Paleontology - Spring 2001

Brachiopods

Brachiopods were a diverse and ubiquitous group of benthic organisms during the Paleozoic. They are often given the briefest of treatments by biologists because of their subsequent decline to the present (they never recovered from being decimated at the end of the Permian). However, the basic biology, ecology, functional morphology, and classification has been intensely studied by paleontologists because of the amazing abundance of brachiopods in the geological past.

Clarkson's treatment can be broken down into a few basic sections: morphology and ontogeny (p. 158-179), classification (p. 179-183), and evolutionary history, ecology, faunal provinces (p. 183-93). You will want to focus on the first two sections for Wednesday, and leave the (more interesting?) final sections for Friday.

Morphology

Clarkson's treatment of brachiopod morphology is rather long in this edition. Thankfully, he starts with a description of three representative species of articulated brachiopods (i.e., from the subphylum Rynchonelliformea) that provide you an opportunity to encounter most of the major features. Then, the text turns to the "major features of brachiopod morphology" and the terminology comes fast and furious. The treatments of the other subphylums are brief. It is worth considering the reason for all this terminology. This is part of the basic vocabulary (see the Treatise if you want more) used to describe the variations in brachiopod shells. Almost all these variations are used (at some level) in classification. Unfortunately, it is all too easy to get entangled in the morphology and lose track of the ecology.

So, what to do? It is important not to get too overwhelmed by the terms. We can focus on the most important morphological features used to understand brachiopod ecology and to identify the major groups. Here are some of the major features:

• Two valves: pedical (a.k.a., ventral) valve and brachial (dorsal) valve
• The anterior part of the shell is at the hinge whereas the posterior part is at the commissure
• Attachment: pedical and pedical foramen
• Feeding: lophophore and brachidium
• Hinge features: teeth and socket; hinge line
• Anterior part of shell
  • pedical valve: umbo, interarea, delthyrium and deltidial plates
  • brachial valve: notothyrium
• Posterior part of shell: commissure
• General shell features
  • fold and sulcus
  • strophic versus non-strophic
• Overall shape: biconvex, plano-convex, concavo-convex

Classification

Clarkson incorporates the classification scheme developed by Williams and others (1996). They used a cladistic analysis to try to sort out the relations between the varied brachiopod groups. Figure 7.16 in Clarkson (p. 180) illustrates the durations and postulated relationships between the orders - look carefully for the light dotted lines (particularly at the base of the diagram). This paper was a true landmark in brachiopod systematics. However, you may notice one interesting feature if you look at the figure and compare it to the classification. Most of the groups are paraphyletic: pentamerids gave rise to rhynchonelids which gave rise to atrypids, which gave rise to spiriferids and terebratulids; the Subphylum Rhynchonelliformea evolved from Subphylum Linguliformae. This is an excellent example of how a cladistic analysis has been used without the adoption of a full-fledged cladistic taxonomy. In fact, the data in the original paper indicates that the evolutionary pattern was actually something like this:

You need only worry about the major groups. So with that in mind, here is a simplified classification of the phylum with annotations for shell features:

Subphylum Linguliformea (L. Camb. - Rec.) chitonophosphatic shells; 2 classes with 5 orders.

• Subphylum Craniiformea (M. Ord. - Rec.) calcareous shells without articulated hinge; 3 orders in one class.
• Subphylum Rynchonelliformea (L. Camb. - Rec.) calcareous shells; 5 classes with many orders. Only the most numerous groups are briefly described below.
  • Class Strophomenata
    • Order Strophomenida (Ord. - Jur.) Plano-convex to concavo-convex or rarely biconvex, strophic shells with a long hinge line, pedicle foramen usually closed. Generally larger than orthids.
    • Order Productida (L. Dev.-Perm., Trias?) contains cup-like and cemented vase-shaped forms. Plano-convex shells with deep mantle body cavity, usually with spines. Delthyrium and notothyriun frequently closed.
  • Class Rhynchonellata
    • Order Orthida (L. Camb. - Rec.) Generally simple, small, semi-circular, unequal biconvex, strophic shells, delthyrium usually open. Oldest articulate group.
    • Order Pentamerida (Camb. - Dev.) Smooth, biconvex, non-strophic shells, generally with a pentagonal shape. Internal muscles attached to large platforms.
    • Order Rhynchonellida (Ord. - Rec.) Small, non-strophic shells with zigzag commissure, strong fold and sulcus, functional pedicle, delthyrium partially closed.
    • Order Spiriferida (Ord. - Jur.) Biconvex, strophic to non-strophic shells, usually with prominent fold and sulcus, commonly ribbed, delthyrium open or closed. Elongate hinge line often gives a winged appearance.
    • Order Atrypida (Ord. - Dev.) Compact, biconvex shells with narrow hinge line, pronounced fold and sulcus, weak ribbing. Umbonal foramen common. Some authors include atrypids under spirifers (for example: Clarkson).
    • Order Terebratulida (Dev. - Rec.) Smooth, biconvex, non-strophic, shells, often elongate in anterior-posterior direction. A functional pedicle emerges through umbonal foramen. Last order to appear and the most diverse today.

Evolutionary history, ecology, faunal provinces

The evolutionary history of brachiopods is well known due to over a century of work by biostratigraphers. Interest in their ecology and environmental distribution was sparked by the work of Rudwick and Zeigler, respectively. This is a rich field of study and the resulting literature is large (and only partly summarized in the text).

The evolutionary history largely reflects the sequential diversification and decline of the various brachiopod orders. As noted by Clarkson, different Paleozoic periods can be distinguished by the relative abundance of the different orders (or subphylums for the Cambrian).

Rudwick's (1970) work on the ecology and functional morphology of brachiopods inspired (or provoked) consideration of brachiopod life habits. Personally, I find some of the more recent terminology used to describe ecological groups (fixosessile, liberosessile, etc.) a bit awkward. Clarkson's text does not really highlight the main functional types. So, more simply, here are some basic morphologic types to consider.

1. The lingulids have a muscular pedical and many muscles connecting their shells (see the laboratory specimen). They move their shells by each other in a scissor-like motion. What is their life style? What kind of environments are they adapted to live in?
2. Most of the articulated forms have a pedical opening. What does this indicate about their life orientation? What environmental limits are likely?
3. Concavo-convex shells (many strophomenids) usually have a closed pedical foramen. What was their life orientation? What does this say about the environment?
4. Pentamerids have large shells, commonly with a lot of internal platforms in the proximal (hinge) part of their shell. Consider their life orientation, and position of the commissure - how might such a large amount of internal structure stabilize the shell? (Pentamerids are commonly in slightly higher energy settings than most brachiopods).
5. Productids were a late Paleozoic group that invaded reef settings. How do some of their rather unusual morphological features (spines, unequal valves, cementation) related to this environmental shift?

I would point out that there are a couple of major adaptive breakthroughs/trends that occurred within the phylum. One is the development of an articulated hinge that defines the Subphylum Rynchonelliformea. A related trend is the progressive development of an efficient tooth-and-sprocket structure and a shift from strophic to non-strophic shells - this is presumably a more efficient articulation structure. Another area involves the pedical area (recall all the terminology). Initially the pedical was between the valves (linguliforms and craniiforms). This was also the case in primitive rynchonelliforms (recall some have both a delthyrium and a notothyrium) - in Cambrian orthids, the size of the opening was fixed. However, this changed during the evolution of the group. Later brachiopods were able to modify the geometry around the pedical foramen (thus the terms) and shifted the pedical opening to the pedical valve; modern terebratulids reabsorb shell around the foramen to enlarge the opening as needed.

The study of "benthic assemblages" dates back to Zeigler (1965) and Zeigler and others (1968) - Boucot (1975) summarized the major findings. Your text gives an abbreviated summary (p. 188-192). After you read it, read over Zeigler's (1965) short paper - it will give you a good sense of what this is about. Finally, Clarkson closes with an useful and straightforward summary of the uses of brachiopods to define faunal provinces and biostratigraphic zones (p. 192-194).