An Early Devonian lake and its associated biota in the Midland Valley of Scotland

Abstract

The Tillywhandland fish bed of the Lower Old Red Sandstone in the Strathmore area of the Scottish Midland Valley accumulated in a lake, here called Lake Forfar, which was created suddenly following a period of fluvial deposition. Lake creation may have been due to basin faulting or the disruption of drainage patterns by contemporaneous volcanic activity. The fish bed laminites accumulated in a hydrologically open lake under a seasonal climatic regime. When fully developed, laminites comprise repeated quadruplets of clastic silt/carbonate/organic/green clay–shale laminae averaging 0·5 mm in thickness. Following 2000 years of laminite deposition an increasingly silty succession with thin current-rippled sandstones provided the lake-fill.The fish fauna is dominated by Mesacanthus and Ischnacanthus with rare Euthacanthus, Parexus, Climatius, Vernicomacanthus and Cephalaspis. Most fish carcasses were partially decayed before deposition in the laminites on the poorly oxygenated lake floor. Abundant coprolites are the result of predation on Mesacanthus and small Ischnacanthus, probably by larger Ischnacanthus. Arthropods present include eurypterids (Pterygotus), washed in as near complete exuviae and fragments, and millipeds which were washed in from surrounding terrestrial environments along with plants, of which Parka and Zosterophyllum are common. Bioturbation indicates that conditions were not permanently anoxic during deposition of the laminites.Comparison of our collections with the Mitchell Collection accumulated in the 19th century indicates that Tillywhandland Quarry was the main source of specimens in laminite lithologies labelled ‘Turin Hill’.

Four new Early Devonian ischnacanthid acanthodians from the Mackenzie Mountains, Northwest Territories, Canada: an early experiment in dental diversity

“…Ischnacanthus gracilis has only been found in freshwater lake deposits (Trewin and Davidson, 1996), whereas MOTH specimens are preserved in marine sediments (Gabrielse et al, 1973;Dineley and Loeffler 1976;Adrain and Wilson 1994;Hanke, 2001;Hanke et al, 2001b;Zorn et al, 2005). This further supports the hypothesis that Ischnacanthus gracilis was unlikely to have lived at the MOTH locality during the Early Devonian.…”

Exceptionally Preserved Fossil Assemblages Through Geologic Time and Space

“…The decline in marine cluster count, however, began in the Cambrian (Fig. 7c, d), prior to the rise of terrestrial animal life (Shear and Selden, 2001), and although the oldest unambiguous non-marine clusters first appear in the Devonian (Trewin, 1985;Trewin and Davidson, 1995), they remain infrequent until the Carboniferous (Fig. 7g, h).…”

“…In light of this relationship, the results indicate that exceptional preservation occurs within non-marine settings in response to local and provincial conditions, largely immune to global secular environmental changes. As non-marine assemblages occur in clusters throughout the middle Devonian-Neogene interval (Bradley, 1964;Huber and McDonald, 2003;Trewin, 1985;Trewin and Davidson, 1995;Wang et al, 2012), these phenomena must include regional scale factors, like topography, sedimentation rate, climate, primary productivity, salinity, and paleogeography (Allison and Briggs, 1993). In general, exceptional preservation may be favored in regional settings with bottom water stagnation, episodic burial, pore water anoxia, and early-diagenetic sealing of sediments in addition to low levels of animal scavenging, bioturbation, and sediment reworking (Allison and Briggs, 1991;Seilacher et al, 1985).…”

PTERYGOTUS ANGLICUS AGASSIZ (CHELICERATA: EURYPTERIDA) FROM ATHOLVILLE, LOWER DEVONIAN CAMPBELLTON FORMATION, NEW BRUNSWICK, CANADA

“…Chlupáč (1994) noted that pterygotids, including species of Acutiramus and Pterygotus from the Czech Republic, belong to typical marine faunas ranging from near‐shore BA1 to deep marine BA5. Alternatively, Trewin and Davidson (1996) considered that P. anglicus in the Midland Valley of Scotland lived in fresh water. Scottish Early Devonian fish beds of the Midland Valley are considered to be lacustrine (Trewin and Davidson 1996; Braddy 2000, 2001) with P. anglicus at sites such as Tillywhandland sharing ‘Lake Forfar’ with acanthodians ( Mesacanthus , Ischnacanthus and Euthacanthus ) and ostracoderms ( Cephalaspis pagei ).…”