The weird and wonderful of the Ordovician (Part 1): Climacograptus wilsoni – a slender sentinel of the Scottish Ordovician
Jon Trevelyan (UK)
This is the first in my series of short articles on fossils of the Ordovician. Among the many graptolites recovered from the classic Dob’s Linn section of the Southern Uplands, Climacograptus wilsoni is one of the most elegant. Its slim, saw-toothed profile captures a phase in graptolite evolution where uniserial forms (consisting of one series or row), with dense thecal spacing, became the dominant plankton of mid-Ordovician oceans. In graptolites, thecae are the individual tubular or cup-like structures that housed the organism’s individual members, called zooids. The shape, arrangement and number of these thecae are crucial features for classifying and identifying different graptolite fossils.
Discovery and appearance
Climacograptus wilsoni was first described by Lapworth from the richly fossiliferous shales at Dob’s Linn, a site now world-famous as the GSSP for the Ordovician-Silurian boundary. In the dark, finely laminated mudstones of the Lower Hartfell Shale, the species commonly occurs as narrow, pyritised ribbons only a few millimetres wide, but often exceeding 2-3cm in length. Its striking preservation – golden to brassy against the grey shale – makes it one of the more recognisable Scottish graptolites.
Morphologically, C. wilsoni is a uniserial monograptid, with a single row of thecae projecting from one side of a straight to gently curved stipe. (A stipe is a branch of the colony’s overall structure, known as a rhabdosome.) Each theca forms a forward-leaning “saw-tooth” aperture, and the heavy overlap between successive thecae gives the rhabdosome its characteristic serrated outline. The thecal density is high, at around 8-10 thecae per 10mm, with remarkably consistent spacing.
The rhabdosome typically tapers only slightly towards the distal end, and although the sicula is rarely seen in most specimens, its position is marked by a subtle change in stipe outline near the base. In life, the colony drifted freely in the water column as part of the oceanic zooplankton, held in suspension by a long thread-like extension (the nema) that trailed behind it.
Interpretation and classification
Climacograptus wilsoni belongs to the family Climacograptidae, one of the most successful uniserial graptolite lineages of the Middle and Late Ordovician. Members of this group share a distinctive combination of characters: steeply inclined apertures, heavy overlap and a rigidly linear colony architecture.
Its appearance coincides with an interval of significant planktonic restructuring during the late Darriwilian to early Sandbian (Fig. 3). As oceanic circulation patterns changed and productivity declined in some regions, graptolites shifted towards morphologies that improved feeding efficiency in more oligotrophic conditions (typically waters that are low in nutrients). The narrow, densely spaced thecae of Climacograptus wilsoni reflect exactly this adaptation, that is, a design optimised for filtering diffuse organic particles as the colony drifted in open water.
Because its stratigraphic range is well constrained within the wilsoni Biozone, the species is especially useful for correlating mid-Ordovician sequences across the Southern Uplands, and into Scandinavia and eastern North America, where similar faunas are known.
Significance
Although smaller and less architecturally flamboyant than the later branching dicranograptids, Climacograptus wilsoni plays an important role in understanding Ordovician plankton evolution. Its abundance at Dob’s Linn provides a stable biostratigraphic marker in a section otherwise dominated by structural complexity and turbiditic sedimentation.
The species also illustrates the early stages of an evolutionary trend towards streamlined, high-efficiency graptolites, a pattern that would culminate in the spectacular monograptids of the Silurian. In this sense, C. wilsoni captures a moment when uniserial design reached a refined, functional optimum.
Conclusion
Climacograptus wilsoni is an understated, but highly informative inhabitant of the mid-Ordovician oceans. Its slim, delicately serrated rhabdosomes offer a window into the ecological pressures shaping planktonic life during a time of global oceanographic change. As one of the signature fossils of Dob’s Linn, it continues to help geologists tie together disparate successions and reminds us that sometimes the simplest graptolites are among the most revealing.