Saturday, September 19, 2015

Graptolites: gone yesterday, here today?

Graptolites are one of the less-heralded members of the local Ordovician menagerie, probably because they don't seem to be all that common around here. Graptolites were colonial animals (and I use "were" and other past-tense terminology with implied quotation marks, because of the shocking twist yet to come). The colonies, called "rhabdosomes", are made up of branches called "stipes", which support cup-like structures called "theca" that housed the individual animals. (See also here for official terminology.) The overall effect is that stipes often resemble saw blades, with the "teeth" being the theca. Early graptolites were apparently attached to the seafloor and formed densely branching (dendritic) colonies (see for example several of the specimens here and here), whereas later forms were apparently planktonic and attached to floats of their own device or other floating things, such as seaweeds. The rapid taxonomic turnover of graptolites, coupled with the ease of distribution for planktonic forms, make them excellent index fossils. Planktonic distribution also has the neat side effect of getting graptolites into rock formations that otherwise lack much for fossils, usually because of low oxygen levels in the water column precluding a great deal of life while the formation was being deposited. While low oxygen would limit bottom dwellers, it wouldn't stop things from drifting through higher up the water column. Fossils of graptolites are found from the Cambrian into the Carboniferous. This may not be where the story ends, though: it has been known for a while that graptolites are most similar to pterobranchs, a living group of tube-dwelling often-colonial worm-like hemichordates. There is evidence that pterobranchs are, in fact, living graptolites (e.g. Mitchell et al. 2013; Discovery Magazine ran an article on this topic back in 1993). Should this be the case, it would appear that only the planktonic forms truly disappeared; seafloor forms persisted, albeit in much reduced circumstances (only a handful of pterobranch genera and species are known). All of the species reported from the Twin Cities appear to be of the planktonic persuasion.

As mentioned, graptolites are an uncommon component of the local fauna. They do pop up from time to time, though, and their thin dark saw blade fossils are quite distinctive if you get a large enough chunk. Reports of graptolites in the Twin Cities go back almost to the beginning of local geology, actually. Shumard (1852) reported graptolites from a bluff about a half-mile downstream from Fort Snelling, in what would now be considered the lower Platteville Formation. Winchell and Schuchert (1895) provided illustrations for three species, Climacograptus typicalis, Diplograptus pristis?, and Diplograptus putillus. All of their cited specimens came from outside the metro, and apparently from higher stratigraphically than virtually all metro rocks, with C. typicalis from what would now be the Cummingsville Formation and the other two from what would now be the Dubuque Formation, per the stratigraphic table in Winchell and Ulrich (1897). Stauffer (1930) found abundant black fragments of graptolites in a particularly calcareous layer low in the Decorah Shale in rocks recovered from a heating shaft dug for Northrop Auditorium. Stauffer and Thiel (1941) observed C. typicalis in what would now be the Mifflin Member at Lock and Dam 1, on the Ramsey County side. Their lists reported three following species in the Platteville Formation and Carimona Member (their Spechts Ferry). All three were listed under Hydrozoa, the group that includes relatives of corals and jellyfish, which was a reasonable enough hypothesis at the time (before the discovery of pterobranch affinities, graptolite relationships were a free-for-all):

Climacograptus typicalis (Pl, Ca)
Climacograptus (Mesograptus) putillus (Pl)
Diplograptus amplexicaulis? (Pl)

(More graptolites are known from older rocks a bit farther afield in Minnesota. Ruedemann [1933] described several species from the Upper Cambrian St. Lawrence Formation of Afton.)

From Winchell and Schuchert (1895). This looks kinda like at least one of the examples in the following photo, but you should always be wary of diagnosing your fossils via photos and plates.

A University of Minnesota piece with a few graptolites (look for the things that look like saw blades on sticks. Mouse-sized hacksaws.).

References:

Mitchell, C. E., M. J. Melchin, C. B. Cameron, and J. R. Maletz. 2013. Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite. Lethaia 46(1):43–56.

Ruedemann, R. 1933. The Cambrian of the upper Mississippi Valley, part III, Graptolitoidea. Milwaukee Public Museum Bulletin 12(3):307–348.

Shumard, B. F. 1852. Geological report of local, detailed observations, in the valleys of the Minnesota, Mississippi, and Wisconsin rivers, made in the years 1848 and 1849, under the direction of David Dale Owen, United States Geologist, by B. F. Shumard, head of subcorps. Pages 481–531 in Owen, D. D. Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Lippincott, Grambo & Co., Philadelphia, PA. Available at http://archive.org/details/mobot31753000174885 (plates not included), https://archive.org/details/reportofgeologi00owen (full plates) or http://books.google.com/books?id=Y_ZYAAAAYAAJ.

Stauffer, C. R. 1930. Conodonts from the Decorah Shale. Journal of Paleontology 4(2):121–128.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Bulletin 29.

Winchell, N. H., and C. Schuchert. 1895. Sponges, graptolites, and corals from the Lower Silurian in Minnesota. Pages 55–95 in Lesquereux, L., C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(1). Johnson, Smith & Harrison, state printers, Minneapolis, MN.

Winchell, N. H. and E. O. Ulrich. 1897. The lower Silurian deposits of the Upper Mississippi Province: a correlation of the strata with those in the Cincinnati, Tennessee, New York and Canadian provinces, and the stratigraphic and geographic distribution of the fossils. Pages lxxxiii–cxxix in L. Lesquereux, C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

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