FOSSIL SITES IN IRELAND - Chapter 2 - Fossils from different ages

Contents - Fossils sites in Ireland

Section 1-1 - Fossil collecting in Ireland
Section 1-2 - Where are the dinosaurs?
Section 1-3 - The good news

Section 2-1 - Precambrian/Cambrian
Section 2-2 - Ordovician
Section 2-3 - Silurian
Section 2-4 - Devonian
Section 2-5 - Carboniferous/Mississippian
Section 2-6 - Carboniferous/Pennsylvanian

Chapter 3 - Fossil sites in Ireland
Section 3-4 - County Sligo - Streedagh Point - Mississippian 

Resources 

Chapter 2 - Fossils from different ages

by Jutta Kruse, version April 2025

This is a brief overview of time periods and examples of fossils typically found from those periods in Ireland.

The information is arranged in chapters and sections. The short citations in the text refer to the sources detailed in the chapter Resources.

If you spot any errors or omissions, please let me know, Jutta at info@fossils.ie

Images ⓒ Jutta Kruse unless otherwise noted.

This is the latest international chart of time periods at time of writing. It is included here to provide the context for the below sections relevant for Irish fossils.

Fig. 2.1 - International Chronostratigraphic Chart 2024.
(International Commission on Stratigraphy, 2024)



Section 2-1 - Precambrian/Cambrian

Fig. 2-1.1 - The Cambrian period.
(adapted from: International Commission on Stratigraphy, 2024)


Fig. 2-1.2 - The Precambrian period.
(adapted from: International Commission on Stratigraphy, 2024)


The Ediacaran is the most recent period of the Precambrian eon (International Commission on Stratigraphy, 2024). Some of the Ediacaran life forms extend into the next eon - the Cambrian. The systematic classification of these fossils is still unclear. The youngest Ediacaran fossils from the late Cambrian are found in county Wexford (Sleeman et al., 2004, p. 31) (Meere et al., 2020, pp 307-334) (Holland & Sanders, 2009, p. 101) (GSI, Booley Bay, undated). 

There are, however, doubts about their exact nature (fossils, trace fossils or sedimentary structures?) and whether they can be classed Ediacaran (MacGabhann et al., 2007). Here is a rare example of this trace fossil (or not?) from the late Cambrian of Booley Bay in county Wexford. 

Fig. 2-1.3 - Ediacaran biota (or not?) from the Cambrian of county Wexford.


In county Wicklow, early and middle Cambrian rocks contain trace fossils from deep-sea sediments such as Oldhamia (Sleeman et al., 2004, pp 17, 29). For a finer calibration of Cambrian Oldhamia see (Herbosch et al., 2011).

Fig. 2-1.4 - Trace fossil Oldhamia radiata (Forbes) from the Cambrian of county Wicklow.
(a) marks the prostrate form. Image credit: Kinahan, 1856, Fig. 5.


More background information on the Ediacaran can be found in a video by PBS Eons The other explosion you should know about and on the Cambrian in this video From the Cambrian Explosion to the Great Dying.

Section 2-2 - Ordovician

Fig. 2-2.1 - The Ordovician period in context.
(adapted from: International Commission on Stratigraphy, 2024)


Ordovician fossils in Ireland have helped to trace the origin of the island's currently united landmass.

The north-western half of what is now the island of Ireland was part of the ancient continent of Laurentia. The south-eastern half was part of the ancient continent of Avalonia. In the Ordovician period, thousands of kilometers of the Iapetus Ocean lay between them; far enough that the faunas could not cross the ocean and therefore they evolved separately. 
A palaeogeographic map of the Ordovician indicates the approximate positions of the parts which were to make up the modern island of Ireland as we know it [Fig. 2-2.2].

Fig. 2-2.2 - Ordovician palaeogeograpic positions of north-western (Laurentia) and south-eastern (Avalonia) parts of the modern island of Ireland (Geological Survey Ireland/3-Ordovician).


When the continents pushed together, one continental plate was subducted beneath the other and the Iapetus Ocean closed. Part of the Iapetus suture now runs across Ireland, roughly from the Shannon estuary in the south-west to Clogherhead, county Louth in the north-east. 

Fossils of different species, for example trilobites and graptolites, are now found close together, although they lived far apart in the Ordovician period, and their differences reveal their origins (Sleeman et al., 2004, p. 33), (Harper et al., 1996), [Fig. 2-2.3].

Fig. 2-2.3 - Map of the Iapetus Suture and the trilobites and graptolites from former continents Laurentia (Pacific) and Avalonia (Atlantic). Image credit: adapted from Woudloper, Iapetus fossil evidence.


Section 2-3 - Silurian

Fig. 2-3.1 - The Silurian period in context.
(adapted from: International Commission on Stratigraphy, 2024)


The Iapetus Ocean finally closed during the Silurian and the continental margins of Avalonia and Laurentia met as described above. Graptolites are index fossils of this period and during the Ordovician and Silurian periods about forty time periods can be distinguished with their aid (Sleeman et al., 2004, p. 37).

Many of the rocks of the mountains in counties Galway and Mayo formed in the Silurian. This is also where Ireland's first fossil brittle star was found: Crepidosoma doylei [Fig. 2-3.2] (Doyle, 1989) (Blake et al., 2017).

Fig. 2-3.2 - Ireland's first fossil brittle star Crepidosoma doylei. Image credit: Dr S.K. Donovan.


Other fossils from the Silurian seas reflect the ever-changing environments and their dynamic biodiversity (Sleeman et al., 2004, p. 39). Coastal shifts can be reconstructed from the changing communities: brachiopods, corals, crinoids, graptolites and trilobites (Sleeman et al., 2004, p. 39). On the Dingle Peninsula in county Kerry, for example, there is an abundance of corals, bivalves and trace fossils from a shallow, warm sea (Higgs & Williams, 2018), (Holland & Sanders, 2009), (Sleeman et al., 2004, p. 39) (Video: Higgs, Silurian Rocks).

Fig. 2-3.3 - Silurian trace fossil Chondrites in siltstone, interpreted as feeding burrows in silty mud, Clogher Strand, Dingle peninsula, county Kerry. Width of view c. 30cm.


Section 2-4 - Devonian

Fig. 2-4.1 - The Devonian period (adapted from: International Commission on Stratigraphy, 2024).


During the Devonian, habitats changed due to a landward coastal shift from the south. The fluvial sediments which had been deposited in an arid, warm climate and are preserved as the Old Red Sandstone (ORS) and conglomerates, were gradually replaced by marine sediments.

The mostly fossil-poor ORS of the early Devonian originates in the dry climate of the Laurussia continent (also called the Old Red Continent). This continent was formed by the closing of the Iapetus Ocean and the union of the two land masses. The generally arid conditions with periodic fluvial flooding events resulted in the oxidation of the sediments and thus to their red and purple discoloration (Sleeman et al., 2004, p. 49).

Fig. 2-4.2 - Old Red Sandstone at Dunmore East, county Waterford.


One of the few types of fossils in the ORS are Beaconites trace fossils, interpreted as crustacean burrows in river channels or flood plains [Fig. 2-4.3].

Fig. 2-4.3 - Beaconites trace fossils in Devonian ORS at Lumsdin’s Bay, county Wexford.


The famous tetrapod trackways on Valentia Island in county Kerry date from the middle or late Devonian. They occur in the Valentia Slate Formation, which is part of the Old Red Sandstone. The tracemakers were early amphibians, possibly Ichthyostega (Stössel, 1995), (Stössel et al., 2016) (Meere et al., 2020, Ch. 4), (Sleeman et al., 2004, p. 54), (Video: @tirulero1, Valentia Tetrapod). These rare trace fossils probably originated in sediments on a river bank, or perhaps in a flooded coastal zone.

At present only a few other such traces are known: from Tarbat Ness in Scotland; Genoa River in New South Wales, Australia; Świętokrzyskie Voivodeship in Poland; and Glenisla in Victoria, Australia. 

Fig. 2-4.4 - Tetrapod tracks in Devonian ORS, Valentia Island, county Kerry.
(Image used with permission © Geological Survey Ireland/James Bannon)


Well-preserved fossils of terrestrial plants (e.g. Archaeopteris hibernica) from the late Devonian period were collected from the old Kiltorcan quarry in county Kilkenny. Bivalve, crustacean and fish fossils from the fluvial sandstones and green siltstones indicate ancient freshwater habitats (GSI, Kiltorcan, undated).

Fig. 2-4.5 - Plant fossil, probably Cyclostigma kiltorkense, from the upper Devonian, old Kiltorcan quarry, county Kilkenny (Research Images by Jutta Kruse ⓒ National Museum of Ireland, Geological Collection, specimen F20902). Scale: grid divisions = 1㎠.


Fig. 2-4.6 - Coccosteus, placoderm cephalic plate, from the upper Devonian, old Kiltorcan Quarry, county Kilkenny (Research Images by Jutta Kruse ⓒ National Museum of Ireland, Geological Collection, specimen F27352). Scale: grid divisions = 1㎠.


Section 2-5 - Carboniferous/Mississippian

Fig. 2-5.1 - The Carboniferous period, divided into Pennsylvanian and Mississippian epochs (adapted from: International Commission on Stratigraphy, 2024).


In Ireland, the Mississippian (Lower Carboniferous) epoch is divided into three stages: the Tournaisian, the Viséan and the Serpukhovian. Fossils from the Tournaisian and Viséan were formed mostly in tropical, shallow, light-filled seas and therefore comprise corals, crinoids, sea urchins, brachiopods, gastropods, bryozoans, nautiloids, trilobites, conodonts, microscopic foraminifera and many algae (Holland & Sanders, 2009, p. 216) (Sleeman et al., 2004, p. 61).

There are too many sites from this period to mention, as Mississippian rocks (mostly limestone) make up a large proportion of the bedrock in Ireland. Fossils from this epoch can be found at the sites described below at Hook Head (county Wexford), the Burren (county Clare) and Streedagh (county Sligo). For more on the Serpukhovian, see Pennsylvanian/Namurian below.

Fig. 2-5.2 - Fossil mollusc (c. 5cm diameter) in a limestone gate pillar on the Aran island of Inishmore, county Galway.


The limestones of the Aran Islands group, to which Inishmore belongs, date from the Mississippian epoch (GSI, Aran Islands, undated).

Section 2-6 - Carboniferous/Pennsylvanian

Fig. 2-6.1 - The Carboniferous period, divided into Pennsylvanian and Mississippian epochs (adapted from: International Commission on Stratigraphy, 2024).


⇒ Namurian

The Namurian is a stratigraphic regional stage of the Carboniferous for northwest Europe (RSS). It straddles the Lower- and Upper Carboniferous boundary and corresponds to the Serpukhovian and the Lower Bashkirian of the international classification (GSS). It follows the Viséan stage and is followed by the Westphalian stage (STDK 2022 [German Stratigraphic Commission, DSK]), (Gradstein et al., 2021), (Sleeman et al., 2004, p. 73). 

Used here are the regional stratigraphic divisions for Northwest Europe (RSS) as well as those of the global stratigraphic scale (GSS), see [Fig. 2-6.2]. I thank Professor Hans-Georg Herbig and Dr Bettie Higgs for the explanations of the current stratigraphy of the Carboniferous. All errors in understanding are mine.

Sevastopulo uses Heckel and Clayton's more recent stratigraphical framework for north-west Europe (Holland & Sanders, 2009, Ch. 11), which replaces the Namurian, Westphalian and Stephanian stages with newer divisions. However, as many publications on these time periods are older or still use the older divisions, the 'Namurian' is used here to define the stage described above.

Fig. 2-6.2 - White arrows indicate the Namurian (German: Namurium) in the chronostratigraphic division of the Carboniferous in Europe (table adapted from: Menning & Hendrich, STDK, 2022. Used with permission of the DSK).


In Ireland, the Lower Namurian is characterized by deep-sea shale and turbidite sediments. This is followed by sandstones originating in shallower seas and deltaic estuaries. A period of the Lower Namurian is characterized by cyclothems, divided by marine bands (or 'condensed sections'), resulting from sea-level fluctuations, particularly in shelf areas or basin interiors (Holland & Sanders, 2009, Ch. 11), (Sleeman et al., 2004, p. 73). 

Fig. 2-6.3 - Dunbarella from a Namurian marine band in mid-west county Clare.


The upper Namurian and lower Westphalian contain coal beds. The upper Westphalian is rarely found in Ireland and Stefanian rocks are not found at all, so I end this review of Irish fossil periods here (Sleeman et al., 2004, p. 73).