A fossil carabid beetle from Colorado.

The Paleontological Gulf

A fossil carabid beetle from Colorado.
“Harpalus” redivivus (USNM-63444, holotype), a fossil ground beetle (Family Carabidae) from the late Eocene Florissant Formation of Colorado, USA. Photo: Herbert W. Meyer/U.S. National Museum of Natural History.

Today’s post is something a little different. Normally I avoid weighing in publicly on the business of doing science; I just have a Master’s in Museum Studies, and not a single paper to my name. But I came across an illustration today of a scientific issue near to my heart. I’m very interested in other people’s thoughts on this issue and–perhaps–ways to solve it in the interest of better scientific communication and collaboration.

A coworker recently pointed me at Yves Bousquet’s new monograph on the Geadephaga (comprising three–or two, depending on who you ask–families of beetles), Catalogue of Geadephaga (Coleoptera: Adephaga) of America, north of Mexico, an incredible and delightfully Open Access resource that I am very grateful for, especially as it came out at a really useful time for me at work [1].

But I’m going to pick a little paleontological nit, because it provides an apt example of the disconnect between the two disciplines most connected to paleontology: modern biology and earth sciences. In Appendix 1, Bousquet provides a list of known fossil species of Geadephaga. And I came across this:

Nebria occlusa Scudder, 1900a: 17. Type locality: «Florissant [Teller County], Colorado» (original citation), from the early Oligocene period. Holotype [by monotypy] in MCZ [# 1755]. Note. 1. Although noted originally as from the Miocene period, most authors agree that the shales of the Florissant Basin are from the Early Oligocene (Wilson 1978: 17). 2. According to Kavanaugh (1978: 275), the holotype probably belongs to the genus Amara.

“Most authors agree”? I thought, with some shock. You see, I spent three years working part-time at Florissant Fossil Beds National Monument, and another three years in graduate school working on Florissant plant fossils for part of my thesis. I’ve read a large chunk of the Florissant literature, and since my thesis in particular dealt with the estimated temperatures and ages of the Florissant Formation and several other Colorado fossil deposits, I can say with confidence that most authors now agree that the Florissant Formation was deposited 34.07 million years ago, at the end of the Eocene Epoch [2].

I looked up the reference: Wilson MVH (1978) Paleogene insect faunas of western North America. Quaestiones Entomologicae 14: 13-34.

And that explains why the reference is from 1978–it’s not a reference to a geoscience article about stratigraphy or radiometric dating, as I would expect in a paleontological context, but a reference to an entomological journal. In the early days of taxonomic work at Florissant–and on many fossil floras and faunas–a lot of workers who described non-vertebrate fossils were both paleontologists and modern taxonomists, and often the latter first. Consequently, they published in general science journals or in botanical and entomological journals, and this trend continued well into the 20th century.

At some point however, a great deal of the paleontological literature shifted to earth science and specialized paleontology journals. Paleontologists began collaborating more and more with stratigraphers and other geoscientists; many have relatively little experience with the modern analogues of their fossil organisms of interest. These days, I get the impression that more paleontologists come from geology backgrounds than biology backgrounds. In one sense this is good: today’s paleontologists tend to have a much stronger grasp of stratigraphy and geological context, as well as taphonomy (how fossils are or are not preserved). On the other hand, less background in biology comes with drawbacks; paleoecologists have not adopted quantitative methods as quickly as their counterparts working on living species, and taxonomy of fossil organisms is often an afterthought in need of major revisions.

I am not surprised that a taxonomist who works on modern groups might not have searched geology journals for more recent publications on the age of the Florissant Formation; I suspect many paleontologists wouldn’t necessarily think of digging through back issues of biology journals for information on fossil species.

In the case of Florissant, a number of things have happened since 1978 that make that paper not the best choice to cite for the age of the Florissant fossils.

  1. In 1993, the stratotype (in biology, a type specimen defines a species; in stratigraphy, a type section of rock defines a geologic boundary) for the Eocene-Oligocene boundary was formally defined by Premoli Silva and Jenkins in Massignano, Italy, with the boundary itself estimated via radiometric dating at 34 million years ago [3]. The boundary is more precisely understood than it was in the 1970s.
  2. Paleontologists (particularly paleobotanists) and other geoscientists have produced a lot of papers trying to pinpoint the Eocene-Oligocene boundary and describe the transitions of floral and faunal communities across it. Most of these papers have been published in paleontology or geoscience journals.
  3. Specifically, a number of different methods have been used to determine the age of the Florissant Formation itself. Radiometric dating using 40Ar/39Ar produced a date of 34.07 ± 0.10 million years ago [4, 5]. This was the first radiometric age published for the deposition of the Florissant Formation since Epis and Chapin 1975 (an age of 34.9 million years ago, which I believe at the time was considered early Oligocene) [6]. Examination of the fossil mammal assemblage indicated a North American Land Mammal Age of Chadronian, latest Eocene [7, 8]. The latest Eocene age for Florissant has also been corroborated by some paleomagnetic work, but I can’t find the reference at the moment.

I don’t think anyone currently working on Florissant would argue that it’s an early Oligocene deposit: the floral and mammalian assemblages fit pretty clearly into what you’d expect for late Eocene North America. The plants of the slightly younger (33.7-30.5 Ma) Antero Formation, while not as abundant for comparison as I would have liked when I was working on my thesis, are already showing some shift towards a more temperate-adapted flora, corroborated by recent pollen evidence [9]. Certainly they are very different from the solidly Oligocene Creede flora (26.9-26.3 Ma).

But why does this matter? We’re only talking about a couple million years, not much in geological time. Is the difference between latest Eocene and earliest Oligocene really that important?

Yes. The Eocene-Oligocene transition marked a time of relatively abrupt global cooling, according to marine records (terrestrial records, at least in North America, may indicated slightly more gradual cooling, geologically speaking–but there’s a reason this has been such a major paleobotanical research topic in the last 30 years). So if, for example, someone wanted to look at how composition communities of carabid beetles in North America changed during this transition, perhaps trying to correlate this with climatic shifts, it would be important to know whether a particular fossil assemblage was deposited before or after the Eocene-Oligocene boundary.

In addition to the old citation for the age of the Florissant Formation, Bousquet did not appear to be aware of Frank M. Carpenter’s 1992 contribution to the Treatise on Invertebrate Paleontology, covering Hexapoda, which might have provided some additional information about the most recent thinking on the taxonomy of the Geadephaga of Florissant and other North American fossil localities. While I don’t have this publication myself, Florissant paleontologist Herbert W. Meyer, in his 2003 book The Fossils of Florissant, does discuss Carpenter’s 1992 compilation of fossil insects, and notes that Carpenter questioned the generic assignments of many of the fossil insects at Florissant, including nearly all of the carabids [10].

I don’t blame Bousquet for not digging into the more recent paleontological literature for a footnote to an appendix; science has become a network of subdisciplines that are often opaque to scientists in different subdisciplines, and the art of literature searching doesn’t necessarily carry over from one to another. I think it’s great that Bousquet decided to include an appendix of known fossil species at all; I don’t think this is standard practice in monographs of this nature. I wonder, though, if he might have missed some other species, or more recent paleoentomological papers, perhaps ones hidden in geoscience journals outside his scope of expertise. His appendix of fossil species is a fine starting point–but what if there’s more?

I think it’s unfortunate that paleontology has become so embedded in the geosciences that there’s less communication with modern biology–unfortunate for both disciplines. Modern ecological and taxonomic techniques have a great deal to offer paleontology, which at least in some subdisciplines has been slow to adopt quantitative methods. At the same time, fossil evidence can provide another set of data to phylogenetic research, complementing taxonomic and molecular studies of living species. The gulf between biological study of living species and paleontological study of fossil species isn’t ideal for either field.

In order for information and techniques to be exchanged, scientists have to be able to access each other’s literature, to have cross-discipline discussions, to share methods and figure out how to adapt them to different scenarios–and this doesn’t seem to be happening as well as it might.

References

[1] Bousquet, Yves. 2012. Catalogue of Geadephaga (Coleoptera: Adephaga) of America, north of Mexico. In: ZooKeys, volume 245, special issue, 1-1722 p.

[2] Barton, Melissa A., and Dena M. Smith. 2009. Floral diversity and climate change in central Colorado during the Eocene and Oligocene. In Geological Society of America Abstracts with Programs, Vol. 41, No. 7, p. 561.

[3] Premoli Silva, Isabella, and D. Graham Jenkins. 1993. Decision on the Eocene-Oligocene boundary stratotype. In Episodes, volume 16, number 3, pp. 379-382.

[4] Evanoff, Emmett, William C. McIntosh, and Paul C. Murphey. 2001. Stratigraphic summary and 40Ar/39Ar geochronology of the Florissant Formation, Colorado. In Fossil Flora and Stratigraphy of the Florissant Formation, Colorado, Proceedings of the Denver Museum of Nature & Science, Series 4, number 1, pp. 1-16.

[5] McIntosh, William C., and Charles E. Chapin. 2004. Geochronology of the central Colorado volcanic field. New Mexico Bureau of Geology & Mineral Resources Bulletin 160, pp. 205-237.

[6] Epis, R.C., and C.E. Chapin. 1975. Stratigraphic nomenclature of the Thirtynine Mile volcanic field, central Colorado. U.S. Geological Survey Bulletin 1395-C, 23 p.

[7] Worley-Georg, M. P. and J.J. Eberle. 2006. New additions to the Chadronian mammalian Fauna, Florissant Formation, Florissant Fossil Beds National Monument, Colorado. In Journal of Vertebrate Paleontology volume 26, issue 3, pp. 685-696.

[8] Lloyd, Karen J., Marie P. Worley-George, and Jaelyn J. Eberle. 2008. The Chadronian mammalian fauna of the Florissant Formation, Florissant Fossil Beds National Monument, Colorado. In: Paleontology of the Upper Eocene Florissant Formation, Colorado, Geological Society of America Special Paper 435, p. 117-126.

[9] Meyer, H.W., E.B. Leopold, D.M. Smith, and M.A. Barton. 2012. Paleobotanical and pollen evidence from the Antero Formation (Colorado, USA) for climate and floral change during the Eocene-Oligocene transition. In: Abstracts, 13th International Palynological Conference and 9th International Organisation of Palaeobotany Conference, Japanese Journal of Palynology, Special Issue 58, p. 150-151.

[10] Meyer, Herbert W. 2003. The Fossils of Florissant. Washington DC: Smithsonian Books, 258 p.

Note: Herb Meyer and Dena Smith have both been my supervisors at various points; Smith was my graduate advisor and Meyer was on my thesis committee. I have also met or corresponded with a number of the other authors I cite. The opinions in this blog post are my own, and do not represent those of my employers past or present or those of any of the authors cited here.

I believe, as a photograph of a specimen owned by the U.S. government, taken by a U.S. government employee on official duty, that the fossil carabid image at the top of the post is free to use for non-commercial purposes. If I am incorrect, I will be happy to take it down.

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