 | 
| 
| 
|
Over the last 10 years the disused quarry at East Kirkton has been studied in great detail, this is because of the extremely important vertebrate discoveries that have been made there, firstly by Stan Wood (Wood et al 1985), and subsequently by an international team headed by National Museums of Scotland (NMS) (Rolfe et al 1994).
In this essay I will first present a broad picture of the Bathgate region at the time, I will then focus in on the rocks found at East Kirkton, using the broad picture as a framework. I will look at the stratigraphic column and discuss points of interest within it and then go on to look at the fossils and their preservation within the volcanogenic sequence. I will conclude with a brief palaeoenvironmental reconstruction and look at it's worldwide significance for future discoveries of early reptiles and amphibians.
The rocks at East Kirkton provide a window into a rare environment and fauna of the lower Carboniferous in the Midland Valley of Scotland. They were first described, during the last stages of quarrying in 1836 by Dr Hibbert (Hibbert 1836) he correctly worked out that the sequence contained a freshwater limestone formed under volcanic influence in the Carboniferous, and he found part of a eurypterid - Hibbertopterus Scouleri within it. Hibbert likened the relations of volcanic and sedimentary material to what he had seen in the Auvergne. In 1925 H.M Cadell proposed that the rocks were related to hot spring activity, such as is seen in Yellowstone National Park and New Zealand based upon the occurrence of spherulitic growths in the limestone. Like modern interpretations, he recognised that volcanicity was only dormant at this time. He proposed that there were occasional ash eruptions into this environment, which under modern scrutiny has proven not to be the case.
These early ideas fit into the regional environmental picture very well. The Midland Valley in Dinantian times was a low lying block in an essentially arid landscape. Rivers drained into this scenery from mature and high areas to the north on the Highland Boundary Fault. There was an ocean 500-1000km to the south.
Upton (1994) proposes Dinantian magmatism was probably related to extension leading to melting by adiabatic decompression. Initial volcanoes were fissural and basaltic, where magmas encountered wet sediments we get phreatomagmatic deposits, leading to the interfingering seen between sand, shale and limestone in the East Kirkton section. Upton suggests that low basaltic cones, a few hundred metres high, developed in the area producing subaerial lavas (boles seen elsewhere). Rare, longer lived volcanoes would have been larger with associated persistent geothermal activity, this would be different to hot springs. Fumeroles would be related to more active areas. Upton draws a parallel between the Midland Valley in lower Carboniferous times and the modern day East African Rift.
At it's simplest the sequence in the East Kirkton quarry can be divided into a lower sequence of limestone- the East Kirkton limestone, overlain by blue-grey shale with lenticular limestone- the Little Cliff Shale, overlain by bedded tuffs- the Geikie Tuffs (Rolfe et al 1990).
In more detail the section has been excavated by a team under the direction of NMS between 1987 and 1992 (Rolfe et al 1994). On the basis of this work the 14.75m section in the quarry has been divided into 88 units. These units were numbered as excavations went downwards. (See also figure 1).
East Kirkton limestone- units 88-36 (~8.70m). Well laminated fine grained limestone, often with calcareous spherulites. Intercalations of black shale, coarse grained, graded tuffaceous clastic limestone and tuff. Locally chert bands. Several horizons with charcoal present. Gypsum pseudomorphs are found in places. Different fauna to that in younger beds. Complete amphibian and reptiliomorph skeletons in black shale. Bones found throughout the limestone. Scorpion and eurypterid remains, wood and plants fairly abundant. Several horizons of ostracodes. No fish.
Little Cliff Shales- units 36-32 (1.85m). This unit thins rapidly northwards and is reportedly absent from the southern part of the quarry. The unit is of blue-grey and blue-black shales intercalated with fine bedded tuffs. There are some continuous nodular ironstone horizons. Plants, spores, fish scales, spines, bones, ostracodes and arthropod cuticle are found. A bivalve was fond at the bottom of the unit. Specifically Rhizodus, fish scales are found.
Geikie Tuffs- units 31-1 (4.2m). Bedded yellow/green tuffs varying in grainsize from fine grained to some units with 20mm rounded fragments. Some lapilli tuffs. Fossils are of fish scales, plants ostracodes, these are widely distributed throughout the units but do concentrate in some horizons. Rootlets occur in two horizons, principally near the bottom of the unit. There are ironstone nodules present in some units.
Durant (1994) in detailed work on the tuffs of the entire section reports that they are of basaltic origin although they have been highly altered since deposition. He points to subaqueous deposition for the tuffs, as the result of reworking of previously subaerial tuffs. As evidence for this he cites lateral variation in thickness, the poorly sorted nature of the tuffs, heterogeneous lithological composition, well rounded fragments juxtaposed to angular ones and the presence of fossils including ostracodes within tuffs. This all provides good evidence for there not to have been active volcanism into this environment during this period of deposition.
I will now examine the East Kirkton limestone in more detail in order to extract more information about the lake environment at the time of deposition.
It is the East Kirkton limestone that has provided interesting insight into the lake water chemistry. The limestone is laminated on the millimetre scale and is interpreted as being of freshwater origin because of the fine laminae (typical of low energy, restricted circulation environments) and due to the presence of freshwater fossils in many horizons (McGill et al, 1990). The limestone has abundant radial-fibrous calcite (RFC) spherules (~1mm) and less common, larger stromatolite-like bodies of laminated botryoidal RFC (Walkden et al, 1994). Walkden shows that carbon and oxygen stable isotope studies of these features point to a freshwater origin with them forming under relatively cool conditions. Trace element signatures show that these freshwaters were under a volcanic influence but stable isotope data indicate they were removed from hot-spring activity. Walkden recognised an association with biogenic activity, especially of the botryoids suggesting that variation in water chemistry either directly controlled the precipitation of either kind of calcite or controlled it through other factors such as the biogenic activity.
Within the East Kirkton Limestone there is also silica, preserved as cherty layers. Walkden et al (1994) dismiss this as secondary, however McGill et al (1990, 1994) suggest that some is of primary origin. McGill uses as evidence the slumped nature of some of the silica laminae, showing plastic deformation prior to diagenesis. He envisages a gel-like stage of silica deposition. The primary silica, tested using stable isotope d18O shows the silica formed at about 60° from water of mainly meteoric origin. McGill et al suggest that the silica laminae are the result of hot spring activity at times in the lake.
McGill et al also studied the pyrite deposits associated with some laminae. Based on sulphur isotopic analyses they concluded two basic origins for sulphur; firstly that associated with plant material was isotopically light indicating anoxic bacterial sulphate reduction; secondly pyrite in nodules and in chert had sulphur values indicating a magmatic/hydrothermal origin, this could have been leached from local lithologies by a hydrothermal system.
Looking at the evidence from the limestone we get a picture of a freshwater lake which is deep enough, at least at times, to have an anaerobic bottom. The lake is of changing chemistry, possibly related to climatic or seasonal changes (Clarkson et al, 1994). The lake had algae, in some places forming the nucleii for calcite growth. It has been suggested that there were stromatolites growing in places (Clarkson et al, 1994), the evidence for this is tenuous, relating I believe to the shape of some of the mammalian calcite accretions. There is no reason why they shouldn't be found here, however. There is a little dispute over whether there was active hot spring activity, although I feel that the evidence favours McGill using both his Silica and sulphur results. I cannot see any reason why the cool deposition model for calcite and hot spring model for silica and pyrite have to be mutually exclusive. As Clarkson et al point out, there can be a temperature gradient away from the vent. This could also explain the observation by McGill et al (1990) that calcite and silica laminae grade into one another in places. The chemical activity of the lake may also explain the lack of certain fauna within it, such as the absence of fish from the East Kirkton Limestone.
The charcoal, which is found in many horizons in the limestone unit is thought to represent evidence for periodic forest fires (eg. Clarkson et al, 1994). These would be expected in a warm environment with strong evaporative power (gypsum pseudomorphs indicate some evaporite formation).
I will now go on to look at the exceptional terrestrial fauna found associated with the limestone unit.
Eurypterids
As already mentioned it was Hibbert (1836) who discovered the first eurypterid in the quarry, Hibbertopterus scouleri. Under more recent scrutiny there have been three genera of generally poorly preserved eurypterid found in the East Kirkton Limestone.
Jeram and Selden (1994) suggest that these appear
to have been washed into the depositional area, due to the lack
of juveniles in the assemblage. They believe that the eurypterids
found here are semi aquatic. They cite the rarity of complete
specimens and abundance of cuticle fragments in horizons rich
in pulverised plant debris as evidence for the eurypterids living
immediately around the lake and not permanently within it. Jeram
and Selden propose that the eurypterids preyed on small invertebrates
(one genera using paddle-like filters to extract them from the
sediment) and possibly brine shrimps which would have inhabited
pools around the lake.
Scorpions
East Kirkton material has also shown the first direct
evidence for terrestrial scorpions due to the excellent 3D preservation
of a few articulated specimens. Jeram (1994) describes what he
believes is a new genera- pulmonoscorpinus, restructuring what
is a complex infraorder based upon very few specimens. Jeram asserts
that pulmonoscorpinus had compound eyes and shows no adaptations
to suggest it was nocturnal. From this evidence he concludes that
scorpions were more active by day in the lower Carboniferous than
they are today. He has speculated that they ate insect material
in the absence of any preserved.
Myriapods
Myriapod millipedes are also found although these are generally poorly preserved. These are the earliest Carboniferous myriapods known. Shear (1994) describes the millipedes found as 'novel', different from earlier and later forms. He also reports the first occurrence of an oozipore- a repugnatory gland opening, from millipedes from the site. Myriapods are important to a palaeozoic ecosystem in Shear's view as they reduce floral litter and act as food for preying animals eg. tetrapods?
Also found were the earliest known harvestman spider and a parasitic mite found in the tail of a scorpion.
From the evidence seen so far in the invertebrates we can see that there was a fairly complex trophic web developed in this environment. We can also see that East Kirkton was exceptional in it's preservation potential and possibly as an isolated area for peripheral isolates to develop. This can be see further in the tetrapod discoveries.
Temnospondyl Amphibians
These are the most common tetrapods found in the East Kirkton assemblage. Over 30 complete and partial skeletons were found of Balanerpeton woodi, a previously unknown species. it grew to 50 cm in length. Milner and Sequeira (1994) concluded that this was a terrestrial form on the basis of bony carpals and tarsals, possible presence of eyelids and the presence of a large tympanic ear (suggesting the ability to hear high frequency airborne sound).
Ribs of another possible temnospondyl were found indicating it might have been 1-2m in length.
The presence of temnospondyls here proves to Milner
and Sequeira that they were diversifying in the Viséan
and that there were other Temnospondyls elsewhere at this time
(footprints and vertebrae found in Nova Scotia). Clarkson et al
(1994) suggest that Balanerpeton was a lake marginal form as it
was abundant in the lake sediments. They also suggest that the
teeth are adapted to preying on larger animals.
Aïstopod Amphibians
Early forms of the elongate, limbless Aïstopod
amphibians are found here in 5 specimens. Milner, AC. (1994) proposes
that they had a snake-like mode of life and were terrestrial due
to their shortened tail and 200 vertebrae. Clarkson et al (1994)
suggest that it's primitive form, having few large teeth allowed
it to prey on animals as large as it's head eg. arthropods or
small vertebrates.
The earliest Reptiliomorph
East Kirkton's most headline grabbing discovery was in 1988 by Stan Wood, of the earliest known stem-amniote or reptiliomorph (Smithson, 1989, Smithson et al, 1994). Westlothiana Lizziae contains many intermediate characters between primitive tetrapods and true amniotes.
Smithson et al (1994) suggest that the skull of Westlothiana
resembles that of true amniotes and individual vertebrae resemble
those of true amniotes. It has small but definitely boned limbs.
The humerus resembles that of early amniotes, it has 3 proximal
tarsals as primitive tetrapods, but the same number of phalanges
as amniotes. Large dorsal and ventral scales are thought even
more primitive than in anthracosaurs. Smithson et al conclude
that Westlothiana is less primitive than Permian Seymouria.
Anthracosaurs
A new, primitive anthracosaur species was found in the quarry, Eldeceeon rolfei. Smithson (1994) describes it as a 30-40cm terrestrial form, known from two specimens. It's dentition indicates a carnivorous nature. Clarkson et al (1994) suggest it filled a similar niche to Balanerpeton (amphibian) but not living as close to the lake.
It has been suggested that the relative abundance of terrestrial fossils in the lake deposits is caused by forest fires on land chasing animals into the water (Clarkson et al 1994). This may create some of the better preserved, whole specimens in the absence of many aquatic predators. Amphibian preservation probably happened as they swam and died in the lake. At least at one interval amphibians would have been feeding on ostracodes and other aquatic organisms that were abundant enough to sequester much of the oxygen and nutrient, forming a black shale layer. Poorer, incomplete specimens and bones were probably washed in during the deposition of the many reworked tuff bands.
In the lower Carboniferous at East Kirkton there was a freshwater lake, situated in a dormant volcanic terrain. The fast weathering of the lavas provided nutrient rich soils for a forest to grow (Scott et al, 1994) around the perimeter. This flora provided habitat for millipedes and early harvestman spiders, probably other insects too. Scorpions moved around in this environment by day. In the absence of any fish as predators amphibians bred in large numbers in the lake, feeding off ostracodes which lived there when the water chemistry was not too hostile. Eurypterids occasionally lived at the edge of the lake, feeding by scooping infauna from mud using adapted paddles, possibly having made the journey from a nearby lake such as lake Cadell, or being swept in. A few early reptiliomorphs lived here, not needing to breed in the water few are preserved. Frequently rainfall brought ephemeral rivers depositing tuffs from the surrounding low volcanoes as sheet deposits. It was possibly at these times water chemistry allowed more aquatic organisms to flourish, forming black shale deposits. Around the edge of the lake strong evaporation brought gypsum growth for short periods. At times of hostile water chemistry stromatolites possibly grew, with the lack of grazing organisms. Under the water, chemistry was controlled by occasional hot springs, precipitating silica directly. It was the volcanic rocks underlying the whole area that provided the leached calcium carbonate for spherulitic deposition.
The significance of the Geikie Tuffs and Little Cliff Shales is in the history of the lake after limestone deposition ended. Clarkson et al (1994) propose that these represent the subsequent joining up of the East Kirkton lake and nearby lake Cadell, bringing in fish and a more normal Oil Shale population.
If we look at the palaeogeography for the Dinantian
we can see that all tetrapod finds to date have come from a small
area (Figure 2), stretching from the Tournaisian of Nova Scotia
and Viséan of West Virginia through the Midland Valley
into Namurian of the Rhur (Milner, AR et al, 1986). All are represented
by only a few, generally poor specimens. Is the record biased
against tetrapod preservation, or does it represent a very limited
distribution, due to for example special conditions being necessary
for early tetrapod proliferation? We can see from the fossil evidence
collated that the East Kirkton Limestone preserves an innovative
terrestrial fauna, in my view this is inherently related to the
isolated and unusual nature of the area.
|
|
|
|