"A geologist's song 05" joke

Graptolites (Melody: Danny Boy)
by Brenna Lorenz

Oh, graptolites -
Your stipes, your stipes are calling
From every shale in every ancient land;
Oh, graptolites -
Through dream's dark oceans falling,
Your rhabdosomes with grace in every strand.

Oh, take me back to Cambro-Ordovician days,
With all your youth and glory in full blaze -
You lived to see a mighty ocean wax and wane,
But modern oceans spread for you in vain.

Dendroidea -
Your autothecae smiling,
Through Tremodocian trials they would last -
Dendroidea -
Your bithecae are crying,
For when you fade, alas, they too must pass.

Why did you leave the ocean bottom safe and wide
To drift with plankton on the roving tide?
Oh, Dictyonema, from thy thecal loins emerged
Proud Graptoloids, that from their past diverged.