Introduction and biology

Corals have been around since at least the Ordovician. They come in both colonial and solitary flavours – colonies comprise many (genetically identical) individuals in close proximity, and can form really large reefs. Solitary corals are single individuals living on the sea floor. Let's learn more about them!


Key points to take away from this video are:

  • The corals are members of the phylum Cnidaria: they are animals with radial symmetry, and one opening through which they do everything. They excrete a hard skeleton.
  • Corals are marine, and comprise three major groups – the Rugosa, Tabulata, and Scleractinia.
  • Rugose and tabulate corals appear in the Ordovician and die out at the end of the Permian period. Scleractinian corals evolved in the Triassic and are the corals we see today.
  • Reef ecosystems are really important – and have been throughout geological time.The groups that create reefs differs through time and with different environmental conditions.

Meet a member of each coral group.

To serve as a gentle introduction to the corals, below is a member of each major subdivision.

Rugose corals

Here is an example from the order Rugosa, which was around from the Ordovician to the end Permian. It's typical of the solitary members of this group from the mid-Palaeozoic. What external features can you see?

A fossil rugose coral (Heliophyllum halli). This is Middle Devonian in age, sourced from the Moscow Formation of Erie County, New York. Specimen ~11cm long.

Tabulate corals

To contrast that, check out this tabulate coral. This groups was also around from the Ordovician to the end Permian. This is a colonial form (tabulate corals are all colonial).

A tabulate coral (Emmonsia emmonsii) – also Devonian in age, but from the Onondaga Limestone of Genesee County, New York. Longest dimension of specimen ~8.5 cm.

Scleractinian corals

Finally, here is a a member of the Scleractinia. These are the corals that are still around today (they are both solitary and colonial, this is a colonial form).

This is a scleractinian coral Septastrea marylandica, which is Pliocene in age, sourced from the Tamiami Fm. (Pinecrest Beds) of Sarasota County, Florida. Maximum length of specimen is 4.6 cm.


So, that is our three groups. We'll now go on to learn some terms about coral anatomy, and about how to tell these groups apart. Some key words to look out for are:

  • Corallum
  • Corallite
  • Calice
  • Tabulae
  • Septa
  • Theca/epitheca
  • Dissepiments
  • Rugae/growth lines
  • Apex

You could label these on a diagram as the video goes along, but there is a model with some – but not all of these – labelled below.


Key points to take away from this video are:

  • Corals come in a range of forms, but all have septa (radial structures) and tabulae (transverse ones). These are developed to differing degrees depending on the group.
  • Rugose corals have strong septae, and are robust, calcitic corals (but have a wide range of forms).
  • Tabulate corals are colonial, calcitic and have well-developed tabulae. Septae are reduced.
  • Scleractinian corals are aragonite, and have a relatively light, porous skeleton.

The three groups: Redux.

It's really useful to differentiate these three major coral groups from each other when in the field. Let's have a look at the three groups in 3D, to get a better feel for how they differ. This is, to an extent, a bit repetitious – you have my apologies, but, in general the more fossils you see the better you become at identifying them. So with that in mind…


This group I tend to associate more with Upper Palaeozoic rocks, based on UK fieldwork. Rugose corals have strong septae, as you can see in this typical solitary form:

A solitary rugose coral (Zaphrentites spinulosum), Lower Carboniferous and found in Tennessee. ~3.5 cm long.

If you find something like the above in a Palaeozoic rock, it's a rugose coral (tabulate ones are colonial only, remember). But rugose corals can also be colonial! Here is an example:

A colonial rugose coral. This one is Acrocyathus floriformis, and it's Lower Carboniferous in age (Mississippian St. Louis Limestone of Monroe County, Illinois).

In particular, note that you can still see the septae of each corallite quite clearly – then compare to those tabulate corals below.


Tabulates were amongst the earliest corals, and are our Palaeozoic, colonial only group. In contrast with the above colonial (rugose) coral, check out the one below. Note that it doesn't really have septa when looking down the calice. This is typical of the tabulate corals.

Tabulate coral Favosites tuberosus. Devonian in age, and sourced from the Onondaga Limestone, Erie County, New York. Longest dimension of specimen~12 cm.

On the sides of the rock you can make out the tabulae within the coral where the corralites are split. Sometimes there are chain like, branching and tubular forms of tabulate coral. Here's an example of the latter:

Tabulate coral Thamnopora limitaris from the same site as the last fossil. Maximum dimension ~17.5cm.


Let's finish with a cheeky scleractinian. This time a solitary one. See if you can get a feel for the less chunky skeleton than you see in solitary rugose corals – or spot the difference in the septal arrangement.

Extant solitary coral Flabellum moseleyi. This lived at ~250 m depth off the southwest coast of Florida. Specimen ~5.5cm long.

A quick quiz.

I think you know what's coming next. You've looked at the models, so let's do a little quiz. It's called name the coral.

Why should we care?

Let's finish by looking at how geologists can make use of corals, and what they look like in rocks!


Key points to take away from this video are:

  • Corals are not too difficult to ID in a rock, especially if cracks in the rock split them at an angle.
  • Coral morphology, if in life position, or level of fragmentation if not, can tell you where you are on a reef.
  • As long as you're looking at a scleractinian coral with photosynthetic symbionts (called zooxanthellae, FYI), then you're looking at a depositional environment that is:
    • In the photic zone
    • Warm
    • Probably agitated
    • Relatively low on clastic sediment input
  • But you need to be more careful with this assumption the older rocks get.
  • We can tell using corals that Earth's rotation is slowing (!?).

A chain coral

So you can get an idea of what they look like in rocks, here is one of those chain corals in 3D for you:

A tabulate “chain coral” Halysites catenularia from the Silurian. Longest dimension ~24 cm.

Fancy seeing corals in more detail, in section?

If so, here you can find a lovely Carboniferous Limestone with lots of corals in it, courtesy of the UK Virtual Microscope and the Open University. The rock dates from the Lower Carboniferous, is a limestone, and was collected South-East of Dunbar, North Berwickshire, Scotland. In it you can see both Koninckophyllum corals and brachiopod shell fragments.

To view either of the two spots marked under PPL and XPL, just click on the icons. The slider on the bottom zooms; scale at top.