The Huenellidae

Researchers who deal with the modern marine fauna are used to thinking of brachiopods as a marginal group, their diversity greatly overshadowed on a global scale by the superficially similar bivalves. However, modern brachiopods are but a shadow of their former selves; for much of the Palaeozoic era, their relationship with the bivalves was the inverse of today. Many are the brachiopod lineages that came and went over this time.

External views of ventral (left) and dorsal valves of Huenella triplicata, from Walcott (1924).

The Huenellidae were an assemblage of brachiopods that lived during the late Cambrian and early Ordovician (Amsden & Biernat 1965). They represent early representatives of the Pentamerida, a Palaeozoic order of fairly generalised-looking brachiopods. Within the Pentamerida, they fall within the suborder Syntrophiidina. Syntrophiidinans as a whole are rarely found in the fossil record and as a result remain poorly known. Members of the suborder share a distinctive shape with biconvex valves marked by a dorsal fold and ventral sulcus. That is, the midline of the shell is raised above either side with the ventral valve forming a 'valley' to match the raised 'hill' of the dorsal valve. What, if anything, was the purpose of this arrangement I wouldn't know but modern brachiopods often inhabit locations with a lot of organic silt and/or fine sediment. Perhaps the uneven level of the syntrophiidinan shell helped protect it from burial by a shifting substrate.

Interior view of ventral valve of Radkeina taylori, from Laurie (1997), with scoop-shaped spondylium at upper midline.

Families of Syntrophiidina may be distinguished based on the development of the spondylium, an internal projection at the base of the ventral valve that provided an attachment site for the shell muscles. Members of the Huenellidae possessed either a sessile spondylium or a pseudospondylium, a spondylium-type structure rising from the internal surface of the valve itself rather than from the hinge. Amsden & Biernat (1965) recognised a division of the huenellids between two subfamilies based on the development of the brachiophore plates, projections on the inside of the dorsal valve that would have supported the lophophore. Members of the Huenellinae possessed more developed brachiophores than members of the Mesonomiinae. Outer ornament of the huenellid shell varied from more or less smooth with weak concentric ridges to costate with distinct radiating ridges.

Phylogenetic relationships within the Syntrophiidina do not seem to have been established in detail but the early appearance in the fossil record of huenellids at least raises the question of whether they included the ancestors of later families. As well as other families of the Syntrophiidina, candidates for descent would include members of the suborder Pentameridina as well as of the related order Rhynchonellida. This latter order includes species which survive to the present day so the possibility exists that while the huenellids themselves may be long gone, their legacy may yet live on.

REFERENCE

Amsden, T. W., & G. Biernat. 1965. Pentamerida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 2 pp. H523–H552. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas Press: Lawrence (Kansas).

The Oligorhynchiidae

Dorsal view of Oligorhynchia subplana gibbosa, from Cooper (1935).

From Oligochiton, we move onto Oligorhynchia. The Oligorhynchiidae are a family of very small brachiopods known from the Middle and Late Ordovician. They were among the earliest representatives of the Rhynchonellida, a major group of brachiopods that survives to the present day. Rhynchonellidan shells are usually characterised by a strong beak associated in life with a well-developed pedicel. In oligorhynchiids, this beak is suberect and the shell as a whole is an elongate subtriangular shape. The valves of the shell are folded into coarse plicae (ridges). At least towards the base of the shells, the major folds are in what is called an inverted arrangement, with a ridge in the dorsal valve matched by a valley in the ventral valve (Schmidt & McLaren 1965). Other structural features defining the group include small plates projecting into the pedicel opening, distinct vertical dental plates and divided hinge plates in the valve articulation, and the usual absence of a median septum or cardinal process inside the shell (Savage 1996).

The oligorhynchiids first arose in the east of what was then the continent of Laurentia (corresponding to modern North America). They subsequently spread across the Iapetus Ocean to the continents of Baltica and Kazakhstan (Jin 1996). The end of the Ordovician saw their replacement by other rhynchonellid families. Nevertheless, their genetic lineage would continue for some time yet as they have been identified as ancestors of later families: the Trigonirhynchiidae and Camarotoechiidae (Jin 1989). The brief oligorhynchiid spark would blossom into later rhynchonellid success.

REFERENCES


Jin, J. 1989. Late Ordovician–Early Silurian rhynchonellid brachiopods from Anticosti Island, Quebec. Biostratigraphie du Paléozoïque 10: 1–127, 130 pls.

Jin, J. 1996. Ordovician (Llanvirn–Ashgill) rhynchonellid brachiopod biogeography. In: Copper, P., & J. Jin (eds) Brachiopods pp. 123–132. CRC Press.

Savage, N. M. 1996. Classification of Paleozoic rhynchonellid brachiopods. In: P. Copper, & J. Jin (eds) Brachiopods pp. 249–260. CRC Press.

Dalmanellidae

The photo above (copyright Dave), may or may not show Dalmanella, a brachiopod originally described from the later Ordovician of Sweden. Dalmanella belongs to the Orthida, one of the earliest groups of articulate brachiopods to appear in the fossil record ('articulate' meaning that the two valves of the shell are hinged together, not that they are particularly well spoken). The Dalmanellidae, the family to which Dalmanella belongs, are known from the lower Ordovician to the lower Carboniferous (Williams & Wright 1965).

Over the years, numerous fossil brachiopods from Europe and North America have been assigned to Dalmanella, leading Jin & Bergström (2010) to describe it as "perhaps one of the most commonly reported orthide brachiopods". However, if truth be told, the main reason Dalmanella is so widely recognised is because of how perfectly unremarkable it is. It is small and unspecialised, and the genera within Dalmanellidae have mostly been separated by somewhat vague characters such as shell shape and ribbing pattern. Some studies of variation in dalmanellid populations have questioned whether characters used to separate genera can even be used to separate species or whether they may vary within a single population.

This uncertainty lead Jin & Bergström (2010) to restudy the original type species of Dalmanella, D. testudinaria. Their conclusion was that D. testudinaria was morphologically distinct from North American species attributed to the genus: for instance, the midline of the dorsal valve bore an interspace (the furrow between two costae) in D. testudinaria but a raised costa in the American species. The myophore, a process associated with the hinge to which the muscles responsible for opening the shell would have attached in life, is much narrower in D. testudinaria than in the American species. Not only were the morphologically distinct, they were ecologically distinct as well: D. testudinaria being found in cooler, deeper waters while the American species basked in tropical shallows. Not for the first time, it appears that an external sameyness masks an internal divergence.

REFERENCES

Jin, J., & J. Bergström. 2010. True Dalmanella and taxonomic implications for some Late Ordovician dalmanellid brachiopods from North America. GFF 132 (1): 13–24.

Williams, A., & A. D. Wright. 1965. Orthida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 1 pp. H299–H359. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas Press: Lawrence (Kansas).