Mackinnonia anabarica, from Parkhaev (2005).

Belongs within: Helcionellida.

Who knows which way the water flows?
Published 3 November 2016
Dorsal and lateral views of specimens of Trenella bifrons, from Parkhaev (2001).

There is no question that the molluscs are one of the most significant groups of animals in the marine environment. And thanks to the production by many species of mollusc of a hard shell, they are also one of the best-known groups in the fossil record. A rich and detailed picture of molluscan evolution is available to us as far back as the earliest Cambrian. But, of course, the further back in time we go the more questions we have about what the picture means. And it is in the earliest part of their history that the picture becomes the most opaque.

The Trenellidae are part of that early picture. This family of molluscs is known from the early Cambrian (Parkhaev 2002). They are part of the assemblage of early molluscs referred to as the helcionelloids, whose overall position in the molluscan family tree is very much open to question. Helcionelloids are simple, more or less cap-shaped or cone-shaped shells that are usually also tiny. The type species of the Trenellidae, Trenella bifrons, for instance, is only about 1 to 1.5 millimetres along the longest axis, and only one-half to one millimetre tall (Parkhaev 2001). This all adds up to a general shortage of morphological details that might help us pin down which, if any, modern molluscan groups helcionelloids are connected to. Possession of a undivided dorsal shell has lead many to compare them to gastropods. Others have pointed to the monoplacophorans like the modern Neopilina. In both cases, though, the resemblance is fairly superficial and confirming things one way or another would depend on identifying features of the soft anatomy, such as torsion, that are difficult if not impossible to infer from features of the shell alone.

Within the helcionelloids, trenellids are characterised by having the lower rim of one end of the shell’s long axis drawn out into a siphonal groove. It seems likely that this groove was somehow involved in the passage of water around the gill(s), but whether its position indicates the front end or the back end of the shell, and whether it was used to draw water in or expel water out, depends again on what each author expects its original soft anatomy to have been. Unfortunately, evidence for the latter in trenellids is almost completely non-existent; while muscle scars have been identified in some helcionelloids, they remain unknown for this family.

The Trenellidae are closely related to, and probably include the ancestors of, the Yochelcionellidae in which the siphonal groove become raised and closed ventrally, turning it into a snorkel-like structure. However, comparing trenellids to yochelcionellids raises something of a question in my mind. In general, mollusc shells grow through secretion from the mantle around the shell’s rim only, meaning that once shell growth has passed a certain section the mollusc usually cannot go back and rearrange it. Assuming that helcionelloids grew in the usual molluscan manner, surely yochelcionellids would have gone through a stage in their development before the lower part of the ‘snorkel’ was closed off where they looked a heck of a lot like a trenellid? Is it even possible to distinguish a mature trenellid from a juvenile yochelcionellid?

Systematics of Securiconidae
Securiconidae [Trenellidae]
| i. s.: ‘Helcionella’ terraustralis Runnegar & Jell 1976P02
| ‘Latouchella’ merino Runnegar & Jell 1976P02
| ‘Latouchella’ accordionata Runnegar & Jell 1976P02
| ‘Latouchella’ comma Geyer 1986P02
| ‘Latouchella’ holmdalense Peel 1988P02
| ‘Latouchella’ pearylandica Peel 1988P02
| ‘Securiconus’ costulatusP08
|--Udzhella Vassiljeva 1990P02
|--Oelandia Westergård 1936P02
| |--O. acutacostaK52
| |--O. pauciplicataRJ76
| `--O. rugosaW01
|--Horsegullia horsegulliensisP08
|--Prosinuites Poulsen 1967P08, P02
|--Parailsanella Zhegallo in Voronina et al. 1987 [incl. Bemellina Vassiljeva 1998]P02
| `--P. murenicaP08
|--Trenella Parkhaev 2001P02
| `--*T. bifrons Parkhaev 2001BR17
|--Xianfengella He & Yang 1982 [incl. Obscurella Vassiljeva 1990, Rugaeconus Vassiljeva 1990; Rugaeconidae]P02
| `--*Rugaeconus’ ipatovi Vassiljeva 1990BR17
|--Securiconus Jiang 1980BR17, P02
| `--*S. simus Jiang 1980BR17
|--Leptostega Geyer 1986P05
| |--*L. irregularis Geyer 1986P05
| `--L. hyperborea Parkahev 2005P05
`--Mackinnonia Runnegar in Bengtson et al. 1990P02
|--M. rostrata (Zhou & Xiao 1984) (see below for synonymy)P05
|--M. anabarica Parkhaev 2005P05
`--M. plicata (Missarzhevsky 1989) [incl. Leptostega corrugata Runnegar in Bengtson et al. 1990]P05

Mackinnonia rostrata (Zhou & Xiao 1984) [=Mellopegma rostrata; incl. Bemella anhuiensis Zhou & Xiao 1984, B. costa Zhou & Xiao 1984, *Mackinnonia davidi Runnegar in Bengtson et al. 1990, Ma. obliqua Landing & Bartowski 1996]P05

*Type species of generic name indicated


[BR17] Bouchet, P., J.-P. Rocroi, B. Hausdorf, A. Kaim, Y. Kano, A. Nützel, P. Parkhaev, M. Schrödl & E. E. Strong. 2017. Revised classification, nomenclator and typification of gastropod and monoplacophoran families. Malacologia 61 (1–2): 1–526.

[K52] Knight, J. B. 1952. Primitive fossil gastropods and their bearing on gastropod classification. Smithsonian Miscellaneous Collections 117 (13): 1–56, 2 pls.

Parkhaev, P. Yu. 2001. Trenella bifrons: a new helcionelloid mollusk from the Lower Cambrian of South Australia. Paleontological Journal 35 (6): 585–588.

[P02] Parkhaev, P. Yu. 2002. Phylogenesis and the system of the Cambrian univalved mollusks. Paleontologicheskii Zhurnal 2002 (1): 27–39 (translated: Paleontological Journal 36 (1): 25–36).

[P05] Parkhaev, P. Yu. 2005. Two new species of the Cambrian helcionelloid mollusks from the northern part of the Siberian Platform. Paleontologicheskii Zhurnal 2005 (6): 43–46 (translated: Paleontological Journal 39 (6): 615–619).

[P08] Parkhaev, P. Y. 2008. The Early Cambrian radiation of Mollusca. In: Ponder, W. F., & D. R. Lindberg (eds) Phylogeny and Evolution of the Mollusca pp. 33–69. University of California Press: Berkeley.

[RJ76] Runnegar, B., & P. A. Jell. 1976. Australian Middle Cambrian molluscs and their bearing on early molluscan evolution. Alcheringa 1 (2): 109–138.

[W01] Wagner, P. J. 2001. Gastropod phylogenetics: progress, problems and implications. Journal of Paleontology 75 (6): 1128–1140.

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