Introduction and biology

Graptolites were very common during the Palaeozoic, especially early in the era. We commonly find fossils of these colonial critters in black shales: almost universally those fossils represent the remains of the organic material that housed a colony. We find out more about the group as a whole in our first video.

Summary

Key points to take away from this video are:

  • Graptolites are members of the hemichordates, which is a small phylum of soft bodied animals comprising the acorn worms, and a group of small, mainly colonial creatures called the pterobranchs.
  • The graptolites are a group ot pterobranchs that were highly successful in the early Palaeozoic. They may still have a living representative.
  • Hemichordates are deuterostomes, and the graptolites are sublcass Graptolithina within class Pterobranchia.
  • There are a few taxonomic schemes, but the simplest splits the graptolites into two orders, Order Dendroidea (=dendroid graptolites), and order Order Graptoloidea (=graptoloid graptolites).
  • Graptolites started as benthic animals, and evolved planktonic forms (the graptoloids) which went on to be particularly successful between the Ordovician and the Devonian.

Meet a graptolite.

Here is a typical graptolite. Note the rock it is in, as well as how it appears on the surface. This is a dead giveaway and makes grapolites super easy to spot in the field, even at arm's distance!

This is a typical graptolite fossil (Glossograptus whitfieldi) in a black shale - from the Ordovician, Ledbetter Slate in Washington. Maximum dimension ~11cm.


Morphology

Words. Why are there always words? I guess in this group having a bunch of new words makes more sense than in many others, as the individual bits are hard to homologise with other animals (because many of the names refer to bits of a colony, not of individual anatomy). But still, there are a bunch I am afraid:

  • Tubarium
  • Sicula
  • Dissepiments
  • Thecae
  • Zooids
  • Nema
  • Stipes

Summary

Key points to take away from this video are:

  • A colony is housed in a tubarium which has arms (stipes).
  • Individuals (zooids) live in thecae.
  • Dendroid graptolites have many stipes, joined by struts called dissepiments.
  • Graptoloid graptolites have fewer stipes, and evolve from having simple to complex thecae.
  • The evolution of their attitude is really useful in dating Ordovician/Silurian rocks in the field.
  • When it comes to the mode of life of this group, we can be most confident in the statement that we know very little (how socratic).

Let's compare the dendroids and graptoloids.

Dendroidea

This dendroid graptoloid doesn't actually have obvious dissepiments, rather it shows something called anastomosis which means stipes join or share thecae. It's relatively rare, and not very well understood. But a key thing to note in this species is the number of stipes, which is typical of the dendroid graptolites.

This graptolite is a member of the Dendroidea (species is Palaeodictyota anastomotica, FYI). Silurian in age, from Niagara County, New York. Specimen ~8.5 cm in diameter.


Graptoloidea

Cool beans. Now let's meet some graptoloids. First, here is a Didymograptus – so in our generally two-stiped but still pendent or declined stage of evolution.

Didymograptus artus from the Ordovician of Idaho (Phi Kappa Formation). Rock ~4.5cm max.

Note here the thecae on the inside of the stipe, facing each other - so the entire thing looks a bit like a zip. Compare this with the 3D model we met before video two – where they were on the outside. That one was the scandent form, which evolved from the pendent/declined form we see in the Didymograptus specimen. Now if we look at the model below, we can see a monograptid:

Monograptus clintonensis: Silurian in age (from the Williamson Shale of Rochester, New York). Max dimension of rock ~11.5 cm.

This is that stage where you end up with only one stipe – note the zig zags occur on only one side of the fossil. So this is typical of our later Silurian into Devonian species of graptoloid.

A quick quiz.

Of course! We have two groups! Let's see if we can tell them apart via a cheeky quiz!

Why should we care?

Let's learn why graptolites are helpful for geologists, and see some in a rock.

Summary

Key points to take away from this video are:

  • We associate graptolites with anoxic black shales.
  • They are usually compressed, carbonised lines in said rocks, but when weathered the shales can become less black.
  • Graptolites are useful for regional biostratigraphy, and can also tell us about water depth.
  • All of that is overprinted by palaeogeography: the group has been useful in working out what was going on in Ordovician and Silurian continents.

And that, my friends, brings us to the end of our foray through invertebrate fossil diversity, other than our concluding material! I hope it has been interesting.