ICHTHYOSAUR COPROLITE WITH NAUTILOID: NEW DATA ON THE DIET OF SHONISAURUS

In the first direct confirmation that shonisaurs ate cephalopods, a large Shonisaurus coprolite (Heteropolacopros ichnosp.) collected near Berlin‑Ichthyosaur State Park (Late Triassic, Shaly Limestone Member, Luning Formation, Nevada, USA) contains the remains of a coiled cephalopod. The chambered shell is 27 mm in greatest dimension (in a coprolite fragment 6 cm long) and represents an approximate 150° arc sector of the shell. Although the shell is partly crushed, the position of the siphuncle and the presence of smoothly curved septa suggest that the ingested cephalopod is a nautiloid (cf. Cenoceras). The dietary preferences of the giant ichthyosaur Shonisaurus popularis can now be specified with greater precision. C. Camp (1980, Palaont. Abt. A. 170:139-200) inferred that Shonisaurus fed on "(?)fishes, and cephalopods." J. Mawby (1980, Nev. St. Jour., Aug. 10) argued that "small fish and squid‑like cephalopods" constituted the shonisaur diet. McMenamin and Schulte McMenamin (2011, Geol. Soc. Am. Abst. Prog. 43:310) suggested that shonisaurs ate "smaller cephalopod prey" by analogy with cetaceans. In an analysis of the contents of a second large coprolite, McMenamin and Hussey (2015, Geol. Soc. Am. Abst. Prog. 47:827) and McMenamin (2016, Dynamic Paleontology, Springer, Cham, Switzerland) identified fish skull plates preserved inside the coprolite as belonging to a whiteiidid coelacanth. This interpretation is in accord with Camp's (1980) account of J. G. Marr's discovery in Camp's Quarry No. 6 of "countless minute brachiopods mixed with an extraordinary lot of small bones . . . possibly [from the] stomach contents of ichthyosaurs." We interpret Marr's "minute brachiopods" as coelacanth scales, whose circuli running across the anterior field of each scale can mimic brachiopod growth lines. It is now possible to say with some confidence that Shonisaurus ate both coelacanth fishes and coiled cephalopods, including nautiloids. As both coelacanths and nautiloids are associated with deeper marine settings, these findings support the concept (as per the original Triassic Kraken hypothesis) that shonisaurs dove deeply in blue ocean waters to pursue their prey. Such feeding behavior would take them into the realm of potentially larger and more dangerous cephalopods.