In both articulate and inarticulate species, the shape, extent and position of the the hingeline may vary. It may be long or short, and straight or curved, and at the back edge of the dorsal shell or set forward from it. In articulate species, when the hingeline is straight and parallel to the posterior margin of the shell, the species is referred to as strophic. This condition is especially evident in species belonging to the order Strophomenida (see image, right). All other forms are saisd to be astrophic.
|
Left: a stophic species (of the order Strophomenida). Right, an astrophic species (a Terebratulid)
|
Another common and important feature is the presence of a sulcus on the ventral valve (image, right). This is a deep and wide groove, running from the beak of shells and splaying out towards the rim. Species that show the presence of a sulcus are known as sulcate. The sulcus may be bounded by raised areas (folds). In some species matching sulci (the plural of sulcus) form opposite each other on either shell, in which case the commissure remains straight. In others, the sulcus is matched by a corresponding fold on the opposite (dorsal) shell, in which case the commissure will be undulating. Several species also carry spines, both for protection and for achorage in the soft sediments in which they lived. On fossils, the spines have often been lost, but the stubs may be evident, and in some species can be diagnostic. In addition, some species display small 'pimples' on their surface, usually arranged in a more-or-less regular way (see image right). These are known as punctae, and can be extremely useful in identification, but they are not always easy to see and are best viewed with a hand lens. |
Ribs, sulci and folds
Punctae on a brachiopod shell
|
A key feature of any brachiopod species is, of course, the shape of the two valves, and of the organism as a whole. In cross section, shapes vary from almost circular to bi-convex (both shells convex), to plano-convex (one flat shell and one convex shell) to concavo-convex (one convex and one concave shell) - see image right.
|
The outline (i.e. viewed from above or below) varies from almost circular, through oval (with the longer direction either from front to back or side to side), to elongate shapes almost like a pair of wings (see image right). In many orders of brachiopod, shape is a useful diagnostic, but in some species the shape is naturally variable because of the impact of environmental conditions. Shape also varies between the ventral and dorsal valves (a defining characteristic of brachiopods) so this needs to be allowed for when trying to identify species.
Size varies, too, of course, but is often not reliably diagnostic because juvenile specimens are inevitably relatively small. It is most helpful when several specimens are available, giving some indication of the average size of the population (at least at the stage at which they die). Warning also has to be given about the apparent sizes often implied by drawings or photographs of bivalves (as in the image to the right). It helps when a measure of magnification is given - but we need to remember that the image may not have bern reproduced at its original scale! |
Left: Rhynconella (Recent), showing exterior (top) and interiors of the ventral (middle) and dorsal (bottom) valves. Note the presence of teeth and the distinct foramen.
Below: Stropheodonta (Devonian), showing the ventral (top) and dorsal (bottom) valves. Note the straight hingeline and the well-developed interarea on the ventral valve. |
Order: Athyridida
Athyrids lived from the Ordovician to the Jurassic, and in New Zealand were especially common in the Triassic, when considerable morphological development occurred, and several new species emerged. The distinctions between the Triassic species has not been fully resolved, however, and they are not always easy to tell apart. They've thus provided considerable challenge, and much entertainment, in developing this collection. Athyrids are articulate species and are usually oval to round in shape, with plano-convex profiles and smooth shells. They are have a complex spiral brachidium and retain their pedicle after maturation, so the foramen is present throughout their life (note the small hole in the specimens shown to the right). |
Order: Terebratulida
Terebratulid species first appeared in the Devonian and have survived to the present day. They are articulate species, typically smooth-shelled and bulbous in shape, with biconvex shells. They are pedunculate (i.e. have a pedicle) and the pedicle opening (foramen) is often clearly defined (see image, right). They have a straight or folded commissure, and the brachidium, when visible, can be seen to be looped and may be either long or short. |
Order: Rhynchonellida
Rhynchonella (Rhynchonelliformea) species are amongst the most long-surviving brachiopods in the world, having developed in the Ordovician and continued to the modern day. They are quite distinctive. They are bulbous (bi-convex) and usually have strong ribbing, with a deep sulcus that gives them a markedly zig-zag (plicated) commissure. They are articulate species, and their hingeline is curved (astrophic), short and often comes to a point. They lack a brachidium. They are usually quite small (<15 mm in diamater) but their shells are robust, so they survive well as fossils. |
Order: Spiriferida
The Spiriferida is a highly variable order of brachiopods, but also includes some of the most interesting and beautiful species we are likely to find. The Spiriferida are articulate species, and many have a long, straight (strophic) hingeline, giving them a winged shape. Ribbing is often distinctive and may be deep. Some also have a deep sulcus and corresponding fold. |
Order: Strophomenida
Strophomenida were a relatively short-surviving order of brachiopods, appearing and disappearing during the Palaeozoic. During their brief reign, however, they were for some time one of the most proliific groups in the world. As their name suggests, they are characterised by a very straight - i.e. strophic - hingeline. They tend to have shallow, plano-convex or concavo-convex profiles, with large, semicircular to oval outlines. Their ribbing is delicate and fine. They are articulate species, and internally show distinctive dental socket patterns and a well-defined interarea. Juveniles had a pedicle, and thus show a foramen, but as they matured the pedicle was lost and the foramen closed. |
Order: Productida
Productida (also known as Strophemonata) lived during the Palaeozoic, surviving from the Ordovician through to the Permian. In New Zealand, they make their first appearance in Devonian rocks, but flourished during the Permian, when almost fifty species emerged. The shells are large, and concavo-convex or plano-convex, or more rarely conical. They are articulate species, usually with a straight (strophic) and elongated hingeline. Most had spines either on the ventral valve or on both vales, Some were infaunal - i.e. buried below the surface; others anchored themselves to hard surfaces. |
Order: Orthida
Orthids were amongst the first of the rhynchonelliform brachiopods to diversify, and first appeared in the Cambrian. They were globally at their peak in the Ordovician and survived until the Permian. In New Zealand, however, few records have been made before the Devonian, when they became relatively common. They are articulate species with a biconvex profile and a sub-circular to oval outline. The shells are often strongly ribbed with a strophic (straight) hingeline. Many are sulcate, and internally they display a well-developed interarea. |