Tuesday, June 29, 2021

Latest Smithian Ammonoids in northern Utah

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

Available online 18 June 2021


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.

Sunday, May 23, 2021

Dinwoody Ammonoids & Nautiloids



Abstract—We document a relatively small but very important late Griesbachian ammonoid and nautiloid assemblage from the Dinwoody Formation at Crittenden Springs, Elko County, Nevada. This discovery represents the first significant report of late Griesbachian ammonoids in the low-paleolatitudes of eastern Panthalassa, and it also signifies the first report of Wordieoceras wordiei and two co-occurring taxa outside of the Boreal Realm. This similarity in ammonoid faunas, irrespective of paleolatitude, provides support for the concept of weak latitudinal diversity gradients following the end-Permian extinction. The finding is even more noteworthy given the Dinwoody Formation’s reputation for poor fossil preservation and a near complete absence of documented and identifiable ammonoid and nautiloid occurrences. Consisting of four taxa of which two are newly described, the ammonoid fauna includes Wordieoceras wordiei (Spath), Kyoktites cf. K. hebeiseni Ware and Bucher, Wordieoceras mullenae n. sp. and a new taxon belonging to the Mullericeratidae family, Ophimullericeras paullae n. gen., n. sp. The nautiloids are attributed to a newly described species, i.e., Xiaohenautilus mulleni n. sp., a genus heretofore unknown in eastern Panthalassa but commonly reported from the late Griesbachian of South Primorye and the late Griesbachian/early Dienerian of South China.

New Mexico Museum of Natural History & Science Bulletin 86

Monday, July 6, 2020

Some Prionitids Lumped

With a few very narrowly defined species, The Prionitidae is a family of very closely related ammonoids.  However most collections, especially from the western US, always contained "Prionitid" or "Prionitid indet.", Prionitids with bullae or ribs, maybe nodes or tubercles, that just didn't fit with any of the other described genera or species.  With new collections from the Smithian (Early Triassic) Sinbad Formation around the Utah/Arizona border, we were able to synonymize all these with ammonoids described in 1929 by Asa Mathews.  By priority, Gurleyites smithi Mathews 1929 replaces Arctoprionites resseri (Mathews 1929).  With a very wide range of intraspecific variation, the new taxon will fit just about anything that doesn't fit in one of the other defined taxa.

See our new paper here.

New middle and late Smithian ammonoid faunas from the Utah/Arizona border: New evidence for calibrating Early Triassic transgressive-regressive trends and paleobiogeographical signals in the western USA basin



Tuesday, April 16, 2019

Sexual Dimorphism in Ammonites

Sexual dimorphism as represented by the ammonite Yezoites frontierense (Cobban 1952)

Macroconch (female) 38mm on the left, microconch (male) 18mm on the right
Previously referred to Scaphites frontierense, presumably, the female was much larger to carry eggs.  Many extant cephalopods (the Blanket Octopus for example) show the same relative (?) dimorphism with the males much smaller than the females.

Thursday, March 28, 2019

Lower Ferron Sandstone and Juana Lopez Members of the Mancos Shale around the north side of the San Rafael Swell

In an attempt to show that the Juana Lopez Member of the Mancos Shale really does expose itself on the west side of the San Rafael Swell, I introduce some photos to help.

Lower Ferron east of Huntington (visible upper right).
Lower Ferron east of Huntington, these beds contain Prionocyclus hyatti near MoundsThey also form a small cliff under the Ferron Sandstone south of here from Ferron south to Last Chance Creek.

Juana Lopez Member east of Huntington (visible upper left)  Prionocyclus macombi concretions in middle of cliff  (right) and P. wyomingensis concretions at top of cuesta across wash.
Juana Lopez Member east of Huntington.  The same thin platy siltstone beds that appear on the east side of the swell appear here with the same platy rubble littering the slopes.  The concretion beds are also the same.

Two beds of the Lower Ferron Sandstone with large round concretions just north of Mounds.  Lower bed contains the ammonite Prionocyclus hyatti along with many other mollusks.
Around the north side the Juana Lopez is still there above the Lower Ferron.

Juana Lopez Member just west of Cat Canyon where Highway 6 and 191 cut through the Farnham Dome just east of Wellington.  Prionocyclus wyomingensis beds at top of cuesta in middle distance (Light cliff), P. macombi concretions in middle of slope.  Farnham Unit of Lower Ferron Sandstone in foreground.  Some minor faults also in foreground, one duplicating the orange bed in center.
The same thin siltstone beds, the same platy rubble.

Lower Ferron Sandstone just below dark Juana Lopez Member, 1 mile southwest of Grassy Siding.  Coon Spring Sandstone Bed at base.  Brown concretion to left center is what's left of the P. hyatti beds at Mounds.
On the east side of the Swell, the beds above the Lower Ferron have been refered to the Juana Lopez.  The only real difference I can see is a couple (2) of thin (2cm max) calcarenite beds.  South and east of Green River the Juana Lopez has more and thicker calcarenite beds.

Juana Lopez Member just west of Grassy Siding, Mt. Elliot in right distance.  Prionocyclus macombi concretions form edge of middle cuesta, P. wyomingensis concretions form orange lip of upper cuesta.

 Here on the east side are the same thin bedded siltstone beds, the same platy rubble, and the same concretion beds with the same fossils.  The siltstone beds on the west side maybe get a bit thicker, up to 3 or 4 cm, even more reason not to refer them to the Blue Gate Shale.


Dane, C. H., Cobban, W. A., and Kauffman, E. G., 1966, Stratigraphy and Regional Relationships of a Reference Section for the Juana Lopez Member, Mancos Shale, in the San Juan Basin, New Mexico, U.S.G.S. Bulletin 1224-H

Edwards, C., D. Hodgson, S. Flint, and J. Howell, 2005, Contrasting styles of shelf sediment transport and deposit in a ramp margin setting related to sea-level change and basin floor topography, Turonian (Cretaceous) Western Interior of central Utah, USA: Sedimentary Geology, v. 179

Molenaar, C. M., and Cobban, W. A., 1991, Middle Cretaceous Stratigraphy on the South and East Sides of the Uinta Basin, Northeastern Utah and Northwestern Colorado,   U.S.G.S. Bulletin 1787-P

Wednesday, February 14, 2018

Giant Cephalopod Midden or Mosasaur Feces

Back in 2011, and again in 2013,  Mark and Dianna McMenamin postulated a giant cephalopod created a midden with the bones of Icthyosaurs.  The internet came alive with much fanfare,  comments, news, etc. probably because of the size the cephalopod would have had to been to form such a midden.

Well, back in 1954 two well-known cephalopod paleontologists, John B. Reeside and William A. Cobban (one prominent then, and one then becoming prominent), postulated  that concretions in the Mowry and Aspen Shales were possibly "fecal matter of some large carnivore--reptile, fish or cephalopod", and "The diet of the carnivore would have had to be almost entirely ammonites and fish, and the carnivore would perhaps, like the living octopus, have had to frequent a sort of lair, to which it repaired and in which the mucous-bound fecal matter could accumulate" (Reeside & Cobban 1954, 1960).

Some of these concretions were over 2 meters in diameter, so you have a mosasaur like that in the movie Jurassic World, a very large fish, or another Kraken.  Though nowhere near the size of the Triassic Kraken.

I wonder, had there been the internet in 1954, would there have been the same or similar response?

Plate 28 from Reeside & Cobban 1960
The fossils were found in fossiliferous concretions containing thousands of ammonites, along with a few bivalves and bones of a few fish and a few pterodactyls in some.  These fossiliferous concretions were rare among the many non-fossiliferous concretions.  The authors reported that some concretions contained well over 4200 ammonites, from 10mm to 400mm in diameter, and these probably represented less than half the original content.  Most fossils were preserved in 3D with most of the living chambers crushed or missing, and some had damage to the phragmocone.

It was these fossils that Reeside and Cobban used in their 1960 paper to show the advantages of a population approach in taxonomy to a strictly typological one, especially for ammonoids with a large amount of intraspecific variation.  A significant moment for the lumpers of the world.


McMemamin, M. A. S., and McMenamin, D. L. S., 2011.  Triassic Kraken: The Berlin Icthyosaur Death Assemblage Interpreted as a Giant Cephalopod Midden: Geological Society of America Abstracts with Programs, Vol. 43, No. 5, p. 310

McMemamin, M. A. S., and McMenamin, D. L. S., 2013.  The Kraken's Back: New Evidence Regarding Possible Cephalopod Arrangement of Icthyosaur Skeletons: Geological Society of America Abstracts with Programs, Vol. 45, No. 7, p. 900

Reeside, J. B., Jr., and Cobban, W. A., 1954. Ammonite accumulations in the Cretaceous Mowry and Aspen shales: Science, v. 119, p. 255.

Reeside, J. B., Jr., and Cobban, W. A., 1960. Studies of the Mowry Shale (Cretaceous) and Contemporary formations in the United States and Canada: U.S. Geological Survey Professional Paper 355, 126 p.