Didymograptus

Quick facts

Didymograptus was an Ordovician graptolite whose colony commonly carried two diverging branches. Flattened on shale, it resembles a tuning fork or the letter V edged with fine teeth. Each “tooth” marks a tubular dwelling that held one zooid. The soft animals themselves are almost never preserved.

Growth from one initial tube

Development began with a conical sicula, the housing of the first zooid. Budding added new tubes, called thecae, and produced two branches or stipes. Detailed study of the proximal region of the type species D. murchisoni revealed a particular sequence in the construction of the first theca. Such characters distinguish genera more securely than the overall V-shaped outline.

The colony was not a random bundle of tubes. Budding direction, thecal order and spacing were repeated during growth. The angle between branches, their width and the orientation of thecae varied among species. Revision has moved many graptolites once broadly called Didymograptus into other genera, so an old museum label may not match current classification.

What remains in the rock

The shared organic skeleton was thin. Compaction and heating turned it into a dark flat film, sometimes with slight relief. Individual zooids, tentacles and internal soft parts are not normally visible. Filter feeding is reconstructed from colony design and relationship to living hemichordates, not from preserved mouthparts of Didymograptus.

Compression direction can change the apparent branch angle and obscure the first growth stages. Palaeontologists therefore compare many specimens, expose impressions on both halves of split rock and measure spacing between thecae. A fragment of one branch is much harder to identify to species than a complete proximal region.

Graptoloids are generally interpreted as planktonic or passively drifting inhabitants of the open sea. Models differ over the living orientation of the two branches because a flat impression cannot show how the colony hung in water. The more net-like Dictyonema followed a different branching plan rather than representing a larger Didymograptus.

Clocks for Ordovician seas

Some species evolved rapidly and spread widely, so their sequence divides Ordovician strata into biozones. The D. murchisoni Zone, for example, has been used to correlate Darriwilian rocks. Identification needs the proximal region and theca form; the silhouette of a tuning fork alone is insufficient.

Graptolites date a bed alongside other evidence. Conodonts such as Acodus lived in the same broad marine world and are known from tooth-like phosphatic elements. Agreement between groups reduces the risk caused by reworking or misidentification.

Although Didymograptus is Ordovician rather than Cambrian, it occupies its established place in the paired catalogue. The profile states the age directly so navigation never becomes a biological claim.

Evidence, inference and reconstruction