Scotland is made up of discrete fragments, or terranes, that were assembled progressively by strike-slip movements and were welded together during the collision of Laurentia with Avalonia and Baltica during the Caledonian Orogeny. The Caledonian Orogeny or epoch of mountain building is named after Scotland where evidence for this ancient upheaval is well exposed. This episode can be broken up into a number of separate orogenic events which spans the period from the early Ordovician to the early Devonian, a period of about 100 million years. The movements were all caused by the subduction zones on either side of the Iapetus Ocean. During the Caledonian Orogeny fold mountains of Himalayan proportions, as evidenced by the deep rock structure and fold amplitudes now exposed, were formed over Scotland.

The volcanic island arc, which must have occurred to the northwest, is represented by rare volcanics of early Ordovician age in the northern Southern Uplands and further north in the Highland boundary Fault area. The well known Ballantrae Volcanics of the Ayrshire coast contain pillow lavas  and associated volcanics and sediments and represent an ophiolite. This is a fragment of the early Ordovician oceanic crust, which rather than be subducted has been wedged up on to the continental margin.

At this time the seas were teeming with life and Trilobites are found in Ordovician mudstones in Ayrshire and in Southern Scotland near Moffat fossil Graptolites can be found. With the closure of the Iapetus Ocean there was also extensive volcanism with island arc chain volcanoes. These island arcs were active possibly in the Midland valley area but younger sediments cover evidence of this.

The end of the Ordovician period abiut 439 million years ago was marked by a mass extinction. This was severe but not nearly as severe as the end Permian or the end Cretaceous extinction events. The trilobite dominated communities of the Cambrian dissapeared during the Ordovician radiation, diluted by the rapid growth of the dominant Paleozoic communities with articulate brachopods, crinoids, bryozoans and some Paleozoic corals. These groups lived attached to the sea floor and filtered food out of the water. There was no, or very little, life on the land at this stage.

There were two discrete extinction events separated by perhaps 500,000 to 1 million years. They extinguished 60% of marine genera and 26% of marine families.

It is thought that glaciation and global cooling were the most likely cause of this event. The extinction began with a rapid glaciation and consequent drop in sea levels, and then as the glaciers melted, sea levels rose rapidly and delivered cold anoxic water into shallow seas and thus caused the second pulse of extinction. Brachiopods suffered particularly heavy extinction, as did trilobites, echinoderms, bivalves and some corals. One of the most enduring curiosities of this episode is why it had such little long-term ecological impact. The various surviving groups produced many new species over the following 1-2 million years, and the Earths Silurian communities do not appear to be much different from those before the extinction.

The Girvan Fossils

Since their discovery well over a hundred years ago the fossils of the Girvan area have been of great interest to both professionals and amateur geologists. During the late Ordovician what was to become the south of Scotland and the Girvan area were on the southern margin of the continent of Laurentia. As the Iapetus Ocean closed by subduction the areas that were to become Scotland and England moved towards one another eventually to collide with the Caledonian Orogeny. The fauna and flora that would was fossilised in the Girvan area lived in the shallow waters at the edge of the Iapetus Ocean near a volcanic arc chain of islands.

Most of the fossils are the remains of the hard-shelled parts of the organisms such as corals, seashells, trilobites, starfish and worm-tubes. The diversity of the fossils found here, particularly in an area known as the Starfish Bed, is thought to represent more than one paleo-environment. It is thought that the accumulation of such a number of different organisms is a least partly due mass transport or slumping of an unstable submarine slope and rapid burial of live organisms from a number of ecosystems.

Due to the eventual, closure of the Iapetus ocean there is a marked difference in most of the species of trilobites to be found at Girvan and trilobites of a similar age from England and Wales. Some of the Girvan trilobites and brachiopods show affinities with those of similar age in North America, particularly Newfoundland, while those in England and Wales are much more “European”.

I have illustrated here some of the more common fossils from this most remarkable locality in the south of Scotland. 

In 2003 there was an exhibition at the Hunterian Museum in Glasgow of fossils collected by George Rae. I knew George well and have collected with him at Girvan many times. George died of cancer on the 10th November 1998 but bequeathed his fabulous collection to the Hunterian Museum where a lot of it is presently on display More >>>>>>>>  

Dob’s Linn

Dob’s Linn near Moffat in the South of Scotland has been defined as the Global Boundary Stratotype Section and Point [GSSP], which marks the boundary between the Ordovician and Silurian periods on the geologic time scale. The area was first studied by the famous 19th century geologist, Charles Lapworth, whose work established graptolites as a method of understanding stratigraphic sequences. This work later established the importance of graptolites as “zone fossils” for correlating Ordovician and Silurian rocks the world over.  Thus graptolites are excellent zone fossils worldwide for a number of reasons:

1. Rapid evolution
2. Easy recognition
3. Numerical abindance
4. Mode of life
5. They occur in a wide range of rock types

The rock at Dob’s Lin is a black shale thought to have been deposited in deep water in the Iapetus Ocean which separated England and Wales from Scotland during the Ordovician before closing in the Silurian. Similar shales, greywackes and graptolites almost identical to those of the south of Scotland are found in central Newfoundland in eastern Canada, proving that the two areas were once in close proximity before the opening of the present day Atlantic during the Mesozoic.

The rocks at Dob’s Lin are a sequence of black shales and grey/black mudstones. With the black shale being the most fossiliferous. The entire sequence is estimated to be 100m thick.
It is composed of four formations:

• Glenkiln Shale [Llandeilo – Caradoc]
• Lower Hartfell Shale [Caradoc – Ashgill]
• Upper Hartfell Shale [Caradoc – Ashgill]
• Birkhill Shale [Ashgill – Llandovery]

The shales contain chitinozoa and conodonts but neither are well preserved and the predominant fossils are graptolites.

• Glenkiln Shale – Nemagraptus gracilis, Climacoraptus peltifer

• Lower Hartfell Shale – Climacograptus wilsoni, Dicranograptus clingani, Pleurograptus linearis

• Upper Hartfell Shale – Dicellograptus complanatus, Dicellograptus anceps, Climacograptus extraordinarius

• Birkhill Shale – Glyptograptus persculptus, Coronograptus gregarious, Rastrites maximus

Graptolites are usually found flattened along the bedding plane of the host rocks, although occasionally they may occur three dimensionally especially if replaced by iron pyrite. They vary in shape usually being dendritic or branching, saw blade like or tuning fork shaped. Different species of graptolites formed colonies of different shapes, some straight, others curved or even coiled. They are essentially preserved as a black or grey carbonised film on the rock.

Graptolites were less ecologically specific than most other Ordovician and Silurian organisms and were able to live in a wide range of water temperatures. They lived exclusively in marine environments and were widespread in the oceans at any particular time. Most graptolites were planktonic, floating near the surface of the sea attached to floating seaweed or the like from the early Ordovician until their extinction in the early Devonian. Recent research has focused on reconstructing how graptaloid colonies might have moved through the water column. There are two possibilities, they either simply drifted along with the currents or Graptoloid colonies actively moved themselves around.

One group of graptolites known as Dendroids, due to their richly branched form, lived on the sea floor and survived longer until the early Carboniferous.

A graptolite actually represents a colony of interconnected animals, or zooids, with each zooid occupying an individual theca. These thecae form the “teeth” along the “saw-blade” or stipe, of fossil graptolites. The evolution of graptolites, other than the Dendroids which barely changed, shows a general trend towards increasing simplicity starting with many branched and often curved forms early in their history and ending with unbranched straight forms throughout their last 50 million years until their extinction. The dead planktonic graptolites having sunk to the sea floor would eventually become entombed in the oxygen deficient sediment and thus would be well preserved.

Graptolites area generally thought to be allied to the pterobranchs, a rare group of modern marine animals belonging to the phylum Hemichordates. Comparisons are drawn with the modern hemichordate Cephalodiscus and Rhabdopleura. They are thus more closely related to the vertebrates than many other types of soft-bodied animals.

What caused their extinction when they had been such a widespread and adaptable species is unknown. It has been linked to the expansion of the Gondwana ice sheets, global cooling, sea level fall and subsequent rise and changes in oceanic upwelling, nutrient abundance and water chemistry.