Crossopholis magnicaudatus
GRI #765
This relatively rare species has been found only in the Fossil Lake basin of the Green River Formation of Wyoming. It grew up to 1.5 m in length, and fed on other fish, such as Knightia, as documented by many specimens with fish skeletons in their stomach area (Grande, 2013). This species was closely related to the extant American paddlefish, Polyodon, which lives in the Mississippi River drainage basin. Polyodon has a long rostrum with thousands of electrosensors that are tuned to detect the electric currents produced by water movement. These sensors are tuned to about 1 millivolt, which is the voltage produced by the movements of their prey, plankton (Wilkens & Hofmann, 2007). Crossopholis magnicaudatus was not a filter feeder, but had electrosensory organs in its rostrum (Grande & Bemis, 1991), which were probably used to locate prey in dark, muddy waters. The characteristic stellate bones of the rostrum are not that apparent in this specimen (some barely distinguishable on the anterior side), because they are either missing or crushed on the median rostral bones in this lateral view. Like extant paddlefish, the vertebral column of C. magnicaudatus was not heavily ossified, but had a row of ossified supraneurals extending all the way back to the caudal region (as seen in this specimen). Supraneurals are not actually part of the neural arch, as are neural spines. Supraneurals form as an acipenseriform grows. In Crossopholis, they are fully formed and ossified by the time it has grown to about 385 mm in length (Grande & Bemis, 1991). The slab also contains at least three other fish specimens (Knightia eocaena: two closely associated on the ventral side of the C. magnicaudatus specimen and one, only partly preserved, on its dorsal side), and several small coprolites.
References:
Grande, L., 2013. The lost world of Fossil Lake: Snapshots from deep time. University of Chicago Press. p. 425.
Grande, L. and Bemis, W.E., 1991. Osteology and phylogenetic relationships of fossil and recent paddlefishes (Polyodontidae) with comments on the interrelationships of Acipenseriformes. Journal of Vertebrate Paleontology, 11(S1), pp. 1-121.
Wilkens, L.A. and Hofmann, M.H., 2007. The paddlefish rostrum as an electrosensory organ: a novel adaptation for plankton feeding. BioScience, 57(5), pp. 399-407. doi:10.1641/B570505.