Rostroconchia

 Conocardium sp., from here.

Belongs within: Mollusca.
Contains: Conocardioida, Scaphopoda.

The Rostroconchia are a group of molluscs known from the Cambrian to Late Permian. They possess a pseudobivalved shell, with the valves joined by an anterior partition called the pegma that in life also provided the attachment site for the pedal retractor muscles; a posterior pegma may also be present (Prothero 1998). Some authors regard the rostroconchs as ancestral to the modern Scaphopoda; others place them closer to the bivalves (suggestions that they may be ancestral to both are inconsistent with the tendency for most recent phylogenetic studies to not place the two lineages as closely related).

The beak-shells’ legacy
Published 8 August 2011
Specimen of Conocardium japonicum, from here.

The Rostroconchia were a group of molluscs that lived during the Palaeozoic, being definitely found from the Late Cambrian to the latest Permian. The name means, roughly, ‘beak-shell’, and refers to the shape of the shell. Similar to bivalves (with which rostroconchs were classified prior to the 1970s), rostroconchs had a shell divided into left- and right-hand valves, which in many species were elongate in one or both directions forewards and backwards. The shells of different species gaped to varying degrees at the posterior and anterior ends. Rostroconchs differed from bivalves, however, in that the original larval shell was not divided. Instead, the shell of a rostroconch was initially cap-shaped and entire. As the animal developed past the larval stage, the shell developed lateral lobes that would eventually become the two valves. As a result, rostroconchs did not have a toothed hinge connecting the valves like that of bivalves. Nevertheless, in many species, the pressure of the valves growing outwards eventually caused the larval shell to break through (Pojeta & Runnegar 1976). Like many bivalves, rostroconchs would have been infaunal, living buried in the sediment; however, the absence of a toothed hinge or the adductor muscles that connect the valves in bivalves means that the shell of rostroconchs would have been less flexible.

The Ordovician scaphopod Rhytiodentalium (topmost image) and the scaphopod-like rostroconch Pinnocaris (lower two images), from here.

The largest work to date on rostroconchs was that of Pojeta & Runnegar (1976), who suggested that rostroconchs included the ancestors of two living classes of molluscs, the bivalves and the scaphopods. Scaphopods, tusk-shells, are a group of tubular molluscs with an opening at each end of the tube that also live buried in sediment. According to Pojeta & Runnegar’s suggestion, the bivalves evolved through the evolution of two calcification centres in the larval stage. The scaphopods would have evolved through the fusion of the valves along the ventral margin (as embryological studies have shown the scaphopod shell does develop), together with the restriction of growth to the anterior direction only.

However, more recent studies of molluscan phylogeny (Wilson et al. 2010) have mostly indicated that bivalves and scaphopods are not closely related relative to other molluscs. Bivalves are probably a relatively basally derived group, while scaphopods are closer to cephalopods. The upshot of this for rostroconchs is that they may be related to bivalves or scaphopods, but not to both. Of the two, it seems more likely that rostroconchs are related to scaphopods (or, as some authors have put it more bluntly, scaphopods are living rostroconchs). As well as the anatomical arguments that have been made in favour of such a relationships, a scaphopod connection has the advantage over a bivalve one of stratigraphy. The oldest definite rostroconchs, as previously noted, are known from the Late Cambrian. However, the oldest definite bivalves come from the Early Cambrian—that is, some 40 or 50 million years earlier than the first known rostroconchs. Scaphopods, on the other hand, do not appear until the Ordovician or Devonian (depending on whether the earlier forms are accepted as scaphopods), well after the known appearance of their supposed ancestors. Pojeta & Runnegar (1976) did not recognise a stratigraphic conflict in deriving bivalves from rostroconchs as they had identified the Early Cambrian Heraultipegma as a rostroconch. This identification, however, was disputed by MacKinnon (1985), who held that the characters cited by Pojeta & Runnegar in support of their assignation had been misinterpreted and were not truly present in Heraultipegma.

The conocardioid rostroconch Arceodomus longirostris, from here.

Pojeta & Runnegar (1976) recognised three orders of rostroconch: the Ribeirioida, Ischyrinioida and Conocardioida. This division has been followed by all subsequent authors, though it should be noted that Pojeta & Runnegar recognised the Ribeirioida as explicitly paraphyletic with regard to the other orders. Pojeta & Runnegar (1976) regarded the scaphopods as derived from ribeirioids (specifically related to the somewhat scaphopod-like Pinnocaris), but Peel (2004) stated that protoconch characters indicated a derivation from conocardioids. The ribeirioids and ischyrinioids both became extinct at the end of the Ordovician, leaving only the conocardioids and their descendent scaphopods until the former also became extinct at the end of the Palaeozoic.

Systematics of Rostroconchia
<==Rostroconchia
    |--+--ConocardioidaHP03
    |  `--+--ScaphopodaP04
    |     `--JinonicellinaBR17
    |          |--Jinonicella Pokorný 1978 [Jinonicellidae]PR79
    |          |    `--*J. kolebabai Pokorný 1978BR17
    |          `--Janospira Fortey & Whittaker 1976 [Janospiridae]PR79
    |               `--*J. nodus Fortey & Whittaker 1976BR17
    `--RibeirioidaP04
         |  i. s.: Myona Kobayashi 1935RJ76
         |           |--*M. flabelliformis Kobayashi 1935RJ76
         |           `--M. queenslandica Runnegar & Jell 1976RJ76
         |--TechnophoridaeSB93
         |    |--TechnophorusSB93
         |    `--Oepikila cambrica (Runnegar & Pojeta 1974)SB93
         `--RibeiridaeP04
              |--Pinnocaris Etheridge 1878C04, P04
              |    |--P. lapworthi Etheridge 1878P04
              |    |--P. robustaBRW98
              |    `--P. wellsiPR79
              `--Ribeiria Sharpe 1853C04
                   |--R. apusoides Schubert & Waagen 1903HMY02
                   |--R. bassleriCT83
                   |--R. junior Runnegar 1996P04
                   `--R. manchuricaCT83
                        |--R. m. manchuricaCT83
                        `--R. m. pennataCT83
Rostroconchia incertae sedis:
  Isitella recta (Missarzhevsky 1989)D94
  Ischyriniidae [Ischyrinioida]SB93
    |--Pseudotechnophorus typicalis Kobayashi 1933SB93
    |--Eoischyrinia billingsi Kobayashi 1933SB93
    `--IschyriniaSB93
         |--I. schmidti Teichert 1930SB93
         `--I. winchelli Billings 1866SB93
  Euchasma Billings 1865SB93
    |--E. eopteriformeCT83
    `--E. wanwanenseCT83
  Heraultia varensalensis Cobbold 1935RJ76
  WanwaniaCT83
    |--W. ambonychiformisCT83
    `--W. compressaCT83
  WanwanellaCT83
    |--W. altaCT83
    |--W. asiaticaCT83
    |--W. striataCT83
    |    |--W. s. striataCT83
    |    `--W. s. auriculataCT83
    `--W. tumidaCT83
  WanwanoideaCT83
    |--W. delicataCT83
    `--W. trigonalisCT83
  Euchasmella multistriataCT83
  Pseudoeuchasmella typicaCT83
Nomen nudum: Heraultia sibiricaRJ76

*Type species of generic name indicated

References

[BRW98] Beesley, P. L., G. J. B. Ross & A. Wells (eds) 1998. Fauna of Australia vol. 5. Mollusca: The Southern Synthesis. Part A. Australian Biological Resources Study: Canberra.

[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.

[CT83] Chen J.-Y. & C. Teichert. 1983. Cambrian Cephalopoda of China. Palaeontographica Abteilung A: Paläozoologie—Stratigraphie 181 (1–3): 1–102.

[C04] Cope, J. C. W. 2004. Bivalve and rostroconch mollusks. In: Webby, B. D., F. Paris, M. L. Droser & I. G. Percival (eds) The Great Ordovician Biodiversification Event pp. 196–208. Columbia University Press.

[D94] Dzik, J. 1994. Evolution of ‘small shelly fossils’ assemblages of the early Paleozoic. Acta Palaeontologica Polonica 39 (3): 247–313.

[HMY02] Hoare, R. D., R. H. Mapes & T. E. Yancey. 2002. Structure, taxonomy, and epifauna of Pennsylvanian rostroconchs (Mollusca). Journal of Paleontology 76 (Suppl.): 1–30.

[HP03] Hoare, R. D., & L. P. Plas Jr. 2003. Permian rostroconchs (Mollusca) from Nevada. Journal of Paleontology 77 (5): 873–875.

MacKinnon, D. I. 1985. New Zealand late Middle Cambrian molluscs and the origin of Rostroconchia and Bivalvia. Alcheringa 9 (1): 65–81.

[P04] Peel, J. S. 2004. Pinnocaris and the origin of scaphopods. Acta Palaeontologica Polonica 49 (4): 543–550.

Pojeta, J., Jr & B. Runnegar. 1976. The paleontology of rostroconch molluscs and the early history of the phylum Mollusca. U.S. Geological Survey Professional Paper 968: 1–88.

[PR79] Pojeta, J., Jr & B. Runnegar. 1979. Rhytiodentalium kentuckyensis, a new genus and new species of Ordovician scaphopod, and the early history of scaphopod mollusks. Journal of Paleontology 53 (3): 530–541.

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

[SB93] Skelton, P. W., & M. J. Benton. 1993. Mollusca: Rostroconchia, Scaphopoda and Bivalvia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 237–263. Chapman & Hall: London.

Wilson, N. G., G. W. Rouse & G. Giribet. 2010. Assessing the molluscan hypothesis Serialia (Monoplacophora + Polyplacophora) using novel molecular data. Molecular Phylogenetics and Evolution 54 (1): 187–193.

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