NMITA Molluscs: Identification and taxonomic consistency

Identification and taxonomic consistency

Jonathan A. Todd

Department of Palaeontology, The Natural History Museum,
Cromwell Road, London, U.K. SW7 5BD
J.Todd@nhm.ac.uk

Introduction

In today’s oceans the phylum Mollusca is second only to the Arthropoda in numbers of described species. As many molluscs have well-calcified and taphonomically robust shells, it is unsurprising that they represent the largest phylum of macro-invertebrates in the Panama Palaeontology Project collections, Recent and fossil. To date (October 2000), I have recognized 980 genera and subgenera in 164 families, with this breakdown by class:

Class	Number of families	Number of genera and subgenera
Bivalvia	68	317
Gastropoda	92	647
Scaphopoda	4	16

The classes Cephalopoda and Polyplacophora are currently known from just a couple of genera.

Why genera and subgenera, not species ?

The choice of hierarchical ranks with which to study neotropical molluscan diversity has been dictated to us by an almost total lack of comprehensive systematic revisions of the fossil and Recent members of higher systematic groups across the whole region, or even just within the (present) East Pacific or Caribbean. The numerous papers of E.Vokes on the systematics of Neogene and Recent West Atlantic muricid gastropods and Jung’s (1989) revision of the ‘Strombina group’, gastropod family Columbellidae, stand as two of the very few exceptions. Without group-by-group revisions it is impossible to quantify species-diversity; attempts to do so will only result in false precision and provide misinformation rather than usable data. Regionally, the majority of the species of the gastropod Polystira (Todd, unpublished data) and glycymerids (Tschudin, unpublished data) consist of undescribed species. While Polystira seems to consist of a huge endemic radiation, within neotropical bivalve families the glycymeridids are probably fairly unremarkable in terms of species diversity. Therefore, conservatively we have estimated that less than half the species in PPP collections have been described (Jackson et al., 1999). Indeed, even our Recent collections contain a surprising number of undescribed species.

What are our genera and subgenera ?

We have followed the supraspecific systematics employed in authoritative systematic revisions for recently revised taxa. For example, we follow Ponder (1985) for the genera and subgenera of the gastropod family Rissoidae. However, such revisions are absent for most of the families treated here, with many taxa remaining essentially unrevised or with piecemeal systematic treatment. Consequently, I continuously review the systematics and nomenclature that we use in the light of published systematic works, not just on Neotropical taxa, but those occurring worldwide, Recent and fossil. For both recently revised and unrevised groups my taxonomic philosophy has been to recognize as genera and subgenera the smallest easily diagnosable and consistently recognizable, putative supraspecific clades (at least within the geographic and temporal limits of the study). In doing so I try to ensure that the taxa we list and recognize are monophyletic with respect to other taxa in our collections or that are known to occur fossil or alive in the region. However, in a few families a handful of clearly paraphyletic taxa remain due to lack of recent systematic revision, e.g., “Cantharus”, “Terebra” and “Turritella”. These may be recognized as such from both the inverted commas around the name and the listing of closely related taxa (often subgenera of the genus concerned) in the current systematic listings of gastropods and bivalves. Of course, cladistic analysis of many traditional taxa is likely to show that they are paraphyletic but, to date, very few rigorous studies have been made on the phylogenetic relationships of neotropical molluscs. In choosing morphologically tightly circumscribed groups I have tried to balance our need for maximally useful genealogical units (as close to species proxies as possible) with the pragmatic concerns of allowing relatively rapid and accurate identification.

The need to ensure systematic consistency

Sorely lacking, even at the generic and subgeneric level, are taxonomic compendia covering the whole of the region, even more so the Neogene taxonomic diversity of any one ocean basin. The most comprehensive Recent taxonomic compendia are those for the molluscs of the Tropical West Atlantic (Keen, 1971), and the Atlantic and Pacific coasts of North America (Abbott 1974), both of which are showing their age, and Olsson’s 1961 study of the tropical Eastern Pacific bivalves. For PPP fossil malacofaunas the most recent monograph is the final (sixth) part of Woodring’s ‘Geology and Paleontology of Canal Zone and adjoining parts of Panama’ (1982). Unfortunately, these works show significant discrepancies in both their nomenclature and their recognition and delimitation of supraspecific taxa.

The case of Anadara

A good example is the systematic treatment of species assigned to the ark shell genus Anadara. This genus is regionally abundant and widespread in shallow water sediments from the Miocene through to the Recent. One subgenus (of the 11 neotropical subgenera I currently recognize) is the sixth most abundant mollusc in the PPP Caribbean fossil collections (Jackson et al., 1999). Today, throughout the region, a number of species are harvested for food. Considering that economically important taxa are usually better studied than those which are not, let us compare the systematic treatment of 16 East Pacific species common to the works of Olsson (1961), Keen (1971) and Bernard (1983). Of these only 8 (50%) are assigned to the same subgenus in the three works. This falls to 7 (43%) if we compare the treatments with the systematics adopted by me, which takes account of various published revisions made since (e.g., Noda 1966, Woodring 1973) as well as my own systematic surveys of fossil taxa worldwide. Bernard recognizes 6 subgenera, Keen recognizes 7, Olsson 8 and I recognize 10 for the same species. However, these inconsistencies are trifling compared to those we would obtain if we subjected the same species to the classification used by Newell in the authoritative 1969 volume, ‘Treatise on Invertebrate Palaeontology’ — here it seems only 4 taxa would be necessary.

Systematic consistency: is a single system necessary ?

Though the example of Anadara may seem extreme, the problems of not having a usable consistent systematic framework cannot be underestimated; they extend across all taxa. Simply using the nomenclature employed in regional systematic works without attempting to unify usages through resystematization appears untenable. In 1995 I re-identified the molluscs of some randomly chosen PPP fossil collections and then compared my identifications, which used up-to-date systematics, with those previously made by another identifier using standard regional compendia (e.g., Abbott, Woodring). Typically, I found 30-40% discordance between the entries in the two lists. To a small extent this reflected genuine disagreement on the affinities of particular molluscs, but overwhelmingly it represented inconsistencies in the assigned taxonomic rank (e.g., genus versus subgenus), relative inclusiveness of taxon names, and discrepant current supraspecific systematics and nomenclatures. To minimize all of these problems, we decided to institute an ‘in-house systematics’.

PPP ‘In-House’ systematics, reference collections, and systematic accountability

To reconcile consistency of identification with maintenance of a current systematic treatment of molluscs in PPP collections, we have instituted our own in-house systematics. This follows the latest authoritative revision of any particular taxon at any level, though necessarily there will be a time lag before this is reflected in all our occurrence databases, analysis databases and ‘current’ systematic lists. We recognize a number of putative undescribed taxa at the generic/subgeneric level and these are indicated by means of open nomenclature. I decided to try to maintain up-to-date systematic treatment for a number of reasons;

1) So analytical work based on databased information would use the highest quality data possible.
2) To limit the discrepancies between taxa submitted to NMITA by research workers at any level (e.g., species) with the systematic concepts used elsewhere in the molluscan data in NMITA. A molluscan name or taxonomic concept ideally should be able to be followed throughout NMITA and maintain its content.

In Naturhistorisches Museum Basel and the Smithsonian Tropical Research Institute, Panama City, taxonomic reference collections have been established. These were developed following discussion between myself and Antoine Heitz as the best practical method of ensuring rapid and accurate identifications. The reference collections contain specimens of all taxa currently recognized in the respective collections (Basel: fossil; Panama City: Recent) at a generic/subgeneric level. Where there is a very wide phenotypic variability within a taxon, specimens have been chosen to try to cover the range of morphology. When identifying problematical taxa, the ability to physically compare the specimen at hand with a voucher specimen has proved invaluable. I am drawing-up lists of diagnostic characters delimiting these taxa and accompanying the specimens. These will provide the basis for taxonomic keys which will be developed later. Starting with the bivalves we are imaging the taxonomic reference collections to provide a NMITA ‘virtual museum’, to illustrate current PPP taxonomic concepts. The images will be linked to various molluscan databases, so providing occurrence (stratigraphical and geographical) data for each taxon starting at the generic/subgeneric level and later extending to the species level for recently studied and systematized taxa.

We intend this openly accessible data to provide systematic accountability for the taxonomic names used in analytical papers. I am developing, and the PPP is currently using, the first unified systematic treatment of fossil and Recent, East Pacific and Caribbean molluscs yet attempted.

Identification and coordination

All identifications of Recent molluscs are currently being made by Marcos Alvarez (STRI, Panama) and all fossils by Antoine Heitz (NMB, Switzerland), who is also revising all older identifications to the same standard. I act as systematics coordinator, ensuring that we are all using the same consistent nomenclature and pointing Marcos and Antoine to the authoritative systematic revisions we use wherever possible. I maintain systematics master-lists which are regularly updated and circulated between Basel, London and Panama to minimize nomenclatural inconsistency. In turn, I am provided with lists of previously unrecorded molluscs (with source of the identification) and sometimes images, and I decide upon the nomenclature to be used. A couple of times a year, I visit the labs in Basel and Panama to examine and confirm the identifications of previously unrecorded taxa and to offer opinions on ‘difficult’ and previously unrecorded taxa.

References

ABBOTT, R. T. 1974. American Seashells; the marine Mollusca of the Atlantic and Pacific coasts of North America. Second edition. Van Nostrand Reinhold, New York, 663 pp.

BERNARD, F. R. 1983. Catalogue of the Living Bivalvia of the Eastern Pacific Ocean: Bering Strait to Cape Horn. (Canadian Special Publication of Fisheries and Aquatic Science, 61), vii, 102 pp.

JACKSON, J. B. C., TODD, J. A., FORTNATO, H. and JUNG, P. 1999. Diversity and assemblages of Neogene Caribbean Mollusca of Lower Central America. Pp. 193-230 In, Collins, L. S. & Coates, A. G. (eds). A paleobiotic survey of Caribbean faunas from the Neogene of the Isthmus of Panama. (Bulletins of American Paleontology, 357).

JUNG, P. 1989. Revision of the Strombina-Group (Gastropoda: Columbellidae), fossil and living. Schweizerische Palåontologische Abhandlungen, 111, 298 pp.

KEEN, A. M. 1971. Sea shells of Tropical West America; marine mollusks from Baja California to Peru.. Second edition. Stanford University Press, Stanford. xiv, 1064 pp.

NEWELL, N. D. 1969. Superfamily Arcacea Lamarck, 1809. Pp. 250-264, In Cox, L.R., Newell, N.D., Boyd, D.W., Branson, C.C., Casey, R., Chavan, A., Coogan, A.H., Dechaseaux, C., Fleming, C.A., Haas, F., Hertlein, L.G., Kauffman, E.G., Keen, A.M., LaRocque, A., McAlester, A.L., Moore, R.C., Nuttall, C.P., Perkins, B.F., Puri, H.S., Smith, L.A., Soot-Ryen, T., Stenzel, H.B., Trueman, E.R., Turner, R.D., and Weir, J., eds, Treatise on Invertebrate Zoology. Part N. Volume 1 (of 3). Mollusca, Bivalvia. Lawrence, The Geological Society of America, Boulder and the University of Kansas.

OLSSON, A. A. 1961. Mollusks of the Tropical Eastern Pacific; particularly from the southern half of the Panamic-Pacific Faunal Province (Panama to Peru). Panamic-Pacific Pelecypoda. Paleontological Research Institution, Ithaca. 574 pp.

NODA, H. 1966. The Cenozoic Arcidae of Japan. The Science Reports of the Tohoku University, Sendai, Japan; Second Series (Geology), 38(1): 161 pp. + 14 pls.

PONDER, W. F. 1985. A review of the genera of the Rissoidae (Mollusca: Mesogastropoda: Rissoacea). Records of the Australian Museum, Supplement 4, 221 pp.

WOODRING, W. P. 1973. Geology and Paleontology of Canal Zone and adjoining parts of Panama. Description of Tertiary mollusks (Pelecypods: Propeamussidae to Cuspidariidae; additions to Families covered in P 306-E; additions to gastropods; cephalopods). U.S. Geological Survey Professsional Paper, 306-F: iii, 541-759 + pls 83-124.

________________. 1982. Geology and Paleontology of Canal Zone and adjoining parts of Panama. Description of Tertiary mollusks (additions to gastropods, scaphopods, pelecypods: Nuculidae to Malleidae). U.S. Geological Survey Professsional Paper, 306-E: 453-539 + pls 67-82.