The weird and wonderful of the Ordovician (Part 2): Didymograptus murchisoni – a classic marker of the Middle Ordovician

Jon Trevelyan (UK)

This is the second in my series of short articles on fossils of the Ordovician. Among the many colonial organisms that chart the course of Ordovician seas, Didymograptus murchisoni remains one of the most recognisable. A key graptolite of the Middle Ordovician, it combines elegant morphology with exceptional biostratigraphic value. Its paired, gently diverging stipes (a stipe is a branch of the colony’s overall structure, known as a rhabdosome), and widespread distribution, make it one of the most important fossils for understanding both the evolution of planktonic life and the refinement of Ordovician time scales.

Discovery and appearance

Didymograptus murchisoni was first recognised in the mid-nineteenth century from classic British Ordovician black-shale successions, particularly in Wales and the English Lake District. These were the same beds examined by early workers such as Roderick Murchison, Henry Alleyne Nicholson, Charles Lapworth and John Edward Marr, whose combined studies – both within and beyond the Geological Survey – helped establish this species as one of the key graptolites of the Ordovician.

In life, the colony consisted of two slender stipes diverging from a common origin. Each stipe bore rows of minute cups, or thecae, housing individual zooids. The overall form was typically 2-5cm long, with the stipes held at a relatively shallow angle. These colonies floated freely in the water column, supported by a long nema, which is a narrow thread-like extension that functioned as a stabilising structure.

The fine preservation seen in many black shale sequences allows details of the thecal spacing and stipe curvature to be resolved, key features that help distinguish between closely related species within the didymograptid lineage.

Interpretation and classification

Didymograptus murchisoni belongs to the Diplograptidae, a family of planktonic graptolites that diversified rapidly during the Darriwilian (Fig. 3). Its two-stiped colony reflects a relatively early stage in the evolution of more complex, multiply branching forms that would appear later in the Ordovician.

As graptolites are hemichordates – close relatives of modern acorn worms and pterobranchs (tiny, tube-dwelling colonial animals that live attached to the seafloor) – their colonies provide an important window into the evolution of early deuterostomes (a major group of animals that includes echinoderms and, ultimately, vertebrates). D. murchisoni, in particular, helps trace how simple branching strategies gave rise to the spectacular elaboration of Late Ordovician forms.

Significance

Few Ordovician fossils are as central to global correlation as Didymograptus murchisoni. Its short stratigraphic range, rapid dispersal and widespread occurrence make it a near-perfect index fossil. Entire biozones are named for it, and its appearance marks a reliable horizon in many classic shale sequences of Britain, Scandinavia and China.

Beyond biostratigraphy, the species charts the rise of open-ocean planktonic ecosystems. Its success reflects a world in which nutrient-rich upwelling, expanding continental shelves, and increasing ecological complexity drove major radiations of planktonic life.

Conclusion

Elegant and instantly recognisable, Didymograptus murchisoni captures a turning point in Ordovician ecology. As one of the most valuable graptolites for correlation and evolutionary study, it continues to illuminate both the diversification of colonial hemichordates and the structure of Middle Ordovician oceans.