To the FWGNA group:
The Planorbidae is the most diverse of the basommatophoran pulmonate
families, including hundreds of species and dozens of higher taxa
worldwide. Planorbids are the most successful freshwater
pulmonates in the topics, the notorious Biomphalaria, Bulinus, and Indoplanorbis serving as the
intermediate hosts of schistosomiasis in both the Old World and the
New. It is a point of pride for us in North America, therefore,
that the classification of this family generally recognized around the
world today was heavily influenced by the work of a hometown boy, Frank
Collins Baker of Urbana, Illinois (1).
Here we pick up a thread left dangling somewhat over a year ago, the
life and career of F. C. Baker (1867-1942). Baker cultivated a
comprehensive knowledge of freshwater pulmonate taxonomy, great skills
as an anatomist, and a tremendous feel for the living organisms to
which he devoted his life. And his (1945) "The Molluscan Family
Planorbidae" was a masterwork (2).
Baker originally conceived of his monograph in two parts - a systematic
review of the anatomy and classification of all higher planorbid taxa
worldwide, recent and fossil, and a survey of the nominal species
inhabiting the Americas. Although only a skeletal 16 pages of
text for the second half of the project had been completed at the time
of his death, together with a phenomenal 140 plates, the first half was
sufficiently complete for his editor (H. J. van Cleave) to carry to
publication posthumously.
Baker left us lovely anatomical illustrations and detailed
morphological observations for 81 species and races of planorbids
representing diverse taxa worldwide, together with ranges (both
geological and geographical) and species lists. On this basis he
proposed a classification of the family recognizing 4 subfamilies, 36
genera and 18 subgenera.
We have noted previously that Baker's taxonomy remained firmly rooted
in 19th-century typology throughout his career. The quality of
the science in his 1945 monograph of the Planorbidae was not
substantially different from that in his 1911 treatment of the
Lymnaeidae. But what made the 1945 work so special was its
worldwide scope. Baker offered detailed anatomical observations
for Pingiella and Polypylis from China, Intha and Indoplanorbis from India, and Planorbis, Anisus, Segmentina and Hippeutis from Europe, and included
seven higher taxa known only as fossils. The work should be
better known today than it is.
For our modern understanding of planorbid
systematics has developed in close parallel to (but lagging slightly
behind) our understanding of the Lymnaeids. And just as Baker's
(1911) lymnaeid monograph was supplanted by Bengt Hubendick's (1951)
masterpiece (3), so too was
Baker's (1945) monograph of the Planorbidae supplanted by Hubendick in
1955 (4).
With his greater access to the African and Eurasian faunas and his
skill with thin-section microscopy, the Riksmuseum's Bengt Hubendick
was able to explore planorbid anatomy down to the finest detail.
The classification he proposed ten years after Baker's was explicitly
evolutionary - postulating ancestral and derived character states and
hypothesizing phylogenetic relationships (At left, note 5). But rather
than setting aside all the earlier classifications and starting afresh,
as he had done with the lymnaeids four years previously, Hubendick
built directly upon the foundation that F. C. Baker had laid. He
wrote, "It is not my intention to give a complete account of the
morphology of the different planorbids. Baker (1945) has already
presented a comprehensive monograph on the subject." So taking
"the recent genera accepted by him as a starting point," Hubendick was
able to re-monograph the entire family Planorbidae in just 90
pages. Hubendick's monograph of the Lymnaeidae, a much smaller
family with but two genera, had required 223 (6).
Hubendick criticized Baker rather severely for neglecting two major
planorbid groups, the African Bulinus
and the South American Plesiophysa.
And indeed Hubendick's classification began by recognized three
subfamilies - the Planorbinae, the Bulininae, and the Plesiophysinae -
the latter two of which Baker had omitted. But within the
Planorbinae, Hubendick recognized 31 genera gathered into 6 - 9
"tribes," broadly agreeing with Baker's genera and subfamilies.
Hubendick did not advocate subgenera.
Later in his life, Hubendick (1978) suggested that the ancylid limpets
be might united with the planorbids into a gigantic
"Ancyloplanorbidae," but this idea never caught on, at least in its
proposed form (7). The
concept has recently been revived by a number of molecular phylogenetic
studies, which tend to confirm that the planorbids and the ancylids are
both paraphyletic and interdigitated (8).
The classifications implied by molecular data are jarringly different
in some respects from those that have been established by common
practice over the last 50 years (9),
and time will be required to see what new system stabilizes.
Meanwhile, back in North America, Burch (10) adopted the Hubendick (1955)
classification with a couple minor tweaks and one big shove. He
subsumed the genus Armiger
under Gyraulus following
Meier-Brook (11), and substituted the older name Vorticifex for the younger synonym
used by Hubendick, Parapholyx
(12). In addition, Burch
also advanced a rather dramatic change to the genus Helisoma that has baffled me and
many of our colleagues for quite a few years.
Both Hubendick and Baker recognized the North American genus Helisoma as a large, natural
group. Baker sorted roughly 77 species and subspecies of Helisoma (too many!) into four
subgenera: Planorbella, Seminolina, Pierosoma, and Helisoma (ss). Burch trimmed
the specific nomina down to about 17. Then without explanation or
attribution, he raised Planorbella
to the genus level and diverted 16 of the species (from three of
Baker's former subgenera) into it. To the single species of Helisoma left behind (H. anceps) Burch added the single
species of the Baker/Hubendick genus Carinifex,
C. newberryi. No
rationale was offered for any of the Helisoma
rearrangements whatsoever.
During the course of my research for last month's essay, however, I
stumbled across a clue to Burch's "mystery of the exploded Helisoma" - the 1966 monograph that
Dwight Taylor published on the Plio/Pleistocene mollusk faunas of the
American West (13). At
the end of that lengthy work, in his section entitled "taxonomic
notes," Taylor raised Baker's Planorbella
to the genus level and removed the Helisoma
exactly as Burch was to advocate ten years later, offering as his
rationale the apparent axis of shell coiling (14). Baker's 530 pages of
anatomical observations seem to have been dismissed by Taylor, and he
was apparently unaware of Hubendick's monograph entirely (15). Variance on a single
shell character was sufficient for D. W. Taylor to explode the genus Helisoma, and apparently Burch
found Taylor's evidence convincing.
The FWGNA project has adopted a (slightly updated) version of the
Hubendick (1955) system for the classification of the (roughly 45)
species of planorbids inhabiting North America, preserving Helisoma and Carinifex as recognized by
Baker. We do not mean to imply that a 50-year-old hypothesis is
(or could be!) the definitive model of planorbid evolution.
Indeed, we think it quite likely that future malacologists may adopt
some classification that combines the ancylids and planorbids along the
lines that Hubendick himself foresaw in 1978. But for now, this
is it:
The Classification of
the Planorbidae
And we'll keep in touch,
Rob
Notes
(1) The Legacy of Frank Collins Baker
(2) Baker, F. C. (1945) The Molluscan Family Planorbidae.
University of Illinois Press, Urbana. 530 pp.
(3) The Classification of the Lymnaeidae
(4) Hubendick, B. (1955) Phylogeny in the Planorbidae.
Trans. Zool. Soc. London 28: 453-542.
(5) Hubendick's "synoptic diagram" is depicted above (click the image
for an enlargment).
Although one certainly sees evolutionary trees proposed for fossil taxa in
papers of this era, I do think that Hubendick's construction of
phylogenetic trees for entirely modern taxa was much ahead of his time.
(6) But in fairness, the Hubendick (1951) lymnaeid monograph went down
to the species level. Neither Baker nor Hubendick seems to have
contemplated reviewing the immense worldwide diversity of planorbids
any lower than the genus.
(7) Hubendick, B. (1978) Systematics and comparative
morphology of the Basommatophora. pp 1 - 47 in "Pulmonates,
Volume 2A," (V. Fretter & J. Peake, eds). Academic Press, New
York.
(8) Morgan, J.A.T. et al. (2002) A phylogeny of planorbid
snails, with implications for the evolution of Schistosoma parasites. Mol.
Phylogenet. Evol. 25: 477-488.
Jorgensen, A., T. K. Kristensen & J. R. Stothard (2004) An
investigation of the "Ancyloplanorbidae" (Gastropoda, Pulmonata,
Hygrophila): preliminary evidence from DNA sequence data. Molec.
Phylogenet. Evol. 32: 778-787.
Albrecht, C., K. Kuhn & B. Streit (2007) A molecular
phylogeny of Planorboidea (Gastropoda, Pulmonata): Insights from
enhanced taxon sampling. Zoologica Scripta 36: 27 - 39.
(9) Albrecht and colleagues propose that Bulinus and Indoplanorbis be split out into a
separate Bulinidae, and that the most of the ancylid limpets might best
be considered a subfamily within an enlarged Planorbidae.
(10) Burch originally proposed his classification for the North
American freshwater gastropods in 1978 (Journal de Conchyliologie 115:
1-9). His "North American Freshwater Snails" was published as an
EPA manual in 1982, as three volumes of Walkerana (1980, 1982, 1988),
and as a stand-alone book in 1989.
(11) Meier-Brook, C. (1979) The planorbid genus Gyraulus in Eurasia.
Malacologia 18: 67 - 72.
Meier-Brook, C. (1983) Taxonomic studies on Gyraulus (Gastropoda:
Planorbidae). Malacologia 24: 1 - 113.
(12) Parapholyx (Hanna 1922)
was apparently a bit more prominent in Hubendick's day, but Vorticifex (Meek 1870) clearly has
priority. See Burch's Note #60.
(13) Taylor, D. W. (1966) Summary of North American Blancan
nonmarine mollusks. Malacologia 4: 1 - 172.
(14) While understanding that the group he called "Planorbella" bore sinistral shells,
Taylor considered the shells of Carinifex
and Helisoma anceps to be
dextral. In point of fact, all planorbids are embryonically
sinistral, but the shells of some species flip over their backs
("hyperstrophically") so as to appear more or less dextral in adults.
(15) Actually, I suspect this was an intentional snub. See the
illuminating anecdote about Hubendick in the D. W. Taylor obituary
upcoming in Malacologia.
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