The specimen in the following photographs is a slab of Ordovician March (Theresa) Formation sandstone collected in Lanark County, Ontario about half way between Perth and Smiths Falls. The slab is about 14 cm long by 8.5 cm wide by 1.8 cm thick. The sand that makes up the bulk of the slab is much coarser and a darker colour than the fine grained tan surfaces of the top and bottom of the slab.
The fine grained surface of one side of the slab preserves a number of discoidal impressions with diameters from 5 to 16 mm. The slab was loose on the ground when I picked it up, so I can’t be certain that the discoidal impressions are preserved on a bed top or a bed sole. However, I believe this to be a bed sole, largely because the other surface looks more like a bed top. The first two photos show the same side of the slab. In the second photo I’ve place white circles over some, but not all, of the discoidal impressions. The circle that I’ve marked with the number ‘1' is interesting because it is more of a dark colouration than an impression. The gradations on the ruler are in millimeters.
These non-mineralized discoidal impressions resemble each of the following:
- [A] scyphozoan medusae [true jellyfish or "true jellies"] (Hagadorn, Dott and Damrow (2002, Figure 3, D, E), Hagadorn and Belt (2008, Figure 6); Hagdorn ( 2015), Young & Hagadorn, (2020); Lacelle, Hagadorn & Groulx (2008))
- [B] blisters and fluidization structures produced by dewatering or degassing of underlying sediment or organic matter, often trapped by a microbial mat (Bottjer and Hagadorn (2007, Figure 4(a), 1, 2), Dornbos, Noffke, and Hagadorn (2007, Figure 4(d)-2, C, D, E , F), Hagadorn and Miller (2011, Figure 4 b))
- [C] fossil eldonids (MacGabhann and Murray (2010); Schroeder, Paterson and Brock (2018))
The three options appear equally likely, because weak radial furrows are present at the edges of some of the discs in the top photo, but lack definition. Interestingly, if the discs record eldonids, true jellies, or similar creatures, then a linear feature shown in the photographs could be a tentacle such as one sees in present day jellyfish or on some jellyfish fossils. One ‘tentacle’ comes out of the black disc near the bottom of the photographs.
I considered and rejected both discoidal Siphonophores and dicoidal Chondrophorina (capitate) as they are just weird - Lieberman et al. (2017). (The most familiar siphonophore is the Blue Bottle or Portuguese Man-o-war.)
The paper by Schroeder, Paterson & Brock (2018) includes photographs of two disc-shaped fossils, one with preserved concentric corrugations (called concentric lines, rays, radial furrows, or ridges by other authors), from the lower Cambrian Emu Bay Shale of South Australia, which they identify as Eldonioids Their more complete specimen has a diameter of 20.9 mm. Their more complete specimen looks like the disc-shaped specimens in my photographs, but with better corrugations . The disc that I've marked '2', and the disc on its left, have the best corrugations, but the corrugations are hard to see in my photographs.
Below is a photograph of the side of the slab that I believe is the bed top.
Christopher Brett
Ottawa
References and Suggested Reading
Bottjer, D., and Hagadorn, J. W., 2007
Mat growth features, chapter 4(a), P. 53-71, in Atlas of Microbial mat features preserved within the Clastic Rock Record. Elsevier, 450 pages. See Figure 4(a), 1, 2.
Brett, Christopher, 2019 A Concentric Circular Structure in Rocks of the Ottawa Embayment that are Mapped as the Ordovician Gull River Formation. Blog posting Tuesday, 22 October 2019.
http://fossilslanark.blogspot.com/2019/10/a-concentric-circular-structure-in.html
An occurrence of radially symmetric sedimentary structures in the basal Ediacaran cap
dolostone (Keilberg Member) of the Otavi Group, EarthArXiv, pages 26-38
http://www.mme.gov.na/files/publications/Crockford_et_al2021_Keilberg%20Member%20sedimentary%20structures.pdf
https://eartharxiv.org/repository/view/2393/
Dornbos, S.Q., Noffke, N., and Hagadorn, J. W., 2007,
Mat-decay features, chapter 4(d), P. 106-110, in Atlas of Microbial mat features preserved within the Clastic Rock Record. Elsevier 450 pages . See Figure 4(d)-2, C, D, E , F.
Fryer, Geoffrey and Stanley, George D. Jr., 2004
A Silurian porpitoid hydrozoan from Cumbria, England, and a note on porpitoid relationships. Palaeontology, Vol 47:1109–1119. doi: 10.1111/j.0031-0239.2004.00402.x.
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.0031-0239.2004.00402.x
https://www.readcube.com/articles/10.1111%2Fj.0031-0239.2004.00402.x
Hagadorn, James W. and Randall F. Miller, 2011
Hypothesized Cambrian medusae from Saint John, New Brunswick, reinterpreted as sedimentary structures. Atlantic Geology Volume 47, 2011, p. 66–80
https://www.erudit.org/en/journals/ageo/1966-v2-n1-ageo47/ageo47art02/
Hagadorn, J.W., and Belt, E.S., 2008
Stranded in upstate New York: Cambrian medusae from the Potsdam Sandstone.
Palaios, 23, pp. 424–441. doi:10.2110/palo.2006.p06-104r
Hagadorn, J.W., Dott, R.H., Jr., and Damrow, D. , 2002.
Stranded on a Late Cambrian shoreline: medusae from central Wisconsin. Geology, 30, pp. 147–150. doi:10.1130/0091-7613(2002)030<0147:SOALCS>2.0.CO;2
Hagdorn, Hans, 2015
Wirbellose des Lettenkeupers. [photos of Hydrozoen-Medusen „Medusina“ atava
(POHLIG, 1892)] In book: Der Lettenkeuper -- Ein Fenster in die Zeit vor den Dinosauriern (pp.107-140)Edition: Palaeodiversity SonderbandChapter: 7. Publisher: Staatliches Museum für Naturkunde Stuttgar
http://www.palaeodiversity.org/pdf/08Suppl/07Palaeodiversity_SB_Hagdorn.pdf
Kimmig, Julien; Helena Couto, Wade William Leibach, Bruce Lieberman, 2019
Soft-bodied fossils from the upper Valongo Formation (Middle Ordovician: Dapingian-Darriwilian) of northern Portugal. The Science of Nature 106(5-6):27 DOI:10.1007/s00114-019-1623-z
Lacelle, M.A., Hagadorn, J.W., & Groulx, P. (2008)
The Widespread Distribution of Cambrian Medusae: Scyphomedusae Strandings in the Potsdam Group of Southwestern Quebec. Geological Society of America Abstracts with Programs. 2008;40:369
Lieberman,Bruce; Richard Kurkewicz, Heather Shinogle, Breandán Anraoi MacGabhann 2017
Disc-shaped fossils resembling porpitids or eldonids from the early Cambrian (Series 2: Stage 4) of western USA. PeerJ 5(6):e3312 DOI:10.7717/peerj.3312
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5463991/
Lucas, Spencer G., and Allan J. Lerner, 2017
The rare and unusual pseudofossil Astropolithon from the Lower Permian Abo Formation near Socorro, New Mexico . New Mexico Geology, Volume 39, Number 2, pages 40-42 [CPB: probably fossil medusa; somewhat like Seputus pomeroii MacGabhann and Murray, and Discophyllum peltatum Hall. See figures in MacGabhann, 2012 ] https://geoinfo.nmt.edu/publications/periodicals/nmg/39/n2/nmg_v39_n2_p40.pdf
MacGabhann, B. A., 2012
A Solution to Darwin's Dilemma: Differential Taphonomy of Ediacaran and Palaeozoic Non-Mineralised Discoidal Fossils. Earth and Ocean Sciences, National University of Ireland, Galway, Ireland 1, 657 pages
MacGabhann, Breandán Anraoi and John Murray, 2010
Non-mineralised discoidal fossils from the Ordovician Bardahessiagh Formation, Co. Tyrone, Ireland. January 2010 Irish Journal of Earth Sciences 28:1-12. DOI:10.3318/IJES.2010.28.1
https://www.jstor.org/stable/25780702
https://www.researchgate.net/publication/235764014_Non-mineralised_discoidal_fossils_from_the_Ordovician_Bardahessiagh_Formation_Co_Tyrone_Ireland
MacGabhann, B.A., Murray, J., and Nicholas, C. 2007.
Ediacaria booleyi: weeded from the Garden of Ediacara?. Geological Society of London Special Publication, 286, pp. 277–295.
MacGabhann, Breandán ; Schiffbauer, James; Hagadorn, James ; Van Roy, Peter ; Lynch, Edward ; Morrsion, Liam ; Murray, John, 2015
The taphonomy of unmineralised Palaeozoic fossils preserved as siliciclastic moulds and casts, and their utility in assessing the interaction between environmental change and the fossil record
EGU General Assembly 2015, held 12-17 April, 2015 in Vienna, Austria. id.15384
2015EGUGA..1715384M
MacGabhann,B. A., J. Schiffbauer, James W.Hagadorn, PeterVan Roy, Edward P.Lynch, Liam Morrison, John Murray, 2019
Resolution of the earliest metazoan record: Differential taphonomy of Ediacaran and Paleozoic fossil molds and casts. Palaeogeography, Palaeoclimatology, Palaeoecology Volume 513, 1 January 2019, Pages 146-165 https://par.nsf.gov/servlets/purl/10125488
DOI:10.1016/J.PALAEO.2018.11.009Corpus ID: 135003752
Pickerill, R.K., and Harris, I.M., 1979
Reinterpretation of Astropolithon hindii Dawson 1878. Journal of Sedimentary Petrology,
49, pp. 1029–1036.
https://doi.org/10.1306/212F78AB-2B24-11D7-8648000102C1865D
Schroeder, Natalie; John Paterson and Glenn A Brock, 2018
Eldonioids with associated trace fossils from the lower Cambrian Emu Bay Shale Konservat-Lagerstätte of South Australia. Journal of Paleontology , Volume 92 , Special Issue 1: Cambrian Explosion , January 2018 , pp. 80 - 86 DOI: https://doi.org/10.1017/jpa.2018.6
https://www.researchgate.net/figure/Eldonioid-from-the-early-Cambrian-Emu-Bay-Shale-Kangaroo-Island-South-Australia-SAM_fig1_322611581
Walcott, Charles D., 1914.
No. 3 Middle Cambrian Holothurians and Medusae, pp. 41 -68, pls 8-13, in Cambrian geology and paleontology, II.Smithsonian Miscellaneous Collections, Volume 57, Smithsonian publication 2136
https://archive.org/details/smithsonianmisce571914smit/page/n164
Young, Graham A. & James W. Hagadorn, 2020
Evolving preservation and facies distribution of fossil jellyfish: a slowly closing taphonomic window. Bollettino della Società Paleontologica Italiana, 59 (3), 2020, 185-203. Modena
http://paleoitalia.org/media/u/archives/02_Young__Hagadorn_2020_BSPI_593_WJoiJAU.pdf
Zhu, Mao-yan, Yuan-Long Zhao, Jun-Yuan Chen, 2002
Revision of the Cambrian discoidal animals Stellostomites eumorphus and Pararotadiscus guizhouensis from South China. Geobios 35 (2002) 165–185
https://www.academia.edu/17380660/Revision_of_the_Cambrian_discoidal_animals_Stellostomites_eumorphus_and_Pararotadiscus_guizhouensis_from_South_China
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