Paper No. 407-12
Presentation Time: 4:20 PM

THE KRAKEN'S BACK: NEW EVIDENCE REGARDING POSSIBLE CEPHALOPOD ARRANGEMENT OF ICHTHYOSAUR SKELETONS


MCMENAMIN, Mark A.S. and SCHULTE MCMENAMIN, Dianna L., Geology and Geography, Mount Holyoke College, South Hadley, MA 01075, mmcmenam@mtholyoke.edu
In 2011, we hypothesized that extremely large Triassic cephalopods may be responsible for certain anomalous aspects of an unusual assemblage of giant ichthyosaur skeletons in the Luning Formation of Nevada. The hypothesis has been criticized by researchers who do not accept the ichnological evidence suggesting that the skeletons were deliberately arranged rather than being deposited by currents.

Hydrodynamic considerations regarding the probability of displacement (PD) of ichthyosaur vertebral centra arrays (n=12) show that three different biserial arrangements have PDs of 17%, 89% and 100% respectively by currents strong enough to displace a single centra. The critical Specimen U array at Berlin‑Ichthyosaur State Park has PD=~100, indicating that it is highly unlikely that the biserial pattern was imparted by submarine currents. The unwinnowed wackestone matrix confirms that competent water velocities did not frequently occur in this deep-water depositional environment. The Luning Formation also hosts Protopaleodictyon ichnosp. and supergiant amphipods.

We recently obtained photographs of a retired exhibit formerly on display at the University of Nevada, Las Vegas, Museum of Natural History. The display reconstructed a well‑preserved Shonisaurus skeleton as it was found in the field. The exhibit is well documented by photographs from a variety of vantage points. The skeleton appears to have been partly disassembled during the Triassic, and a biserial array of centra very similar to the Specimen U array occurs adjacent to the nearly complete skeleton. The UNLV array has a PD=~100, again indicating that the biserial pattern was not the result of current assembly. Finally, at least three of these centra show what may be triangular bite marks removed from their margins.

Handouts
  • KrakensBackGSA1.pptx (22.0 MB)
  • TriassicKraken21stCtScan.pdf (338.6 kB)