Dr. Rob Dillon, Coordinator

Monday, February 10, 2020

Malacological Mysteries: What was Marstonia ozarkensis?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023b)  The SNHTHICACBW Marstonia VI: ozarkensis.  Pp 261 – 268 in The Freshwater Gastropods of North America Volume 6, Yankees at The Gap, and Other EssaysFWGNA Project, Charleston, SC.

All I know about the late Marstonia ozarkensis is what I’ve read in the newspapers.  The diminutive hydrobiid, small-bodied even by Marstonia standards, was originally described as “Pyrgulopsis ozarkensis” by A. A. Hinkley in 1915 [1] from the North Fork of the White River in north-central Arkansas, about 20 miles from the Missouri line.  Hinkley’s single-paragraph description focused entirely on the 3 mm shell (H, below), offering no anatomical observations on the animal itself, indeed no biological notes of any sort, beyond “found in shallow water on the bedrock.”

The taxon receded into utter obscurity (as opposed to mere obscurity) for 80 years, listed by Burch [2] but not figured.  Hershler [3] essentially reprinted Hinkley’s original description in his 1994 monograph with no additional observations, stating “A limited survey of this region in 1991 – 1992 did not yield this species.”  The scanning electron micrograph image of an ANSP paratype published by Hershler (a, below) was a poor likeness [4].

Marstonia ozarkensis: Hinkley [1], Hershler [3], INHS cotype.
In 1997 Wu and colleagues [5] reported the discovery of a population of P. ozarkensis in the North Fork of the White River in southern Missouri, about 45 km upstream from Hinkley’s type locality.  In 2002 Thompson & Hershler [6] resurrected the genus Marstonia and assigned Hinkley’s ozarkensis to it.  And in 2007 Christian & Hayes [7] reported a population of Marstonia ozarkensis in Mud Creek, a tributary of the Black River about 115 km east of Hinkley’s type locality.

Marstonia ozarkensis was one of the 404 “aquatic, riparian and wetland species from the Southeastern United States” listed in the megapetition filed by the Center for Biological Diversity in 2010 [8].  In response, the US Fish & Wildlife Service declared Marstonia ozarkensis “presumed extinct” in December of 2018 [9]

The authors of the FWS “Species Status Assessment” filed in August of 2018 [10] were unable to confirm the 1997 report of Wu and colleagues, finding that “the museum records cited for this population are not present in the museum database.”  And after some hemming and hawing about high levels of endemicity in North American hydrobioids, they concluded that the Mud Creek population “may be a morphologically similar undescribed species (D. Hayes, pers. comm.).”  Thus, to quote the 18Dec18 FWS press release [9] verbatim: 
Following rigorous, science-based surveys, the Ozark pyrg, a small snail native to Arkansas and Missouri, is presumed extinct, according to the U.S. Fish and Wildlife Service. No Ozark pyrgs have been confirmed in surveys since their first discovery in 1915. As a result of today’s finding, the pyrg will not be listed under the Endangered Species Act (ESA).
That sounds like the end of the story, doesn’t it?  For the Feds, it probably is.  But for us, not so much.

In late February of last year I was pleased to receive an email from Ms. Rachel Vinsel, the Manager for the Illinois Natural History Survey mollusk collection, with attached tif images of two hydrobiid shells collected in a wetland south of Chicago.  They were clearly Marstonia, but not of any morphology with which I was personally familiar.  So I pulled my trusty copy of Hershler (1994) out of the files and started thumbing through it, looking for clues to the identity of Rachel’s unknowns.  And my eyes were drawn to the poor likeness [4] of Marstonia ozarkensis reproduced at the top of this blog post.  Hershler’s figure looked like a possible match to the images Rachel had sent me.  And heck, it isn’t all that far from Chicago to Arkansas, is it?

But if I have learned anything from many years of misadventure as a jpeg naturalist, it is that there is no substitute for the actual specimens in hand.  And I happened to have a copy of the INHS freshwater gastropod holdings on my hard drive.  And I discovered that there are several lots of bona fide Marstonia ozarkensis in the INHS collection, collected by Hinkley himself in 1914.

So the next day I replied to Rachel, suggesting that her Chicago-area unknowns might represent a rediscovery of M. ozarkensis, but simultaneously emphasizing that she dig some of the bona fide M. ozarkensis out of the INHS collection and compare.  And on 25Feb19 she sent me a tif file depicting a really interesting series of little hydrobiids, featuring both her Chicago area unknowns and the INHS cotypic M. ozarkensis.

The “INHS” image at the far right of the photomontage that opened this essay was clipped from the photo Rachel sent me 25Feb19.  Three revelations struck me almost simultaneously.  First, and least importantly, Hershler’s figure, the only illustration I had seen to that point, didn’t look anything like bona fide M. ozarkensis.  Second, our Chicago-area unknowns didn’t look anything like bona fide M. ozarkensis either [11].  But thirdly and most importantly, the bona fide M. ozarkensis in the INHS did look a whole lot like something I had seen before.

And Rachel was not done.  There were additional tif files attached to her email of 25Feb19, including one that would challenge even that tiny little bit of knowledge I thought I might have been able to glean about the late Marstonia ozarkensis.  Tune in next time for, “Is Marstonia ozarkensis extinct?”


[1] Hinkley, A.A. (1915) New Fresh-water Shells from the Ozark Mountains. Proceedings of the United States National Museum, 49:587-589.  This is actually the 1916 volume of the PUSNM, but Hinkley’s date of publication is given as “December 23, 1915” in the index.

[2] This is a difficult work to cite.  J. B. Burch's North American Freshwater Snails was published in three different ways.  It was initially commissioned as an identification manual by the US EPA and published by the agency in 1982.  It was also serially published in the journal Walkerana (1980, 1982, 1988) and finally as stand-alone volume in 1989 (Malacological Publications, Hamburg, MI).

[3] Hershler, R. (1994) A review of the North American freshwater snail genus Pyrgulopsis (Hydrobiidae).  Smithsonian Contributions to Zoology 554: 1-115.

[4] Actually, to be fair.  It seems entirely possible to me that M. ozarkensis may have demonstrated a variety of shell form, to include the relatively robust form depicted by Hershler as well as the gracile depicted by Hinkley.  And possibly carinate forms as well, like Pyrgophorus or Potamopyrus?  Maybe even to the extreme of Marstonia scalariformis, perhaps?  But let's not get ahead of ourselves.

[5] Wu, S-K, R. D. Oesch & M. E. Gordon (1997) Missouri Aquatic Snails.  Missouri Department of Conservation, Jefferson City. 97 pp.

[6] Thompson, F. G. & R. Hershler (2002) Two genera of North American freshwater snails: Marstonia Baker, 1926, resurrected to generic status, and Floridobia, new genus (Prosobranchia: Hydrobiidae: Nymphophilinae).  The Veliger 45: 269 - 271.

[7] Christian A. D. & D. M. Hayes (2007) Diversity and distribution of freshwater gastropods from the Ozark region of Arkansas.  Report submitted to the Arkansas Game and Fish Commission.  34 pp.

[8] Center for Biological Diversity. 2010. Petition to list 404 aquatic, riparian and wetland species from the Southeastern United States as threatened or endangered under the Endangered Species Act.  For more, see:
  • Megapetitions of the Old West [14July09]
  • Megapetitions II: Armistice Day?  [18May11]
[9] The 12/2018 findings of the USFWS on Marstonia ozarkensis can be read here:
  • Federal Register: Endangered and threatened wildlife and plants; 12-month findings on petitions to list 13 species as endangered or threatened species [19Dec18]
  • Press Release: Ozark snail species presumed extinct following science-based surveys [18Dec18]
[10] U.S. Fish and Wildlife Service. 2018. Species status assessment report for the Ozark pyrg (Marstonia ozarkensis). 6Sept18. Atlanta, GA.  [pdf]

[11] Those two little shells of which Rachel initially sent me photos are not the subject of this essay.  I’m still not 100% sure what they were, but I think Rachel and I have settled on weirdly-fat and weirdly-dark Marstonia lustrica, and it doesn’t matter for our purposes here, anyway.