Identification and taxonomic consistency
Jonathan A. Todd
Department of Palaeontology, The Natural History Museum,
Cromwell Road, London, U.K. SW7 5BD
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:
||Number of families
|| Number of genera and subgenera
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(Todd, unpublished data) and glycymerids (Tschudin, unpublished data) consist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
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
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
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
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.
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.
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In Cox, L.R., Newell, N.D., Boyd, D.W., Branson, C.C., Casey, R.,
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R.C., Nuttall, C.P., Perkins, B.F., Puri, H.S., Smith, L.A., Soot-Ryen,
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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.
Last updated on March 28, 2001-tsa.