Paleonet: Unidentified microfossils?

Timothy Palmer [tjp] tjp at
Thu Mar 30 08:35:15 GMT 2017

...and you might like to look at papers by Rutzler and Macintyre from the late 1970s.  I recall work on the Belize reef, and one of the papers was in Marine Biology.  The genus was Placospongia.  They showed the gradual etching of these microscleres down to bean-shaped selenasters.  I later encountered them in micropal samples picked by a long-forgotten student

Tim Palmer

From: paleonet-bounces at <paleonet-bounces at> on behalf of Trenton J. Ryan <tr86 at>
Sent: 30 March 2017 06:35
To: Joseph Botting; paleonet at
Subject: Re: Paleonet: Unidentified microfossils?

Thank you for the suggestions everyone. Gwen got me pointed in the right direction, and Joe confirmed my findings. Thanks Gwen and Joe!! They are "adult" sponge sterrasters from the Geodia genus of sea sponges

On Wed, Mar 29, 2017 at 9:48 PM Joseph Botting <acutipuerilis at<mailto:acutipuerilis at>> wrote:
Hi Trenton,
    Yes, indeed it is a sponge - these are sterrasters from a geodiid demosponge. Very distinctive microscere spicules!
See, e.g.,
All the best,

From: Trenton J. Ryan <tr86 at<mailto:tr86 at>>
To: paleonet at<mailto:paleonet at>
Sent: Thursday, 30 March 2017, 0:36
Subject: Paleonet: Unidentified microfossils?


I am working on a masters thesis and have found numerous examples of a thus far unidentified microfossil? and have taken some SEM photomicrographs of them, found here:

The specimens in question were found in Northern California in the late Miocene Saint George Formation  (sandy mudstone). Occurrence of them seems to be limited to the shelly deposits within the formation, as they are not found in the strata between prominent shell beds. The macrofossils in the shell beds are mostly made up of Macoma inquinata, Macoma secta, and less frequent Solen sp., Clinocardium sp., and Nassarius sp.. The only forams that I have found in the shell beds are Elphidium sp., which are rare.

Under a regular light microscope and when wet, the unidentified specimens appear translucent with a more opaque, milky-white central 'nucleus'. This 'nucleus' accounts for ~20% of the total volume per specimen and is not apparent (visually) when dry. All of them have an ovate form, and do not vary much in terms of size. They are likely siliceous, as they do not react to HCL.  As you can see in the SEM images, they also exhibit an intricate stellate pattern.

Any ideas on what they could be? Some suggestions thus far are octocoral sclerites, dinoflagellate cysts, or fossilized pollen.

Trenton Ryan
tr86 at<mailto:tr86 at>
Humboldt State University
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