Tag Archives: Jurassic

When dinosaurs roamed the Western Isles

Cuillin Hills, Isle of Skye, Scotland, UK

Cuillin Hills, Isle of Skye, Scotland, UK (credit: Wikipedia)

The Isle of Skye off the northwest coast of Scotland  is known largely as a prime tourist destination, such as Dunvegan Castle with a real clan chief (The MacLeod of MacLeod) and its Faerie Flag; Britain’s only truly challenging mountains of the Black Cuillin; and, of course, the romantic connection with the Young Pretender, Charles Edward Stuart and his escape, in drag, from the clutches of the Duke ‘Butcher’ Cumberland, hence the Skye Boat Song. Geologists know it best for its flood basalts with classic stepped topography and the exhumed guts of a massive central volcano (the Cuillin), relics of the Palaeocene-Eocene (62 to 54 Ma) North Atlantic Large Igneous Province. The spectacular Loch Coruisk, a glacial corrie drowned by the sea, exposes the deepest part of the main magma chamber. It is also the lair of Scotland’s lesser known Monster, the dread Each Uisge (Water Horse). Yet evidence is emerging for the former presence in the Hebrides of other, more tangible monsters.

Skye’s great volcanic edifice rests on Mesozoic sedimentary rocks including shallow-water muddy limestones of the Great Estuarine Group of Middle Jurassic (Bathonian, 174–164 Ma) age. For dinosaur specialists this is of the time when meat-eating theropods and herbivorous sauropods began growing to colossal sizes. Yet the Bathonian is notable for its global paucity in well exposed terrestrial and near-shore sedimentary sequences. Easily accessible, the Skye Bathonian sequence is much visited and has yielded a rich, though generally fragmentary fauna. A group of recent visiting palaeontologists from the University of Edinburgh, the Chinese Academy of Sciences and Skye’s Staffin Museum have discovered an extensive tract of wave-cut platform on the east shore of the Trotternish Peninsula where lagoonal carbonate muds were trampled by several dinosaurs that left around 50 tracks (dePolo, P.E. et al. 2018. A sauropod-dominated tracksite from Rubha nam Brathairean (Brothers’ Point), Isle of Skye, Scotland. Scottish Journal of Geology, online; doi:10.1144/sjg2017-016).

Dinosaur foot prints from Skye. Left example of a sauropod rear-foot print; right theropod. (credit dePolo, P.E. et al. 2018, modified from Figs 8 and 9a)

Some are of medium-sized sauropods (either Parabrontopodus or Breviparopus – both names for footprints rather than any genus of dinosaur) whose crudely elephant-like footprints are up to 0.5 m across (the largest, from Western Australia, are about 1.7 m across). Although there are fragmentary dinosaur bones from the same strata, assigning the footprint to a known species is not possible. However, foot size can be used to estimate how high the creatures’ hips stood (2 to 2.5 m): hefty beasts but not the true giants of later times A variety of three-toed, clawed, somewhat bird-like, footprints also occur. They are assigned to probably bipedal carnivores or theropods. Variation in foot size suggests a range of hip-height from about 0.9 to 2 metres, so these carnivores would have been pretty formidable.


Age calibration of Mesozoic sedimentary sequences: can it be improved?

Relative age sequences in sequences of fossiliferous sediments are frequently intricate, thanks to animal groups that evolved quickly to leave easily identifiable fossil species. Yet converting that one-after-the-other dating to absolute values of past time has been difficult and generally debateable. Up to now it has relied on fossil-based correlation with localities where parts of the sequence of interest interleave with volcanic ashes or lavas that can be dated radiometrically. Igneous rocks can provide reference points in time, so that age estimates of intervening sedimentary layers emerge by assuming constant rates of sedimentation and of faunal speciation. However, neither rate can safely be assumed constant, and those of evolutionary processes are of great biological interest.

Setting Sun at Whitby Abbey

Sunset at St Hilda’s Abbey, Whitby NE England; fabled haunt of Count Dracula (credit: epicnom)

If only we could date the fossils a wealth of information would be accessible. In the case of organisms that apparently evolved quickly, such as the ammonites of the Mesozoic, time resolution might be extremely fine. Isotopic analysis methods have become sufficiently precise to exploit the radioactive decay of uranium isotopes, for instance, at the very low concentrations found in sedimentary minerals such as calcium carbonate. So this prospect of direct calibration might seem imminent. Geochemists and palaeontologists at Royal Holloway University of London, Leicester University and the British Geological Survey have used the U-Pb method to date Jurassic ammonites (Li, Q. et al. 2014. U–Pb dating of cements in Mesozoic ammonites. Chemical Geology, v. 376, p. 76-83). The species they chose are members of the genus Hildoceras, familiar to junior collectors on the foreshore below the ruined Abbey of St Hilda at the small port of Whitby, in NE England. The abundance and coiled shape of Hildoceras was once cited as evidence for the eponymous founder of the Abbey ridding this choice locality of a plague of venomous serpents using the simple expedient of divine lithification.

English: Hildoceras bifrons (Bruguière 1789) L...

Hildoceras from the Toarcian shales of Whitby (credit: Wikipedia)

The target uranium-containing mineral is the calcite formed on the walls of the ammonites’ flotation chambers either while they were alive or shortly after death. This early cement is found in all well-preserved ammonites. The Hildoceras genus is found in one of the many faunal Zones of the Toarcian Age of the Lower Jurassic; the bifrons Zone (after Hildoceras bifrons). After careful selection of bifrons Zone specimens, the earliest calcite cement to have formed in the chambers was found to yield dates of around 165 Ma with precisions as low as ±3.3 Ma. Another species from the Middle Jurassic Bajocian Age came out at 158.8±4.3 Ma. Unfortunately, these precise ages were between 10-20 Ma younger than the accepted ranges of 174-183 and 168-170 Ma for the Toarcian and Bajocian. The authors ascribe this disappointing discrepancy to the breakdown of the calcium carbonate (aragonite) forming the animals’ shells from which uranium migrated to contaminate the after-death calcite cement.

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