Introduction and biology

Brachiopods were really successful. They were the coolest kids of the Palaeozoic. Kinda. They are still alive today, but those species that survive pale in comparison to the diverse glory of brachiopods that were around in the Palaeozoic. How did we get here? What are brachiopods? Find out in the video below.

Summary

Key points to take away from this video are:

  • The brachiopods are a phylum of twin‐valved, often sessile, shellfish.
  • They are marine, and filter feed using an organ called the lophophore, placing them in the clade Lophotrochozoa.
  • Their diversity in the geological past – especially the Palaeozoic, following their Lower Cambrian origins – is far greater than today.
  • Correlation does not imply causation.

Meet your first brachiopod

Below you can find a 3D model - the first shows what I tend to think of as a typical brachiopod based on my fieldwork within the UK (this will differ based on where you go in the world!). Take a look at the lack of symmetry between the valves.

Brachiopod Peregrinella peregrina, Late Cretaceous, France. ~8 cm long.

And here, for good measure, is an extant brachiopod, a member of a rather famous genus called Lingula.

Extant brachiopod Lingula anatina from the Phillipines. Longest dimension of specimen is approximately 6 cm.

Morphology

Cool cool cool. Now let's look at the morphology of these critters. As per., words:

  • Pedicle
  • Dorsal/ventral valve
  • Lophophore
  • Foramen
  • Umbo
  • Adductor/diductor muscle scar
  • Rib
  • Growth line
  • Cardinal process

As I mention in the video, you may want to label these from the 3D model below. A model showing the 3D form was difficult to come by, so I've also put an image of an aritculate brachiopod below to help out..

Summary

Key points to take away from this video are:

  • Brachiopods have a smaller dorsal, and ventral larger valve through which the pedicle protrudes.
  • Rhynchonelliformeans are articulated (have a hinge), and have muscles that open and close the valves.
  • Linguliformeans aren't hinged, and open their valves through shifting their body position.
  • Craniiformeans have neither a hinge, nor a pedicle, and often cement to a surface.

I introduce some more facts about these three groupings below, which will flesh out the lecture material.

The three groups

That's all great. Let's dig a little deeper into these thre groups of brachiopods, whilst also learning about their morphology. All three groups have their origins in the Cambrian, and are still around today.

Rhynchonelliformea

We met this group in the video. It is actually a really important one, because it represents the greatest diversity of brachiopods, both today and in the fossil record. The group maps to the more traditional Articulata in older taxonomies. The model below shows a typical example, with many of the bits labelled for you.

A modern brachiopod, Laqueus sp. from Japan, a member of a major group called the terebratulids. Specimen maximum length ~4cm

To help you get an idea of the internal anatomy, which is hard to see on 3D models, I've placed an image below.

Source.

The diductor muscles, which leave the diductor scars, open the valves, whilst the adductors close them. Some of these species have a structure supporting the lophophore (called the brachidium, FYI). Interestingly, this group lacks an anus (they just shoot waste out of their mouth), and are generally epifaunal. Some members of this group have crenulations - where the commissure zig-zags, increasing the area of the opening for feeding, but not its width, and strengthening the shell. This group also has a median fold and sulcus, which helps to separate incoming from outgoing water. You can see both of these features in the fossil below, also a rhynchonelliform:

A fossil brachiopodtypical of the order order Spiriferida: Mucrospirifer mrkonensis. This specimen is Devonian in age (it's from the Arkona Shale of Ontario, Canada). Maximum dimension ~5cm.

Gymnotoceras beachi, which you met above, is actually a ceratite, if you want to contrast it with these two!

Linguliformea

Let's meet this cool little group then. The modern example of a brachiopod at the top of this page is an example of Lingula which typifies this group (you may remember from the lecture that they have no hinge, and make their shell out of organophosphate). They have a large, and muscular pedicle, and adductor muscles to open and close the valves, which are often quite rectangular. Modern forms are burrowers, and can tolerate brackish waters - in fact this gorup specialises in inter- to subtidal habitats.

The model below shows members of this clade in a rock - see if you can get an idea of their shell shape from what you can see:

This is actually the brachiopod Obolus matinalis. It's Cambrian, and was found in Polk County, Wisconsin. Specimen ~7 cm long.

Craniiformea

Let's finish with these critters. They are inarticulate brachiopods, but have calcareous valves which are often rather circular in outline. These are actually an arrangement called punctate (they are perforated). They generally lack a pedicle and instead cement to a hard surface, often another brachiopod.

I'm afraid I've not been able to find a single 3D model of a brachiopod from this group, so instead I've put another image of the group so you can get an idea of what they look like.

Isocrania costata, a member of the family Craniidae from the Upper Maastrichtian (Upper Cretaceous). Source: Mark A. Wilson.

Bivalves vs Brachiopods

Next, I suggest you load the two models below, and satisfy yourself as to which is a bivalve, and which is a brachiopod. I've chosen fairly similar looking species, so look out for those planes of symmetry.

Brachiopod


Bivalve

A quick quiz.

Think you can tell brachiopods and bivalves apart? Let's do a quiz and see!

Why should we care?

Let's finish by looking at why these brachs are of use to geologists, and what they look like in rocks!

Summary

Key points to take away from this video are:

  • As long as you have enough of a shell to see symmetry, you can usually identify a brachiopod in hand specimen.
  • If not, they often have a fibrous fabcric in cross section.
  • Brachiopods are useful for studying evolution, palaeoecology, and aspects of Earth history.
  • This is particularly true of the Palaeozoic fossil record of the group.

I also promised to put the image of brachiopod modes of life, and the associated morphologies on this site! Here it is:

Image from (and copyright) Benton, M.J. and Harper, D.A., 2020. Introduction to paleobiology and the fossil record. John Wiley & Sons.

Want to search for a brachiopod in section?

This rock is a Lower Carboniferous limestone courtesy of the UK Virtual Microscope and the Open University. It is chock full of criniods - members of the Echinodermata, which we will be learning about later in this lecture series. But there are also some brachs in there - see if you can find any based on their fibrous appearance in section.

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.