East Kirkton Quarry

East Kirkton Quarry
Geologic site
An overgrown limestone outcrop at East Kirkton, 2013
East Kirkton Quarry Show map of Scotland East Kirkton Quarry Show map of West Lothian
Coordinates: 55°54′14″N 3°37′00″W / 55.903975°N 3.616536°W / 55.903975; -3.616536
Location	Bathgate, West Lothian, Scotland, United Kingdom
Age	Viséan, Carboniferous

East Kirkton Quarry, or simply East Kirkton, is a former

The East Kirkton Limestone in particular has produced numerous well-preserved fossils of tetrapods (four-limbed vertebrates) and arthropods (multi-legged chitinous invertebrates like millipedes and arachnids). East Kirkton had been ignored by paleontologists since the 1840s, but Scottish fossil collector Stan Wood managed to procure the land in 1985, sparking a rush of scientific interest. New species from East Kirkton have been named on a regular basis since 1990, and nearly all of these species have been found nowhere else. Notable discoveries include Westlothiana (one of the most reptile-like Mississippian tetrapods), Balanerpeton (a common early representative of amphibians in the group Temnospondyli), and Pulmonoscorpius (the largest known terrestrial scorpion). The East Kirkton area represents an unconventional environment: dry woodlands and mineral-rich lakes nestled among volcanic cinder cones. Aquatic animals, though not uncommon, are less diverse than those found in the swampy coal forests and coastal sediments prevalent at other Scottish Carboniferous fossil sites. The prevalence of terrestrial organisms represents a broader trend of decreasing reliance on an amphibious lifestyle during the Carboniferous Period.

History

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

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.

The East Kirkton Quarry preserves up to 19 m (62 ft) of strata from the mid-lower part of the Bathgate Hills Volcanic Formation. It is equivalent in time to rocks from the upper part (Hopetoun Member) of the West Lothian Oil-Shale Formation, exposed north of Linlithgow.^[28] Both formations are part of the broader Strathclyde Group^[29] (informally termed the Oil-Shale Group)^[10] found throughout the Midland Valley of Scotland.^[30][29][31]

By comparison to equivalent oil shale strata, the East Kirkton is estimated to belong to the

East Kirkton is one of many geographically restricted limestone deposits cropping out on a north-south tangent between Bathgate and Linlithgow.[8]^[28] The nearby West Kirkton Quarry is a slightly younger marine limestone deposit.^[7][28] West Kirkton failed to produce any significant fossils, as it had been filled in and redeveloped by the time Stan Wood began his excavations in 1985.^[14]

The rocks of East Kirkton

Three distinctive geological intervals can be found at the quarry: the Geikie Tuff (youngest / highest), Little Cliff Shale (middle), and East Kirkton Limestone (oldest / lowest). The most well-exposed area is a 15 m (49 ft) thick outcrop at the northwest edge of the quarry. Here, the three intervals are subdivided further into a series of thin units, labelled 1 to 88 from the top of the outcrop (the youngest point) to the bottom of the outcrop (the oldest point).[10]

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]

The East Kirkton Limestone (units 36–88) is by far the thickest, most fossiliferous, and most geologically diverse sequence in the quarry.

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]

Palaeoenvironment

During the Dinantian, the Midland Valley would have been a subtropical lowland rift zone. Faults and cooled lava flows would have contributed to the landscape by diverting rivers and damming lakes,^[39] as would the progradation of deltaic systems further east.^[40] Some of the lakes may have filled in volcanic craters, forming low crater lakes (also known as maars). Most individual volcanoes were small but numerous, emphasizing basaltic and phreatic eruptions alongside hot springs. Modern equivalent environments include the system of volcanoes around the Mozambique Channel and the Chaîne des Puys in France.^[39]

Towards the north and east was a large lake or estuary system known to geologists as Lake Cadell. It was responsible for the deposition of most sediments comprising the West Lothian Oil-Shale Formation. An expanse of dry volcanic uplands were located west of East Kirkton. These uplands, formally known as the Clyde Plateau, completely lacked any form of limestone deposition. The East Kirkton Limestone was deposited at a time and place at the intersection between these two environments, as the expanding dry plateau began to displace the brackish lake, enabling the development of a riparian ecosystem in the Bathgate area.^[29][35]

East Kirkton was located close to the equator in the Viséan. The climate was warm and semi-arid, gradually becoming wetter as the Carboniferous progressed.^{[41][39][28][42]} Rainfall was prevalent enough to support woodlands and lake ecosystems, but infrequent enough to dissuade coal forests and allow gypsum deposition.^[39][28][42] The more humid conditions of the Geikie Tuff may correspond to the expansion of a larger lake into the East Kirkton area.^[28] Chert and pyrite, alongside permineralized plant fossils, may indicate that hot springs were prevalent during the deposition of the East Kirkton Limestone.^[28][43]

Paleobiota and paleoecology

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

Tetrapods of East Kirkton
Genus / Taxon	Species	Strata	Material	Notes	Images
Balanerpeton[18]	B. woodi	East Kirkton Limestone	Over 30 specimens, including skulls and nearly complete skeletons[18]	A medium-sized terrestrial temnospondyl similar to Dendrerpeton.[18]
Eldeceeon[45]	E. rolfei	East Kirkton Limestone	Four partial skeletons, two of which are nearly complete[46]	A medium-sized "anthracosaur" with proportionally large hindlimbs and proposed terrestrial habits.[45][46] Initially identified as an unnamed "Eoherpeton"-like form.[15]
Embolomeri	Indeterminate	East Kirkton Limestone	Skull fragments (pterygoid, quadratojugal, lower jaw)[24]	A large indeterminate embolomere with an estimated skull length of 20 cm (7.8 inches), the largest reported for a Mississippian embolomere.[24]
Eucritta[47]	E. melanolimnetes	East Kirkton Limestone	Four partial skeletons and an isolated skull roof[48]	A small loxommatid.[15]
Kirktonecta[26]	K. milnerae	East Kirkton Limestone	Single nearly complete skeleton[26]	A very small newt-like aquatic microsaur, with soft tissue of a deep tail fin.[26]
Ophiderpeton	O. kirktonense[49]	East Kirkton Limestone	Five partial skeletons, including skull material[49]	A small aistopod, a type of snake-like legless tetrapod.[49]
Silvanerpeton[50]	S. miripedes	East Kirkton Limestone	Nine partial skeletons, including skull material[51]	A small "anthracosaur" with short limbs.[51]
Westlothiana[20]	W. lizziae	East Kirkton Limestone	Two nearly complete skeletons[21]	A small, slender stem-amniote tetrapod close to the ancestry of amniotes[21] and/or lepospondyls.[22]
Temnospondyli	Indeterminate	East Kirkton Limestone	Isolated ribs[18]	A large (~2 meter, 6.6 feet) indeterminate temnospondyl, possibly related to Edops.[18][23]
Termonerpeton[25]	T. makrydactylus	East Kirkton Limestone	Single partially articulated posterior torso[25]	A large "anthracosaur"-like tetrapod with a foot similar to Eldeceeon and amniotes.[25]

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]

Bony fish

Cartilaginous fish

Cartilaginous fish (Chondrichthyes and Acanthodii) of East Kirkton
Genus / Taxon	Species	Strata	Material	Notes	Images
Acanthodidae?	Indeterminate	Little Cliff Shale, Geikie Tuff	Three specimens: a disarticulated skeleton and fragments of a fin spine and pectoral girdle[52]	A tiny acanthodiform similar to Acanthodes,[23] Acanthodopsis, or Howittacanthus.[52]
Climatiidae?	Indeterminate	Geikie Tuff	Single bony plate[52]	A possible late-surviving climatiid similar to Climatius.[52]
Diplodoselache	D. woodi?	East Kirkton Limestone, Little Cliff Shale	Three teeth[54]	A xenacanth "shark" (elasmobranch).[54]
Tristychius	T. arcuatus?	East Kirkton Limestone, Little Cliff Shale	Fin spines (two complete and six fragmentary) up to 4 cm (1.6 inches) in length[54]	A hybodont "shark" (elasmobranch).[54]

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]

Arthropods of East Kirkton
Genus / Taxon	Species	Strata	Material	Notes	Images
Brigantibunum[17]	B. listoni	East Kirkton Limestone	Single compressed partial specimen[23]	A small harvestman (Opiliones) with very long thin legs, possibly a member of the suborder Eupnoi.[17]
Hibbertopterus	H. scouleri	East Kirkton Limestone	Partially articulated specimens and isolated fragments[55]	A massive semi-terrestrial Dunsopterus.[55][23]
Myriapoda	Unnamed	East Kirkton Limestone	Multiple specimens, six of which are complete enough to describe[56]	At least three species of many-legged arthropods, including a possible glomeridesmid (slug millipede), another millipede with ozopores (scent glands), and a putative non-millipede myriapod.[23][56]
Pulmonoscorpius[57]	P. kirktonensis	East Kirkton Limestone, Little Cliff Shale	16 complete specimens and hundreds of cuticle fragments[57]	A large basal scorpion up to 70 cm (28 inches) in length, the largest terrestrial scorpion known. Two distinctive specimens may represent additional species of Pulmonoscorpius.[57] Initially reported as Gigantoscorpio.[15]
Scorpionida	Indeterminate	East Kirkton Limestone, Little Cliff Shale	Cuticle fragments[57]	At least two species of indeterminate scorpions, including an aquatic "archaeoctonoid" and a terrestrial "orthostern". One piece of scorpion cuticle apparently sheltered a mite.[57]

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]

Gymnosperms

Lycopsids

Ferns

Horsetails

See also

• List of Sites of Special Scientific Interest in Edinburgh and West Lothian
• Ballagan Formation
• Stan Wood

References