Wooster’s Fossil of the Week: An early bryozoan on a Middle Ordovician hardground from Utah

October 10th, 2014

ORBIPORA UTAHENSIS (Hinds, 1970) 072014Last week I presented eocrinoid holdfasts on carbonate hardgrounds from the Kanosh Formation (Middle Ordovician) in west-central Utah. This week we have a thick and strangely featureless bryozoan from the same hardgrounds. It is very common on these surfaces, forming gray, perforate masses that look stuck on like silly putty. Above you see one on the left end of this hardground fragment. (The circular object to the right is another eocrinoid holdfast.)
Kanosh bryo eo 072014Here is a closer view of the bryozoan, again with one of those ubiquitous eocrinoids encrusting it. The holes are the zooecial apertures. Each zooecium is the skeletal component of a living bryozoan individual (zooid). Note that the walls are thick and granular between the zooecia. All the zooecia look pretty much the same, and there are no other structures like spines, pillars or maculae. This is about as simple as a bryozoan gets.

It is impossible to be certain without a thin-section or acetate peel showing the interior, but I’m pretty sure this Kanosh bryozoan is Orbipora utahensis (Hinds, 1970). It matches fairly well the description in Hinds (1970), who named it Dianulites utahensis, and it fits within the redescription by Ernst et al. (2007).

Several years ago we would have called this a trepostome bryozoan and left it at that. These are, after all, the “stony bryozoans” with thick calcite skeletons and long zooecia. However, the group to which Orbipora belongs is unusual because they have no polymorphs (small zooecia different from the primary zooecia) and have granular skeletal textures rather than laminated. We think the granular walls may be because the original skeletons were made of high-magnesium calcite that later altered to low-magnesium calcite and dolomite, losing details of the microstructure. Orbipora is thus in an as yet undescribed new order of bryozoans. [Update: See comment below from Paul Taylor.]

The Kanosh hardgrounds and their attaching faunas are important in geological and biological history because they are telling us something about the geochemical conditions of the seawater when they formed. We think this was a peak time of Calcite Seas, when low-magnesium calcite was a primary marine precipitate and carbon dioxide levels were high in the atmosphere and seawater. Hardgrounds would have formed rapidly because of early cementation, and aragonite and high-magnesium skeletons would have altered soon after death. The abundant Kanosh communities and substrates are critical evidence for these conditions that were superimposed on the Great Ordovician Biodiversification Event (GOBE). We thus have a delightful combination of seawater geochemistry (and, ultimately, the tectonics that controls it) and evolution intertwined in the history of these rocks and fossils.

References:

Ernst, A., Taylor, P.D. and Wilson, M.A. 2007. Ordovician bryozoans from the Kanosh Formation (Whiterockian) of Utah, USA. Journal of Paleontology 81: 998-1008.

Hinds, R.W. 1970. Ordovician Bryozoa from the Pogonip Group of Millard County, western Utah. Brigham Young University Research Studies, Geology Series 17: 19–40.

Marenco, P.J., Marenco, K.N., Lubitz, R.L. and Niu, D. 2013. Contrasting long-term global and short-term local redox proxies during the Great Ordovician Biodiversification Event: A case study from Fossil Mountain, Utah, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 377: 45-51.

Wilson, M.A., Palmer, T.J., Guensburg, T.E., Finton, C.D. and Kaufman, L.E. 1992. The development of an Early Ordovician hardground community in response to rapid sea-floor calcite precipitation. Lethaia 25: 19-34.

2 Responses to “Wooster’s Fossil of the Week: An early bryozoan on a Middle Ordovician hardground from Utah”

  1. Paul Tayloron 11 Oct 2014 at 9:48 am

    Once again, great to see bryozoans feature on the excellent Wooster Blog.

    Just a couple of points.

    ‘Zooecium’ ( I really hate that term) is actually the skeletal component of the zooid, not the container within which the bryozoan zooid sits. My old mentor Pat Cook was neurotic about viewing bryozoans as animals in boxes.

    Secondly, the Order Esthonioporata, within which Orbipora can be classified, was introduced earlier this year by Ma et al. (2014. Studi Trent. Sci. Nat. 94: 153–161).

  2. Mark Wilsonon 12 Oct 2014 at 9:55 am

    Thank you as always, Paul. I modified the definition of a zooid to make it more clear, and I noted in the text that the reader should see your update on the new order. Very helpful!

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