Saturday, September 10, 2022
Thursday, July 21, 2022
Friday, April 15, 2022
Wednesday, March 9, 2022
|An internal mold of an ammonite chamber. Left, lateral view, right, apertural view|
Two views of an internal mold of one of the chambers from the phragmocone of an ammonite (Prionocyclus macombi Meek 1876). The chamber was hollow and probably filled with gas and a little liquid while the animal was alive, and was probably empty of any solids after it died. How long it sat empty on the seafloor, and while it was being buried under sediment is unknown. Eventually, it was filled with what we have here, dark sparry calcite crystals. I imagine the crystals grew from the inner walls of the chamber towards the center until it was completely filled. You can see the siphuncle at the top of the chamber (the keel here is missing), and on the bottom, in the apertural view, you can see a notch where the keel of the preceding whorl would have fit.
Ammonites preserved in limestone or concretions are more often found this way because the surrounding shell was preserved uncrushed as the matrix around the shell hardened. Those found in sandstone or shale many times have the phragmocone and most of the chambers crushed unless the chambers were filled with sediment.
Sunday, December 5, 2021
Because the Cephalopods were so different, Rousseau H. Flower wrote proposing the Canadian System with four divisions of unspecified rank (Flower 1957 p. 17). Quoting from Flower:
"Faunally, the Canadian is relatively isolated from the Ordovician above and is characterized by stocks of its own. In the cephalopods..., the Canadian-Ordovician boundary involves changes never lower than of generic rank, and generally of the rank of families and orders."
Someone looking at only, or mostly Cephalopod fossils could see the column this way. The four divisions are areas where Flower collected cephalopods. The Cassinian from the Ft. Cassin area in Vermont, the Jeffersonian from the Adirondacks in NY, the Demingian from the Florida Mts. in New Mexico, and the Gasconadian from the Ozark Mt. area of Missouri. I don't think his later collections in Utah and Nevada changed anything. His boundary is just above Zone K, which is basically the Hesperonomiella minor bed (e.g. Li and Droser 1999) or the base of the Whiterockian Stage of the Ordovician.
|Chart modified from Flower 1964 (p. 23, fig. 3) Ross-Hintze Zones on the right.|
These days, the Canadian System of Flower (later referred to as the Canadian Series) has been replaced with the Ibexian Series (Ross et al., 1997) and the Lower Ordovician Series of the ICS.
Flower, R.H., 1957, Studies of the Actinoceratida, Memoir 2, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.
Flower, R.H., 1964, The Nautiloid Order Ellesmeroceratida (Cephalopoda), Memoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.
Li, X., and Droser, M.L., 1999, Lower and Middle Ordovician Shell Beds from the Basin and Range Province of the Western United States (California, Nevada, and Utah) PALAIOS, V. 14.
Ross, R.J., Jr, Hintze, L.F., Ethington, R.L., Miller, J.F., Taylor, M.E. & Repetski, J.E., 1997, The Ibexian, lowermost series in the North American Ordovician. United States Geological Survey Professional Paper, 1579-A.
Monday, November 8, 2021
Needless to say, I now have to re-arrange the thoughts of horizontal nautiloids having smiles and put the smile on the vertical nautiloid.
This might be just as hard as giving up the Geosynclinal Theory I was taught back in 1972.
Friday, October 8, 2021
Smith, C.P.A., Laville, T., Fara, E., Escarguel, G., Olivier, N., Vennin, E., Goudemand, N., Bylund, K.G., Jenks, J.F., Stephen, D.A., Hautmann, M., Charbonnier, S., Krumenacker, L.J., & Brayard, A., 2021, Exceptional fossil assemblages confirm the existence of complex Early Triassic ecosystems during the early Spathian. Scientific Reports 11, 19657 (2021). https://doi.org/10.1038/s41598-021-99056-8
The mass extinction characterizing the Permian/Triassic boundary (PTB; ~ 252 Ma) corresponds to a major faunal shift between the Palaeozoic and the Modern evolutionary fauna. The temporal, spatial, environmental, and ecological dynamics of the associated biotic recovery remain highly debated, partly due to the scarce, or poorly-known, Early Triassic fossil record. Recently, an exceptionally complex ecosystem dated from immediately after the Smithian/Spathian boundary (~ 3 myr after the PTB) was reported: the Paris Biota (Idaho, USA). However, the spatiotemporal representativeness of this unique assemblage remained questionable as it was hitherto only reported from a single site. Here we describe three new exceptionally diverse assemblages of the same age as the Paris Biota, and a fourth younger one. They are located in Idaho and Nevada, and are taxonomic subsets of the Paris Biota. We show that the latter covered a region-wide area and persisted at least partially throughout the Spathian. The presence of a well-established marine fauna such as the Paris Biota, as soon as the early Spathian, indicates that the post-PTB biotic recovery and the installation of complex ecosystems probably took place earlier than often assumed, at least at a regional scale.
Tuesday, June 29, 2021
Corroborating the existence of a UAZ6 (Unitary Association Zone, a means of biostratigraphic correlation). See the paper linked below. Characterized by the ammonoid Psudosageceras augustum (Brayard & Bucher, 2008) in association with Xenoceltites subevolutus Spath 1930. Found in rocks previously assigned to the Induan (lower Lower Triassic) Dinwoody Fm., now assigned to the Olenekian (upper Lower Triassic, top of Smithian) Thaynes Group.
|Pseudosageceras augustum left and Xenoceltites subevolutus right|
|Xenoceltites subevolutus left and Pseudosageceras augustum right|
In press, Pre-Proof Article in GEOBIOS
The late Smithian extinction represents a major event within the Early Triassic. This event generally corresponds to a succession of two and possibly three successively less diverse, cosmopolitan ammonoid assemblages, which when present, provide a robust biostratigraphic framework and precise correlations at different spatial scales. In the western USA basin, known occurrences of latest Smithian taxa are rare and until now, have only been documented from northeastern Nevada. Based on these restricted basinal occurrences, a regional zone representing the latest Smithian was postulated but not corroborated, as representative taxa had not yet been reported from outside Nevada. Here we document two new ammonoid assemblages from distant localities in northern Utah, overlying the late Smithian Anasibirites beds and characterized by the unambiguous co-occurrence of Xenoceltites subevolutus and Pseudosageceras augustum. The existence of a latest Smithian zone in the western USA basin is therefore validated, facilitating the identification of the Smithian/Spathian boundary and intra-basin correlation. This zone also correlates with the latest Smithian zone recognized from southern Tethyan basins. Additionally, these new data support other observed occurrences of Xenoceltites subevolutus throughout most of the late Smithian.