnerae, in honour of the site.^[26] East Kirkton Quarry has been designated as both a Local Geodiversity Site (LGS) of West Lothian and a Site of Special Scientific Interest (SSSI).^[27]

Geikie Tuff

The Geikie Tuff (units 1–31), as the name implies, consists mainly of yellowish-green volcaniclastic tuff. Fossils, though abundant in a few layers, are limited in diversity and preservation. They mainly include fish scales, plant fragments, and shells of ostracods (tiny bivalved crustaceans). Ironstone nodules are also present. The Geikie Tuff is more than 4 m (13 ft) thick at the main outcrop, though it can reach 8 m (26 ft) thick in exploratory boreholes.^[10] Stratigraphic sections acquired via boreholes have demonstrated that the tuff is overlain by basalt.^[10][12]

Pseudomorphs of olivine and plagioclase crystals indicate that the tuff is basaltic in nature. The volcanic grains are coarse and rounded, sorted into discontinuous lenticular layers with graded bedding. This suggests that the volcanic material was not directly supplied by a pyroclastic flow, base surge, or ash fall. Rather, it was washed down as debris from older ash deposits, settling underwater alongside wood and other non-volcanic fragments. The area most likely experienced small intermittent cinder cone eruptions, providing basaltic ash or lava as a predecessor to the tuff material.^[15][34]

Little Cliff Shale

The Little Cliff Shale (units 21–36) is the thinnest interval exposed at the site. It reaches its greatest thickness (about 1.85 m or 6.07 ft) near the middle of the quarry's west wall. The sediments of the Little Cliff Shale include blue-gray shale interspersed with greenish tuff. Fossils and ironstone are more common than in the Geikie Tuff, with the addition of scorpion cuticle and a greater diversity of plant and fish remains.^[15][10]

East Kirkton Limestone

The East Kirkton Limestone (units 36–88) is by far the thickest, most fossiliferous, and most geologically diverse sequence in the quarry.^[10] Most layers are laminated (thinly layered) limestone, with a fine-grained texture of calcareous spherulites (bead-like grains). Black shale, coarse tuffaceous limestone, silica (chert and chalcedony), pyrite, gypsum, and tuff beds may occur in some layers.^[9][15][10] Tetrapod, arthropod, and plant fossils are abundant throughout the limestone. Conversely, fish are absent beyond unit 36 and ostracods are mostly restricted to black shale-bearing horizons. Many laminated beds are deformed or warped, and stromatolitic crusts and algal filaments are common.^[10] Small slumping features can be seen in some layers, likely corresponding to the slope between shallow and deep parts of a lake bed.^[9][35] The lake bed was probably oxygen-deprived, according to trace metal geochemical markers and an absence of bioturbation (animal-mediated disturbance).^[15][36] The true thickness of the limestone is unknown, but about 9 m (30 ft) are exposed on the main outcrop.^[10]

Stable isotope data help to pin down the origin of minerals like silica and pyrite in the limestone. Hydrogen and oxygen isotope ratios indicate that the chert beds precipitated from meteoric water heated to around 60 °C (140 °F), with a minor amount of mineral recycling after deposition. Sulfur molecules in pyrite are rather lightweight, arguing that crystal formation was mediated by bacterial activity. Yet the pyrite was also heavy enough to imply a constant supply of sulfur-34, likely from older gypsum beds heated by magmatic activity.^[37] Both the silica and pyrite support historical interpretations of a hydrothermal influence on the East Kirkton Limestone.^[15][37] Another hypothesis, based on strontium isotope trends, is that heat and alkaline groundwater are byproducts of chemical reactions between tuff minerals and carbon dioxide molecules seeping in via meteoric water (such as rain).^[38]

Contrary to earlier suggestions,^{[5][8][7][15]} the calcareous minerals of East Kirkton Limestone (mostly calcite) are probably not directly hydrothermal in origin.^[9][36][35] Nevertheless, East Kirkton is a useful model for the formation of spherulite-rich freshwater limestone in volcanic settings.^[9][35] Most limestones with spherulites are dependent on a high proportion of clay particles, but clay makes up only a small portion of the East Kirkton Limestone. The prevailing interpretation is that spherulite formation at East Kirkton follows a complex mineralization pathway.^[9][36][35]

In the reconstructed sequence of spherulite formation, freshwater lakes are supplied with high concentrations of dissolved alkaline minerals and microbial acids. These conditions encourage the precipitation of fibrous calcite, which accumulates on strands of organic material such as algae or cyanobacteria In the shallow part of a lake. The fibrous calcite acts as a base for radiating balls of botryoidal (lumpy) calcite. Even once the algae die and settle, the calcite continues stacking up into crusts and domes on the lake bed. Fluctuating mineral concentrations lead to constant corrosion and reprecipitation of the calcite. Wave action breaks up the calcite crusts into smaller grains, which are periodically washed into deeper parts of the lake. This periodic supply of spherulite grains is responsible for the laminated appearance of the limestone, alternating between microscopic bands of dark clay and organic material, and thicker (though still narrow) bands of calcite.^[36][35] Diagenesis (underground heat and pressure) fractures and warms the calcite after deposition, introducing voids and further reprecipitation within the molds of algal strands.^[9][35]

Tetrapods

East Kirkton tetrapod fossils have all been preserved in aquatic sediments of the East Kirkton Limestone. A majority of the fossils come from laminated spherulitic limestone slabs (units 70 and below), though the best-preserved specimens come from a thin black shale layer (unit 82). Fish fossils do not occur alongside tetrapod fossils, suggesting that the tetrapods lived in or around shallower bodies of water than those supporting the local freshwater fish.^[44]

The tetrapod species of East Kirkton are all endemic, though some broader groups (dendrerpetontids, aistopods, "anthracosaurs", etc.) can be found at other Carboniferous locales. Notably absent are adelogyrinids, Crassigyrinus, Doragnathus, and lysorophians, all of which are considered fully aquatic.^[15][44] The absence of lysorophians could be explained by the age of East Kirkton, which is much older than the known aquatic members of the group.^[44] True amniotes are also absent, and only a single microsaur fossil has been discovered.^[26] The rarity of microsaurs may be due to geographic rather than environmental factors, since few fossils of the group are known from other British sites. Overall, East Kirkton has a more terrestrial character than other Scottish fossil sites, though a few aquatic or semi-aquatic species certainly inhabited the area as well.^[44]

Fish

Though few of East Kirkton's fish species have been formally described, available information suggests that the East Kirkton lake hosted a diverse freshwater ecosystem. Body types seen in East Kirkton fish include fusiform (tuna-shaped) generalist predators and deep-bodied durophages (Eurynotus). The fish fauna is comparable to other sites in the Mississippian Oil Shales of Scotland. Only the Granton shrimp bed, a fully marine environment, lacks freshwater fish and tetrapods.^[52]

Beside direct body fossils of fish, coprolites are also found in several layers near the Little Cliff Shale-East Kirkton Limestone transition. Small pellet- bullet- and cigar-shaped coprolites are common, often containing ostracod shells, and in one case, bone fragments. One irregular mass, containing ostracods and plant fragments, may be a regurgitate. Elongated spiralling coprolites are rarer and would have been produced by elasmobranchs akin to modern sharks. Irregular or strand-like coprolites may have been produced by tetrapods, though fish cannot be excluded. Massive clusters of phosphatized grains were probably produced by large omnivorous eurypterids like Hibbertopterus, or less likely large rhizodont fish.^[53]

Invertebrates

Apart from arthropods, other identified invertebrates include freshwater ostracods (Carbonita) and bivalves (Curvirimula?).^[10] Ostracods likely formed the base of the aquatic food chain, since they have been found as stomach contents in several fish and tetrapod fossils.^[23]

Plants

The plant fossils of East Kirkton are fragmentary but diverse, and small fragments of fusain (fossilized charcoal) are common in the East Kirkton Limestone.^[15][42] The area was likely forest or open woodland frequented by wildfires. There are few aquatic plants apart from algal laminations, and lycopsid fragments (an indicator of swamp forests) are only common in the Geikie Tuff. The surrounding woods were probably drier during the deposition of the Little Cliff Shale and East Kirkton Limestone. The East Kirkton flora is fairly typical among Viséan Scotland, most species found here have been reported previously from the Midland Valley.^[42]

Gymnosperm wood and fern leaves (particularly Spathulopteris and Sphenopteridium) are the most common plant fossils at East Kirkton. These plants are also abundant at Weaklaw, a Viséan-age volcanic ash deposit in East Lothian. It is uncertain whether this similarity betrays a preference for volcanic areas or simply broader biostratigraphy.^[42]

Plant fossils have been fossilized through several different chemical pathways. In the East Kirkton Limestone, robust plant parts such as gymnosperm branches and Stigmaria roots are often preserved by permineralization (petrification). During permineralization, the original organic material is sequentially replaced with silica and/or carbonate carried by alkaline groundwater. More commonly, heat and pressure compresses organic matter into carbonaceous films. These coalified compressions of wood and foliage can be found in practically every layer of the site. Some fossils are both compressed and permineralized, particularly in the upper layers (units 44–48) of the East Kirkton Limestone.^[15][43]