Wednesday, 30 September 2015

Talking Grap(tolites)

As a vertebrate palaeontologist, I struggle sometimes to understand what motivates invertebrate workers. Sure, the stuff they study is often copious. It provides vital biostratigraphic information, allowing us to date rock more accurately, often inter-continentally. But it's all so... boring.

Having said that, it turns out you can get pretty enthused about even the least charismatic of fossils, providing you found it yourself. I recently accompanied the Edinburgh University 3rd year undergrads on the first of a series of field trips for their Palaeontology class. We travelled an hour and half on roller-coaster roads south of the city to Dob's Linn: a ravine near Moffat in the Scottish borders. There, we found graptolites.
The road to Dob's Linn (Image: author's own)
Although not far from the narrow road, it quickly felt remote as we descended into the small glen.
Sheep startled and scattered at the approach of forty sets of undergraduate boots, tramping along the river to the first of severel sites known for their graptolite fossils.

The short walk down to the mouth of the ravine. (Image: author's own)
Graptolites are somewhat underwhelming fossils, but very common in Palaeozoic shales. Their resemblance to writing led to their name: graptos = written, lithos = rock. They look like small needles, fern fronds or leaves (depending on the species), but in life they would have been attached to one another in colonies and were more closely related to animals than plants - their common ancestor is thought to be a Cambrian worm-like creature. The earliest filter-feeding graptolites were secured to the seafloor, but free-floating forms soon evolved, possibly with the ability to move around in the water column as jellyfish do today, following food in seasonal or perhaps nightly cycles.

Reconstruction of a floating graptolite colony from the
Redpath Museum, Montreal*
A linn is where water has cut a deep feature through the landscape. At Dob's Linn the river leads up a modest ravine to a waterfall that pours down a sheer rock face: a hidden reward for the graptolite-hunting palaeontologist. Dob's Linn is named for Halbert Dobson, a covenanter who hid from English persecutors for six weeks during "The Killing Time" by squirreling away in a cave above the waterfall.
The waterfall cascades over greywackes into the
shale ravine. (Image: author's own)
The most famous person associated with Dob's Linn is the eminant English geologist, Charles Lapworth. After moving to the Scottish borders where he fell in love with a girl from Galasheils (there's surely a song lyric in there somewhere), Lapworth recognised the importance of Dob's Linn in biostratigraphy, carrying out pioneering work on these little graptolite fossils. The site is now a Global Boundary Stratotype Section and Point (GSSP) for the Ordovician-Silurian boundary.
Amazing stratigraphy in the ravine (Image: author's own)

Although not the only creatures alive in the seas during the Palaeozoic, graptolites were composed of collagen and polymers that preserved in anoxic shales where other organic materials did not. As a result they've become vital in rock dating: beds from the Ordivician and Silurian are often dated based on assemblages of these strange creatures, sometimes with exceptional precision.
Little silvery graptolie fossils in shale from Dob's Linn. (Image: author's own)
Our group stopped at two sites along the ravine, finding many fossils of varied quality. Even students who had struggled to generate any enthusiasm for the sample graptolites we'd shown them in class were soon scrambling high and low with their bums in the air, picking through the shale scree. Once they realised how easy it was to find them, they raised the bar: everyone was trying to find the best graptolite, and the highest number of species.

Violets growing in the shale scree, as students hunt for fossils. (Image: author's own)

We all returned to the bus with our pockets filled with fossils (and for one unlucky student, shoes filled with peaty stream water...) and returned to the university ready to get out the books and identify our species.

Nothing beats fossil hunting, even for "rubbish" fossils.

*UPDATE - I've since been told the reconstruction of a graptolite colony in the Redpath Museum I've included here is innacurate, there is no evidence they had floats. Thanks David Bapst (@dwbapst) for the heads up!

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